SYMPOSIUM ON SURFACE SCIENCE 2011 Baqueira Beret, Lleida ...

SYMPOSIUM ON SURFACE SCIENCE 2011 

Baqueira Beret, Lleida, Spain 

March 6-12, 2011 

CONTRIBUTIONS 

EDITORS 

Andrés Arnau and Pedro Miguel Echenique 

UPV/EHU, DIPC and CFM-CSIC

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CONTRIBUTIONS 

EDITORS 


UPV/EHU, DIPC and CFM-CSIC

This symposium is organized by: 


Donostia International Physics Centre (DIPC) 

Departamento de Física de Materiales (UPV/EHU) 

Centro de Física de Materiales (CFM-CSIC) 

Paseo Manuel de Lardizabal 4 

20018 Donostia-San Sebastian 

Spain 

International Scientific Committee Organizing Committee 

A. Arnau, Donostia, Spain Nora Gonzalez Lakunza 

F. Aumayr, Vienna, A Marisa Faraggi 

E. Bauer, Tempe, USA Pepa Cabrera Sanfelix 

P. M. Echenique, Donostia, Spain Aran García Lekue 

R. Fasel, Dübendorf, CH Thomas Frederiksen, 

A. Ichimiya, Nagoya, J Andrés Arnau Pino 

T. Koshikawa, Osaka, J Pedro Miguel Echenique Landiríbar 

D. Menzel, Munich/Berlin, D 

K. Morgenstern, Hannover, D 

P. Müller, Marseille, F 

F. Netzer, Graz, A 

A. Saul, Marseille, F 

W. D. Schneider, Lausanne, CH 

G. Thornton, London, UK 

I. Tsong, Tempe, USA 

P. Varga, Vienna, A

PREFACE 

We would like to welcome all participants and accompanying persons to the 24 th 

Symposium on Surface Science (3S). Since the foundation of 3S in 1983 as a winter 

school by members of the Institut für Allgemeine Physik (renamed recently to Institute 

of Applied Physics) of the Vienna University of Technology (TU Wien), its format has 

been kept following the spirit of the Gordon Conferences. This means that there is 

ample time for discussions and joint outdoor activities for the participants. The 

attendance of the symposium was kept below 100 participants so that active 

communication between all members could be guaranteed. The conference seeks to 

promote the growth of scientific knowledge and its effective exchange among scientists 

in the field of surface physics and related areas, including applied topics. At the 

beginning, 3S took place every second year exclusively in Austria, but from 1990 on 

every year switching between France and Austria. During the years 1998 - 2001 3S 

became a real global conference, with venues in the US and Canada, Bulgaria and Japan 

before going back to Austria and France again. In 2009, 3S was hold for the first time in 

Switzerland and last came back to Austria. This year 2011 it is the first time that 3S 

takes place in Spain. 

We hope that all participants enjoy a lively and successful meeting in this beautiful 

mountain region of the Pyrenees called Val d’Aran. 

Andrés Arnau and Pedro Echenique 

Dates and locations of 3S conferences: 

1983 (31.01.-04.02.) Obertraun A 

1985 (27.01.-02.02.) Obertraun A 

1988 (22.05.-28.05.) Kaprun A 

1990 (11.03.-17.03.) La Plagne F 

1991 (10.02.-16.02.) Obertraun A 

1992 (15.03.-21.03.) La Plagne F 

1993 (09.05.-15.05.) Kaprun A 

1994 (06.03.-12.03.) Les Arcs F 

1995 (23.04.-29.04.) Kaprun A 

1997 (26.01.-31.01.) Aussois F 

1998 (29.03.-04.04.) Park City US 

1999 (21.02.-27.02) Pamporova BUL 

1 

2000 (15.02.-18.02.) Kananaskis Village CAN 

2001 (07.01.-13.01.) Furano J 

2002 (03.03.-09.03.) St.Christoph/Arlberg A 

2003 (30.03.-05.04.) La Plagne F 

2004 (29.02.-06.03.) St.Christoph/Arlberg A 

2005 (13.03.-19.03.) Les Arcs 1800 F 

2006 (05.03.-11.03.) St. Christoph/Arlberg A 

2007 (11.03.-17.03.) Les Arcs 2000 F 


2009 (08.03.-14.03.) St. Moritz CH 


2011 (06.03.-12.03.) Baqueira Beret/Lleida S


Sunday, 6th March 2011 



Time Schedule 

16:30-20:30 Registration at Hotel Tuc Blanc 

17:30 Shuttle Bus departure from Toulouse airport 

20:30- Dinner 

Monday, 7th March 2011 

07:45-08:30 Breakfast 

08:00-08:50 Registration 

08:50-09:05 Opening 

SESSION-I 

Chairperson: Wolf-Dieter Schneider 

09:05-09:25 Harald Brune 

Giant Lattice Distortions of Graphene on Ru(0001) 

09:25-09:45 Michael Altman 

Moiré Twist in Graphene on Ru(0001) 

3

SESSION-II 

Chairperson: Pedro Miguel Echenique 

16:30-16:50 Dietrich Menzel 

Ultrafast charge transfer at graphene surfaces 

16:50-17:10 Norbert Müller 

Interplay between Electronic correlations and Coherent Structural 

Dynamics during the Monoclinic Insulator-to-Rutile Metal Phase 

Transition in VO2 

17:10-17:30 Ulrich Höfer 

Time-resolved two-photon photoemission of metal/organic interface 

states 

17:30-17:50 Wolf Widdra 

Electronic properties of NiO thin films: A combined STM, STS, and 

2PPE study 

17:50-18:20 COFFEE BREAK 

SESSION-III 

Chairperson: U. Diebold 

18:20-18:40 Claudia Ambrosch-Draxl 

A Epitaxy of Organic Nano-Fibers on Sheet Silicates: A Growth Model 

Based on Experiments and Simulations 

18:40-19:00 Armin Gölzhäuser 

Janus nanomembranes: Surfaces without bulk, functionalized on both 

sides 

19:00-19:20 Manfred Buck 

Electrochemical Generation of Low Dimensional Metal Structures on 

Top of Self-Assembled Monolayers 

19:20-19:40 Christof Wöll 

Charge Transport Through and Within Self-Assembled Monolayers: New 

Insights from Nanofabricated Model Devices 

20:20 DINNER 

Tuesday, 8th March 2011 


SESSION-IV 

Chairperson: E. V. Chulkov 

08:30-08:50 Matthias Scheffler 

Gold clusters at finite temperature in vacuo and in a CO plus O2 

atmosphere: ab initio studies towards gold catalysis 

08:50-09:10 Geoff Thornton 

Defects on room temperature ultra-thin film CeO2 with STM 

4

SESSION-V 

Chairperson: E. Lundgren 

16:30-16:50 Peter Varga 

High Island Densities in Pulsed Laser Deposition: Causes and 

Implications 

16:50-17:10 Yuriy Yanson 

Copper electrodeposition on fast time scale: from underpotential 

deposition to bulk growth 

17:10-17:30 Phil Willmott 

Buckling under tension – LaAlO3 on SrTiO3 

17:30-17:50 Fabien Cheynis 

Dewetting dynamics of crystalline thin films 


SESSION-VI 

Chairperson: M. Scheffler 

18:20-18:40 D. Stradi, “The role of dispersion forces in the structure of graphene 

monolayers over the Ru(0001) surface” 

18:40-19:00 Gustavo Ceballos 

Growth of graphene nanoislands on a Ni(111) surface 

19:00-19:20 Goucai Dong 

Kinetics of graphene growth on Rh(111) 

19:20-19:40 Gilberto Teobaldi 

Structure and properties of surface and subsurface defects in graphite 

accounting for van der Waals and spin polarization effects 

20:20 DINNER 

Wednesday, 9th March 2011 


SESSION-VII 

Chairperson: G. Thornton 

08:30-08:50 Ulrike Diebold 

STM investigations of pure and Sn-doped In2O3 surfaces 

08:50-09:10 Luca Gragnaniello 

Fabrication of a NiOx nanodot superlattice 

5

SESSION-VIII 

Chairperson: F. Himpsel 

16:30-16:50 Aitor Mugarza 

Tuning the magnetic moment of individual molecules at the metallic 

interface 

16:50-17:10 Ulrich Heinzmann 

Spin-resolved Photoelectron Spectroscopy of Mn6Cr Single-Molecule- 

Magnets and of Manganese Compounds as Reference Layers 

17:10-17:30 Daniel Sánchez Portal 

Magnetism of Covalently Functionalized Graphene 


SESSION-IX (poster session) 

Chairperson: H. Brune 

18:00-18:30 Poster presentations (14 posters) 

M. Faraggi, “Characterization of an oxalic acid layer on Cu(111)” 

Q. Liu, “Study of NO reduction by H2 on a Pt(110) model catalyst in a 

High-Pressure STM” 

V. M. Silkin, “Ultrafast screening of a point charge at a metal surface” 

A. García-Lekue, “Plane-wave based Electron Tunneling trough Au 

Nanojunctions” 

S. J. Leake, “Structural studies of the metal-insulator transition in LaNiO3 

thin films” 

N. Gonzalez Lakunza, “Structure and electronic properties of TCNQ-F4 

deposited on clean Au(111)” 

V. Navarro, “ Cobalt catalyst in action followed at high pressures with 

STM and SXRD during hydrocarbon synthesis” 

S. Blomberg, “The high pressure oxidation and reduction by CO of Rh – 

from single crystal to nanoparticles” 

P. Cabrera-Sanfelix, “ Water Adsorption on Clean and Oxygen 

Decorated Metal Substrates” 

Johannes V. Barth, “Assembly and manipulation of supramolecular 

dynamers and rotatable sandwich complexes on a surface” 

6

Ulrich Heinzmann, “Preparation of Monolayers of Mn6Cr Single 

Molecule- Magnets on different Substrates and characterization by 

means of nc-AFM” 

T. Passanante, “Thermal decomposition of oxidised Silicon-on-insulator 

thin film” 

M. E. Messing, “Generation of Pd model catalyst nanoparticles by spark 

discharge” 

Nabil Berkaïne, “Theoretical study of O2 adsorption on Al4Cu9(110) 

surface” 

18:30-20:30 Poster Session 

20:30 DINNER 

Thursday, 10th March 2011 


SESSION-X 

Chairperson: E. Taglauer 

08:30-08:50 F. Aumayr 

Nano-craters formed by impact of individual highly charged ions on 

PMMA surfaces 

08:50-09:10 P. Bauer 

Information depth in Low Energy Ion Scattering 

SESSION-XI 

Chairperson: D. Menzel 

16:30-16:50 F. J. Himpsel 

Magnetism at Stepped Silicon Surfaces 

16:50-17:10 J. I. Cerdá 

CoPc adsorption on Cu(111): Origin of the C4 to C2 symmetry reduction 

17:10-17:30 E. V. Chulkov 

Electronic structure of topological insulators 

17:30-17:50 J. E. Ortega 

Tailoring interactions in supramolecular networks by fluorination 


7

SESSION-XII 

Chairperson: J. V. Barth 

18:20-18:40 W.D. Schneider 

Supramolecular self-assembly driven by electrostatic repulsion: The 1D 

aggregation of Rubrene pentagons on Au(111) 

18:40-19:00 K. Morgenstern 

Preferred Pathway for a Molecular Photo Switch inContact with a 

Surface 

19:00-19:20 H. Daimon 

Wide acceptance angle photoelectron spectrometer for stereophotograph 

of atomic arrangement 

19:20-19:40 E. Lundgren 

Probing a surface reconstruction with anomalous X-ray diffraction 

19:40-20:00 Giant Slalom Ceremony Award 

20:30 CONFERENCE BANQUET 

Friday, 11th March 2011 


SESSION-XIII 

Chairperson: K. Morgenstern 

08:30-08:50 T. Stempel 

Bridging the Pressure Gap - Developments and Challenges for Ambient 

Pressure Photoelectron Spectroscopy 

08:50-09:10 M. Maier 

High Precision local electrical Probing at T< 5K: Potential and 

Limitations for the Analysis of Nanocontacts and Nanointerconnects 

SESSION-XIV 

Chairperson: F. Aumayr 

16:30-16:50 T. Koshikawa 

Dynamic magnetic domain observation with novel highly spin polarized 

and high brightness LEEM 

16:50-17:10 J. Gustafson 

Methane oxidation over Pd and Pt: linking surface science and industrial 

catalysis 

17:10-17:30 S. Yu. Krylov 

Atomic scale friction: Physically nontrivial problems 

17:30-17:50 F. Salvat-Pujol 

Contribution of surface excitations to secondary-electron emission 

observed by secondary-electron-energy-loss coincidence spectroscopy 

8


SESSION-XV 

Chairperson: P. Muller 

18:20-18:40 M. Dürr, “ Fast and with atomic precision – real-space investigation of 

hydrogen diffusion on Si(001) using nanosecond laser heating and STM” 

18:40-19:00 F. Tabak 

Fast scanning with piezo/counter-piezo elements and MEMS scanners: a 

comparison 

19:00-19:20 F. J. Giessibl 

Sensing Atomic Forces 

19:20-19:40 T. Frederiksen 

Atomic-scale engineering of electrodes for single-molecule contacts 

19:40-19:50 Closing 

20:20 DINNER 

Saturday, 12th March 2011 

07:30-08:00 Shuttle bus departure to Toulouse Airport 


9

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CONTENT 

Giant Lattice Distortions of Graphene on Ru(0001) 22 

Harald Brune 

Moiré Twist in Graphene on Ru(0001) 23 

K.L. Man, T. Greber, and M.S. Altman 

Ultrafast charge transfer at graphene surfaces 25 

Silvano Lizzit, Rosanna Larciprete, Paolo Lacovig, Krassimir L. Kostov, and Dietrich Menzel 

Interplay between Electronic correlations and Coherent Structural Dynamics 27 

during the Monoclinic Insulator-to-Rutile Metal Phase Transition in VO2 

H. Dachraoui, N. Müller, G. Obermeier, C. Oberer, S. Horn, V. Eyert, and U. Heinzmann 

Time-resolved two-photon photoemission of metal/organic interface states 29 

U. Höfer, M. Marks, C. H. Schwalb, B. Schmidt, S. Sachs, A. Schöll, F. Reinert 

Electronic properties of NiO thin films: A combined STM, STS, and 2PPE study 31 

Mario Kiel, Stephan Großer, Anke Höfer, Klaus Duncker, and Wolf Widdra 

A Epitaxy of Organic Nano-Fibers on Sheet Silicates: A Growth Model Based on 

Experiments and Simulations 

33 

C. Ambrosch-Draxl, 

C. Simbrunner, G. Hernandez-Sosa, D. Nabok, M. Oehzelt, T. Djuric, 

R. Resel, L. Romaner, P. Puschnig, I. Salzmann, G. Schwabegger, I. Watzinger, and H. Sitter 

Janus nanomembranes: Surfaces without bulk, functionalized on both sides 35 

Zhikun Zheng, Christoph T. Nottbohm, Andrey Turchanin, Heiko Muzik, André Beyer, 

Mike Heilemann, Markus Sauer and Armin Gölzhäuser 

Electrochemical Generation of Low Dimensional Metal Structures on Top of 37 

Self-Assembled Monolayers 

Christophe Silien and Manfred Buck 

Charge Transport Through and Within Self-Assembled Monolayers: New 39 

Insights from Nanofabricated Model Devices 

Christof Wöll 

Gold clusters at finite temperature in vacuo and in a CO plus O2 atmosphere: ab initio 42 

studies towards gold catalysis 

Elizabeth C. Beret, Luca M. Ghiringhelli, Merel M. van Wijk, and Matthias Scheffler 

Defects on room temperature ultra-thin film CeO2 with STM 43 

D.C. Grinter, R. Ithnin, C.L. Pang, G. Thornton 

High Island Densities in Pulsed Laser Deposition: Causes and Implications 45 

Peter Varga 

Copper electrodeposition on fast time scale: from underpotential deposition to bulk growth 46 

Y. Yanson, J. W. M. Frenken, M. J. Rost

Buckling under tension – LaAlO3 on SrTiO3 48 

Phil Willmott, Stephan Pauli, Steven Leake, Bernard Delley, Christof Schneider, 

Jochen Mannhart, and Stefan Paetel 

Dewetting dynamics of crystalline thin films 50 

E. Bussmann, F.Leroy, F.Cheynis, O.Pierre-Louis and P.Müller 

The role of dispersion forces in the structure of grapheme monolayers over the 51 

Ru(0001) surface 

D. Stradi, S. Barja, C. Díaz, M. Garnica, B. Borca, J. J. Hinarejos, D. Sánchez-Portal, M. Alcamí, 

A. Arnau, A. L. Vázquez de Parga, R. Miranda, and F. Martín 

Growth of graphene nanoislands on a Ni(111) surface 53 

G. Ceballos, M. Ollé. P. Gambardella 

G. Dong, D.W. van Baarle, M.J. Rost and J.W.M. Frenken 

Kinetics of graphene growth on Rh(111) 55 

Structure and properties of surface and subsurface defects in graphite 57 

accounting for van der Waals and spin polarization effects 

G. Teobaldi, K. Tanimura, and A. L. Shluger 

Ulrike Diebold, Daniel Hagleither and Michael Schmid 

STM investigations of pure and Sn-doped In2O3 surfaces 60 

Fabrication of a NiOx nanodot superlattice 61 

Svetlozar Surnev, Luca Gragnaniello, 

Francesco Allegretti, Teng Ma, A. Fortunelli, 

G. Barcaro and Falko P. Netzer 

Tuning the magnetic moment of individual molecules at the metallic interface 62 

A.Mugarza, C.Krull, Roberto Robles and P.Gambardella 

Spin-resolved Photoelectron Spectroscopy of Mn6Cr Single-Molecule-Magnets 64 

and of Manganese Compounds as Reference Layers 

Andreas Helmstedt, Aaron Gryzia, Niklas Dohmeier, Norbert Müller, Armin Brechling, 

Marc Sacher, Ulrich Heinzmann, Veronika Hoeke1, Thorsten Glaser, Mikhail Fonin, 

Ulrich Rüdiger, and Manfred Neumann 

Magnetism of Covalently Functionalized Graphene 65 

Elton J. G. Santos, Andrés Ayuela and Daniel Sánchez-Portal 

Characterization of an oxalic acid layer on Cu(111) 67 

M. N. Faraggi, M. Trelka, C. Isvoranu, C. Martí-Gastaldo, E. Coronado, J. Schnadt, 

J. M. Gallego, R. Otero, R. Miranda and A. Arnau 

Study of NO reduction by H2 on a Pt(110) model catalyst in a High-Pressure STM 69 

Q. Liu, C.T. Herbschleb, J.W. Bakker, V. Navarro, M.E. Cañas Ventura, 

P.C. van der Tuijn, A. Ofitserov, G.J.C. van Baarle, J.W.M. Frenken 

Ultrafast screening of a point charge at a metal surface 70 

V. M. Silkin, V. Despoja, E. V. Chulkov and P. M. Echenique

A. Garcia-Lekue and L.W. Wang 

Plane-wave based Electron Tunneling trough Au Nanojunctions 71 

Structural studies of the metal-insulator transition in LaNiO3 thin films 73 

S. J. Leake, S. A. Pauli, M. Schmitt, M. Garcia-Fernandez, P. Aebi, 

R. Scherwitzl, P. Zubko, J-M. Triscone and P. R. Willmott 

Structure and electronic properties of TCNQ-F4 deposited on clean Au(111) 74 

N. Gonzalez-Lakunza, N. Jiang, A. Langner, S. Stepanow, H-J. Gao, K. Kern, and A. Arnau 

Cobalt catalyst in action followed at high pressures with STM and SXRD 76 

during hydrocarbon synthesis 

V. Navarro, S.B. Roobol, R. van Rijn, Q. Liu, O. Balmes, D. Wermeille, A. Resta, 

R.Felici and J.W.M. Frenken 

The high pressure oxidation and reduction by CO of Rh – from single 

crystal to nanoparticles 

77 

S. Blomberg, 

J. Gustafson, R. Westerström, J. N. Andersen, M. Messing, K. Deppert, 

N. Martin, M. E. Grass, Z. Liu, H. Bluhm and E. Lundgren 

Water Adsorption on Clean and Oxygen Decorated Metal Substrates 79 

P. Cabrera-Sanfelix, 

S. Maier, I. Stass, Byoung-Young Choi, Yu Shi, A. Arnau, 

M. Salmeron and D. Sánchez-Portal 

Assembly and manipulation of supramolecular dynamers and rotatable 81 

sandwich complexes on a surface 

Dirk Kühne, Florian Klappenberger, Wolfgang Krenner, Svetlana Klyatskaya, 

Mario Ruben, David Écija, Willi Auwärter, Saranyan Vijayaraghavan, 

Knud Seufert, Felix Bischoff, Kentaro Tashiro and Johannes V. Barth 

Preparation of Monolayers of Mn6Cr Single Molecule- Magnets on different 

Substrates and characterization by means of nc-AFM 

83 

Aaron Gryzia, Armin Brechling, Hans Predasch, Ulrich Heinzmann and Thorsten Glaser 

Thermal decomposition of oxidised Silicon-on-insulator thin film 

T.Passanante, 

F.Leroy, E. Bussmann, F.Cheynis and P.Müller 

84 

Generation of Pd model catalyst nanoparticles by spark discharge 

M. E. Messing, 

R. Westerström, B. O. Meuller, S. Blomberg, J. Gustafson, 

85 

J. N. Andersen1, R. van Rijn, O. Balmes, H. Bluhm, E. Lundgren and K. Deppert 

Theoretical study of O2 adsorption on Al4Cu9(110) surface 87 

N. Berkaïne, C. Lacaze-Dufaure and J. Morillo 

Nano-craters formed by impact of individual highly charged ions on PMMA surfaces 90 

R. Ritter, G. Kowarik, R. Ginzel, R. Heller, R. M. Papaléo, W. Rupp, 

J. R. Crespo López-Urrutia, J. Ullrich, S. Facsko and F. Aumayr 

Information depth in Low Energy Ion Scattering 92 

D. Primetzhofer, M. Spitz, S. Rund, D. Goebl, D. Roth, R.C. Monreal, D. Valdés, 

E. Taglauer, and P. Bauer

Magnetism at Stepped Silicon Surfaces 93 

F. J. Himpsel, S. C. Erwin, P. C. Snijders, N. Guisinger, and I. Barke 

CoPc adsorption on Cu(111): Origin of the C4 to C2 symmetry reduction 94 

J. I. Cerdá, R. Cuadrado, Y.Wang, X. Ge and R. Berndt 

Electronic structure of topological insulators 96 

E.V. Chulkov, T.V.Menshikova, M. Vergniory, S.V Eremeev, G.Bihlmayer, 

Yu.M. Koroteev, J.Henk and A.Ernst 

Tailoring interactions in supramolecular networks by fluorination 98 

E. Goiri, A. El-Sayed, D. G. de Oteyza, M. Matena, C. Rogero, J. M. Lastra, 

D. Mowbray, A. Rubio, Y. Wakayama and J. E. Ortega 

Supramolecular self-assembly driven by electrostatic repulsion: The 1D aggregation 99 

of Rubrene pentagons on Au(111) 

Giulia Tomba, Massimiliano Stengel, Wolf-Dieter Schneider, Alfonso Baldereschi, 

and Alessandro DeVita 

Preferred Pathway for a Molecular Photo Switch in Contact with a Surface 101 

Jörg Henz, Maciej Bazarnik, Ryszard Czajka, Andreas Schaate, Boris Ufer, 

Peter Behrens and Karina Morgenstern 

Wide acceptance angle photoelectron spectrometer for stereophotograph of 103 

atomic arrangement 

Hiroyuki Matsuda, Laszlo Toth, Kentaro Goto, Fumihiko Matsui, Tomohiro Matsushita, 

Mie Hashimoto, Chikako Sakai, Hideo Nojiri, and Hiroshi Daimon 

Probing a surface reconstruction with anomalous X-ray diffraction 105 

E. Lundgren, R. Westerström, J.N. Andersen, X. Torrelles, C. Quiros, S. Ferrer, 

I. Popa, D. Wermeille and R. Felici 

Bridging the Pressure Gap - Developments and Challenges for Ambient Pressure 108 

Photoelectron Spectroscopy 

T. Stempel 

M. Maier, 

B. Günther, J. Koeble, D. Jie, Ch. Joachim, F. Matthes, C.M. Schneider, A. Feltz 

High Precision local electrical Probing at T< 5K: Potential and Limitations 

for the Analysis of Nanocontacts and Nanointerconnects 

110 

Dynamic magnetic domain observation with novel highly spin polarized and high 111 

brightness LEEM 

T.Koshikawa,M.Suzuki, T.Yasue and E.Bauer 

Methane oxidation over Pd and Pt: linking surface science and industrial catalysis 

J. Gustafson, 

A. Resta, S. Blomberg, N. Martin, R. van Rijn, O. Balmes, D. Wermeille, 

M. E. Messing, K. Deppert, P.A. Carlsson, A. Hellman, H. Grönbeck, M. Skoglund, 

X. Torrelles, J. N. Andersen and E. Lundgren 

112 

Atomic scale friction: Physically nontrivial problems 114 

S. Yu. Krylov and J. W. M. Frenken

Contribution of surface excitations to secondary-electron emission observed 116 

by secondary-electron-energy-loss coincidence spectroscopy 

F. Salvat-Pujol, W.S.M. Werner, W. Smekal, and R. Khalid 

Fast and with atomic precision – real-space investigation of hydrogen diffusion on 

Si(001) using nanosecond laser heating and STM 

M. Dürr, 

C. H. Schwalb, M. Lawrenz, and U. Höfer 

118 

Fast scanning with piezo/counter-piezo elements and MEMS scanners: a comparison 120 

F.C. Tabak, P.C. van der Tuijn, H. Borsboom, G.J.C. van Baarle1, J.W.M. Frenken 

and W.M. van Spengen 

Sensing Atomic Forces 122 

F. J. Giessibl 

Atomic-scale engineering of electrodes for single-molecule contacts 124 

T. Frederiksen, G. Schull, R. Berndt, A. Arnau1, and D. Sánchez-Portal

CONTRIBUTIONS

Monday 7 th March 2011

Giant Lattice Distortions of Graphene on Ru(0001) 

Harald Brune 

Institute of Condensed Matter Physics (IPMC), Ecole Polytechnique Fédérale de Lausanne (EPFL), 

Station 3, CH-1015 Lausanne 

harald.brune@epfl.ch 

Graphene (g) forms moiré structures on lattice mismatched close-packed metal surfaces. They involve 

periodic transitions between three stacking areas with different substrate binding energies. This creates 

a periodic potential acting on the 2D electron gas and is expected to create lateral lattice distortions. 

We show real-space evidence for these distortions in graphene mono- and bilayers on Ru(0001). In 

addition, we show that a superlattice of metal clusters grown g/Ir(111) strongly increases the amplitude 

of the periodic electron potential leading to a band gap opening and to strongly asymmetric group 

velocities. We discuss perspectives for further band-gap engineering of g/metal surfaces. 

22

Moiré Twist in Graphene on Ru(0001) 

K.L. Man 1 , T. Greber 2 , and M.S. Altman 1 

1 Department of Physics, Hong Kong University of Science and Technology, Hong Kong 

(corresponding author: M.S. Altman, e-mail: phaltman@ust.hk) 

2 Physik-Institut der Universität Zürich, CH-8057 Zürich, Switzerland 

Detailed knowledge of the structure of graphene (g) layers supported by crystalline 

substrates is important for understanding the electronic, physical and chemical properties of 

these systems that could underpin potential future applications. The structure of a single layer 

of graphene on the Ru(0001) surface has been controversial, beginning with the elementary 

matter of its registry. It was eventually shown using surface x-ray diffraction (SXRD) that a 

superstructure forms from the superposition of (25�25) graphene hexagons on (23�23) Ru 

units [1]. Although a moiré-like (2�2) periodic corrugation was detected within the supercell 

using SXRD, the predominant origin of corrugation in this system, whether physical [1-3] or 

electronic [4,5], has been disputed. Intriguing evidence from SXRD was also put forth that 

g/Ru(0001) exhibits chirality, whereby the weakly bound, protruding regions of a physically 

corrugated graphene layer are rotated in-plane by up to two degrees [3]. 

We have investigated single layer g/Ru(0001) using low energy electron microscopy 

(LEEM) and micro-low energy electron diffraction (�LEED) in order to verify the existence 

of chirality by these alternative approaches. Chirality should give rise to differences between 

the intensities of diffraction spots mirrored across the high symmetry directions, for example, 

the (-2/23,25/23) and (2/23,1) spots indicated in Fig. 1(a). However, this broken mirror 

symmetry will be very difficult to detect using laterally averaging techniques because of the 

presence of two chiral enantiomers and two terminations of the hcp substrate. The different 

terminations of adjacent terraces separated by a single atomic height step are evident in darkfield 

LEEM images that are formed using diffraction spots along the high symmetry 

directions, as shown in Figs. 1(b),(c). The advantage of performing �LEED measurements in 

a LEEM instrument over LEED measurements in a conventional apparatus or any other 

laterally averaging measurement for that matter is that �LEED can provide diffraction 

information from selected areas with ~250nm dimensions, i.e. localized on a single terrace. 

However, using this approach, we have not detected any evidence of the broken mirror 

symmetry that should be produced by chirality. Therefore, the �LEED measurements do not 

(yet) support the existence of chirality in g/Ru(0001) of the form proposed in ref. 3. 

A surprising observation that was made in the course of the �LEED measurements is 

that the graphene superstructure diffraction spots are seldom aligned exactly with the 

substrate reciprocal lattice. Instead, the entire ensemble of satellite spots around each integer 

order diffraction spot is rotated as a group about their respective stationary foci. When the 

�LEED selected area is scanned across the surface, the rotation angle, �, undulates randomly 

23

about a mean that is aligned optimally with the substrate (Fig. 1(a)). The standard deviation of 

the misorientation is on the order of two degrees. This behavior may be explained by 

orientational variations or twist of the moiré-like superposition over long length scales. This 

effect is much more dramatic than the 0.5º rotation of graphene relative to the underlying 

Ru(0001) that was observed with STM recently [5]. The moiré orientational undulations that 

are observed here over sub-micron length scales might also give rise to signatures in laterally 

averaging measurements similar to those that would be caused by chirality on the short length 

scale within the unit cell. 

a 

(0,25/23) 

(-2/23,25/23) (2/23,1) 

(0,1) 

(2/23,-2/23) 

(-2/23,0) 

�� 

(1,0) 

b 

c 

1 �m 

Fig. 1: (a) �LEED pattern of single layer g/Ru(0001). Contrast in dark-field LEEM 

images formed using the (b) (2/23,-2/23) and (c) (-2/23,0) superstructure spots 

identifies terraces with different terminations separated by single atomic height steps. 

[1] D. Marticcia, P.R. Willmott, T. Brugger, M. Björck, S. Günther, C.M. Schlepütz, A. Cervellino, S.A. Pauli, 

B.D. Patterson, S. Marchini, J. Wintterlin, W. Moritz and T. Greber, Phys. Rev. Lett. 101, 126102 (2008). 

[2] W. Moritz, B. Wang, M.L. Bocquet, T. Brugger, T. Greber, J. Winterlin and S. Günther, Phys. Rev. Lett. 

104, 136102 (2010). 

[3] D. Martoccia, M. Björck, C.M. Schlepütz, T. Brugger, S.A. Pauli, B.D. Patterson, T. Greber and P.R. 

Willmott, New J. Phys. 12, 043028 (2010). 

[4] A.L. Vázquez de Parga, F. Calleja, B. Borca, M.C.G. Passeggi, Jr., J.J. Hinajeros, F. Guinea and R. Miranda, 

Phys. Rev. Lett. 100, 056807 (2008). 

[5] B. Borca, S. Barja, M. Garnica, M. Minniti, A. Politano, J. M. Rodriguez-Gracia, J.J. Hinarejos, D. Faria, 

A.L. Vázquez de Parga and R. Miranda, New J. Phys. 12, 093018 (2010). 

24

Ultrafast charge transfer at graphene surfaces 

Silvano Lizzit 1 , Rosanna Larciprete 2 , Paolo Lacovig 1 , Krassimir L. Kostov 3 , 

and Dietrich Menzel 4 

1 ELETTRA, Sincrotrone Trieste S.C.p.A, 34149 Trieste, Italy 

2 CNR-Institute for Complex Systems, 00133 Roma, Italy 

3 Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria 

4 Physik-Dept. E20, Techn. Univ. München, 85748 Garching, Germany, and 

Fritz-Haber-Institut der MPG, Dept. CP, 14195 Berlin, Germany 

(corresponding author: dietrich.menzel@ph.tum.de) 

The dynamics of charge transfer (CT) between adsorbates and surfaces are important for the 

electronic response of surfaces and for photochemistry on surfaces [1]. The corresponding 

times, for instance investigated by the lifetime of excess electrons on an adsorbate, are very 

short – for weak to medium coupling they lie in the low femtosecond range, and for strong 

coupling they can be in the sub-fs range [1]. One possibility for its measurement is the socalled 

core hole clock (CHC) method, in which the transfer time of an excited electron 

(created by a resonant core hole excitation) is determined by a quantitative analysis of the 

core hole decay spectra which differ for the cases when the localized charge persists longer or 

shorter than the core hole life time [2,3]. The method makes use of the so-called Auger 

resonant Raman effect [4]. The rate of charge transfer depends on the overlap of the excited 

electron’s orbital with empty surface states of proper symmetry [5]. Qualitative connections 

appear to exist to the tunneling probability seen in STM and to conduction through atomic 

chains and molecular bridges; these have not yet been pursued in detail quantitatively. While 

it is likely that the main contribution to the transfer time of an initially localized excited 

electron comes from the transfer to the first neighbor which is normally embedded in a threedimensional 

conductor, it cannot be safely assumed that this is always so; the spreading of 

charge after transfer may also contribute to the time scale. Crudely speaking this can be 

expressed as imagining 3D vs. 2D final states of CT. Graphene (Gr) monolayers [6], which 

can be produced on metal surfaces in various degrees of coupling and geometric adaptation 

[7], as well as on insulators like SiC [8] (i.e. decoupled from the substrate), may offer an 

access to these questions. Also, the contrast seen in STM scans of Gr on metal surfaces and 

controversially discussed in terms of geometric and electronic contributions to tunneling [8,9] 

should become visible in our CT times as well. 

We have therefore measured CT times for resonantly core-excited Ar (2p3/2 >> 4s) atoms 

adsorbed on various graphene layers on metal and SiC surfaces with the well-established 

CHC method [1,2,5], using narrowband synchrotron light at the SuperESCA beam line of 

25

ELETTRA. The results can be expressed as CT times. We have also devised ways to 

separately measure the CT times for the regions of corrugated Gr layers (e.g., on Ru(0001) 

[8,9]) seen by STM as hills and valleys. The decay spectra are uniquely well defined and 

devoid of the background usually encountered on metal surfaces [3]. We attribute this to the 

very low coupling to low energy electron-hole excitations on Gr monolayers. 

The main results are: 

- Generally, the CT times are quite short (low fs range) and span a considerable range. 

- The CT times on Gr/SiC are up to 7 times longer than on Gr/metal (for the Ru case; for 

the more weakly interacting Pt case only a factor of 2 is found). 

- There is a clear discrimination between the hills (“top“) and the valleys (“bottom“ sites) 

of the corrugated layers. For Gr/Ru(0001) the CT in the bottom sites is about a factor 2 

faster than on the top sites. 

In view of the fact that no strong changes of adsorption sites and energies of Ar on Gr are 

expected for the various cases, these results show an unexpectedly large dispersion of CT 

times in the various Gr situations. However, we believe that the strong difference between the 

essentially decoupled Gr layer (on SiC) and the Gr layers on metal substrates is not due to a 

“2D vs. 3D” situation of the spreading of the transferred charge, but rather is indicative of 

strong hybridization of the Gr empty states with the surface states of the metal substrates in 

the “3D” cases, in particular on Ru – the metal appears to ‘reach through’ the Gr monolayer, 

leading to improved overlap with the Ar * 4s level. As to the electronic corrugation observed, 

it is opposite to the STM corrugation under normal imaging conditions, but agrees with the 

reversed contrast reported by some authors [9] at high positive bias which corresponds to the 

situation in our measurements. Both points could be further clarified by appropriate 

calculations which we hope to stimulate with our report. 

This work, which has been carried out in an international collaboration, has been supported by an EC general 

grant to ELETTRA and by EC travel grants to KLK and DM. DM is grateful to the German Fonds der 

Chemischen Industrie for general support. 

[1] For a recent survey, see D. Menzel, Surf. Interface Anal. 38, 1702 (2006) 

[2] P.A. Brühwiler, O. Karis, and N. Martensson, Rev. Mod. Phys. 74, 703 (2002) 

[3] D. Menzel, Chem. Soc. Rev. 37, 2212 (2008), and references therein 

[4] F. Gel’mukhanov and H. Agren, Phys. Rep. 312, 87 (1999) 

[5] D. Sanchez-Portal, D. Menzel, and P. Echenique, Phys. Rev. B76, 235406 (2007) 

[6] See, e.g., A.K. Geim et al., Science 324, 1530 (2009), and references therein 

[7] See, e.g., J. Wintterlin and M.L. Bocquet, Surf. Sci. 603, 1841 (2009) 

[8] Th. Seyller et al., Surf. Sci 600, 3906 (2006) 

[9] B. Borca et al., New J. Phys. 12, 093018 (2010) 

26

Interplay between Electronic correlations and Coherent 

Structural Dynamics during the Monoclinic 

Insulator-to-Rutile Metal Phase Transition in VO2 

H. Dachraoui, N. Müller, G. Obermeier 1 , C. Oberer, S. Horn 1 , 

V. Eyert 2 , and U. Heinzmann 

Molecular and Surface Physics, Faculty of Physics, Bielefeld University, D-33615 Bielefeld,Germany 

(cooresponding author:N. Müller, e-mail: nmueller@physik.uni-bielefeld.de 

1 Experimental Physics II, Institute of Physics, University of Augsburg, Germany 

2 Center for Electronic Correlations and Magnetism, 

Institute of Physics, University of Augsburg, D- 86135 Augsburg, Germany 

Materials exhibiting photoinduced transitions from an insulating to a metallic phase are 

attractive candidates for ultrafast electrooptic applications because of the significant changes 

in the resistivity and extremely fast optical switching upon the transition[1]. Vanadium 

dioxide (VO2) is one example of these solids with strongly correlated electron systems: It 

undergoes a first-order transition from a high-temperature metallic to a low-temperature 

insulating phase at almost room temperature Tc = 340 K [2]. The IMT is accompanied by a 

structural phase transition from a monoclinic (M1) unit cell in the insulating phase to 

tetragonal (rutile) symmetry in the metallic phase. Two major mechanisms have been 

proposed to describe the isolator metal transition in VO2: the strong dimerization of V-V 

atoms in the low temperature phase, as well as the fact that this phase is nonmagnetic, have 

lead to the proposition that this transition is connected to a Peierls-like transition [3]. 

However, the discovery of a second insulating phase (M2), in which only half of the V atoms 

form pairs while the other half form zig-zag chains that behave as magnetic insulators [4], 

suggested that the isolator metal transition should rather be regarded as a Mott transition 

[5, 6]. Recent theoretical and experimental work has shown that the VO2 phase transition 

results from the interplay of electronic correlations and structural distortions and can thus be 

regarded as a correlation-assisted Peierls transition [7, 8]. 

We have directly probed the time evolution of electronic states in VO2 across the insulatormetal 

transition in VO2 by using near-infrared / EUV pump probe technique [9]. The 

femtosecond EUV pulses with photon energy of about 95 eV have been generated by means 

of High Harmonic Generation (HHG) and the phase transition has been initiated by the 50 fs 

fundamental near-infrared pump pulses. Due to direct EUV photoemission the V3p core 

levels and the hybridized V3d O2p valence band states are accessible. While the valence band 

states directly reflect their excitation by the near infrared pulses, the core level states reflect 

local changes of the valence band states resulting from the primary near infrared excitation as 

well as from structural changes. 

27

Different temporal behaviors of lattice and electronic degrees of freedom were observed 

across the near infrared initiated insulator-metal transition of VO2, These changes establish 

that the monoclinic-rutile transition in VO2 involves both an electronic and a structural 

transition. Moderate excitation fluences result in a temporally decoupled electronic transition 

and lattice distortion, which indicates the existence of a monoclinic metallic state within the 

first-order insulator to metal transition and that transition is primarily driven not by structural 

transitions but by transitions inside the valence electron system. Moreover, we have identified 

two different types of transitional structures: one is connected with atomic motions on a fs 

time scale, the other with atomic motions on a ps time scale. 

The present data answer a standing question in isolator metal transition in VO2 and 

demonstrate the potential of the time-resolved electron spectroscopy to study light induced 

processes, not only of chemical reactions on surface adsorbates but also in complex materials. 

This work was financially supported by the German Science Foundation (DFG) within the SFB 613. 

[1] Cilento et al., Appl. Phys. Lett. 96, (2010) 

[2] F. J. Morin, Phys. Rev. Lett. 3, 34 (1959) 

[3] V. Eyert, Ann. Phys. (Leipzig) 11, 650 (2002) 

[4] B. L. Chamberland, Journal of Solid State Chemistry 7, 377 (1973). 

[5] D. Paquet and P. Leroux-Hugon, Phys. Rev. B 22, 5284 (1980). 

[6] T. M. Rice, H. Launois, and J. P. Pouget, Phys. Rev. Lett. 73, 3042 (1994). 

[7] S. Biermann, A. Poteryaev, A. I. Lichtenstein, and A. Georges, Phys. Rev. Lett. 94, 026404 (2005). 

[8] T. C. Koethe, Z.Hu, M. W. Haverkort, C. Schüßler-Langeheine, F. Venturini, N. B. Brookes, O. Tjernberg, 

W. Reichelt, H. H. Hsieh, H.-J. Lin, C. T. Chen and L. H. Tjeng, Phys. Rev. Lett. 97, 116402 (2006). 

[9] H. Dachraoui, M. Michelswirth, P. Siffalovic, P. Bartz, C. Schäfer, B. Schnatwinkel, J. Mattay, W. Pfeiffer, 

M. Drescher and U. Heinzmann, Phys. Rev. Lett., in press (2011) 

28

Time-resolved two-photon photoemission of metal/organic 

interface states 

U. Höfer 1 , M. Marks 1 , C. H. Schwalb 1 , B. Schmidt 2 , S. Sachs 2 , A. Schöll 2 , F. Reinert 2 

1 Fachbereich Physik und Zentrum für Materialwissenschaften,Philipps-Universität Marburg, 

Renthof 5, D-35032 Marburg, Germany 

2 Universität Würzburg, Experimentelle Physik II, Am Hubland, D-97074 Würzburg, Germany 

(corresponding author: U. Höfer, e-mail: hoefer@physik.uni-marburg.de) 

Time-resolved two-photon photoelectron spectroscopy (2PPE) can provide very detailed information 

about the dynamics of electron transfer processes at surfaces and interfaces. In order to exploit the full 

power of 2PPE for organic/metal interfaces it is desirable to investigate highly ordered epitaxial thin 

films on single-crystal surfaces as model systems. Such a system is 3,4,9,10-perylene-tetracarboxylic 

acid dianhydride (PTCDA) grown on Ag(111). The first monolayer of PTCDA/Ag(111) is 

chemisorbed with long-range order consisting of two flat lying molecules per unit cell that are 

arranged in a herringbone structure, similar to the crystal planes in the PTCDA bulk [1,2]. Therefore 

the higher molecular layers grow epitaxially on top of the first layer under certain conditions. 

The 2PPE spectra for PTCDA layers of 

various thicknesses display an unoccupied 

dispersing state between the metallic 

Fermi level and the lowest unoccupied 

molecular orbital (LUMO) of 

PTCDA (Fig. 1). The state corresponds 

to the one first observed by scanning 

tunnelling spectroscopy for a monolayer 

coverage [2]. Its energetic position in 

the band gaps of both the Ag(111) substrate 

and the PTCDA overlayer and the 

exponential weakening of the 2PPE 

intensity with film thickness identify it 

as a genuine interface state [3,4]. The 

lifetime of electrons excited into this 

interface state is 55 fs. This is a relatively 

small value for an unoccupied 

state located only 0.6 eV above the 

Fermi level. It is indicative for a large 

penetration of the wavefunction into the 

metal. Both, the large overlap with the 

Fig. 1 Measured dispersion of the interface state for 

1 ML (circles) and 2 ML (diamonds) of PTCDA 

together with the projected Ag(111) bulk bands 

(gray shaded area) and the Shockley state of the 

clean surface (dotted line). The borders of the 

PTCDA Brillouin zone for the two unit cell vectors 

are indicated by X1 and X2. (From Ref. [3], realspace 

structure after Ref. [1]). 

29

metal substrate and its small effective mass of 0.4 me suggest that the interface state originates mainly 

from the Shockley surface state of the bare Ag(111) surface which is strongly upshifted by the 

adsorption of PTCDA [3]. Recent DFT calculations show that the small molecular admixture of the 

state stems from of the LUMO+1 orbital of PTCDA that this admixture increases with increasing 

parallel momentum [5]. 

Interface states that are located between the Fermi level of the metal and the LUMO level of organic 

molecules, such as the one observed for PTCDA/Ag(111), are expected to have a decisive influence 

on the charge carrier injection across a metal-organic interface. In order to gain insight into the 

mechanisms that lead to the formation of such interface states and to understand their properties we 

have investigated the smaller sister molecule of PTCDA, 1,4,5,8-naphtalene tetracarboxylic acid 

diandydride (NTCDA). We find a similar dispersing interface state that forms upon interaction with 

Ag(111). In agreement with the weaker interaction of this molecule with the metal substrate we 

observe a smaller upshift of the Shockley state and a longer electron lifetime as compared to 

PTCDA/Ag(111). In the disordered low-temperature phase of PTCDA, however, where the carboxyl 

groups bend down and interact more strongly with the metal surface than in the ordered PTCDA phase 

[6], we do not observe a further upshift of the interface state. Moreover, its lifetime is increased as 

compared to the ordered phase. We interpret these findings as an indication that the bending 

of the carboxyl groups causes the interface state to attain more overlap with the organic 

molecule. 

Funding by the Deutsche Forschungsgemeinschaft through SPP1093, SPP1121, GK790, and GK1221 

is gratefully acknowledged. 

[1] E. Umbach, M. Sokolowski, R. Fink, Appl. Phys. A, 63, 565 (1996) 

[2] R. Temirov, S. Soubatch, A. Luican, F. S. Tautz, Nature 444, 350 (2006) 

[3] C. H. Schwalb, S. Sachs, M. Marks, A. Schöll, F. Reinert, E. Umbach, U. Höfer, Phys. Rev. Lett. 101, 

146801 (2008) 

[4] S. Sachs, C.H. Schwalb, M. Marks, A. Schöll, F. Reinert, E. Umbach, U. Höfer, J. Chem. Phys. 131, 

144701 (2009) 

[5] M. S. Dyer, M. Persson, New J. Phys. 12, 063014 (2010) 

[6] L. Kilian et al., Phys. Rev. Lett. 100, 136103 (2008) 

30

Electronic properties of NiO thin films: A combined STM, 

STS, and 2PPE study 

Mario Kiel, Stephan Großer, Anke Höfer, Klaus Duncker, and Wolf Widdra 

Institute of Physics, Martin-Luther Universität Halle, 06120 Halle, Germany 

(Corresponding author: W. Widdra, e-mail: wolf.widdra@physik.uni-halle.de) 

By combining scanning tunneling microscopy and spectroscopy (STM, STS) with two-photon 

photoemission (2PPE) the electronic structure of ultrathin NiO(001) films has been 

investigated in the region of unoccupied states. 

For the 2PPE experiments a fiber-based laser system (IMPULSE, Clark-MXR) which drives 

two non-collinear optical parametric amplifiers at a repetition rate of 1.5 MHz is used. It 

delivers independently tunable laser beams in the 

range of 500 – 670 nm and 700 – 950 nm with pulse 

widths of 20 - 30 fs after compression. The photoelectron 

spectra have been recorded with a 

hemispherical electron energy analyzer (Phoibos 

150, Specs) which is equipped with a 2D 

channelplate detector. The STM and STS 

experiments have been performed in a different 

UHV chamber with a homebuilt instruments which 

is operated at 80 K. 

Based on STM for the NiO monolayer which has 

been grown by reactive metal deposition in an O2 

atmosphere on a Ag(001) substrate, an uniaxially 

compressed (2×1) structure is found as shown in 

Fig.1 (a). The NiO monolayer is characterized by 

two well-developed peaks in the corresponding STS 

spectra at 2.25 and 3.8 eV (Fig.1 (b)). For NiO films 

from 2 ML to 9 ML, STM and LEED show an 

Fig. 1: (a) STM image of 1.2 ML NiO deposited on 

Ag(001) at RT. (b) STS spectra in the region of 

unoccupied states for local NiO thickness of 1 and 2 

ML recorded at 80k. (c) Time-resolved two-color 

2PPE spectra for 2 ML NiO as function of pumpprobe 

delay. 

31

unreconstructed (1x1) structure as is expected for the NiO(001) rocksalt structure. With 

increasing layer thickness the STS spectra develop distinct variations as depicted in Fig. 1 (b) 

for the 2 ML film. Additionally for thicker layers a Ni3d-derived surface state develops 

within the band gap on NiO starting from 3 ML. It shifts in energy depending on the film 

thickness from 1.5 to 2.0 eV for 3 and 8 ML, respectively, as based on STS. 

Using two-color 2PPE the unoccupied electronic states are addressed in a alternative way as 

compared to STS. For the NiO monolayer the 2PPE spectra reveal a Ni 3d state at 3.73 eV 

above the Fermi energy which can be compared to STS spectra for defect-free NiO monolayer 

islands. Time-resolved data reveal a lifetime of approx. 35 fs for this state. A second 

unoccupied Ni 3d-derived state is identified in the 2PPE spectra at an energy of 2.4 eV for 

which the STS spectra exhibit again a clear feature. Additional features within the 2PPE 

spectra are assigned to photoemission from an occupied state at -0.34 eV. The time-resolved 

2PPE spectra for a 2 ML NiO film are depicted in Fig. 1 (c). Whereas the photoemission 

intensity which is visible at 3.2 eV in Fig. 1 (c) might be related to the unoccupied Ni 3d state 

at 2.8eV seen in STS, the 2PPE feature in the 3.9 – 4.4 eV region are tentatively assign to 

image potential states. The time-dependent intensities of all features show asymmetrical 

profiles which indicate finite lifetimes of the intermediate states. 

Support by the German joint research network Sonderforschungsbereich 762 “Functionality of oxidic interfaces” 

of the Deutsche Forschungsgemeinschaft is gratefully acknowledged. 

[1] S. Großer, C. Hagendorf, H. Neddermeyer, and W. Widdra, Surf. Interface Anal. 40, 1741-46 (2008). 

32

Epitaxy of Organic Nano-Fibers on Sheet Silicates: 

A Growth Model Based on Experiments and Simulations 

Claudia Ambrosch-Draxl1 

, C. Simbrunner2 , G. Hernandez-Sosa2 , D. Nabok1 , 

M. Oehzelt2 , T. Djuric3 , R. Resel3 , L. Romaner1 , P. Puschnig1 , I. Salzmann4 , 

G. Schwabegger2 , I. Watzinger2 , and H. Sitter2 1 Chair of Atomistic Modelling and Design of Materials, 

University of Leoben, A-8700 Leoben, Austria 

cad@unileoben.ac.at 

2 Johannes Kepler University Linz, Institute of Semiconductor and Solid State Physics, 

Johannes Kepler University, A- 4040 Linz Austria 

3 Technical University Graz, Institute of Solid State Physics, 

Graz University of Technology, A-8010 Graz, Austria 

4 Institut für Physik, 

Humboldt-Universität zu Berlin, D-12489 Berlin, Germany 

During the last years self-assembled organic nano-structures have been recognized as a proper 

fundament for several opto-electronic applications. In particular, phenylenes deposited on 

muscovite mica have turned out as an outstanding material combination. Epitaxial growth of 

phenylenes on muscovite mica results in the formation of parallel aligned nano-fibers, 

providing highly polarized emission in the blue spectral range (see Figure 1). Based on these 

optical properties, several applications have been demonstrated, e.g. waveguides, frequency 

doublers, and lasers. 

Currently, the epitaxial relation between muscovite mica as the substrate and organic nanoneedles 

has been explained by an interplay between electric fields and molecule-substrate 

interactions. It has been assumed that the presence of surface-dipole moments cause a fieldinduced 

dipole interaction between organic molecules and muscovite and thus significantly 

influences the molecular alignment during the initial phase of the growth process [1]. 

By a combined theoretical and experimental approach, comprising x-ray diffraction (XRD) 

measurements, atomic-force microscopy (AFM), fluorescence microscopy, and force-field 

simulations, we present an alternative growth model being able to explain molecular 

33

adsorption on sheet silicates in terms of molecule-surface interactions only [2]. We 

demonstrate that geometrical changes in the substrate surface or molecule lead to different 

molecular adsorption geometries and needle directions which can be predicted by our growth 

model. 

We have chosen para-hexaphenyl (p6P) and sexithiophene (6T) as prototypical examples to 

substantiate our findings. To further support the crucial role of surface morphology, we have 

selected two different substrates, muscovite and phlogopite mica. We demonstrate that the 

presented model is able to explain our and also previously obtained experimental results. 

Figure 1: a) Observed needle orientations of organic molecules on muscovite mica with respect to the 

fluorescence emission wavelength. b) Fluorescence images of para-hexaphenyl (left) and sexithiophene (right) 

nano-fibers grown on muscovite mica. 

Support by the Fonds zur Förderung der Wissenschaftlichen Forschung, project S97, is gratefully acknowledged. 

[1] F. Balzer, H.-G. Rubahn, Appl. Phys. Lett. 79, 3860–3862 (2001). 

[2] C. Simbrunner, G. Hernandez-Sosa, D. Nabok, M. Oehzelt, T. Djuric, R. Resel, L. Romaner, 

P. Puschnig, C. Ambrosch-Draxl, I. Salzmann, G. Schwabegger, I. Watzinger, H. Sitter, preprint. 

34

Janus nanomembranes: 

Surfaces without bulk, functionalized on both sides 

Zhikun Zheng 1 , Christoph T. Nottbohm 1 , Andrey Turchanin 1 , 

Heiko Muzik 1 , André Beyer 1 , Mike Heilemann 1 , Markus Sauer 2 , Armin Gölzhäuser 1 

1 Department of Physics, University of Bielefeld, Germany 

2 Biotechnology & Biophysics, Julius-Maximilians-University Würzburg, Germany 

A 1 nm thick free-standing membrane with amino functionalities on its top side and thiol 

functionalities on its bottom side was fabricated from aromatic self-assembled monolayers. 

It is demonstrated that such a bifacial Janus[1] membrane, is in fact a surface without a 

bulk that can act as a two dimensional platform for the selective immobilization of 

functional molecules[2]. We have functionalized both sides with different fluorescent 

molecules: The amino side of the membrane is modified with tetramethylrhodamine 

(TMR), the thiol side is functionalized with ATTO647N, cf. Fig.1. The successful 

immobilization of molecules is proven by XPS. Functionalized Janus nanomembranes 

cover areas up to several mm 2 and can be suspended over metal grids allowing a simple 

fluorescence detection, cf. Fig. 2. The potential of functionalized Janus nanomembranes as 

generic platform for two-dimensional directional chemistry is discussed. 

[1] Janus is a Roman god with two faces, see below. 

[2] Z. Zheng, C. T. Nottbohm, A. Turchanin, H. Muzik, A. Beyer, M. Heilemann, M. Sauer, A. 

Gölzhäuser: Janus nanomembranes: A generic platform for chemistry in two dimensions, Angewandte 

Chemie Intl. Ed. , 49, 8493 (2010). 

35

Fig.1: a) Schematic representation of the nanomembrane made from cross-linked biphenyl selfassembled 

monolayers. The 1 nm thick membrane has amino and thiol functional groups on its top and 

bottom sides, respectively. b) Schematic of the functionalization with fluorescent dyes of the top and 

bottom sides of the nanomembranes. The amino side is functionalized with TMR, symbolized by 

green dots the thiol side with ATTO647N, symbolized by the red dots. Subsequent functionalization 

of the top and the bottom sides yields a Janus nanomembrane. 

Fig. 2: Schematic and fluorescence micrographs of nanomembranes freely suspended over TEM grids: 

(a) amino side functionalized with TMR and (c) thiol side fuctionalized with ATTO647. 

Corresponding SEM micrographs: (b) and (d) show the same pieces of nanomembrane as (a) and (c), 

respectively. 

36

Electrochemical Generation of Low Dimensional Metal 

Structures on Top of Self-Assembled Monolayers 

Christophe Silien + , Manfred Buck* 

EaStCHEM School of Chemistry, University of St Andrews, 

North Haugh, St Andrews KY16 9ST, United Kingdom 

*mb45@st-and.ac.uk 

Low-dimensional metal structures in 

contact with molecules being of central 

importance for molecular electronics, their 

properties are poorly understood at 

present. It is not only the difficulty of a 

well-controlled preparation but also the 

theoretical understanding of the mutual 

influence of molecules and metal on their 

electronic properties which is at its 

infancy. However, studies performed so 

far indicate that new opportunities for 

Fig. 1: Architecture of a SAM illustrated by the 

example of a pyridine (blue) terminated aliphaticaromatic 

(grey) thiol (yellow). 

tailoring electronic properties arise from the 

combination of molecular systems with low 

dimensional metal structures [1-3]. 

One strategy to study metal-molecule systems is 

based on self-assembled monolayers (SAMs) as 

SAMs offer a rich playground for fundamental 

studies due to the combination of properties. 

There is, firstly, the enormous flexibility in the 

tailoring of SAM properties both with regard to 

the surface of a SAM determined by the tail 

group and the design of the spacer unit (Fig. 1). 

Secondly, using a combination of aliphatic and 

aromatic moieties high quality SAMs can be 

generated whose surfaces exhibit a crystallinity 

closely resembling those of organic single 

crystals. Thirdly, charge transfer through SAMs 

Fig. 2: a) Illustration of electrochemical metal 

deposition on top of a SAM via the reduction of 

coordinated metal ions. b) STM image of Pd 

metal clusters deposited on a SAM. 

+ present address: Materials and Surface Science Institute and Department of Physics, University of Limerick, Ireland 

37

can be controlled via the spacer moiety. 

However the preparation of low dimensional metal structures in contact with a SAM is 

challenging as conventional procedures based on e.g. evaporation of metals onto SAMs are 

hard to control. A promising alternative approach is based on electrochemistry [ 4-7 ]. Owing to 

the particularities of the scheme the metal is confined to on top of the SAM, i.e. no shortcuts 

between the deposited metal and the substrate by penetration of metal into the SAM are 

produced. As illustrated in Fig. 2a the on top deposition is accomplished by an 

electrochemical process where the crucial point is that the metal ions are bound to the SAM 

via complexation. At sufficiently negative potentials the ions can be reduced and metal 

clusters are formed by diffusion of the atoms on the SAM surface as shown by the STM 

image in Fig. 2b. That the metal clusters are not in direct contact with the substrate is proven 

by the ease at which the clusters can be moved. The centre of Fig. 2b shows an area which has 

been wiped clean by the STM tip. Another piece of evidence comes from the observation of a 

Coulomb barrier when tunneling through a metal cluster. 

The complexation based deposition protocol allows precise control of the amount deposited 

and, thus, provides a unique way to low dimensional metal structures. The use of thiol SAMs 

of high structural integrity such as the one with the architecture shown in Fig. 1 significantly 

simplifies the previously reported two step scheme of complexation and reduction to a single 

step. 

Support by EPSRC is gratefully acknowledged. 

[1] H. G. Boyen, P. Ziemann, U. Wiedwald, V. Ivanova, D. M. Kolb, S. Sakong, A. Gross, 

A. Romanyuk, M. Buttner, and P. Oelhafen, Nature Materials 5, 394 (2006). 

[2] J. Kucera and A. Gross, Phys. Chem. Chem. Phys. 12, 4423 (2010). 

[3] J. A. Keith and T. Jacob, Chem. Eur. J. 16, 12381 (2010). 

[4] T. Baunach, V. Ivanova, D. M. Kolb, H. G. Boyen, P. Ziemann, M. Buttner, and P. 

Oelhafen, Adv. Mater. 16, 2024 (2004). 

[5] O. Shekhah, C. Busse, A. Bashir, F. Turcu, X. Yin, P. Cyganik, A. Birkner, W. 

Schuhmann, and C. Woll, Phys. Chem. Chem. Phys. 8, 3375 (2006). 

[6] D. Qu and K. Uosaki, J. Phys. Chem. B 110, 17570 (2006). 

[7] C. Silien, D. Lahayee, M. Caffio, R. Schaub, N. R. Champness, and M. Buck, Langmuir 

(in press). 

38

Charge Transport Through and Within Self-Assembled Monolayers: 

New Insights from Nanofabricated Model Devices 

Christof Wöll 

Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology, KIT, 76344 Karlsruhe 

E-mail: Christof.Woell@KIT.edu, www.ifg.kit.edu 

The use of organic molecules as active 

semiconductors in electronic devices is reaching 

the stage where commercial products are arriving 

at the market. There are, however, still a number 

of fundamental issues, e.g. a proper description of Fig. 1: Schematic diagram of an “ideal” organic 

diode with two tunnel contacts [3]. 

phenomena (e.g. charge injection) occurring at the 

electrode/OSC interface. Unfortunately, there are pronounced problems with growing highly 

ordered OSC-films on (strictly clean) metal substrate [1]. An attractive alternative to produce 

well-defined interfaces also suited for a theoretical analysis is to modify the metal electrode by 

adsorbing a well-defined thiolate-based self-assembled monolayer (SAM) [2]. This approach not 

only allows adjusting the work-function of the metal, but also provides structurally perfect 

substrates well suited for the OMBD-process. 

In this talk we will demonstrate that on an Au substrate covered with a decanethiolate SAM, 

pentacene, one of the most interesting molecules to be used as organic semiconductor in OFETs, can 

be grown in a rather perfect, bulk-like structure. The high structural quality and the absence of any 

contaminations make it possible to produce a two-terminal organoelectronic model device as shown 

in Fig. 1 [3]. This “ideal” device is essentially free of imperfections and well suited for a theoretical 

analysis. The results of numeric simulations [3] reveal that, at positive bias, n-transport dominates 

p-transport, an unusual behavior for pentacene-based devices. 

The n-conduction within the OSC pentacene can be suppressed by introducing OH-groups at the 

SAM/OSC interface, a finding which is in accord with related experiments on OFETs [4]. Recent 

experiments indicate that loading of the OH-traps with electrons is a reversible process [5]. 

Fabrication of functioning OFET devices confirms the positive effect of SAM surface modification 

on device properties [6]. 

A recent investigation of nanographene-based SAMs has demonstrated that using the 

SAM-approach relative molecular orientations can be achieved which are different from that present 

in the corresponding bulk phases, with very positive effects on the charge carrier mobilities [7]. 

References 

[1] G.Witte and Ch.Wöll, J.Mater.Res. 19, 1889 (2004) 

[2] M. Kind and C. Wöll, Prog. Surf. Sci. 84, 230 (2009) 

[3] L.Ruppel, A.Birkner, G.Witte, C.Busse, T.Lindner, G.Paasch, C.Wöll, J.Appl.Phys. 102, 

033708 (2007) 

[4] L.Chua, J.Zaumseil, J.Chang, E.Ou, P.Ho, H.Sirringhaus, R.Friend, Nature 434, 194 (2005) 

[5] Z.-H. Wang, D. Käfer, A. Bashir, J. Götzen, A. Birkner, G. Witte, Ch. Wöll, 

PhysChemChemPhys, 12, 4317-4323 (2010) 

[6] C.Bock, D.Pham, U.Kunze, D.Käfer, G.Witte, Ch.Wöll, J.Appl.Phys. 100, 114517 (2006) 

[7] D. Käfer, A. Bashir, X. Dou, G. Witte, K. Müllen, Ch. Wöll, Adv. Mater, 22, 384-388 (2010) 

39

Tuesday 8 th March 2011

Gold clusters at finite temperature in vacuo and in a CO 

plus O2 atmosphere: 

ab initio studies towards gold catalysis 

Elizabeth C. Beret, Luca M. Ghiringhelli, Merel M. van Wijk, 

and Matthias Scheffler 

Fritz Haber Institute of the Max Planck Society, 

Faradayweg 4-6, D-14195 Berlin-Dahlem, Germany 

(corresponding author: M. Scheffler, Scheffler@FHI-Berlin.mpg.de) 

The marked catalytic activity of gold nanoparticles has inspired a large number of scientific 

contributions. However, many questions still lack a satisfying answer, for example, what are 

the structures and stoichiometries of gold particles at realistic temperatures, in the presence of 

(reactive) gases, and how do they change with particle size. We address these issues for gold 

clusters, focusing on AuN with 3 ≤ N ≤ 20, modeled in vacuo as well as in an atmosphere 

containing CO and O2 in variable compositions. We combine all-electron density-functional 

theory, including density-dependent van-der-Waals tail corrections, with finite temperature 

sampling techniques, like Biased Molecular Dynamics (MD) and Parallel Tempered MD. 

The results show that at temperatures as low as 100 K vibrational spectra already display 

noticeable anharmonic features. At room temperature the flexibility of the clusters is already 

significant, and certain AuN clusters may be better described as liquid droplets. This has 

important implications for adsorption of atoms or molecules because the energy released by 

an exothermal adsorption heats up the complex, and in the gas phase the time to reach thermal 

equilibrium with the environment is much longer than the cluster conformational 

rearrangements. Thus, the concept of a preferred adsorption site becomes questionable. 

Taking the CO oxidation reaction as example, we address the above-mentioned questions for 

very small neutral and anionic gold clusters modeled in a gas-phase atmosphere containing 

CO and O2 as function of partial pressures and temperature. To this aim, DFT (PBE)–based 

ab initio atomistic thermodynamics is applied, including the vibrational contributions to the 

free energy. As a result, the preferred cluster+adsorbate structures for different environmental 

conditions are obtained and interpreted as candidate intermediates in the catalytic CO 

oxidation reaction. Furthermore, we discuss the different behavior of the system in two 

different kinds of conditions: a) a situation where the product CO2 is constantly removed 

from the system (like in a flow reactor) and b) a situation where equilibrium with the product 

CO2 is reached. 

42

Defects on room temperature ultra-thin 

film CeO 2 with STM 

D.C. Grinter, R. Ithnin, C.L. Pang, G. Thornton 

London Centre for Nanotechnology and Department of Chemistry, 

University College London, 20 Gordon Street, London, WC1H 0AJ, UK 

(corresponding author, email: g.thornton@ucl.ac.uk) 

The study of ceria (CeO 2) is currently of great interest, with proven technological applications 

arising from its wide-ranging catalytic properties including exhaust gas purification in 

automotive catalytic converters and in the water gas shift reaction to produce H2. 1 The key to 

these properties lies in the high mobility of lattice oxygen leading to excellent oxygen storage 

and release, thought to be determined by the nature, concentration and mobility of lattice 

defects, especially oxygen vacancies. A further potentially important role of vacancies is in 

the binding of catalytically active metals such as Au, recently discovered to be a highly active 

catalyst for the water-gas-shift reaction. 2,3 

As ceria is an insulator with a band gap of 6 eV, ultra-thin films of CeO 2(111) were prepared 

on a conducting substrate of Pt(111) to permit room temperature studies with scanning 

tunnelling microscopy (STM). The ultra-thin films were grown in two ways: by reactive 

deposition in an oxygen atmosphere, and by post-oxidation of Ce/Pt surface alloys. 

Atomically resolved STM images of films prepared by both methods revealed remarkable 

similarities to images recorded in previous studies on CeO 2(111) single crystals using NC- 

AFM and high temperature STM. Surface oxygen vacancies were imaged individually, as 

trimers, and in linear arrays. Subsurface oxygen vacancies were also detected as well as 

adsorbed water molecules and surface hydroxide trimers, again with a similar appearance to 

that seen on native CeO2(111) 4,5 , with agreement for point defects in filled state images of 

6 

thin film CeO2 (see fig 1). As such, we conclude that ultra-thin ceria films supported on 

Pt(111) make excellent topographic models for the native oxide. 

43

Figure 1: Atomically resolved, filled states, STM image (5 x 4 nm 2 , V s = -3.20 V, I t = 0.20 

nA) and structural model of vacancies and adsorbates on an ultra-thin film of CeO 2(111) on 

Pt(111). Bright spots correspond to top layer oxygen termination of surface, with 

identification of adsorbed water, hydroxide trimers and surface oxygen vacancies possible by 

comparison with NC-AFM 4 and high temperature STM 5 . The assignments are highlighted in 

the structural model. Water molecules are observed exclusively above second layer Ce 

atoms. 

(1) Trovarelli, A. Catalysis by Ceria and Related Materials: Imperial College Press: 

London 2000 Vol 2 

(2) Fu, Q et al. Science 2003, 301, 935-938 

(3) Liu, Z. et al. Phys. Rev. Lett. 2005, 94, 196102 

(4) Gritschneder, S.; Iwasawa, Y.; Reichling, M. Nanotech. 2006, 18, 044025; Phys. 

Rev. Lett. 2007, 99, 056101 

(5) Esch, F.; Fabris, S.; Zhou, L.; Montini, T.; Africh, C.; Fornasiero, P.; Comelli, G.; 

Rosei, R. Science 2005, 309, 752-755 

(6) Eck, S.; Castellarin-Cudia, C.; Surnev, S.; Ramsey, M. G.; Netzer, F. P. Surf. Sci. 

2002, 530, 173-185 

44

High Island Densities in Pulsed Laser Deposition: Causes and Implications 

Peter Varga 

Institute for Applied Physics, Vienna University of Technology, Vienna, Austria 

Pulsed laser deposition (PLD) is a versatile deposition method that combines many 

features of technologically relevant processes such as sputter deposition with ultrahighvacuum 

compatibility and easy accessibility by surface science methods. PLD is 

characterized by high energies of the impinging particles (ions) and known to lead to 

higher island densities and better layer-by-layer growth than conventional thermal 

deposition. We have studied homoepitaxy of Pt on Pt(111) as well as heteroepitaxy of Co 

on Pt(111) by PLD with scanning tunneling microscopy (STM) [1]. For heteroepitaxy of 

Co on Pt(111), atomically resolved STM with chemical contrast shows implantation of 

impinging species in the surface. By directly watching diffusion processes, we 

demonstrate that the resulting heterogeneous surface impedes surface diffusion, leading 

to exceptionally high island densities. For homoepitaxy with moderate ion energies (< 

100 eV), comparison with nucleation theory shows that the high island densities are 

merely caused by the high instantaneous flux of arriving particles, while the kinetic 

energy of the impinging particles plays no significant role. At higher ion energies (> 200 

eV), however, additional nuclei are formed by ``failed sputtering'', i.e., formation of adatoms 

by the impinging energetic ions. Layer-by-layer growth observed in PLD 

experiments down to very low temperatures can be explained by the small island size and 

implantation of energetic particles. 

Nucleation of small Pt clusters shows another unexpected phenomenon: We found a large 

number of linear chains, with lengths of up to 9 atoms. This result cannot be explained by 

a simple diffusion-limited-aggregation model, and we suggest that the strain field of the 

substrate plays an important role, steering some of the diffusing adatoms towards the 

ends of linear clusters. As a further ingredient to this phenomenon, a study of the binding 

energies of Pt adatom clusters by DFT shows that bonding of such clusters does not 

follow the rule of "higher coordination means stronger bonding". Instead, short linear 

chains are bound more strongly than more compact configurations. The reason for this 

phenomenon is strong directional bonding via d z2-like orbitals, a mechanism previously 

proposed for binding between the late transition metals and oxygen atoms and explaining 

the preference for O atoms at opposite sides of a metal atom. 

1 M.Schmid, C.Lenauer, A.Buchsbaum, F.Wimmer, G.Rauchbauer, P.Scheiber, G.Betz, 

and P.Varga, Phys.Rev.Lett. 103 (2009) 076101 

45

Copper electrodeposition on fast time scale: from 

underpotential deposition to bulk growth 

Y. Yanson, J. W. M. Frenken, M. J. Rost 

Kameringh Onnes Laboratory, Leiden University,2333CA Leiden , the Netherlands 

(corresponding author: Y. Yanson, e-mail: yansony@physics.leidenuniv.nl) 

Copper electrodeposition has drawn a lot of attention in the last two decades. One reason for 

that is its increasing relevance for industrial applications [1]. Another reason is that Cu 

electrodeposition on substrates from different metals, especially gold, became a „benchmark“ 

system for studying adsorption and deposition phenomena at solid/liquid interfaces. In fact, 

Cu underpotential deposition (UPD), i.e. deposition of a single monoatomic layer, has 

become a system of choice for testing different surface-sensitive techniques [2]. 

Despite the vast amount of literature on Cu electrodeposition, there are still a lot of 

controversies and open questions concerning the system. For example, there are some 

contradictions in the results found in literature on Cu UPD onto metallic substrates covered 

by self-assembled monolayers (SAM) of thiol molecules [3-5]. Also, the more applicationwise 

relevant Cu deposition regime, i.e. Cu bulk deposition, has been merely studied, 

especially at its initial stages [6,7]. The reason for this is that the bulk deposition process 

happens too rapid 1 for the conventional scanning probe techniques to capture it. 

Figure 1. Cu UPD layer formation a) on a clean Au(111) surface and b) on the surface 

covered by a SPS 2 SAM. 

We have developed a new fast electrochemical scanning tunneling microscope (EC-STM), 

which allows us to obtain information about the dynamics of the UPD processes, see Fig. 1, 

and also gives us the possibility to follow the bulk Cu electrodeposition in-situ. From the EC- 

STM measurements combined with electrochemical cyclic voltammetry (CV) measurements 

46

we were able to develop a model, which describes well UPD onto metallic single crystal 

surfaces covered by thiol SAMs. Although our model was developed for a particular case of 

Cu UPD onto the Au(111) surface covered by a SPS 2 SAM, many results on different systems 

found in literature can be well described by it. 

Our EC-STM also gives us the possibility to take a glimpse at the initial stages of the Cu bulk 

deposition. We compare Cu bulk deposition onto Au(111) surface from an additive-free 

solution and a SPS-containing one, see Fig. 2. We demonstrate that our EC-STM can provide 

information on e.g. the influence of the SPS onto the nucleation density and feature size of the 

Cu deposit. Such parameters can not be easily extracted from conventional electrochemical 

measurements. 

a) b) 

Figure 2. Bulk Cu deposition from a solution a) without b) with SPS. 

1 at relevant Cu 2+ ion concentrations in the solution 

2 Bis(3-sulfopropyl)disulfide 

[1] P. C. Andricacos, C. Uzoh, J. O. Dukovic, J. Horkans, H. Deligianni, IBM Jour. Res. and Dev. 42, 567 

(1998) 

[2] M. E. Herrero, J. L. Buller, D. H. Abruna, Chemical Review 101, 1897 (2001) 

[3] M. A. Schneeweiss, H. Hagenstrom, M. J. Esplandiu, D. M. Kolb, Applied Physics A:Materials Science & 

Processing 69, 537 (1999) 

[4] M. Petri, D. M. Kolb, U. Memmert, H. Meyer, Electrochimica Acta 49, 183 (2003). 

[5] C. Silien and M. Buck, Journal of Physical Chemistry C 112, 3881 (2008) 

[6] A. I. Danilov, E. B. Molodkina, A. V. Rudnev, Yu. M. Polukarov, J. M. Feliu, Electrochim. Acta 50, 5032 

( 2005) 

[7] N. Batina, T. Will, M. D. Kolb, Faraday Discussions 94, 93 (1992) 

47

Buckling under tension – LaAlO3 on SrTiO3 

1 1 1 1 1 

Phil Willmott , Stephan Pauli , Steven Leake , Bernard Delley , Christof Schneider , Jochen 

Mannhart 2 , and Stefan Paetel 2 , 

1. Paul Scherrer Institut,CH-5232 Villigen Switzerland 

(corresponding author: P.R. Willmott, e-mail: Philip.willmott@psi.ch) 

2 Lehrstuhl für Experimentalphysik VI, Universität Augsburg, Universitätstr. 1, D-86135 Augsburg, 

Germany 

Since its discovery in 2004 [1], the conducting interface between the band insulators SrTiO3 

(STO) and LaAlO3 (LAO) has been the subject of intense research [2]. Several propositions 

have been made to explain this phenomenon, including oxygen vacancies [3], electronic 

reconstruction [1], intermixing [4], and dipole compensation through buckling [5] 

One of the most intriguing clues to this complex phenomenon was the discovery that 

conductivity at the interface only occurs if four or more monolayers (MLs) of LAO are 

deposited [6]. This would seem to indicate that subtle structural variations might play a 

pivotal role in determining the physical processes at play. The only technique capable of 

determining the atomic structure with sufficient accuracy (of the order of 10 pm or better) is 

surface x-ray diffraction (SXRD) [4]. 

In this presentation, I will discuss systematic changes to the atomic structure of the STO/LAO 

system as one progresses from two to five monolayers, as determined from comprehensive 

SXRD data [7], analyzed using model-free phase-retrieval methods [8]. The interfacial 

conductivity is explained in terms of intermixing, dipole formation as a result of buckling, and 

electronic transfer. DFT calculations will also be presented. 

[1] A. Ohtomo and H.Y. Hwang, “A high-mobility electron gas at the LaAlO3/SrTiO3 heterointerface,” Nature 

427, 423 (2004) 

[2] S.A. Pauli and P.R. Willmott, “Conducting interfaces between polar and non-polar insulating perovskites,” 

J. Phys.: Condens. Matter 20, 264102 (2008) 

[3] Siemons et al., “Origin of charge density at LaAlO3 on SrTiO3 heterointerfaces: possibility of intrinsic 

doping,” Phys. Rev. Lett. 98, 196802 (2007) 

[4] P.R. Willmott et al., “Structural basis for the conducting interface between LaAlO3 and SrTiO3,” Phys. Rev. 

Lett. 99, 155502 (2007) 

[5] R. Pentcheva and W.E. Pickett, “Avoiding the Polarization Catastrophe in LaAlO3 Overlayers on 

SrTiO3(001) through Polar Distortion,” Phys. Rev. Lett. 102, 107602 (2009) 

48

[6] S. Thiel et al., “Tunable Quasi-Two-Dimensional Electron Gases in Oxide Heterostructures,” Science. 313, 

1942 (2006) 

[7] S.A. Pauli et al., “Evolution of the Interfacial Structure of LaAlO3 on SrTiO3,” Phys. Rev. Lett. 106, 

036101 (2011) 

[8] M. Björck et al., “Atomic imaging of thin films with surface x-ray diffraction: introducing DCAF,” J. Phys.: 

Condens. Matter 20, 445006 (2008) 

49

Dewetting dynamics of crystalline thin films 

E. Bussmann 1 , F.Leroy 1 , F.Cheynis 1 , O.Pierre-Louis 2 , P.Müller 1 

1 CINaM, UPR 3118 CNRS, Aix-Marseille Université , 

Campus de Luminy, case 913, F-13288 Marseille Cedex 9, France 

2 LPMCN, Université de Lyon, 69622 Villeurbane Cédex 

cheynis@cinam.univ-mrs.fr 

Thin solid films are the basic components in many devices. However since the deposition 

conditions generally are far from equilibrium, such 2D thin films, when annealed, may break 

up into 3D islands. This so-called dewetting process is observed in many experimental 

systems. If a continuous film is required for applications, dewetting clearly is a limiting 

process. On the other hand it appears to be a cheap and easy-to-implement method to produce 

and control the formation of an assembly of nanocrystals. 

Here we report a quantitative characterization of 

the dewetting dynamics of silicon-on-oxyde (SOI) 

thin-films. From an experimental point of view we 

use Low-Energy Electron Microscopy (LEEM) to 

record real-time in-situ movies of the dynamics of 

the SOI dewetting process leading to the formation 

of 3D compact Si nanocrystals. The complex 

morphological evolution is reproduced by a simple 

Solid-on-Solid Kinetic Monte Carlo (KMC) model 

in which enter only two physical ingredients: a 

wetting parameter and a reduced temperature. As 

shown in the nearby figure the KMC model 

reproduces the qualitative features of the 

morphological evolution (See Fig. a and b) as well 

as the growth dynamics. 

To get insight into the global dynamics we develop 

analytic models that capture the essentials of the 

underlying physics and enable us to clearly identify 

the origin of the driving-force. Our results show (i) 

that the dewetting is a consequence of surface freeenergy 

minimization mediated by surface diffusion 

and (ii) that the velocity of the dewetting front is 

limited by its thickening. We connect the rim 

growth mechanism to the growth dynamics of the 

Fig: Dewetting morphology versus time: (a) 

AFM and LEEM results. (b) KMC 

dewetted area. The front edges instabilities that lead to the formation of local elongated 

structures (then to 3D islands by a pinch-off process) are closely connected to the local height 

instabilities of the rim. 

Finally the dewetting properties of SOI films are compared to those obtained for a few 

other systems. 

Reference: 

E. Bussmann, F.Cheynis, F.Leroy, P.Müller, O.Pierre-Louis, Dynamics of solid thin-film 

dewetting in the silicon-on-insulator system, NJP Submitted 

50

The role of dispersion forces in the structure of graphene 

monolayers over the Ru(0001) surface 

D. Stradi, 1,2 S. Barja, 3,2 C. Díaz, 1 M. Garnica, 3,2 B. Borca, 3 J. J. Hinarejos, 3 D. Sánchez- 

Portal, 4,5 M. Alcamí, 1 A. Arnau, 4, 5, 6 A. L. Vázquez de Parga, 3, 2 R. Miranda, 3, 2 and F. Martín 1,2 

(corresponding author: F. Martín, e-mail: fernando.martin@uam.es) 

1 Departamento de Química, Módulo 13, Universidad Autónoma de Madrid, 28049 Madrid, Spain 

2 Instituto Madrileño de Estudios Avanzados en Nanociencia (IMDEA-Nanociencia), Cantoblanco, 

28049 Madrid, Spain 

3 Dep. Física de la Materia Condensada, Universidad Autónoma de Madrid, 28049 Madrid, Spain 

4 Materials Physics Center (CSIC-UPV/EHU), Paseo Manuel de Lardizábal 5, 20018 San Sebastián, 

Spain 

5 Donostia International Physics Centre (DIPC), Paseo Manuel Lardizábal 4, 20018 San Sebastián, 

Spain 

6 Dep. Física de Materiales (UPV/EHU), Facultad de Química, Apartado 1072, 20080 San Sebastián, 

Spain 

The electronic properties of perfect graphene makes it a promising material for applications in 

microelectronics and sensing. In practice, the synthesis of graphene can be achieved by 

epitaxial growth of carbon monolayers on transition metals [1]. In many cases, such as in 

graphene grown on Ru(0001) (G/Ru for short), the mismatch between the lattice constant of 

graphene and the one of the substrate leads to the appareance of a Moiré pattern [2]. This 

results in substantial modifications in graphene morphology, which is periodically corrugated, 

and in its electron density, which is no longer uniform [3]. Such properties are responsible for, 

e.g., the formation of quantum dots [4,5] or site selective deposition of organic molecules [6]. 

Despite numerous efforts to determine the morphology of the Moiré observed in G/Ru, the 

actual value of the corrugation is still a subject of controversy. Depending on the 

experimental technique employed, very different values are obtained, which fall in the broad 

range 0.4Å – 1.5Å [3,7,8]. On the theoretical side, Density Functional Theory (DFT) 

calculations predict a corrugation in the range 1.5Å – 1.7Å [9,10,11]. 

51

Standard DFT is not capable to account for long range van der Waals (vdW) interactions. 

However, there is strong evidence that these forces play a crucial role in the adsorption of 

aromatic molecules on metal surfaces [12,13,14], leading to a significant increase of the 

calculated adsorption energies and, what is even more important, to significant modifications 

of the adsorption geometry [15]. In many respects, graphene can be considered as an extended 

aromatic system due to the presence of a π electronic cloud above and below the graphene 

plane. Therefore, it is worth studying if vdW interactions play a similar important role in the 

structure of graphene lying on top of a metal substrate. 

In the present work, using DFT calculations in which effect of vdW interactions is included, 

we show that such interactions lead to a substantial reduction of the corrugation with respect 

to standard DFT calculation. Qualitative agreement is obtained with the apparent height 

variations near the Fermi level observed in STM and structural data obtained both from Low 

Energy Electron Diffraction (LEED) and Surface X-Ray Diffraction (SXRD). 

Mare Nostrum BSC and CCC-UAM for computer time are kindly acknowledged. Work supported by the 

MICINN projects FIS2010-15127, FIS2010-18847, FIS2010-19609-C02-00, ACI2008-0777 and 2010C-07- 

25200, the CAM program NANOBIOMAGNET S2009/MAT1726, and the Gobierno Vasco - UPV/EHU project 

IT-366-07. 

[1] J. Wintterlin and M. L. Bocquet Surf. Sci. 603, 1841 (2009) 

[2] S. Marchini and S. Günther and J. Wintterlin Phys. Rev. B 76, 075429 (2007) 

[3] A. L. Vázquez de Parga and F. Calleja and B. Borca and M. C. G. Passeggi Jr. and J. J. Hinarejos and F. 

Guinea and R. Miranda Phys. Rev. Lett. 100, 056807 (2008); Phys. Rev. Lett. 101 099704 (2008) 

[4] B. Borca and S. Barja and M. Garnica and D. Sánchez-Portal and V. M. Silkin and E. V. Chulkov and C. F. 

Hermanns and J. J. Hinarejos and A. L. Vázquez de Parga and A. Arnau and P. M. Echenique and R. 

Miranda Phys. Rev. Lett. 105, 036804 (2010); Phys. Rev. Lett. 105, 219702 (2010) 

[5] H. G. Zhang, H. Hu, Y. Pan, J. H. Mao, M. Gao, H. M. Guo, S. X. Du, T. Greber, and H.-J. Gao, J. Phys.: 

Condens. Matter 22, 302001 (2010); Phys. Rev. Lett. 105, 219701 (2010) 

[6] J. Mao and H. Zhang and Y. Jiang and Y. Pan and M. Gao and W. Xiao and H.-J. Gao J. Am. Chem. Soc. 

131, 14136 (2009) 

[7] D. Martoccia, P. R. Willmott, T. Brugger, M. Björck, S. Günther, C. M. Schlepütz, A. Cervellino, S. A. 

Pauli, B. D. Patterson, S. Marchini, et al., Phys. Rev. Lett. 101, 126102 (2008) 

[8] D. Martoccia, M. Björck, C. M. Schlepütz, T. Brugger, S. A. Pauli, B. D. Patterson, T. Greber, and P. R. 

Willmott, New J. Phys. 12, 043028 (2010) 

[9] W. Moritz, B. Wang, M. L. Bocquet, T. Brugger, T. Greber, and J. Wintterlin, Phys. Rev. Lett. 104, 136102 

(2010) 

[10] B. Wang, M. L. Bocquet, S. Marchini, S. Günther, and J. Wintterlin, Phys. Chem. Chem. Phys. 10, 3530 

(2008); Phys. Rev. Lett. 101, 099703 (2008); New J. Phys. 12, 043041 (2010) 

[11] D. Jiang, M.-H. Du, and S. Dai, J. Chem. Phys. 130, 074705 (2009) 

[12] P. Sony, P. Puschnig, D. Nabok, and C. Ambrosch-Draxl, Phys. Rev. Lett. 99, 176401 (2007) 

[13] N. Atodiresei, V. Caciuc, P. Lazic, and S. Blügel, Phys.Rev. Lett. 102, 136809 (2009) 

[14] G. Mercurio, E. R. McNellis, I. Martin, S. Hagen, F. Leyssner, S. Soubatch, J. Meyer, M. Wolf, P. Tegeder, 

F. S. Tautz, et al., Phys. Rev. Lett. 104, 036102 (2010) 

[15] E. R. McNellis, J. Meyer, K. Reuter, Phys. Rev. B 80, 205414 (2009) 

52

Growth of graphene nanoislands on a Ni(111) surface 

G. Ceballos, M. Ollé. P. Gambardella 

CIN2(ICN-CSIC), Catalan Institute of Nanotechnology, Campus de la UAB 08193 Bellaterra 

(Barcelona) Spain 

Since they first time a layer of graphene was isolated in 2004 [1], the interest in this material 

increased exponentially. The main attraction of this material lies in their unusual and 

surprising electronic, mechanical and magnetic properties such as the anomalous quantum 

Hall effect, the absence of electronic localization, high optical transparency, the high 

electrical conductivity, flexibility and high mechanical strength. These properties make 

graphene a very promising material for applications in electronics and spintronics, and 

therefore it is necessary to control the growth and properties at the nanoscale. 

The growth of graphene layers on the nickel surface by decomposition of hydrocarbons is 

interesting for three main reasons. On the one hand, the lattice constant of the surface of 

Ni(111) coincides almost perfectly with the lattice constant of graphene, which allows it to 

grow in a (1×1) structure. Moreover, due to the catalytic effect of nickel surface, it is a 

autoterminated reaction, i.e., the reaction stops once the graphene monolayer is formed 

avoiding the growth of multilayers [2]. Finally, the Nickel is a ferromagnetic material, which 

opens the door to applications in spintronics. 

Progress in the manufacture of low dimensional structures such as graphene nanoribbons has 

been reported [3]. It shows that the electronic properties of graphene change in a non-trivial 

way going to nanoscopic dimensions mainly due to the contribution of edge effects. Several 

theoretical studies have focused on the properties 

of the edges of graphene nanocrystals [4-7]. In the 

case of zig-zag edges of triangular nanocrystals a 

particularly high magnetic moment has been 

predicted [8]. 

In this work we study by STM the growth of 

graphene on a Ni (111) surface by decomposition 

of hydrocarbons. By varying parameters such as 

the dosage of hydrocarbon, reaction time and 

temperature was possible to obtain in a 

reproducible manner a wide range of coverages. 

For low coverages the carbon atoms organize 

themselves into graphene nanoislands whose size 

and density are related to the reaction parameters. 

These nanoislands, initially with irregular shapes, 

53 

Figure 1: STM image (8x8nm) of a 

graphene nanoisland on a Ni(111 

surface).

may, by further thermal treatment selectively acquire a triangular (Fig. 1) or hexagonal shape 

both with zig-zag edges. In the case of triangular islands all edges have the same packaging 

with the nickel surface underneath while in the case of hexagonal islands edges change 

alternatively the packaging. The optimum conditions to obtain nanoislands of particular size 

and shape are studied by systematic variation of the parameters of the hydrocarbon 

decomposition reaction and the thermal diffusion of carbon on the surface. 

[1] K.S. Novoselov et al, Science, 306, 666-669 (2004). 

[2] J. Wintterlin, M.-L. Bocquet, Surface Science, 603, 1841–1852, (2009) 

[3] M.Y. Han et al., Phys. Rev. Lett. 98, 206805 (2007). 

[4]A. Kuc, T. Heine and G. Seifert, Phys. Rev. B 81, 085430 (2010). 

[5] W.L. Wang, S. Meng, and E. Kaxiras, Nano Lett. 8(1), 241-245 (2008). 

[6] T. Nakajima and K. Shintani, J. App. Phys. 106, 114305 (2009) 

[7] W.L. Wang, O.V. Yazyev, S. Meng, E. Kaxiras, Phys. Rev. Lett. 102, 157201 (2009). 

[8] J.Fernández-Rossier and J.J. Palacios, Phys. Rev. Lett. 99, 177204 (2007). 

54

Kinetics of graphene growth on Rh(111) 

G. Dong, D.W. van Baarle, M.J. Rost and J.W.M. Frenken 

Kamerlingh Onnes Laboratory, Leiden University, PO Box 9504, 2300 RA Leiden, The Netherlands 

(corresponding author: J.W.M. Frenken, e-mail: Frenken@physics.leidenuniv.nl) 

Graphene, single-layer graphite, is drawing much attention because of its special 

properties and potential use in future-generation electronics [1]. A practical and 

promising method for mass production of graphene is hydrocarbon chemical vapor 

deposition (CVD) on metal surfaces. Metal surfaces which are catalysts for 

hydrocarbon decomposition are chosen as the substrate, because newly arriving 

hydrocarbon molecules do not stick or decompose on the area already covered by 

graphene, which guarantees the single layer graphene coverage. In additional, 

methods of transfer graphene from metal surfaces to other substrates have been 

developed [2]. In this way, the growth graphene on metal is being used for 

applications and scientific measurements. However, it is very difficult to follow the 

these processes in experiments, especielly in an atomical level, which should form the 

basis for strategies to improve the quality of graphene. This is because the graphene 

formation on metal surfaces typically takes place at high temperatures, at which the 

interaction between carbon and metals is very complex and most experimental 

method s can not stand. 

Our scanning tunneling microscope (STM), which has been optimized for (fast) 

scanning at high sample temperatures and during substantial temperature variations 

[3, 4], has enabled us to follow the reaction and growth of graphene in-situ. We 

investigated the growth of graphene on a Rh (111) surface. Due to its lattice 

mismatch, graphene forms moiré patterns on Rh(111), which work as a ‘magnifying 

glass’ for atomic defects in the graphene. In this way, we get atomic-scale information 

about the graphene overlayer, even from STM images without direct atomic 

resolution. Our STM movies provide detailed observations of this growth system, 

from which we obtain for example the temperature ranges for graphene and carbide 

formation. Baised on quantitative analysis on the growth of graphene, we also 

revieled some special knowlege about the kinetics of the growth, which will be 

focused on in this talk. For example, from figure 1 we can see the moiré patterns of 

graphene are no longer just interference between to lattices, but really play a role in 

the growth of graphene. We also learned the limiting factor in graphene growth is the 

kink creation process, and it can be influenced by the detailed geography of graphene 

e.g. it is easier to create a kink at small angle corners of graphene than on a straight 

55

edge. Because the observation thin film formation in real space and at high 

temperature is a very new technique, we hope this detailed information not only helps 

understanding the growth of graphene, but also enriches the surface kinetic 

researches. 

Grow speed nm/s 

0.06 

0.05 

0.04 

0.03 

0.02 

0.01 

0.00 

-0.01 

288 290 292 294 296 

Frame 

Figure 1 The grow speed of graphene measured form a vacancy island: (A) The 

grow speed for each frame. The error bars represent the equipment error by STM 

measurement. (B) The STM images from which the (A) was drawn. The pressure of 

ethylene was ~1.4 x 10-8 mbar. The time for acquiring each frame is 26.3 s. The 

temperature of the sample is 975 K. 

[1] R. M. Westervelt, Science 320, 324 (2008). 

[2] K. S. Kim et al., Nature 457, 706 (2009). 

[3] M. S. Hoogeman et al., Rev Sci Instrum 69, 2072 (1998). 

[4] M. J. Rost et al., Rev Sci Instrum 76, 053710 (2005). 

56 

A 

B

Structure and properties of surface and subsurface defects 

in graphite accounting for van der Waals and spinpolarization 

effects 

G. Teobaldi 1,2,3 , K. Tanimura 2 , and A. L. Shluger 3,4 

1 Surface Science Research Centre, Department of Chemistry, 

the University of Liverpool, Liverpool L69 3BX, United Kingdom 

(corresponding author: G. Teobaldi, e-mail: g.teobaldi@liv.ac.uk) 

2 The Institute of Scientific and Industrial Research (ISIR), Osaka University, Osaka 567-0047, Japan 

3 Department of Physics and Astronomy, University College London, 

WC1E 6BT London, United Kingdom 

4 WPI-AIMR, Tohoku University, Sendai 980-8577, Japan 

Defects in graphite, graphene, and related nanostructures are known to alter the chemical and 

physical properties of these materials [1,2]. In particular, radiation damage of graphite has 

been long representing a major concern for nuclear industry [3]. The introduction of defects in 

carbon-based materials is also recognized as a versatile tool for tailoring their properties to 

technologically relevant functions [4]. Controlled introduction of defects in carbon-based 

nanostructures may allow one to tune the properties of carbon-based materials, and could 

potentially lead to applications in nanoelectronics [5], spintronics [6], portable magnetic 

devices [4], and catalysis [7,8]. 

These perceived advantages have stimulated the pursuit of viable routes to nanoengineer 

graphite-based systems via electron irradiation, ion bombardment, plasma oxidation, and 

intense femtosecond (fs) laser pulses [4,9]. The ensuing explosion of experimental 

information has further motivated first-principles studies of intrinsic defects in carbon-based 

nanostructures. Atomic-scale understanding of the structure, energy, and properties of 

intrinsic defects in graphite, graphene, and related structures is essential to assist and direct 

the design and optimization of new materials with technologically relevant applications. 

With the only exception of Refs. [9,10], the currently available atomic-scale models of 

intrinsic defects in graphite originate from local-density approximation (LDA) or semi-local 

generalized gradient approximation (GGA) density-functional theory (DFT) calculations 

[9,11, and references therein], which do not account for long-range van der Waals (vdW) 

57

interactions [12-15]. This deficiency is responsible for the tendency of LDA to favor diamond 

with respect to graphite as the most stable carbon phase [9,11] and for the poor performance 

of GGA in predicting the interlayer distance in graphite [9,11]. It has recently been 

demonstrated that inclusion of the vdW interactions in the simulations provides the 

experimental interlayer distance, which profoundly affects the energy and properties of 

intrinsic defects in the bulk of graphite [9]. Here we extend this work further by calculating 

the geometries, formation energies, and diffusion barriers of intrinsic defects at the surface of 

graphite taking into account the vdW interactions [11]. 

It is shown that the calculated formation energies and diffusion barriers of subsurface 

interstitial (I) atoms deviate qualitatively and quantitatively from those of surface adatoms. 

The same trend is found also for subsurface and adatom clusters (I2, I3). In spite of the semiquantitative 

agreement on the optimized geometries, the formation energies and diffusion 

barriers of surface and subsurface vacancies (V), divacancies (VV), and intimate (I-V) 

Frenkel pairs differ significantly from the values for the analogous defects in the bulk of 

graphite. This suggests limited transferability of the bulk and subsurface defect models to the 

surface of graphite. These findings are rationalized in terms of the balance between the 

covalent and vdW interaction terms at the surface, subsurface, and bulk of graphite. Finally, 

pairing of individual defects (adatoms, I and V) is calculated to be energetically advantageous 

both on the surface and in the subsurface regions. This process is shown to either saturate 

residual dangling bonds or produce singlet spin states, thus contributing to the quenching of 

residual spin polarization from damaged graphite surfaces. 

This work was supported by a specially promoted Grant-in-Aid for Scientific Research (Contract No. 19001002) 

from the Ministry of Education, Culture, Sports, Science and Technology of Japan. 

[1] T. Tanabe, Phys. Scr. T64, 7 (1996) 

[2] J. H. W. Simmons, Radiation Damage in Graphite, Pergamon London (1965) 

[3] L. Arnold, Windscale 1957: Anatomy of a Nuclear Accident, Palgrave MacMillan, London (1995) 

[4] A. V. Krasheninnikov and F. Banhart, Nature Mater. 6, 723 (2007) 

[5] P. Avouris, Z. Chen, and V. Perebeinos, Nat. Nanotechnol. 2, 605 (2007) 

[6] I. Žutić, J. Fabian, and S. Das Sarma, Rev. Mod. Phys. 76, 323 (2004) 

[7] M. S. Kane, L. C. Kao, R. K. Mariwala, D. F. Hilscher, and H. Foley, Ind. Eng. Chem. Res. 35, 3319 (1996) 

[8] G. Mestl, N. I. Maksimova, N. Keller, V. V. Roddatis et. al., Angew. Chem., Int. Ed. 40, 2066 (2001) 

[9] G.Teobaldi, H. Ohnishi, K. Tanimura, and A. L. Shluger Carbon 48, 4145 (2010) 

[10] Y. Ma, Phys. Rev. B 76, 075419 (2007) 

[11] G. Teobaldi, K. Tanimura, and A. L. Shluger, Phys. Rev. B 82, 174104 (2010) 

[12] H. Rydberg, M. Dion, N. Jacobson, E. Schröder, et. al., Phys. Rev. Lett. 91, 126402 (2003) 

[13] D. C. Langreth, B. I. Lundqvist, et. al., J. Phys.: Condens. Matter 21, 084203 (2009) 

[14] A. Tkatchenko and M. Scheffler, Phys. Rev. Lett. 102, 073005 (2009) 

[15] O. A. von Lilienfeld and A. Tkatchenko, J. Chem. Phys. 132, 234109 (2010) 

58

Wednesday 9 th March 2011

STM investigations of pure and Sn-doped In2O3 surfaces 

Ulrike Diebold*, Daniel Hagleitner, Michael Schmid 

Inst. of Applied Physics, Vienna University of Technology, Wiedner Hauptstrasse 8-10, A- 

1040 Vienna, Austria 

Tin-doped Indium Oxide (ITO) is a transparent conducting oxide that finds wide use 

in a variety of industrial applications such as gas sensing, solar cells, and organic 

light-emitting diodes. While the surface properties of this material play a key role in 

these and many other applications, surprisingly little information exists about its 

fundamental surface characteristics. 

We will present LEED, XPS, UPS, RHEED, LEED, and STM results of epitaxial ITO 

films, grown on Yttria-stabilized Zirconia, and discuss geometric models that are 

derived from atomically-resolved STM images and DFT calculations [1, 2]. Surface 

polarity seems to play a major role in the stability and surface structure of ITO films. 

The (111) surface of the bixbyite structure is non-polar and ITO(111) surfaces are 

essentially bulk-terminated with a 1x1 periodicity. The apparent topology in STM 

images is dominated by the physical corrugation. The (100) surface is polar, and 

atomically-resolved STM images show a rich structure that can be reconciled with a 

dimerization of surface O atoms that is predicted by theoretical calculations, as well 

as adsorbates at specific lattice sites. 

First results on (undoped) In2O3 single crystals will also be discussed. The as-grown 

samples have the form of small cubes with (100)-oriented facets. They are pale 

yellow at room temperature and show a reversible darkening upon heating in UHV to 

a temperature below 500°C. While the LEED pattern is sharp and compatible with 

the one from epitaxial thin films, STM shows a significant disorder in the atomicscale 

structure. The preference for double-height steps suggests that one type of 

termination is preferred. 

[1] E. Morales, Y. He, B. Delley, and U. Diebold, New Journal of Physics, 10 (2008) 125030; 

[2] E. Morales and U. Diebold, Applied Physics Letters, 95 (2009) 253105 

______________ 

*diebold@iap.tuwien.ac.at 

60

Abstract 3S´11, 6 March - 12 March, 2011 

Fabrication of a NiOx nanodot superlattice 

1 Svetlozar Surnev, 1 Luca Gragnaniello, 1 Francesco Allegretti, 1 Teng Ma, 2 A. Fortunelli, 

2 G. Barcaro, 1 Falko P. Netzer 

1 Institute of Physics, Surface and Interface Physics, Karl-Franzens University Graz 

A-8010 GRAZ, Austria 

2 Molecular Modeling Laboratory, Istituto per i Processi Chimico-Fisici (IPCF-CNR) 

I-56124 PISA, Italy 

(falko.netzer@uni-graz.at) 

Here we report the fabrication of a NiOx nanodot superlattice, supported on an ultrathin AlOx 

film, with a narrow size distribution. The preparation involves a two-step process: i) 

formation of a superlattice of metallic Ni clusters via PVD followed by seeded selfassembly; 

ii) morphology conserving oxidation of the metallic Ni clusters to NiOx nanodots. The 

alumina substrate is prepared by thermal oxidation of a Ni3Al(111) single crystal surface, 

yielding a 2 layer thick AlOx film which contains a superstructure of holes [1], acting as 

anchoring sites. After seeding with Pd adatoms [2] Ni clusters with a narrow size distribution 

can be grown (Fig. 1): the average size can be varied and controlled by the amount of the Ni 

deposited. The Ni clusters are exposed to oxygen under kinetic conditions which ensure a 

morphology conserving oxidation. The structure and morphology of the Ni and NiOx 

superlattice have been investigated by STM and LEED measurements, whereas the oxidation 

process has been followed by high-resolution photoelectron spectroscopy with use of 

synchrotron radiation (MAX-Lab, Lund). By varying the emission angle in angle-dependent 

photoemission experiments the interaction of the Ni(Ox) clusters with the AlOx substrate has 

been probed: the cluster-AlOx and the AlOx–Ni3Al interfaces can thus be separated 

spectroscopically. The AlOx-Ni3Al interface has been investigated by DFT calculations and 

the influence of the growth of Ni(O) nanoparticles on the buried alumina-alloy interface has 

been modelled. Moreover, the question of the formation of a core-shell Ni-NiOx nanoparticle 

morphology is addressed. 

Work supported by the ERC Advanced Grant SEPON 

[1] M. Schmid et al., Phys. Rev. Lett. 99 (2007)196104 

[2] G. Hamm, C. Becker, C.R. Henry, Nanotechnology 17 (2006) 1943 

61 

Fig.1, left: STM image 

(200x200 nm 2 ) of 1 Ǻ Ni 

clusters deposited on a 

Al2O3/Ni3Al(111) surface; 

right: size distribution of 

the Ni clusters

Tuning the magnetic moment of individual molecules at 

the metallic interface 

A.Mugarza 1 , C.Krull 1 , Roberto Robles 1 and P.Gambardella 1,2 

1 Centre d’Investigació en Nanociència i Nanotecnologia, CIN2 (ICN-CSIC), UAB Campus, Facultat 

de Ciencies Edifici CM7, E-08193 Bellaterra (Barcelona), Spain 

(corresponding author: aitor.mugarza.icn@uab.es) 

2 Institució Catalana de Recerca i Estudis Avançats (ICREA), E-08100 Barcelona, Spain 

Understanding the magnetic structure of molecules at the interface with metals is fundamental 

for their implementation in magnetic or spintronic nanodevices. However, the intricate 

relation between the molecule-metal interaction, magnetic, and transport properties leads to a 

rich variety of magneto-electronic effects that are difficult to predict [1,2]. This interaction 

can be effectively tuned by controlling the coordination between ion and ligand [3,4], 

switching the molecular conformation [5], or modifying chemical bonds [6]. Although in such 

systems the ligand generaly plays the role of tuning the magnetic properties of the ion by 

setting the ligand field environment and controlling the interaction between ion and substrate, 

recent experiments indicate that they can also participate directly in magnetism, providing a 

delocalized spin that couples both to substrate and ion electrons [7]. 

Here we present a combined scanning tunneling spectroscopy (STS) and ab-initio study of 

four different types of MPc complexes (M = Fe, Co, Ni, Cu) adsorbed on the Ag(100) surface. 

Interestingly, we find that the general tendency to quench the magnetic moment upon 

adsorption observed in this class of molecules can be inverted by combining ions with d 

orbitals near the Fermi level that are decoupled from the substrate [8,9], with ligand orbitals 

that can accept electrons [7]. In such way, the molecules can acquire a more complex 

magnetic structure with coupled ion and ligand spins. We further show that their Kondo 

behavior correspond to that of an inverted role of ion and ligand where the ion-substrate 

coupling determines the existence of a spin at a ligand orbital and its interaction with the 

substrate. 

[1] K. Moth-Poulsen and T. Bjørnholm, Nat. Nanotech. 4, 551 (2009) 

[2] H.Wende et al., Nature Materials, 8, 165 (2007) 

[3] P. Wahl et al., Phys. Rev. Lett. 95, 166601 (2005). 

[4] P. Gambardella et. Al., Nat. Mat. 8, 189 (2009). 

62

[5] V. Iancu et al., Nanolett. 6, 820 (2006). 

[6] A. Zhao et al., Science 309, 1542 (2005). 

[7] A. Mugarza et al., submitted. 

[8] S. Stepanow et al., Phys. Rev. B 82, 014405 (2010). 

[9] A. Mugarza et. al., Phys. Rev. Lett. 105, 115702 (2010). 

63

Spin-resolved Photoelectron Spectroscopy of Mn6Cr 

Single-Molecule-Magnets and of Manganese Compounds 

as Reference Layers 

Andreas Helmstedt, Aaron Gryzia, Niklas Dohmeier, Norbert Müller, Armin Brechling, 

Marc Sacher, Ulrich Heinzmann, Veronika Hoeke 1 , Thorsten Glaser 1 , Mikhail Fonin 2 , 

Ulrich Rüdiger 2 , and Manfred Neumann 3 

Faculty of Physics, Bielefeld University, 33501 Bielefeld, Germany 

1 Faculty of Chemistry, Bielefeld University, 33501 Bielefeld, Germany 

2 Department of Physics, University of Konstanz, 33501 Bielefeld, Germany 

3 Department of Physics, Osnabrueck University, 49069 Osnabrück, Germany 

The properties of the manganese-based single-molecule-magnet (SMM) Mn6Cr are studied. 

This molecule exhibits a large spin ground state of ST =21/2. It contains six manganese 

centres arranged in two bowlshaped Mn3-triplesalen building blocks linked by a 

hexacyanochromate. The Mn6Cr complex can be isolated with different counterions which 

compensate for its triply positive charge. The spin polarization of photoelectrons emitted from 

the manganese centres in Mn6Cr SMM after resonant excitation with circularly polarized 

synchrotron radiation has been measured at selected energies corresponding to the prominent 

Mn L3VV and L3M2,3V Auger peaks. Spin-resolved photoelectron spectra of the reference 

substances MnO, Mn2O3 and Mn(II)acetate recorded after resonant excitation at the Mn-L3edge 

around 640eV are presented as well. The spin polarization value obtained from MnO at 

room temperature in the paramagnetic state is compared to XMCD measurements of Mn(II)compounds 

at 5K and a magnetic field of 5T. We additionally report on XAS measurements 

with the adsorbed Mn6Cr SMM layers in cross comparison of three different counterions with 

respect to the stability of the SMM molecules as a function of the x-ray radiation dosis 

absorbed. 

64

Magnetism of Covalently Functionalized Graphene 

Elton J. G. Santos, Andrés Ayuela and Daniel Sánchez-Portal 

Donostia International Physics Center, Paseo Manuel de Lardizabal 4, 20018 San Sebastián, 

Spain. 

Centro de Física de Materiales (CFM-MPC), CSIC-UPV/EHU, Paseo Manuel de Lardizabal 5, 

20018 San Sebastián, Spain. 

sqbsapod@ehu.es 

We have recently applied ab initio density functional calculations to explore the magnetism 

induced by several types of defects in graphene and graphenic nanostructures, including doping 

with transition metals [1,2,3] and vacancies [4]. In the present contribution we concentrate on 

the effect of covalent functionalization on the electronic structure and magnetism of graphene 

[5] and single-walled carbon nanotubes [6]. We have performed calculations of the 

functionalization of graphene layer with alkanes, polymers, diazonium salts, aryl and alkyl 

radicals, nucleobases, amide and amine groups, sugar and some organic acids. We find that, 

independently of the particular adsorbate, whenever a molecule is linked to the carbon layer 

through single C-C covalent bond, a spin moment of 1.0 μB is induced. This is similar to the 

effect of H adsorption, which saturates the pz orbital in the layer, and can be related to the spin 

moment observed for a single carbon vacancy in a simple π-tight-binding description of the 

graphene layer. Consistently with this analogy, the calculated spin moment is almost entirely 

localized in the carbon layer, with an almost negligible contribution from the adsorbate (see 

Figure 1 below). When the electronegativity of the atom bonded to the layer increases, even if 

still linked through a single bond, the value of the observed spin moment is modified and 

eventually goes to zero. 

The magnetic coupling between adsorbates has also been studied, using H and CH3 for 

graphene [5] and only H for the nanotubes [6], and revealed a key dependence on the sublattice 

adsorption site (see Figure 2 below): Only molecules at the same sublattice stabilize a 

ferromagnetic spin order, with exchange coupling decaying quite slowly. When the molecules 

are adsorbed in different sublattices we always converge to non-magnetic solutions, at least for 

the supercell sizes used here. Using our previous analogy with a π-vacancy, we can now 

understand this behavior in terms of the so-called Lieb theorem for bipartite lattices [7]. In the 

case of the carbon nanotubes, exchange interactions are much larger and have a slower decay 

for metallic than for semiconducting tubes. 

[1] Elton J. G. Santos, A. Ayuela and D. Sánchez-Portal, New Journal of Physics, 12 (2010) 053012 

[2] Elton J. G. Santos, D. Sánchez-Portal and A. Ayuela, Phys. Rev. B, 81 (2010) 125433 

[3] Elton J. G. Santos et al. Phys. Rev. B, 78 (2008) 195420 

[6] Elton J. G. Santos, S. Riikonen, D. Sánchez-Portal and A. Ayuela, arXiv:1012.3304v1. 

[5] Elton J. G. Santos, A. Ayuela and D. Sánchez-Portal, submitted 

[6] Elton J. G. Santos, D. Sánchez-Portal and A. Ayuela, submitted 

[7] E. H. Lieb, Phys. Rev. Lett. 62, (1989) 1201 

Figure 1: (a) Scheme of the “on top” adsorption geometry, through single covalent bond to the layer, 

considered in this work. Panels (b)-(e) show the isosurfaces of the magnetization density induced by the 

adsorption of the Adenine group, CH3, Pmma and PTFE on the carbon surface. The cutoff is at 

±0.0191431 e/bohr 3 . Positive and negative spin densities correspond respectively to light and dark 

surfaces, which alternate on graphene atoms with a slow decay length in all cases. Panels (f) and (g) 

show, respectively, the spin polarized band structures for a 8x8 graphene supercell with, respectively, a 

single Adenine radical and a CH3 molecule chemisorbed on top of a carbon atom. The black and red 

lines denote the majority and minority spin bands, respectively. EF is set to zero. 

65

Figure 2: Exchange coupling as a function of the position of two adsorbates, H and CH3, chemisorbed 

on top of a C atom in a 8x8 graphene supercell. One of the molecules is moved along (a) the armchair 

and (b) the zigzag directions, while the other remains at the origin. The filled and empty squares 

correspond, respectively, to H and CH3 at the same sublattice (e.g. AA). Triangles correspond to both 

adsorbates at different sublattices (e.g. AB), where it was impossible to stabilize magnetic solutions. 

The circles correspond to the best fit of the AA data to a Heisenberg model. 

66

Characterization of an oxalic acid layer on Cu(111). 

M. N. Faraggi 1 , M. Trelka 2 , C. Isvoranu 3 , C. Martí-Gastaldo 4 , E. Coronado 4 , J. Schnadt 3 , J. M. 

Gallego 5,6 , R. Otero 2,5 , R. Miranda 2,5 and A. Arnau 1,7 

1 

Donostia International Physics Center (DIPC), P. Manuel de Lardizabal 4, 20018 San Sebastián, 

Spain. 

2 

Dpto. de Física de la Materia Condensada, Universidad Autónoma de Madrid, Cantonblanco, 

28049-Madrid, Spain. 

3 

Dpto. of Synchrotron Radiation Research, Lund University, Box 117, S-22100 Lund, Sweden. 

4 

Instituto de Ciencia Molecular (ICMol) 46980, Paterna, Spain. 

5 

Instituto Madrileño de Estudios Avanzados en Nanociencia (IMDEA-Nanociencia), 

Cantoblanco, 28049-Madrid, Spain. 

6 

Instituto de Ciencia de Materiales de Madrid - CSIC, Cantoblanco, 28049-Madrid, Spain. 

7 

Departamento de Física de Materiales, Facultad de Química UPV/EHU and Centro de Física de 

Materiales CFM-MPC, Centro Mixto (CSIC-UPV/EHU), San Sebastián, Spain. 

In this work, we present a combined experimental and theoretical study of the geometric and 

electronic structure of the self-assembled monolayer nanostructure formed by deposition of 

oxalic acid (C2O4H2) (Fig. 1(a)) on a Cu(111) surface. Both scanning tunneling microscopy 

(STM) and photoemission (XPS) techniques are used to characterize the nanostructure formed 

before and after thermal annealing. First principles density functional theory (DFT) 

calculations permit to perform an energetic analysis that helps in understanding several 

questions that remain unclear in the experiment, like the role of the deprotonation of the 

carboxylic group of the molecules or the possibility of forming coordination bonds with Cu 

adatoms upon adsorption. 

Based on DFT calculations through VASP [1, 2] code we obtain the binding energy for 

different monomers configurations. Energetic results point out to a strong preference of the 

molecule to be absorbed after deprotonation (confirmed by XPS measurements) and bonding 

with cooper adatoms. This allows us to propose a double deprotonated molecule with two 

cooper adatoms as candidate to understand the STM topographical images (Fig. 1 b)). Making 

use of a Bader charge analysis and induced charge density we analyze the chemical 

environment of the proposed structure. The combined study with experimental data unveils 

the coexistence of different moieties of the oxalic acid -produced by dissociative processes- 

that depend on the coverage level. Going further and taking into account the molecular crystal 

structures [3] we propose a plausible geometry description for the oxalic acid monolayer, 

which includes Cu adatoms on the Cu(111) surface and deprotonation of the molecule, given 

place to the simulated STM image, shown in Fig.1 c). 

67

a) b) c) 

C2O4H2 

Figure 1: a) Acid oxalic molecule, STM images (7x7 nm) b) experimental and c) simulation 

[4]. 

References 

[1] G. Kresse, J. Hafner, Phys. Rev. B 49 (20), 14251, 1994. 

[2] G. Kresse, J. Furthmuller, Phys. Rev. B 54 (16), 11169, 1996. 

[3] I. Nobeli, and S. L. Price, J. Phys. Chem. A 103, 6448 (1999). 

[4] I. Horcas, R. Fernandez, J. M. Gomez-Rodriguez, J. Colchero, J. Gomez-Herrero and A. M. Baro, Rev. Sci. 

Instrum. 78 013705 (2007). 

68

Study of NO reduction by H2 on a Pt(110) model catalyst 

in a High-Pressure STM 

Q. Liu, C.T. Herbschleb, J.W. Bakker, V. Navarro, M.E. Cañas Ventura, P.C. van der 

Tuijn, A. Ofitserov, G.J.C. van Baarle, J.W.M. Frenken 

Kamerlingh Onnes Laboratory, Leiden University, P.O. Box 9504, 2300 RA Leiden, 

The Netherlands 

The emission of nitrogen oxide in industrial processes or from motor 

vehicles is an important problem due to its poisonous nature and 

environmental impact. A way to neutralize nitrogen oxides is by reducing 

them through a catalytic reaction. We have studied the NO reduction by 

H2 over a platinum (110) surface at high pressure and elevated 

temperature with a combination of STM and simultaneous mass 

spectrometry. 

In order to study catalytic reactions under realistic conditions, we make 

use of a newly constructed scanning tunneling microscope (STM) that 

allows imaging the surface with atomic resolution while it is active as a 

catalyst, i.e. during the reaction. For this purpose, the STM is integrated 

with a small (0.5ml), high-pressure flow reactor, which is located inside 

an ultrahigh vacuum (UHV) chamber. In this design sample and tip are 

placed inside the high pressure reactor which is separated from the UHV 

chamber by a Kalrez seal. A combination of load-lock system, sample 

transfer bar and wobble stick allows fast sample transfer between 

ambient environment, high-pressure reactor and a separate sample 

preparation chamber, which is equipped with sputter gun, evaporators 

and LEED. This one-of-a-kind instrument operates over a wide pressure 

range, from UHV to several bar, and at elevated temperatures, from 300 

to 600 K, while the reactive gas mixture, in this case NO and H2, is 

flowing over the catalyst surface. 

First measurements with the ReactorSTM will be shown for the 

reduction of NO by H2 on Pt(110), which suggests that the catalyst 

surface structure changes depending on the precise gas composition. 

69

Ultrafast screening of a point charge at a metal surface 

V. M. Silkin 1,2,3, V. Despoja 1, E. V. Chulkov 1,2,4, and P. M. Echenique 1,2,4 

1 Donostia International Physics Center (DIPC), P. de Manuel Lardizabal 4, 20018 San Sebastián, 

Basque Country, Spain 

(corresponding author: V. M. Silkin, e-mail: waxslavs@sc.ehu.es) 

2 Depto. de Física de Materiales, Facultad de Química, Universidad del País Vasco, Apdo. 1072, 

20080 San Sebastian, Basque Country, Spain 

3 IKERBASQUE, Basque Foundation for Science, 48011, Bilbao, Spain 

4 Centro de Física de Materiales (CFM-MPC), Consejo Superior de Investigaciones Cientificas 

(CSIC) – Universidad del País Vasco (UPV-EHU). P. de M. Zardizabal 5, 20018 San Sebastián, 

Basque Country, Spain 

The space-time evolution of the dynamical screening charge density caused by a suddenly 

created point charge at the Cu(111) surface is investigated in the linear response 

approximation. Considering finite-thickness slabs as a model for the Cu(111) surface we 

investigate the confinement effects on dynamical screening as well. The results have been 

obtained on base of self-consistent evaluation of the energy-momentum dependent response 

function taking into account the realistic surface band structure of Cu(111). 

At the initial stage, we observe fast long range charge density oscillations due to excitation of 

the surface plasmon modes. Then we observe the propagation of the shock wave of the 

electron-hole excitations along the slab with velocity determined by the Fermi velocity of bulk 

Cu. At longer times, we have identified the propagation along the two slab surfaces of much 

slower (with velocity ~0.3 a.u., close to the Fermi velocity of the Cu(111) surface state) 

charge disturbance due to acoustic surface plasmon. A role of the energy band gap in the 

direction perpendicular to the surface on the establishing of the screening is also addressed. 

70

Plane-wave based Electron Tunneling 

through Au Nanojunctions 

A. Garcia-Lekue, L.W. Wang 1 

Donostia International Physics Center (DIPC), Paseo Manuel de Lardizabal 4, 

E20018, San Sebastian, Spain 

(corresponding author: A. Garcia-Lekue, e-mail: wmbgalea@lg.ehu.es) 

1 Lawrence Berkeley National Laboratory (LBNL), Berkeley 94720, United States 

Electron tunneling across a nanojunction is an important topic relevant to STM imaging, 

nanoconductance measurements, and nano electronic devices. To understand such tunneling 

phenomena, one needs to comprehend the electron state coupling between the metal electrode 

and the vacuum, the dependence of such coupling on the shape of the electrode tip, and the 

dependence of the tunneling currents on the electrode-electrode distance. Due to the 

experimental difficulty to determine the exact atomic structure of the electrode tip, theoretical 

simulation can play an important role on such studies. This requires high fidelity quantum 

transport calculations for the tunneling system. However, most of the current quantum 

transport calculations are performed using atom centered localized basis sets, which cannot 

adequately describe the wave function in the vacuum region. 

In this work, we present tunneling conductance calculations obtained using the transport 

calculation method introduced in Refs. [1]-[2]. Such method calculates the scattering states in 

quantum transport problems using a plane-wave basis set, is valid for any applied bias voltage 

and can include exact evanescent states, which are also calculated using plane-waves. Since 

the plane-wave basis set is variational enough to describe the wave function in the vacuum 

region, this quantum transport calculation method is expected to be well suited to describe the 

tunneling problem. 

By means of our plane-wave based quantum transport calculation method we have thoroughly 

investigated electron tunneling through Au nanojunctions. Special emphasis has been placed 

on the study of the apparent barrier height, an experimentally measurable quantity strongly 

influenced by electron tunneling mechanisms. We present theoretical results for the tunneling 

current dependence on the electrode separation, which exhibits perfectly exponential decay. 

By fitting the current versus electrode-separation curves we are able to extract theoretical 

values of the apparent barrier height. We have also investigated how the detailed atomic 

structure of the junction and the coupling between the electrode and the vacuum states affects 

the apparent barrier height. Further knowledge on the tunneling mechanisms has been 

extracted from the visualization of the scattering states.[3] 

71

Figure 1: Total transmission at the Fermi level for a broken Au chain as a 

function of the vacuum-gap width obtained using plane waves (solid line), 

atomic orbitals (dashed-line) and atomic orbitals with ghost atoms (dasheddotted 

line). 

Support from the Basque Departamento de Educaci\'on, UPV/EHU (Grant No. IT-366-07), the Spanish 

Ministerio de Ciencia e Innovaci\'on (Grant No. FIS2007-6671-C02-00), the ETORTEK program funded by the 

Basque Departamento de Industria and the Diputacion Foral de Guipuzcoa, and the DMS/BES/SC of the U.S. 

Department of Energy under Contract No. DE-AC02-05CH11231 are gratefully acknowledged. It used the 

resources of the National Energy Research Scientific Computing Center (NERSC). 

[1] L. W. Wang, Phys. Rev. B. 72, 045417 (2005) 

[2] A. Garcia-Lekue and L. W. Wang, Phys. Rev. B. 74, 245404 (2006) 

[3] A. Garcia-Lekue and L. W. Wang, Phys. Rev. B. 82, 035410 (2010) 

72

Structural studies of the metal-insulator transition in LaNiO3 thin films 

S. J. Leake 1 , S. A. Pauli 1 , M. Schmitt 1 , M. Garcia-Fernandez 2 , P. Aebi 2 , R. Scherwitzl 3 , 

P. Zubko 3 , J-M. Triscone 3 and P. R. Willmott 1 

1 

Swiss Light Source, CH-5232 Willigen PSI, Switzerland 

2 

Institut de Physique, Universite de Fribourg, CH-1700, Fribourg, Switzerland 

3 

DPMC, University of Geneva, CH-1211, Geneva, Switzerland 

The RNiO3 perovskites are an exciting family of rare earth (R) nickelates exhibiting a 

sharp metal-insulator (MI) transition with resistance increases of several orders of 

magnitude[1,2]. LaNiO3 has proved to be an exception, it is the only nickelate that does 

not show a MI transition in its bulk form remaining a metal, but was recently observed 

when grown as a thin film up to a critical thickness (tc) of 8 monolayers [3]. The physical 

properties of strongly correlated electron materials, such as LaNiO3, are well known to be 

sensitive to sub-Angstrom structural variations therefore a detailed structural knowledge 

is required to understand the nature of these phenomena. 

Surface X-Ray Diffraction (SXRD) can resolve the entire structure of the film under 

investigation with the required sub-Angstrom resolution. Through the determination of 

several thousand individual structure factors and the application of phase retrieval 

methods to recover the missing phase information lost during the measurement, a detailed 

structural model of the interface can be achieved. This is now possible within a 

reasonable timescale thanks to the increased availability of high brilliance hard x-ray 

synchrotron sources, the realisation of photon counting area detectors and the ongoing 

development of efficient phase retrieval algorithms[4]. 

The initial results of the film structure of LaNiO3 grown on strontium titanate will be 

described as a function of thickness around tc. The film was found to reconstruct 

completely close to tc, expected due to tilting of oxygen octahedra[5,6], and at tc a 

different symmetry of the reconstruction observed. The direct phasing methods 

implemented to deduce the atomic structure will also be discussed. 

[1] P. Lacorre et al., J. Solid State Chem., 91, 225 (1991) 

[2] J. B. Torrance et al., Phys. Rev. B, 45, 8209 (1992) 

[3] R. Scherwitzl et al., Applied Physics Letters, 95, 222114 (2009) 

[4] M Björck et al. J. Phys.: Condens. Matter, 20, 445006 (2008) 

[5] A. M. Glazer, Acta Crysta., B28, 3384 (1972) 

[6] S. J. May et al., Phys. Rev. B, 82, 014110 (2010) 

73

Structure and electronic properties of TC�Q-F4 deposited 

on clean Au(111) 

N. Gonzalez-Lakunza 1 , N. Jiang 2 , A. Langner 2 , S. Stepanow 2 , H-J. Gao 3 , K. Kern 2 , 

and A. Arnau 1,4 

Fisika Aplikatua I Saila, Donostiako UEP, UPV/EHU, Donostia-San Sebastian, Spain 

(corresponding author: �. Gozalez-Lakunza, e-mail: nora.gonzalez@ehu.es) 

1 Donostia International Physics Center, Donostia-San Sebastian, Spain 

2 Max Planck Institute für Festkörperforschung, Stuttgart, Germany 

3 Chinese Academy of Sciences, Beijing, China 

4 Materialen Fisika saila, Kimika Zientzien Fakultatea, UPV/EHU, and Centro de Física de 

Materiales, Centro Mixto CSIC-UPV/EHU, 

Donostia-San Sebastián, Spain 

Organic molecules with strong electron-accepting character such as tetracyanoquinodimethane 

(TCNQ) and its derivatives have been extensively studied in the last few years due to their 

unique properties when they are combined with other materials. For example, in combination 

with electron-donating organic species they form organic charge-transfer complexes that 

exhibit interesting optical properties for Organic Light Emission Diodes (OLEDs), 

photovoltaic response for solar devices, etc. In order to develop these devices, the complexes 

must be combined with metallic surfaces, and thus their performance depends critically on the 

electronic and structural properties of the organic/metal (OM) interface. 

In this work we study the adsorption of fluorinated TCNQ (TCNQ-F4) on the Au(111) 

substrate. We present a combined experimental and theoretical study. Experimentally, it has 

been observed that TCNQ-F4 molecules deposited on clean Au(111) readily self-assemble 

into large domains, where two features can be distinguished with a 1:1 ratio: spheres and 

ellipsoidal features. High resolution low temperature STM images allow proposing a model 

for the adsorption geometry, with ellipses interpreted as TCNQ-F4 molecules and spheres 

associated to Au adatoms released from the surface. DFT based theoretical calculations permit 

us to confirm this model by comparing experimental STM images with theoretical STM 

simulations, as shown in Figure1. This system had been previously described very briefly in 

ref. [1] too, where the proposed geometry do not consider the presence of the Au adatoms. 

In addition to the structural model, STS spectra and dI/dV maps also show very interesting 

electronic features. Spectra taken on the centre of the molecules clearly differ from the spectra 

74

taken on the metal adatoms. Measurements show that at low positive biases the signal from 

the adatoms is weaker than the signal coming from the TCNQ-F4 molecules. However, above 

∼ 800 mV there is a change in contrast and the signal is dominated by the Au adatoms. This 

behaviour is qualitatively reproduced by LDOS calculations. We also use different theoretical 

tools, like PDOS on molecular orbitals, induced densities, band structure calculations etc. to 

investigate the origin of these spectral features and get a detailed insight into the electronic 

structure of the system. 

Figure 1: Composition of experimental STM image, theoretical simulation and structural model of the 

adsorption geometry of TCNQ-F4 on Au(111) including Au adatoms. 

Support by the Spanish Ministerio de Ciencia e Innovación (ref. FIS2010-19609-C02-01) and Basque 

Government – UP/EHU (No. IT-366-07) is gratefully acknowledged. 

[1] F. Jäckel, U. G. E. Perera, V. Iancu, K.-F. Braun, N. Koch, J. P. Rabe and S.-W. Hla; Phys. Rev. Lett. 100, 

126102 (2008). 

75

Cobalt catalyst in action followed at high pressures with STM and SXRD during 

hydrocarbon synthesis 

V. Navarro 1 , S.B. Roobol 1 , R. van Rijn 1,2 , Q. Liu 1 , O. Balmes 2 , D. Wermeille 2 , A. Resta 2 , R.Felici 2 

and J.W.M. Frenken 1 

1 Interface Physics Group, Kamerlingh Onnes Laboratory, Leiden University, The Netherlands. 

2 Structure of Materials, ID03, European Synchrotron Radiation Facility, Grenoble, France. 

Contact email: navarro@physics.leidenuniv.nl 

The Fischer-Tropsch synthesis (FTS) is the catalytic reaction that leads to the production of 

hydrocarbons from a mixture of H2 and CO under certain conditions of high temperature and 

pressure. In the later years it has stimulated renewed interest as an alternative and clean 

source of synthetic fuel. Most of the scientific studies about this reaction have been performed 

from the point of view of the industry. However, basic research towards understanding the 

atomic and molecular aspects of the reaction mechanisms under industrial conditions is limited 

[1]. Under real reaction conditions catalysts are dynamic: they can change the surface 

morphology, structure and composition during the reaction [2]. 

In order to study the FT reaction from the atomic or molecular point of view, we need a system 

with a well defined and controlled amount of defects (steps, kinks, vacancy islands etc.) to study 

their role in the catalytic process. As a model catalyst we have used a Co(0001) single crystal. 

Most of the studies about hydrocarbon synthesis are performed under typical surface science 

conditions. Although surface science techniques can shed light onto the fundamental 

mechanisms that govern the catalytic processes, the conditions in those studies differ 

significantly from the ones at real industrial processes. Working pressures in surface science 

are several orders of magnitude lower than the pressures at which the industrial FTS takes 

place. 

We have performed studies on the FTS with two innovative surface science techniques that 

approach the conditions at which the reaction takes place in industry. Reactor scanning 

tunneling microscopy (STM) [3] and reactor surface X ray diffraction (SXRD) [4] are used to 

study in situ the catalytic surface under conditions of high pressure and high temperature. Both 

techniques are complementary since STM gives information about the local changes in the 

surface, and the SXRD gives information of the changes in long range periodic structures 

present on the catalyst and in particular on its surface. In both cases the instruments are 

configured in the form of small flow reactors that mimic industrial conditions by reaching 

pressures up to 2 bar and temperatures up to 320̊C. Since they are embedded inside a UHV 

chamber, surface science techniques can be used to prepare and characterize the samples. 

These techniques allow us to study the morphological and structural changes on the catalyst 

surface while the reaction takes place as well as the analysis of the reaction products by 

quadrupole mass spectrometry. 

References: 

1- J. Wilson et al., J. Phys. Chem. 99, 7860-7866 (1995). 

2- B. L. M. Hendriksen and J. W. M. Frenken. Phys. Rev. Lett. 89, (2002). 

3- C.T. Herbschleb et al., to be submitted. B.L.M. Hendriksen, et al., Topics in Catalysis, 36, 1–4 

(2005). 

4- R. van Rijn et al., Rev. Sci. Instrum. 81, 014101 (2010). 

76

The high pressure oxidation and reduction by CO of Rh 

– from single crystal to nanoparticles 

S. Blomberg 1 , J. Gustafson 1 , R. Westerström 1 , J. N. Andersen 1 , M. Messing 2 , K. Deppert 2 , 

N. Martin 1 , M. E. Grass 4 , Z. Liu 4 , H. Bluhm 4 , E. Lundgren 1 

1. Division of Synchrotron Radiation Research, Lund University, Lund, Sweden. 

2. Solid State Physics, Lund University, Lund, Sweden 

4. ALS, Lawrence Berkeley National Laboratory, Berkeley, CA, USA. 

sara.blomberg@sljus.lu.se 

The catalytic properties of the Pt-group metals have been studied intensely for many years. 

Because of the material complexity of a real industrial catalyst, model systems have been 

developed such as single crystal surfaces under Ultra High Vacuum (UHV) conditions. This 

effort has resulted in a fundamental understanding of surface reactions by different gases [1]. 

Therefore, UHV and ex situ techniques have been dominating the investigations and for real 

catalysis the results may not reflect realistic conditions. Recently, experimental techniques 

compatible with a higher gas pressure have been developed, which allow in situ monitoring of 

the chemical and structural state of adsorbates and substrates under more realistic pressure 

conditions [2-5]. 

In this contribution we report on the in situ oxidation and reduction by CO of aerosol 

deposited Rh particles and single crystals using High Pressure X-ray Photoelectron 

Spectroscopy (HPXPS). This technique can be used in pressures up to 1 Torr and due to its 

high surface sensitivity small changes of the surface state can be observed in situ. This is 

important for the determination of the active phase of the surface during, e.g., CO or CH4 

oxidation. 

The Rh particles were deposited by an aerosol method [6] onto a Si wafer with a native SiOx 

surface layer. The particle size can be controlled to a high degree as confirmed by SEM 

studies as shown in Figure 1 a) where an even and narrow size distribution of the particles can 

be seen. In Figure 1 b) is a TEM image showing the hexagonal shape of the pristine particles. 

The oxidation of 21 nm Rh particles as well as a Rh(100) single crystal was followed in situ 

with HPXPS at an O2 pressure of 0.1 mbar and increasing temperature. 

a b 

Figure 1 a) SEM image of the pristine 21 nm Rh sample and b) TEM image of the 21 nm Rh 

particle. 

77

The Rh 3d5/2 spectra in Figure 2 shows in situ oxidation series of 21 nm particles and Rh(100) 

single crystal in 0.1 mbar O2. In the spectra it can be seen that the RhO2 surface oxide [7], and 

the surface of the Rh2O3 bulk oxide has similar core level shifts of 0.8 eV and they are fitted 

with a common component, while the bulk of the Rh2O3 oxide is shifted further to 1.12 eV 

relative to the metallic bulk. The oxidation series shown in Figure 2 start with purely metallic 

samples but, as can be seen, the oxidation starts already at 140°C. At this temperature the 

single crystal exhibits only a surface oxide while the bulkoxide formation has started for the 

particles. Not until the temperature has been increased by another 150°C we detect the bulk 

oxidation of the single crystal. When the temperature is increased even further, the oxide 

continues to grow. 

The bulkoxide formation has also been studied with STM, which shows a very rough surface 

under the same conditions as when the bulkoxide is observed in HPXPS. Finally, the 

reduction of the Rh2O3 in 0.1 mbar CO is discussed. SEM images after oxidation and 

reduction cycles show that the particles are still present on the surface without any major 

changes in size or shape. 

Figure 2. In-situ oxidation HPXPS Rh 3d5/2 spectra of a) 21 nm Rh particles and b) Rh(100) 

in 0.1 mbar O2. 

Support by the Swedish Research Council, the Crafoord Foundation, the Knut and Alice Wallenberg Foundation, 

the Swedish Foundation for Strategic Research and the Anna and Edwin Berger foundation is gratefully 

acknowledged. 

References 

[1] G. Ertl, H. Knözinger, and J. Weitkamp, Handbook of Heterogeneous Catalysis. Wiley, New 

York, 1997 

[2] A. Stierle and A. Moelenbroek, MRS Bull. 32, 1000 (2007). 

[3] E. Lundgren and H. Over, J. Phys. Condens. Matter 20, 180302 (2008). 

[4] H.-J. Freund, Surf. Sci. 601, 1438 (2007). 

[5] D.F. Ogletree, H. Bluhm, E.B. Hebenstreit, M. Salmeron, Nucl. Instrum. Methods A 601 (2009). 

[6] M. E. Messing, K. A. Dick, L. R. Wallenberg and K. Deppert, Gold Bull., 20, 42, (2009). 

[7] J. Gustafson et al, Phys. Rev. B, 71, 115442 (2005) 

78

Water Adsorption on Clean and Oxygen Decorated 

Metal Substrates 

P. Cabrera-Sanfelix, 1,* S. Maier, 2 I. Stass, 2,3 Byoung-Young Choi, 2 Yu Shi 2,4 A. 

Arnau, 1,5,6 M. Salmeron 2,4 and D. Sánchez-Portal. 1,6 

1 Donostia International Physics Center (DIPC), P. Manuel de Lardizabal 4, San Sebastian 

20018, Spain 

2 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, 

USA 

3 Institut für Experimentalphysik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, 

Germany 

4 Dep. of Materials Science and Engineering, Univ. of California Berkeley, Berkeley, CA 94720, 

USA 

5 Dep. de Física de Materiales UPV/EHU, Fac. de Química, Apdo. 1072, San Sebastian 20080, 

Spain 

6 Centro de Física de Materiales CSIC-UPV/EHU, Materials Physics Center MPC, P. Manuel de 

Lardizabal 5, San Sebastian 20018, Spain 

swbcasam@sc.ehu.es 

Water growth on metal surfaces has been a topic of debate during many years. 

Nowadays, it is still unclear what are the growth mechanisms and the structures for 

water layers on many substrates. An important ingredient to understand the selforganization 

of water in metallic surfaces is the competition between inter-molecular 

and metal-substrate interactions. This competition determines whether the molecules (i) 

cluster in a 3D-phase, (ii) wet the surface following a well-ordered 2D-adlayer or even, 

in a more complicated scenario, (iii) a significant fraction of water molecules in the 

[1, 2] 

layer undergo partial dissociation (thus, decomposing into H and OH). 

Both, the inter-molecular and metal-substrate interactions, are strongly influenced by 

the substrate itself and by co-adsorbing other adsorbates. [3] In this work we have 

explored both types of interaction in the adsorption of water on clean Cu(110) and 

oxygen decorated Ru(0001) surfaces using Density Functional Theory (DFT) 

calculations motivated by recent scanning tunneling microscopy experiments. 

The wetting of Cu(110) has been recently reported. Experimental results showed that at 

low temperature and coverage water forms one dimensional arrays of side-sharing 

pentagons on Cu(110). [4, 5] These pentagon row structures can evolve to hexagon row 

structures as the coverage of water increases beyond 0.2 layers, according to the 

experimental evidence. [5] It is then proposed that this transformation is mediated by the 

formation of lobe structure (LS) clusters made of n pentagons connected to a central 

hexagonal ring (n-LS). These cluster configurations are characterized by the appearance 

of additional bright spots in the experimental STM images (see Figure 1). The most 

frequent LS configurations are the 4-LS and they are believed to act as a catalyst for the 

propagation of H-bond re-orientation along the chain of pentagons and thus, they could 

be considered the precursors of the transformation to the hexagonal row structure. Using 

DFT calculations, we compare the structures and energetics of pentagon and hexagon 

79

infinite row structures and explore the possible structures for the isolated 4-LS, 

depending merely on the molecules orientation. Finally, we present the several 4-LS 

“defects” into the pentagon row structure. [6] 

We also present results on the wetting of the oxygen decorated Ru(0001) surface at 0.5 

ML oxygen coverage, consistent with the O(2x1)/Ru(0001) surface. We found that the 

adsorption of water causes a shift of half of the chemisorbed oxygen atoms from hcp 

sites to fcc sites, creating a (2x2) honeycomb structure. Our calculations show that the 

energy cost of reconstructing the oxygen overlayer is more than compensated by the 

adsorption of water on the newly created unbounded Ru atoms. The STM images reveal 

a (4x2) water super structure, which is due to the existence of two relative orientations 

of the water molecules. Interestingly, the oxygen honeycomb structure induced by the 

adsorption of water remains metastable after water desorption and does not revert to the 

stable linear 2x1 structure until after temperatures close to 300K. [7] 

Figure 1. Upper panels correspond to experimental images: on the left STM images (30nm × 30nm) of the Cu(110) 

surface covered by 0.2 ML water after annealing to 120 K. The peculiar structures (named n-LS) are found in various 

locations: isolated (I), at the end of chains (E) and in the middle of chains (M 1 and M 2). On the right STM images 

(5nm × 15nm) of a chain composed of three segments offset by one lattice constant. The first image was obtained at 

0.1V and 30pA followed by three successive images at the same bias but higher tunnel current (80, 100 and 120pA). 

The 4-LS can be displaced right and left along the zigzag chain (green arrows). Downward panels correspond to DFT 

images for pentagonal row structure, on the left, and, on the right, the proposed hexagonal row structured based on 

the propagation of 4-LS clusters. 

[1] P. Cabrera-Sanfelix, A. Arnau, A. Mugarza, T. K. Shimizu, M. Salmeron, D. Sanchez-Portal, Physical 

Review B 2008, 78. 

[2] A. Michaelides, A. Alavi, D. A. King, Phys. Rev. B 2004, 69, 113404. 

[3] P. Cabrera-Sanfelix, D. Sanchez-Portal, A. Mugarza, T. K. Shimizu, M. Salmeron, A. Arnau, Phys. Rev. B 

2007, 76, 205438. 

[4] J. Carrasco, A. Michaelides, M. Forster, S. Haq, R. Raval, A. Hodgson, Nature Materials 2009, 8, 427. 

[5] B.-Y. Choi, Y. Shi, M. Salmeron, Submitted 2010. 

[6] Cabrera-Sanfelix P et al., In preparation. 

[7] S. Maier, P. Cabrera-Sanfelix, I. Stass, D. Sanchez-Portal, A. Arnau, M. Salmeron, Physical Review B 

2010, 82, 075421. 

80

Assembly and manipulation of supramolecular dynamers 

and rotatable sandwich complexes on a surface 

Dirk Kühne 1 , Florian Klappenberger 1 , Wolfgang Krenner 1 , Svetlana Klyatskaya 2 , 

Mario Ruben 2,3 , David Écija 1 , Willi Auwärter 1 , Saranyan Vijayaraghavan 1 , Knud 

Seufert 1 , Felix Bischoff 1 , Kentaro Tashiro and Johannes V. Barth 1 

1 Physik Department E20, TU München, James-Franck Str., D-85748 Garching, Germany; 2 

Institut für Nanotechnologie, Karlsruhe Institute of Technology, D-76021 Karlsruhe, 

Germany; 3 IPCMS-CNRS UMR 7504, Université de Strasbourg, 23 Rue du Loess, 67034 

Strasbourg, France; International Center for Materials Nanoarchitectonics, National 

Institute for Materials Science (NIMS, 1-1 Namiki, Tsukuba 305-0044, Japan 

The confinement of guest species in nanoscale environments leads to new dynamic 

phenomena. Notably the organisation and rotational motions of individual molecules were 

controlled by carefully designed, fully supramolecular host architectures. Here we use an open 

2D coordination network on a smooth metal surface to steer the self-assembly of discrete 

trimeric guest units, identified as noncovalently bound dynamers. Each caged chiral 

supramolecule performs concerted, chirality-preserving rotary motions within the template 

honeycomb pore, which are visualized and quantitatively analyzed using temperaturecontrolled 

scanning tunneling microscopy. With increased thermal energies a constitutional 

system dynamics appears, that is revealed by monitoring repetitive switching events of the 

confined supramolecules’ chirality signature, reflecting decay and reassembly of caged units. 

Fig. 1. Rotational motion of the caged supramolecular dynamer. a,b 60° orientational switching 

between a stable 1 and 2 configuration with conservation of the chirality signature; images recorded 

with a time lap of 204 s (VB = 1.0 V, IT = 0.05 nA, TSample = 64.4 K). c Orientation switching event while 

recording the topography at the position indicated with an arrow; the molecular units are exclusively 

imaged with positions reflecting preferred configurations (VB = -1.0 V, IT = 0.05 nA, TSample = 64.8 K). d,e 

Rapid fluctuation of rotator states with event rates largely exceeding imaging frequency. The envelope 

of the intracavity molecular features with different chirality corresponds to a superposition of the two 

respective stable configurations ((d): -rotator, VB = -1.0 V, IT = 0.1 nA, TSample = 78.3 K, (e): -rotator, VB 

= 0.2 V, IT = 0.22 nA, TSample = 81 K). f Model for the collective rotation superimposed on the data in (e). 

81

We furthermore report a novel route to synthesize 

bis(porphyrinato)cerium double-deckers (Ce(TPP)2) and 

tris(porphyrinato) cerium triple-deckers (Ce2(TPP)3) directly on a 

Ag(111) surface under ultrahigh vacuum conditions by exposing a 

porphyrin precursor layer to an atomic beam of cerium followed by 

temperature-programmed reaction and desorption of surplus 

material. The nature of the formed double and triple-decker 

compounds, representing azimuthal molecular rotors, is studied by 

scanning tunneling microscopy and spectroscopy measurements, 

including a comparison with Ce(TPP)2 layers generated by 

molecular beam epitaxy. In particular, we demonstrate the rotary 

motion of double- and triple-decker moieties in specific 

environments: the top porphyrin of each molecular species can be 

Top and side view of a 

Ce-double-decker 

rotated by STM manipulation provided spatial constraints do not interfere. 

Fig. 2. Left : Self-assembly of cerium porphyrinato sandwich compounds on Ag(111): islands of 

double-deckers, with some embedded triple-deckers complexes. The double-deckers islands coexist 

in phase with 2H-TPP domains. Ce dosage was 0.05 ML. Right: Molecular rotation of the top 

porphyrin of Ce(TPP)2 and Ce2(TPP)3 species on Ag(111). a)-c) By lateral manipulation (path of the tip 

from left to right is indicated) the top porphyrin of a CeTPP2 complex embedded in a 2H-TPP island is 

rotated 45º from a) to b) counter-clockwise and from b) to c) clockwise. d)-g) The top porphyrin of a 

Ce2(TPP)3 species embedded in a Ce(TPP)2 island is rotated 57º from position 1: d) and e), to position 

2: f) and g); while scanning at Vb = 0.3 V, with a tunneling current of 0.1 nA. For clarity, in the top-view 

modeling, only the top (red) and medium (green) porphyrins of the Ce2(TPP)3 complex are displayed. 

Images size: a-c) 40 x 40 Å 2 ; d) and f) 71 x 59 Å 2 . Scanning conditions: a-c) It = 0.1 nA, Vb = 1.4 V; d) 

and f) It = 0.1 nA, Vb = 1.4 V. 

Our findings demonstrate the potential of surface-confined self-assembled nanoporous layers 

to control dynamic phenomena of supramolecular species and advance the design of nano- 

architectures to the third dimension, that incorporate adaptive and rotatable homo- and 

potentially heteroleptic multi-decker complexes. 

82

Preparation of Monolayers of Mn6Cr Single-Molecule- 

Magnets on different Substrates and characterization by 

means of nc-AFM 

Aaron Gryzia, Armin Brechling, Hans Predasch, Ulrich Heinzmann, Thorsten Glaser 1 

Faculty of Physics, Bielefeld University, 33501 Bielefeld, Germany 

2 Faculty of Chemistry, Bielefeld University, 33501 Bielefeld, Germany 

The preparation of a highly ordered monolayer of Single-Molecule-Magnets (SMM) is one of 

the main preconditions for a technical application of these molecules. The adsorption of these 

SMMs on surfaces is associated with difficulties due to the often low chemical stability of 

these molecules in the vicinity of a surface. 

The used Mn6Cr-complex [1] has a C3-symmetry and a spin ground state of St = 21/2. This 

complex is a trication and needs therefore counter ions for electrical charge compensation. 

Tetraphenylborate, lactate and perchlorate came into consideration for this function. Mn6Cr- 

SMMs were prepared on different substrates by a droplet technique in air at room 

temperature. The samples were characterized by means of an AFM operating in non-contact 

mode, using tips with cone radii of approx. 2 nm. 

An island-like growth was observed on SiO2- and Si3N4-substrates, whereas on HOPG and 

mica the Mn6Cr-SMM adsorbates preferred a layer growth. Also an influence of the used 

counter ions was observed on different substrates. The measured thicknesses of the layers are 

consistent with the Van der Waals radii of the Mn6Cr-SMMs. 

[1] T. Glaser et al., Angew. Chem., 118, 6179-6183 (2006) 

83

Thermal decomposition of oxidised Silicon-on-insulator thin film 

T.Passanante, F.Leroy, E. Bussmann, F.Cheynis, P.Müller 

CINaM, UPR 3118 CNRS, Aix-Marseille Université , 

Campus de Luminy, case 913, F-13288 Marseille Cedex 9, France 

passanante@cinam.univ-mrs.fr 

Thin silicon films capped by an ultra-thin silicon oxide layer are basic building blocks 

in microelectronic. Microelectronics device fabrication requires thermal annealing steps 

which may induce drastic morphological changes to these building blocks. Here we explore, 

using Low Energy Electron Microscopy (LEEM) the real-time and in-situ annealing 

behaviour of an oxide-capped silicon-on-insulator (SOI) film. Close to 800°C in UHV there is 

a thermal decomposition of the capping oxide then at T >850°C, the Si film spontaneously 

dewets forming an assembly of Si nanoislands (see in this booklet the abstract untitled 

“dewetting dynamics of crystalline thin films” by E.Bussmann et al.). In this paper the 

dynamic evolution of the thermal decomposition of the capping-oxide is studied by Low 

Energy Electron Microscope (LEEM). 

Fig : Sequence of LEEM images taken during thermal decomposition of the oxide (Δt ~ 5 min, FOV : 10 

µm.). In the experimental conditions we use the dark-field mode for imaging the surface. The voids 

appear in bright-grey while the remaining oxide layer appears in dark-grey. Notice that inside the voids 

some individual 1x2 and 2x1 reconstructed Si terraces are resolved. 

The above figure shows a sequence of LEEM images taken during the oxide 

decomposition (at 800°C). The images are typically recorded in dark-field mode using 

electrons in one of ½-order LEED spots associated to the Si(001) surface reconstruction so 

that we have access to the local atomic-scale structure of the clean Si surface after 

desoxidation. Our result show that the oxide decomposition proceeds by heterogeneous 

nucleation of voids on a few local defects, followed by void-opening until coalescence so that 

a clean Si surface is restored at the end of the process. In this paper we report on the time 

evolution of the radius r(t) of individual voids. 

The scenario we propose for the void-opening proceeds in five steps : (i) Si 

monomers creation inside the void, (ii) adatom diffusion towards the void periphery, (iii) 

Si+SiO2 � 2SiO chemical reaction at the triple line, (iv) SiO diffusion, (v) SiO desorption at 

the void edge or on the Si surface if SiO species diffuse on the Si surface. A simple analytical 

model based on this scenario leads to the experimental scaling laws and enable us to discuss a 

few asymptotic behaviours. 

We also develop numerical simulations of the desoxidation process and show how the 

oxide-thickness as well as the anisotropy of surface diffusion on Si(001) play a role on the 

shape of the opening voids. 

84

Generation of Pd model catalyst nanoparticles by spark 

discharge 

M. E. Messing, R. Westerström 1 , B. O. Meuller, S. Blomberg 1 , J. Gustafson 1 , 

J. N. Andersen 1 , R. van Rijn 2,3 , O. Balmes 3 , H. Bluhm 4 , E. Lundgren 1 , and K. Deppert 

Solid State Physics, Lund University, Box 118, S-221 00, Lund, Sweden 

(corresponding author: M. E. Messing, e-mail: maria.messing@ftf.lth.se) 

1 Synchrotron Radiation Research, Lund University, Box 118, S-221 00, Lund, Sweden 

2 Kamerlingh Onnes Laboratory, Leiden University, P.O. Box 9504,2300 RA Leiden, The Netherlands 

3 ESRF, B. P. 220, F-38043 Grenoble, France 

4 Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720 

Heterogeneous catalysts used for industrial purposes or for cleaning of exhaust gases are 

complicated systems of materials usually consisting of an insulating oxide support with 

dispersed metal nanoparticles of the active catalyst, as well as a wide range of additives to 

promote or poisoning specific reactions. Due to the material complexity, atomic scale 

information on the inner workings of the catalyst is at best limited, and as a consequence 

catalyst development is for a large part based on a trial and error approach. 

Because of the need to obtain fundamental information on catalytic reaction pathways, model 

systems have been developed. Studies of processes related to heterogeneous catalysis under 

Ultra High Vacuum (UHV) conditions on well defined single crystal surfaces have been a 

major part of surface science for decades [1]. In recent years, more complex material model 

systems have been developed by depositing pure metal or alloy nanoparticles by Molecular 

Beam Epitaxy (MBE) [2] or by wet chemical methods [3] on a thin oxide film formed on a 

conducting material. 

In this work we present a different route to a model system; size selected aerosol palladium 

particles generated by spark discharge [4]. The system presented consists of Pd nanoparticles 

with a diameter of 15 or 35 nm deposited on HF-etched SiOx or on Al2O3 substrates. The 

conducting SiOx is used for the sake of characterization using X-ray photoelectron 

spectroscopy (XPS), but the type of metal deposit or substrate can be chosen almost 

arbitrarily. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), 

x-ray energy dispersive spectroscopy (XEDS), x-ray photoelectron spectroscopy (XPS) and 

x-ray diffraction (XRD) measurements have been carried out to thorough investigate the asproduced 

catalyst model particles. From these investigations it was found that the particles 

85

consist of a crystalline core surrounded by an amorphous shell. Furthermore, the main 

contaminant of the particles is carbon, most likely contained in the amorphous particle shell. 

The carbon is believed to be incorporated during particle production and can be removed by 

oxidation by O2 of the particles at elevated temperatures. 

The present study [5] shows that aerosol deposition is a useful method for deposition of metal 

nanoparticles on virtually any substrate to be used as a model system for studies of catalytic 

properties. By this method, a well-defined size of the particles can be chosen and a suitable 

well-controlled coverage can be selected, determined by the technique to be used in 

investigations of the catalytic properties. 

Figure 1: (a) SEM image (52 o tilting angle) and (b) TEM image of Pd particles. 

This work was performed within the Nanometer Structure Consortium at Lund University (nmC@LU) and 

supported by the Swedish Research Council, the Crawford Foundation, the Knut and Alice Wallenberg 

Foundation, the Swedish foundation for strategic research (SSF), and the Anna and Edwin Berger Foundation. 

The authors gratefully acknowledge the support by the ESRF and ALS staff. 

[1] R. lmbihl and G. Ertl, Chem. Rev. 95, 697 (1995) 

[2] T. Schalow, B. Brant, M. Laurin, S. Schauermann, J. Libuda and H. –J. Freund, J. Catal. 242, 58 (2006) 

[3] F. Tao, M. E Grass, Y. Zhang, D. R. Butcher, J. R. Renzas, Z. Liu, J. Y. Chung, B. S. Mun, M. Salmeron 

and G. A. Somorjai, Science 322, 932 (2008) 

[4] M. E. Messing, K. A. Dick, L. R. Wallenberg and K. Deppert, Gold Bull. 42, 20 (2009) 

[5] M. E. Messing, R. Westerström, B. O. Meuller, S. Blomberg, J. Gustafson, J. N. Andersen, E. Lundgren, 

R. van Rijn, O. Balmes, H. Bluhm and K. Deppert, J. Phys. Chem. C. 114, 9257 (2010) 

86

Theoretical study of O2 adsorption on Al4Cu9 

(110) surface 

N. Berkaïne and C. Lacaze-Dufaure 

CIRIMAT, 4 allée Emile Monco, BP44362, 31432, Toulouse Cedex 4, France 

J. Morillo 

CEMES, 29 rue Jeanne Marvig, BP 94347, 31055 Toulouse Cedex 4, France 

Abstract 

Corresponding author: nabil.berkaine@ensiacet.fr 

Quasicrystals are very interesting phases since they exhibit exceptional properties 

comparing to conventional alloys [1,2]. We are interested by the surface 

reactivity and more precisely by the first oxidation steps which remain misunderstood. 

In order to add our contribution, we get interested in our study on 

the (110) surface of γ-Al4Cu9 phase which is considered as a good approximant 

of Al-Cu-Fe quasycristal [3]. Thus, oxygen adsorption on the Al4Cu9 (110) surface 

was investigated in the framework of PAW-PBE Density Functional Theory. 

We first considered the adsorption of atomic oxygen and then the interaction 

of the O2 molecule with the metallic surface. Different adsorption sites on the 

Al4Cu9 (110) surface are considered. We investigated on-top, bridge and hollow 

positions. We calculated the O adsorption energies and the charge transfers 

between the adsorbate and the substrate. The O atom is strongly chemisorbed 

on all considered sites as the adsorption energies are strong ranging from -4.5 to 

-7eV. Results also show that the adsorption energy decreases when the distance 

between the O atom and the closest Cu atom is reduced. Moreover, the electronic 

gain on the O atom is proportional to the number of Al neighbour. Thus 

the most favourable adsorption site is a fourfold hole surrounded by 2 Al atoms 

and 2 most distant Cu atoms. The O2 molecule adsorption is therefore considered 

on the fourfold site considering several different starting topologies for 

the O2 molecule. Finally, a comparison between calculated and experimentallymeasured 

STM “Scanning tunneling microscope” images is presented. 

[1]J.-M. Dubois, Useful Quasicrystals World Scientific, Singapore, 2005 

[2]P. A. Thiel, Annu. Rev. Phys. Chem. 59, 129 2008. 

[3]C. Dong, Philos. Mag. A 73, 1519 1996 

1 

87

Thursday 10 th March 2011

Nano-craters formed by impact of individual 

highly charged ions on PMMA surfaces 

R. Ritter 1 , G. Kowarik 1 , R. Ginzel 2 , R. Heller 3 , R. M. Papaléo 4 , W. Rupp 5 , 

J. R. Crespo López-Urrutia 2 , J. Ullrich 2 , S. Facsko 3 and F. Aumayr 1 

1 Institute of Applied Physics, TU Wien - Vienna University of Technology, 1040 Vienna, Austria, EU 

2 Max-Planck Institut für Kernphysik, 69029 Heidelberg, Germany, EU 

3 Helmholtz-Zentrum Dresden Rossendorf, 01314 Dresden, Germany, EU 

4 Faculty of Physics, Cath. University of Rio Grande do Sul, 90619-900 Porto Alegre, Brazil 

5 IMS Nanofabrication AG, 1020 Vienna, Austria, EU 

(corresponding author: F. Aumayr, e-mail: aumayr@iap.tuwien.ac.at) 

We have recently discovered that the impact of individual slow highly charged ions is able to 

cause permanent nano-sized hillocks on the surface of a CaF 2 single crystal [1, 2]. The 

experimentally observed threshold of the projectiles potential energy necessary for hillock 

formation could be linked to a solid-liquid phase transition (nano-melting) [1, 3, 4]. 

Meanwhile a variety of materials has been found, which is susceptible to nano-structuring by 

the impact of slow HCI. The nature, appearance and stability of the created structures, 

however, depend heavily on the properties of the target material and the involved interaction 

processes (determined by the potential and kinetic energy of the projectiles) [4] and can range 

from hillocks (CaF 2, SrTiO 3) pits (KBr [5]), caldera-type structures (TiO 2 [6]) to erasable 

regions of enhanced friction (HOPG [7], mica [8]). 

Fig.1: Nano-craters produced on PMMA after bombardment with Xe 48+ ions (E kin=3.7 keV/amu). 

90

We have systematically studied HCI induced nano-crater formation on polymethylmetacrylate 

(PMMA), a polymer commonly used as a photoresist in the semiconductor industry. Samples 

were irradiated with Xe ions in charge states q = 20 - 50. Final impact energies on the surface 

ranged from 0.35 - 4.0 keV/amu. 

Intermittent contact mode atomic force microscopy (AFM) investigations of the irradiated 

samples revealed that each individual ion creates a nano-sized crater (fig. 1). While the cratervolume 

increases with the potential energy of the incoming ions, a variation of the kinetic 

energy of the HCI seems to solely alter the shape of the pit created. A faster ion leaves behind 

a deeper and slimmer crater compared to a slower ion, while the total volume removed stays 

approximately the same in both cases. 

This work has been supported by the European Projects ITSLEIF (RII3#026015) and AIM (#025646) as well as 

by Association EURATOM-ÖAW and Austrian Science Foundation FWF. 

[1] A.S. El-Said, et al., Phys. Rev. Lett. 100, 237601 (2008). 

[2] A.S. El-Said, et al., Nucl. Instr. and Meth. B 258, 167 (2007). 

[3] C. Lemell, et al., Solid-State Electron. 51, 1398 (2007). 

[4] F. Aumayr, A.S. El-Said and W. Meissl, Nucl. Instrum. Methods B 266, 2729 (2008). 

S. Facsko, et al., J. Phys. C 21, 224012 (2009). 

[5] R. Heller, et al., Phys. Rev. Lett. 101 096102 (2008). 

S. Facsko, et al., J. Phys.: Conf. Ser. 194, 012060 (2009). 

[6] M. Tona, et al. Phys. Rev. B 77 155427 (2008). 

[7] R. Ritter, et al., Vacuum 84 1062 (2010). 

[8] R. Ritter, et al., Nucl. Instr. and Meth. B 268 2897 (2010). 

91

Information depth in Low Energy Ion Scattering 

D. Primetzhofer, M. Spitz, S. Rund, D. Goebl, D. Roth, 

R.C. Monreal 1 , D. Valdés 1 , E. Taglauer 2 , and P. Bauer 

Institut für Experimentalphysik, AOP, JKU Linz, A-4040 Linz, Austria 

(corresponding author: P. Bauer, e-mail: peter.bauer@jku.at) 

1 Departamento de Física Téorica de la Materia Condensada, Universidad Autónoma de Madrid, 

Madrid, Spain 

2 Max-Planck-Institut für Plasmaphysik, EURATOM Association, D-85748 Garching bei München, 

Germany 

The basic idea of Low Energy Ion Scattering (LEIS) is to bombard a sample with noble gas 

ions and analyze the projectiles, which are backscattered into a certain solid angle dΩ. Typical 

incident energies E0 of the ions range from 0.5keV to 10keV. In a common experimental 

geometry, the angle of incidence α, measured with respect to the surface normal, is usually 

smaller than 60◦ and the scattering angle θ is ~140°. 

LEIS is known for its excellent surface sensitivity, when noble gas ions are used as 

projectiles, and for the absence of matrix effects in the yield of scattered ions Y + [1]. The 

extremely high surface sensitivity is based on the large scattering cross section and the very 

efficient neutralization of primary ions. Therefore, quantification of experimental LEIS data 

urgently requires a fundamental understanding of charge exchange. This topic has attracted 

much interest, see, e.g. [2]. 

For He + projectiles, different charge exchange mechanisms are active depending on the 

primary energy and the atomic species of the projectile [3]. Non-local Auger neutralization 

(AN) is possible at all primary energies. Collision induced neutralization (CIN) and 

reionization (CIR) can only contribute for sufficiently close interaction distance between 

projectile and target atom, which promotes the He 1s level to binding energies resonant with 

the conduction band, below or above the Fermi level, respectively. This minimum distance 

corresponds, for a given scattering geometry, to an energy threshold Eth. 

In this contribution we present an analysis of the information depth deduced from P + data 

obtained for He + ions and metal surfaces (Cu, Au, Al) both, in the regimes of reionization and 

where only AN is possible. 

Support by the Fonds zur Förderung der Wissenschaftlichen Forschung (project P20831) is gratefully 

acknowledged. 

[1] H.H. Brongersma, M. Draxler, M. de Ridder, P. Bauer, Surf. Sci. Rep. 62 (2007). 

[2] R. Beikler, E. Taglauer, Nucl. Instr. Meth. Phys. Res. B 182, 180 (2001). 

[3] R. Souda and M. Aono, Nucl. Instrum. Methods Phys.Res., Sect. B 15, 114 (1986). 

92

Magnetism at Stepped Silicon Surfaces 

F. J. Himpsel 1 , S. C. Erwin 2 , P. C. Snijders 3 , N. Guisinger 4 , I. Barke 5 

1 University of Wisconsin Madison, 2 Naval Research Lab., 3 Oak Ridge Natl. Lab., 

4 Argonne Natl. Lab., 5 Universität Rostock 

Semiconductor surfaces exhibit strong re-bonding, particularly when going down in 

dimensionality towards one-dimensional reconstructions at step edges. That produces a 

variety of unexpected effects in the electronic structure [1]. Here we discuss the 

appearance of magnetism at those surfaces. Two types of spin splittings have been found 

at vicinal Si(111) surfaces decorated with Au, one in real space, the other in reciprocal 

space. The latter is due to the spin-orbit interaction (Rashba effect). It was predicted by 

DFT calculations [2] and confirmed by angle- and spin-resolved photoemission ([3] and 

[4]). This type of spin-splitting has been found at several other semiconductor surfaces 

covered with heavy metals [5]. Recently, a completely different type of spin-splitting was 

predicted [6], where individual Si atoms at the step edge become fully spin-polarized. 

This model explains why a predicted step-edge band at EFermi [2] has not been found. The 

magnetic exchange splitting moves the majority and minority bands away from EFermi . 

STM/STS experiments confirm the appearance of a splitting below 50 K, where the 

magnetic reconstruction appears along the step edge. The potential applications of such 

atomically-precise spin structures to single-spin electronics will be discussed. 

[1] I. Barke et al., Low-dimensional electron gas at semiconductor surfaces, Solid State 

Commun. 142, 617 (2007). 

[2] D. Sánchez-Portal, S. Riikonen, and R. M. Martin, Role of Spin-Orbit Splitting and 

Dynamical Fluctuations in the Si(557)-Au Surface, Phys. Rev. Lett. 93, 146803 (2004). 

[3] I. Barke, F. Zheng, T. K. Rügheimer, F. J. Himpsel, Experimental evidence for spinsplit 

bands in a one-dimensional chain structure, Phys. Rev. Lett. 97, 226405 (2006). 

[4] T. Okuda et al., Large out-of-plane spin polarization in a spin-splitting onedimensional 

metallic surface state on Si(557)-Au, Phys. Rev. B 82, 161410(R) (2010). 

[5] Koichiro Yaji et al., Large Rashba spin splitting of a metallic surface-state band on a 

semiconductor surface, Nature Communications 1:17 (2010). 

[6] S. C. Erwin and F. J. Himpsel, Intrinsic magnetism at silicon surfaces, Nature 

Communications 1:58 (2010). 

93

CoPc adsorption on Cu(111): 

Origin of the C4 to C2 symmetry reduction 

1 1 2 2 2 

J. I. Cerdá , R. Cuadrado , Y.Wang , X. Ge, R. Berndt 

1 Instituto de Ciencia de Materiales de Madrid, ICMM-CSIC, 

Cantoblanco 28049 Madrid, Spain 

2 Institut für Experimentelle und Angewandte Physik, 

Christian-Albrechts-Universität, D-24098 Kiel, Germany 

Organic electronic and optoelectronic devices have received enormous attention recently. In 

this context, phthalocyanines (Pc) adsorbed on surfaces have been extensively studied because 

they are widely used as dyes, pigments, liquid crystals, chemical sensors, nonlinear optical 

materials, photodynamic reagents for cancer therapy, sensitizers in photochemical reactions or 

photovoltaic cells and catalysts. At low coverages MPc molecules usually adsorb in a planar 

configuration on surfaces and exhibit four-fold symmetry in STM images, which is consistent 

with their molecular structures. Recently, symmetry reduction from four-fold (C4) to two-fold 

(C2) has been reported for specific cases involving metallic surfaces with a hexagonal lattice 

such as CoPc, FePc and CuPc on Cu(111) or SnPc on Ag(111). The mismatch of molecular 

and surface symmetries leads to different atomic and/or electronic structures along the 

perpendicular molecular axes and, in principle, both could interpret the symmetry reduction in 

STM images. Here the origin of the reduction is clarified from a comprehensive theoretical 

study of CoPc adsorbed to the Cu(111) surface along with experimental STM data. Total 

energy calculations using different schemes for the exchange-correlation energy and STM 

simulations are compared against experimental data. We find that the symmetry reduction is 

only reproduced if van der Waals corrections are incorporated into the formalism. It is caused 

by a deformation along the two perpendicular molecular axis, one of them coming closer to 

the surface by around 0.2 A. An electronic structure analysis reveals: i) the relevance of the 

CoPc interaction with the Cu(111) surface state and, ii) that intramolecular features in dI/dV 

maps clearly discriminate a Co-derived state from the rest of the Pc states, although a direct 

visualization of the molecular orbitals cannot be accomplished. 

Preliminary results for a second monolayer of CoPc grown on top of the first monolayer will 

also be presented. We have identified two different types of adsorption, hollow and top, each 

presenting different contrast in the STM. 

94

Fig. 1: (a) Experimental STM images taken at 0.1 nA. (b) Simulated STM images at +/-1V for 

bridge and hcp CoPc adsorption on Cu(111) employing a sharp Pt(100) oriented tip. The 

calculations are performed for LDA, GGA and GGA+vdW. 

Fig. 2 

: 

T 

o 

p 

r 

o 

w 

: 

M 

a 

p 

s 

o 

f 

d 

I/dV (0.1 nA, 2.0x2.0 nm 2 ) acquired at several voltages --indicated in the inset of each 

map-- for an isolated CoPc on Cu(111). Bottom row: dI/dV simulated maps at similar 

voltages (0.1 nA, 1.5x1.5 nm 2 ). 

95

Electronic structure of topological insulators 

1 1,2 1 2,3 4 

E.V. Chulkov , T.V.Menshikova , M. Vergniory , S.V Eremeev , G.Bihlmayer , 

Yu.M.Koroteev 2,3 , J.Henk 5 , and A.Ernst 5 

Departamento de Física de Materiales, Facultad de Ciencias Químicas, UPV/EH, Apdo 107, 20080 

San Sebastián/Donosita, Basque Country, Spain 

(corresponding author: E.V.Chulkov, e-mail: waptctce@ehu.es) 

1 Donostia international Physics Center (DIPC), Paseo Manuel Lardizabal 4, 20018 

San Sebastián/Donostia, Basque Country, Spain 

2 Tomsk State University, pr. Lenina 36, 634050, Tomsk, Russian Federation 

3 Institute of Strength Physics, and Materials Science, pr. Academicheskii 2/4, 634021 Tomsk, 

Russian Federation 

4 Institut für Festkörperforschung and Institute for Advanced Simulation,Forschungszentrum Jülich 

and JARA,52425 Jülich, Germany 

5 Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2,D-06120, Halle, Germany 

A 3D topological insulator (TI) is a new state of matter which shows exotic surfaces 

properties [1-3]. Being narrow gap semiconductors in bulk, topological insulators exhibit a 

surface state (SS) that makes the surface conducting. This surface state shows linear 

dispersion, forming a Dirac cone with a crossing (Dirac) point at/around the Fermi level. In 

contrast to the Dirac cone in graphene this topological SS is spin-orbit split and carries only 

one electron per momentum with a spin that changes its direction consistently with a change 

of momentum. The topological origin of the SS protects the Dirac cone from surface 

perturbations [1], even under such a strong perturbation as removing of the surface Te(Se) 

atomic layer from tellurium (selenium) chalcogenides this surface state survives [4]. The 

unique surface properties of TIs make these materials important for many applications, in 

particular, in spintronics and quantum computing. 

Here we present ab-initio calculated results for electronic structure of Tl- and In-based narrow 

gap semiconductors [5-7] as well as for binary and ternary compounds with tetradymite-type 

crystal structure [8,9]. We show that some of these compounds are strong 3D topological 

insulators showing non-trivial topological Z2 invariant and a Dirac cone at the centre of the 

surface Brillouion zone – Γ . Peculiarities of charge density behaviour of the Dirac state are 

discussed and compared to that of conventional surface states including Rashba-split surface 

states. The obtained results are also compared to recent photoemission data. 

96

[1] L. Fu and C. L. Kane, Phys. Rev. B 76, 045302 (1985) 

[2] J. E. Moore, Nature 194, 524 (2010) 

[3] M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010) 

[4] S. V. Eremeev, Yu. M. Koroteev and E. V. Chulkov, JETP Lett. 91, 387 (2010) 

[5] K. Kuroda, M. Ye, A. Kimura, S. V. Eremeev, E. E. Krasovskii, E. V. Chulkov, Y. Ueda, K. Miyamoto, T. 

Okuda, K. Shimada, H. Namamate, and M. Taniguchi, Phys. Rev. Lett. 105, 146801 (2010) 

[6] S. V. Eremeev, Yu. M. Koroteev, and E. V. Chulkov, JETP Lett. 91, 594 (2010) 

[7] S. V. Eremeev, G. Bihlmayer, M. Vergniory, Yu. M. Koroteev, T. V. Menshikova, J. Henk, A. Ernst, and 

E. V. Chulkov, Phys. Rev. B 83, xxxxxx (2011) 

[8] A. Kuroda, M. Arita, K. Miyamoto, M. Ye, J. Jiang, A. Kimura, E. E. Krasovskii, E. V. Chulkov, H. 

Iwasawa, T. Okuda, K. Shimada, Y. Ueda, H. Namamate, and M. Taniguchi, Phys. Rev. Lett. 105, 076802 

(2010) 

[9] S. V. Eremeev, Yu. M. Koroteev, and E. V. Chulkov, JETP Lett. 92, 161 (2010) 

97

Tailoring interactions in supramolecular networks by fluorination 

E. Goiri a,b , A. El-Sayed b,c , D. G. de Oteyza d , M. Matena a , C. Rogero b , J. M. Lastra e , D. 

Mowbray a,e , A. Rubio e , Y. Wakayama f and J. E. Ortega a,b,c,* 

a Donostia International Physics Center, 20018-San Sebastian, Spain 

b Centro de Física de Materiales(CFM)_CSIC, UPV/EHU, 20018 San Sebastián, Spain 

c Departamento de Fisica Aplicada I, UPV/EHU, 20018 San Sebastián, Spain 

d The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley (USA) 

e Nano-Bio Spectroscopy group and European Theoretical Spectroscopy Facility, and 

Departamento Fisica de Materiales UPV/EHU, San Sebastián, Spain 

f Dept. of Chemistry and Biochemistry, Fac. of Engineering, Kyushu U., Tsukuba, Japan 

Organic electronics has become an enormously promising field of technology. 

However, many challenges have still to be overcome to make it mature and 

commercially competitive, requiring first a thorough understanding of the basic science 

involved in the operation of organic electronic devices and the physics of organic 

semiconductors. Of particular interest and relevance for the performance of organic 

electronics are donor/acceptor/metal interfaces, where crucial processes as e.g. charge 

injection take place. We investigate the physical-chemical properties of donor-acceptor 

blends in intimate contact with metal surfaces, using STM and a battery of electron 

spectroscopies, namely, X-ray photoemission (XPS), angle-resolved ultraviolet 

photoemission (ARUPS) and near edge X-ray absorption fine structure (NEXAFS), all 

combined with first principles calculations. 

Perfluorination of semiconducting oligomers is an effective route to modify the 

ionization potential and the electron affinity, favoring n-type semiconducting behaviour 

in molecules that otherwise behave as electron donors [1]. When fluorinated and nonfluorinated 

molecules are combined, strong 

intermolecular interactions arise, which steer the 

spontaneous formation of two-dimensional donoracceptor 

assemblies (Fig. 1, [2]). In this work we 

study the electronic structure changes upon 

switching the fluorination, i.e., the donor-acceptor 

character from one molecule species to the other 

in a binary mixture. In fact, we have tested the 

electronic structure changes from pure pentacene 

(PEN) and copper phthalocyanine (CuPc) 

monolayers, to donor/acceptor blends and 

reversed fluorination, namely F16CuPc+PEN and 

PFP+CuPc, using Au(111), Ag(111) and Cu(111) 

as substrates. We will discuss how, upon mixing, 

STM image of a self-assembled 

monolayer of PFP (acceptor) and 

CuPc (donor) on Ag(111) (13.2 nm 2 ). 

donor/acceptor and molecule/surface interactions 

change in a complex way that depends on the 

intimate electronic properties of each system. 

[1] Z. Bao, A. J. Lovinger, and J. Brown, J. Am. Chem. Soc. 120, 207 (1998). 

[2] D. G. de Oteyza, J. M. García-Lastra, M. Corso, B. P. Doyle, L. Floreano, A. 

Morgante, Y. Wakayama, A. Rubio, J. E. Ortega, Adv. Funct. Mater. 19, 3567 (2009). 

* enrique.ortega@ehu.es 

98

Supramolecular self-assembly driven by electrostatic 

repulsion: The 1D aggregation of 

Rubrene pentagons on Au(111) 

Giulia Tomba 1,2,6 , Massimiliano Stengel 3 , Wolf-Dieter Schneider 4 , Alfonso Baldereschi 5 , and 

Alessandro DeVita 1,2 

1 Department of Physics, King’s College London, London, Strand WC2R 2LS, United Kingdom, 

2 INFM-DEMOCRITOS and CENMAT, Trieste, Italy 

3 Institut de Ciència de Materials de Barcelona (CSIC), 08193 Bellaterra, Spain 

4 Institute of Condensed Matter Physics, EPFL, CH-1015 Lausanne, Switzerland 

(corresponding author: W.-D. Schneider, e-mail: wolf-dieter.schneider@epfl.ch) 

5 Institut de Physique Theorique, EPFL, CH-1015 Lausanne, Switzerland 

6 Faculty of Production Engineering and Bremen Center for Computational Material Science, 

University of Bremen, D-28359 Bremen, Germany 

Today, organic molecules are among the best candidate buiding blocks for the construction of 

self-assembling nanoscale devices supported on metal substrates. Control of the formation of 

specific patterns in the submonolayer regime is usually achieved by appropriate choice and/or 

functionalization of the adaorbates. The effect of this intervention, though, is limited by the 

typically short-range character of the bonding. 

We present here a theoretical study on the system rubrene/gold to show that substrate-induced 

molecular charging can instead determine the assembly on larger scales. DFT calculations and 

electrostatic considerations are used to discuss the charge transfer at the metal/organic 

interface[1]. This allows us to rationalize previous puzzling experimental results [2,3,4] and 

in particular, of the unusual molecular gap broadening upon adsorption observedd in scanning 

tunneling spectra[5]. 

The self-assembly process is further studied by means of classical molecular dynamics 

simulations. The charged adsorbates are modeled as mutually repulsive standing dipoles, with 

van der Waals interactions intervening at short distances. The striking resemblance between 

the experimental STM images and the results of our MD simulations shows that this simple 

model is able to capture the key effects driving the assembly in this system. The competition 

between long-range repulsive interactions and short-range attractive forces leads to 

characteristic and easily recognizable 1D patterns. We suggest that experimental evidence of 

99

the presence of similar patterns in other metal/organic systems can provide crucial information 

on the electronic level alignment at the interface, that is, on the occurrence of charge-transfer 

processes between metal and organic adsorbates. 

Support by the Swiss National Science Foundation is gratefully acknowledged. ADV and GT acknowledge 

funding from EPSRC Grant EP/G044864/1 and the ESF-EURCORES SONS Programme. 

[1] G. Tomba. M. Stengel, W.-D. Schneider, A. Baldereschi, and A. De Vita, ACS Nano, in press. 

[2] M.-C. Blüm, E. Cavar, M. Pivetta, F. Patthey, and W.-D. Schneider, Ang. Chem., Int. Ed. 44, 5334 (2005) 

[3] M. Pivetta, M.-C. Blüm, F. Patthey, and W.-D. Schneider, Ang. Chem., Int. Ed. 47, 1076 (2008) 

[4] M. Pivetta, M.-C. Blüm, F. Patthey, and W.-D. Schneider, ChemPhysChem 11, 1558 (2010) 

[5] M.-C. Blüm, M. Pivetta, F. Patthey, and W.-D. Schneider, Phys. Rev. B 73, 195409 (2006) 

100

Preferred Pathway for a Molecular Photo Switch in 

Contact with a Surface 

Jörg Henzl, Maciej Bazarnik 2, Ryszard Czajka 2, Andreas Schaate 1, Boris Ufer 1, and Peter 

Behrens 1 and Karina Morgenstern 

Institut für Festkörperphysik, Abteilung ATMOS, Leibniz Universität Hannover 

Appelstr. 2, 30167 Hannover, Germany 

(corresponding author: K. Morgenstern, e-mail: morgenstern@fkp.uni-hannover.de) 

1 Insitut für Anorganische Chemie, AK Festkörper- und Materialchemie, Leibniz Universität Hannover 

Callinstr. 9, 30167 Hannover (Germany) 

2 Institute of Physics, Poznan University of Technology, Nieszawska 13A, 60-965 Poznan, Poland 

Fictionalisation of metal and semiconductor surfaces with molecular switches is an emerging 

trend in nanotechnology aimed at nanoelectronic applications [1, 2]. Switches should be both 

reversible and bistable. To fulfil these requirements a molecule must have at least two 

different thermally stable forms and a possibility of repeatably triggering externally an 

interconversion between them. The conversion should result in a measurable change of 

molecular properties. Since isomers often differ significantly in their physical and chemical 

properties, isomerization might serve as a molecular switching mechanism. Azobenzene and 

its derivatives are perfect prototypes for photo chromic molecular switches, because they 

isomerise between an elongated trans and a compact cis isomer with a strong change in 

conductivity. Therefore, they have been studied widely on surfaces [3-16]. There are reports 

of cis-trans isomerization induced by tunnelling electrons [6, 7, 10, 11, 13, 14, 16], by the 

electric field in the tip-surface junction [8], or by photons [3-5]. 

Photoisomerization is unlikely for molecules adsorbed on metals because of alternative faster 

excitation channels. Indeed, the photo-isomerization of native azobenzene molecules directly 

adsorbed on a metal substrate is suppressed [3] presumably because of a shorter lifetime of the 

photo excited state on metals. As a consequence the azobenzene molecules were 

functionalised with tert-butyl spacer groups, which are intended to lift the azobenzene 

scaffold away from the surface. Though exposure to light led to isomerization of this TBA 

(tert-butyl azobenzene) on Au(111)[3, 4], the light is adsorbed by the metal's d-band [5]. The 

hole created in this way relaxes to the band top to initiate the isomerization via electron 

transfer from the highest occupied molecular orbital (HOMO) to the hole. The direct photoisomerization 

based on absorbance of the light by the molecule has not been achieved yet for 

supported molecules. 

In this contribution we present two successful attempts for decoupling azobenzene derivatives 

from a metallic surface for enabling direct photoisomerization. 

101

The two isomers of the 4-anilino-4'-nitroazobenzene adsorbed on Cu(111) are physisorbed as 

proven by STM imaging, inelastic electron tunnelling spectroscopy, and a conversion into 

their chemisorbed state. The two isomers of 4,4'-dihydroxyazobenzene azobenzene are 

adsorbed on a thin insulating NaCl layer on Ag(111). 

Switching between the isomers is possible in both cases by tunnelling electrons and via 

exposure to UV light, but not to visible light. This unexpected behaviour is discussed in this 

contribution. 

Support by the Deutsche Forschungsgemeinschaft and from the COST Action CM0601, Electron Controlled 

Chemical Lithography is gratefully acknowledged. 

[1] A. Aviram and M. A. Ratner, Chem. Phys. Lett. 29, 277 (1974). 

[2] C. Joachim, J. K. Gimzewski, and A. Aviram, Nature 408, 541 (2000). 

[3] M. Comstock, et al., Phys. Rev. Lett. 99, 038301 (2007). 

[4] S. Hagen, F. Leyssner, D. Nandi, M. Wolf, and P. Tegeder, Chem. Phys. Lett. 444, 85 (2007). 

[5] S. Hagen, et al., J. Chem. Phys. 129, 164102 (2008). 

[6] J. Henzl, M. Mehlhorn, H. Gawronski, K.-H. Rieder, and K. Morgenstern, Angew. Chem. Int. Ed. 45, 603 

(2006). 

[7] B.-Y. Choi, et al., Phys. Rev. Lett. 96, 156106 (2006). 

[8] M. Alemani, et al., J. Am. Chem. Soc. 128, 14446 (2006). 

[9] G. Füchsel, T. Klamroth, J. Dokic, and P. Saalfrank, J. Phys. Chem. B 110, 16337 (2006). 

[10] J. Henzl, M. Mehlhorn, and K. Morgenstern, Nanotechnology 18, 495502 (2007). 

[11] J. Henzl, T. Bredow, and K. Morgenstern, Chem. Phys. Lett. 435, 278 (2007). 

[12] N. Henningsen, R. Rurali, K. Franke, I. Fernandez-Torrente, and J. Pascual, Appl. Phys. A 93, 241 (2008). 

[13] M. Alemani, et al., J. Phys. Chem. C 112, 10509 (2008). 

[14] K. Morgenstern, Acc. Chem. Res. 42, 213 (2009). 

[15] E. McNellis, J. Meyer, D. A. Baghi, and K. Reuter, Phys. Rev. B 80, 35414 (2009). 

[16] J. Henzl and K. Morgenstern, Phys. Chem. Chem. Phys. 12, 6035 (2010). 

102

Wide acceptance angle photoelectron spectrometer for 

stereophotograph of atomic arrangement 

Hiroyuki Matsuda, Laszlo Toth1, Kentaro Goto, Fumihiko Matsui, Tomohiro Matsushita2, Mie Hashimoto, Chikako Sakai, Hideo Nojiri, and Hiroshi Daimon 

Nara Institute of Science and Technology (NAIST), 8916-5 Takayama, Ikoma, 

Nara 630-0192, Japan 

(corresponding author: H. Daimon, e-mail: daimon@ms.naist.jp) 

1 University of Debrecen, Faculty of Informatics, 4032 Debrecen, Egyetem Tér 1, Hungary 

2 Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, 1-1-1 Koto 

Sayo, Hyogo 679-5198, Japan 

We have developed a new imaging method to take a stereo photograph of the threedimensional 

atomic arrangement around specific atomic species, with which one can view the 

three-dimensional atomic arrangement directly by the naked eyes [1]. The azimuthal shifts of 

forward focusing peaks [2] in a 

two-dimensional photoelectron 

intensity angular distribution 

pattern excited by left and right 

helicity light are the same as the 

parallax in a stereo-view. Taking 

advantage of this phenomenon of 

circular dichroism in 

photoelectron intensity angular 

distribution, one can take a stereo 

photograph of atomic 

arrangement. A display-type 

spherical-mirror analyzer [3] 

installed at BL25SU in SPring-8 

is used to take stereoscopic 

photographs directly on the 

screen without any computeraided 

conversion process. 

(a) (b) 

(c) 

Fig. 1 (a) and (b) are stereo photograph of atomic arrangement 

around In atom in InP(001) for left eye and right eye, respectively. 

(c) is that for red-and-blue glasses (if printed in color). 

Recent results of stereo 

photograph of atomic arrangement around In atoms in InP(001) [4] is shown in Fig. 1. Figure 

1(a) and (b) are stereo photograph for left eye and right eye, respectively. Figure 1 (c) is that 

for red-and-blue glasses (Blue glass is for right eye). One can see the atomic arrangement 

103

around the emitter In atom around [111] direction, the nearest atom A in [111] direction looks 

closer than second nearest atoms B and C atoms in [011] and [101] directions. 

In this experiment of stereophotograph, two-dimensional angular distribution of the 

photoelectron should be measured in a short time. When a commercial electron 

spectrometer is utilized, it is necessary to measure with sweeping the angles of the sample, 

two-dimensionally, because the commercial electron spectrometer can accept the electron in 

only about 10°×1°. Therefore, long measurement time is required. One breakthrough to this 

problem is a display-type spherical-mirror analyzer [3]. This apparatus can measure angular 

distributions over the angle of ±60°×±60° at a time. It has worked effectively. 

Rcently, the 

photoelectron 

emission microscope 

(PEEM) is remarkably 

developed, and it 

brings much 

information of surface 

science. However it 

cannot measure 

Fig. 2 WAAEL and lens system to realize PEEM with wide acceptance angle at 

high kinetic energies. 

angular distribution of photoelectrons at high kinetic energies. Therefore, we have developed 

a wide acceptance angle electron lens (WAAEL) [5]. The WAAEL utilize an ellipsoid mesh 

electrode to remove the spherical aberration completely, and it can accept the electrons in 

±60° even at high kinetic energies. The WAAEL was utilized for hard X-ray photoelectron 

spectroscopy (HAXPES) to overcome the small photoionization cross-section in the hard- xray 

region [6]. Additional lens system is attached to WAAEL to control the image formation 

(Fig. 2) [7]. This apparatus can magnify the sample image and display the angular distribution 

of the selected area by using the selector aperture. This spectrometer is being tested at 

BL07LSU in SPring-8. Angular distribution and magnified image were confirmed. 

Support by the Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Scientific Research (S), 

20224007, 2008 is gratefully acknowledged. 

[1] H. Daimon, Phys. Rev. Lett. 86, 2034 (2001). 

[2] H. Daimon, T. Nakatani, S. Imada, S. Suga, Y. Kagoshima, and T. Miyahara, Jpn. J. Appl. Phys. 32, L1480 

(1993). 

[3] H. Daimon, Rev. Sci. Instrum. 59, 545 (1988). 

[4] T. Matsumoto, F. Matsui, T. Matsushita, K. Goto, Y. Kato, H. Daimon, e-J. Surf. Sci. Nanotech. 7, 181 

(2009). 

[5] H. Matsuda et al.: Phys. Rev. E 71, 066503 (2005); Phys. Rev. E 74, 036501 (2006); Phys. Rev. E 75, 

046402 (2007). 

[6] M. Kobata, et al.: Anal. Sci. 26, 227 (2010). 

[7] H. Daimon, H. Matsuda, L. Toth and F. Matsui: Surf. Sci. 601, 4748 (2007). 

104

Probing a surface reconstruction with anomalous X-ray 

diffraction 

E. Lundgren 1 , R. Westerström, 1 J.N. Andersen 1 , X. Torrelles 2 , C. Quiros 3 , S. Ferrer 4 , I. Popa 5 , 

D. Wermeille 6 , R. Felici 6 

1 Division of synchrotron radiation research, Lund University Sweden 

(corresponding author: E. Lundgren, e-mail: Edvin.Lundgren@sljus.lu.se) 

2 Institut de ciencia de materials de Barcelona, Spain 

3 Departamento de Fisica, Universidad de Oviedo, Spain 

4 ALBA, Barcelona, Spain 

5 ESRF, France 

Surface X-Ray Diffraction (SXRD) is a powerful technique for the determination of surface 

structures. Whereas most traditional electron or ion based surface science techniques are 

limited to vacuum environment, SXRD is able to determine structural parameters of surfaces 

under a gas pressure of several bar, or from a surface immersed in a liquid because of the 

negligible attenuation of hard X-rays. One drawback with SXRD is the often limited data set 

that can be collected during an experiment as compared to a more traditional technique such 

as Low Energy Electron Diffraction (LEED). A second drawback is the lack of chemical 

information of the surface structure or phase under study. The present contribution 

demonstrates how so-called anomalous diffraction (resonant diffraction) can be used to 

increase the number of data points as well as the chemical information from a surface 

reconstruction. 

In the present study we have investigated the surface reconstruction of Sm [1]. In this 

system, the bulk Sm atoms are trivalent (4f 5 (6s6p5d) 3 ) and the surface atoms are divalent 

(4f 6 (6s6p5d) 2 ). This valence transition, which is unique among the pure elements, leads to 

some very unusual surface properties, the most prominent being an expansion of the divalent 

surface Sm atoms by 22%, and leading to a coincidence lattice. We have previously studied 

the resulting surface reconstruction in great detail [1], as summarized in Figure 1a. In 

addition, the structure was supported by ab-initio theoretical calculations. 

In this structure, the different in-plane nearest-neighbour distance in the top-most layer 

as compared to the bulk layers can easily be detected using SXRD, Figure 1b. Thus, it is 

evident that we have an extremely clear signal for differentiating between the single divalent 

surface layer and the deeper trivalent bulk layers. The window free design of the refurbished 

ID03 beamline permitted us to reach the quadrupole (2p – 4f) resonance at 6.716 keV for 

trivalent Sm and perform surface resonant scattering from this single, well defined surface 

layer. In a first experiment we detected the diffracted intensities from the bulk signal at H=0 

and K=1 (L=0.2), and scanned across the L-edge, as shown by the black dotted line in Figure 

1c. A clear dip is seen at a photon energy of 6715 eV. If instead we detect the diffracted 

photons from the surface signal at H=0.82 and K=0, a clear shift of the dip to 6708 eV can be 

observed in the red dotted line in Figure 1c. Thus we find a shift between the bulk and the 

surface anomalous diffraction of approximately 7 eV. This value is very close to the shift in 

the 3d level between divalent and trivalent Sm atoms [2], directly showing that the surface 

105

signal stems from divalent Sm atoms while the bulk signal from trivalent Sm atoms. The 

experiment thus confirms previous surface models for the valence induced surface 

reconstruction of Sm. 

This study shows that anomalous SXRD can be used in a simple way to gain additional, and 

sometimes essential chemical information on an unknown surface structure. We also present a 

detailed structural evaluation based on surface anomalous diffraction including the photon 

energy dependence of the transmission function and the form factors involved. 

Figure 1: (a) Top and side views of the surface reconstruction of Sm(0001). (b) In-plane H-scan 

(K=0) clearly showing the incommensurate surface layer. (c) Photon energy scans across the Sm Ledge 

at (H K L) = (0.82 0 0.2) (red-dotted line) and at (H K L) = (0 1 0.2) (black-dotted line) 

corresponding to diffraction from the surface and the bulk, respectively. The shift between the dips at 

resonance corresponds to the core level binding energy shift between divalent and trivalent Sm atoms 

as found by xps [2]. 

References 

[1] E. Lundgren, J.N. Andersen, R. Nyholm, X. Torrelles, J. Rius, A. Delin, A. Grechnev, O. Eriksson, C. 

Konvicka, M. Schmid, P. Varga, Phys. Rev. Lett. 88 136102 (2002). 

[2] Å. Fäldt and H. Myers, Phys. Rev. B 33, 1424 (1986). 

106

Friday 11 th March 2011

Bridging Bridging the the Pressure Pressure Gap Gap - Developments and Challenges for 

Ambient Ambient Pressure Pressure Photoelectron Photoelectron Photoelectron Spectroscopy 

Spectroscopy 

T. Stempel 

SPECS Surface Nano Analysis GmbH, Voltastr. 5, 13355 Berlin, Germany 

(e-mail: thomas.stempel@specs.com) 

Modern devices are often only functional in environments far away from ultrahigh vacuum, 

still being the standard operation conditions for Photoelectron Spectroscopy (PES) and other 

Surface Science techniques. Many catalytic materials, such as Ru in CO oxidation, only show 

significant reactivity under pressures where chemical potentials become relevant and reactants 

or products accumulate at the surface [1,2]. Materials analysis in future means using PES, 

Scanning Probe Microscopies and related techniques in the generic or near generic device 

environments. This means high, elevated or near ambient pressures of defined working gas 

mixtures, liquid media, potentials or magnetic fields applied. Also extremely low or high 

temperatures might be necessary. 

In PES the electron mean free path in mbar atmospheres is just a few mm, which is too little 

for conventional sample-analyzer distances [3] and count rate decreases exponentially with 

distance. The first implementations of Near Ambient Pressure (NAP) PES Systems included 

differentially pumped apertures in front of the analyser lens, each of them blanking a 

significant part of the electron angle distribution [4]. Only integration of the differential 

pumping into the electrostatic lens, as implemented first at the ALS and BESSY, made the 

technique available to a wide range of applications [5]. Commercially available modular 

differentially pumped lens systems with large acceptance angles and higher count rates form 

the third generation of NAP-PES. Together with ultrafast detector systems, laboratory NAP 

excitation sources and sophisticated manipulator designs working in highest or lowest 

temperatures and in controlled gas volumes, now NAP-PES is a ready to use technique for the 

study of systems under extreme conditions and available for a large scientific community. 

This work summarizes and presents existing solutions nowadays and future development 

routes to new instruments and materials analysis methods being functional under these 

working conditions. Opportunities and limits will be discussed from the perspective of a 

supplier of scientific instruments. 

Finally an exemplary overview on applications and results from NAP-PES for catalysis 

research, surface chemistry and liquid samples will be given. 

108

[1] H. Over, Y. D. Kim, A. P. Seitsonen, E. Lundgren, M. Schmid, P. Varga, A. Morgante, and G. Ertl, 

Science 287, 1474 (2000) 

[2] R. Blume, M. Hävecker, S. Zafeiratos, D. Teschner, A. Knop-Gericke, R. Schlögel, P. Dudin, A. Barinov, 

and M. Kiskinova, Catalysis Today 124, 71 (2007) 

[3] M. Salmeron, and R. Schlögel, Surface Science Reports 63, 169 (2008) 

[4] R. W. Joyner, M. W. Roberts, K. Yatex, Surface Science 87, 501 (1979) 

[5] D. F. Ogletree, H. Bluhm, G. Lebedev, C. S. Fadley, Z. Hussain, and M. Salmeron, Review of Scientific 

Instruments 73, 3872 (2002) 

109

Recent Advances in Surface Science Instrumentation 

High Precision local electrical Probing at T< 5K: Potential and Limitations for the Analysis of 

Nanocontacts and Nanointerconnects 

1 1 1 2 3 4 4 1 

M. Maier , B. Günther , J. Koeble , D. Jie , Ch. Joachim , F. Matthes , C.M. Schneider , A. Feltz 

1 Omicron NanoTechnology GmbH, Germany, 2 Institute of Materials Research and Engineering (IMRE), Singapore, 

3 Nanosciences group, CEMES-CNRS, France, 4 Forschungszentrum Jülich, Germany 

(corresponding author: m.maier@omicron.de) 

Developments in commercial surface science instrumentation typically follow the major trends in science and corresponding 

analysis techniques as invented and established by the leaders in the individual scientific fields. The variety of 

instrumental approaches is as wide-ranged as science itself. Therefore, the identification of relevant analysis techniques 

and their advancement towards ease-of-use and a routinely accessible performance level represent a major challenge for 

enterprises. Beside OMICRON´s major activities in conventional SPM, electron spectroscopy and thin film techniques, 

the class of “multi-technique” instruments represents another important R&D line that is in the focus of this presentation. 

Those instruments combine several conventional methods in order to better address requirements of today´s science. 

One motivation and prominent example in nanotechnology is the development of individual nano-scale devices with a 

tremendous variety of approaches and their fundamental questions. In contrast however, comprehensive concepts towards 

electrically integrated and therefore functional devices are rare. The individual (metallic) contact interface 

represents one of the main challenges and high precision local electrical probing has the potential to increase efficiency 

in evaluating different approaches. 

To meet the involved requirements, we have established and being advancing the UHV NANOPROBE by integrating 

SPM technology with high resolution SEM: (1) Rapid and simultaneous SEM navigation of four local STM probes; (2) 

Localization of nanostructures by sub-4nm UHV Gemini SEM resolution; (3) Individual probe fine positioning by atomic 

scale STM imaging; (4) STM based probe approach for “soft-landing” of sharp and fragile probes and controlled 

electrical contact; (5) approaches towards sharp and clean STM tips; (6) suitable low noise signal re-routing for transport 

measurements; (7) chemical/magnetic analysis by complementary analysis techniques such as SAM, SEMPA, CL 

and others; and (8) structuring by FIB, EBL, EBID. 

We will illustrate achievements (as well as limitations) with the UHV NANOPROBE along the model system Au nanoislands 

on MoS2. These islands represent contact pads, each electrically connected by an individual STM probe. As 

good band gap (approx. 1.3eV transverse gap) semiconductor, MoS2 has the potential to sufficiently decouple nanostructures 

electrically at low voltage. Those Au triangular nano-islands have a lateral size of typically 10-30nm and form 

an “atomically” clean and defined metal-semiconductor interface. We will present measurements on (1) probe navigation 

and electrical contacting with contact distances in the 10nm regime. (2) reproducible Schottky like IV properties for 

the individual STM tip/Au nano-island/substrate contact; (3) surface conductance measurements with variable interisland 

distance down to 17nm; and (4) we also show that the individual STM probe can be employed under SEM to 

manipulate those Au nano-islands with high precision in order to generate arbitrary multi probe planar contact configurations. 

Although the UHV NANOPROBE represents a flexible solution, especially in combination with complementary techniques, 

it´s concept is fundamentally limited in terms of lowest temperature and SPM resolution. Together with the Forschungszentrum 

Jülich, we thus have been developing a completely new design, the Low Temperature UHV 

NANOPROBE. It represents the evolution from a high performance probing system towards 4 simultaneously operating 

and high performing low temperature SPMs, navigated by SEM. The major R&D targets have been (1) equilibrium 

temperature of sample and probes at temperatures T

Dynamic magnetic domain observation 

with novel highly spin polarized and high brightness LEEM 

T.Koshikawa 1*,M.Suzuki 1, T.Yasue 1 and E.Bauer 2 

1. Fundamental Electronics Research Institute, Osaka Electro-Communication Univ. 

2. Dept. of Physics and Astronomy, Arizona State Univ. 

Memory size has been tremendously enlarged after the development of GMR. 

Recently new concept MRAM (magnetic random access memory) has been proposed, in 

which the magnetic domain wall can be driven with current (current-induced domain 

wall motion: CWM) using perpendicular magnetic anisotropy [1]. [Co/Nix] multi-layer 

nano-wires might be an important candidate, which has strong perpendicular magnetic 

anisotropy [2]. Here we have investigated the detailed magnetization process of those 

multi-layer with newly developed very high brightness, highly spin-polarized and long 

life time SPLEEM [3-6]. The magnetic images of Co/Ni2/W(110) and 1 ML Au on 

Co/Ni2/W(110) are shown in Fig.1 which show that Au ultra thin film conducts to the 

strong perpendicular magnetic anisotropy. 

Before Au deposition 

[001] in-plane [110] (easy axis) out-of-plane 

After Au deposition (1ML) 

Fig.1 Magnetic domain images of in-plain and out-of plain of Co/Ni2/W 

(110) and 1 ML Au on Co/Ni2/W (110). 

1. M.Yamaguchi et al., Phys. Rev. Lett. ,92, 077205 (2004). 

2. H.Tanigawa et al., Appl. Phys. Express, 2, 053002 (2009). 

3. N.Yamamoto et al., J.Appl.Phys., 103, 064905 (200). 

4. X.G.Jin et al., Appl.Phys. Express, 1, 045002 (2008). 

5. X.G.Jin et al., J.Cryst. Growth, 310, 5039 (2008). 

6. M.Suzuki et al., Appl.Phys.Express. 3, 026601 (2010). 

*E-mail address: kosikawa@isc.osakac.ac.jp 

111 

FOV=6µm

Methane oxidation over Pd and Pt: 

linking surface science and industrial catalysis 

J. Gustafson 

M. E. Messing 2 , K. Deppert 2 , P.A. Carlsson 3 , A. Hellman 3 , H. Grönbeck 3 , M. Skoglund 3 , A. Resta 

, 

1 , S. Blomberg, N. Martin, R. van Rijn 1 , O. Balmes 1 , D. Wermeille 1 , 

X. Torrelles 4 , J. N. Andersen and E. Lundgren 

Division of Synchrotron Radiation Research, Lund University, Box 118, SE-221 00, Lund, Sweden 

(corresponding author: J. Gustafson, e-mail: johan.gustafson@sljus.lu.se) 

1 ESRF 6, rue Jules Horowitz, 38000 Grenoble, France 

2 Solid State Physics, Lund University, Box 118, SE-221 00 Lund, Sweden 

3 Competence Center for Catalysis, Chalmers University of Technology, SE-41296 Göteborg, Sweden 

4 Institut de Ciencia de Materials de Barcelona (C.S.I.C), 08193 Bellaterra, Barcelona, Spain 

Methane (CH4) produced from a biobased feedstock is a candidate fuel for efficient and 

sustainable automotive propulsion and energy conversion processes. Accordingly, future 

technical applications need efficient exhaust aftertreatment technologies capable of cleaning 

the resulting emissions, which primarily contains methane but also carbon monoxide (CO) 

originating from partially oxidised methane, at as low temperatures as 250°C and below. 

Generally, supported platinum (Pt) and palladium (Pd) catalysts are highly active for methane 

oxidation although the activity needs to be further increased for sufficient conversion. In this 

contribution we have used the flow reactor setup at beamline ID03, ESRF, combining surface 

X-ray diffraction (SXRD) with mass spectrometry, to study the methane oxidation over Pd 

and Pt samples of three different kinds; single crystals, nanoparticles on sapphire and pellets 

of a realistic alumina supported powder catalysts. 

Figure 1 shows the results of an experiment where 4 mbar O2 and 1 mbar CH4 is flowing over 

a Pd(100) crystal while the temperature is ramped from 400°C to 600°C and down again. The 

surface structure is followed in situ using SXRD and the methane oxidation rate is monitored 

by the CO2 mass spectrometry signal. In order to increase the sensitivity, the reactivity was 

measured in batch mode by temporarily stopping the flow to follow the rate of the increase of 

the CO2 signal. In figure 1a, is shown the raw mass spec data and the height of each line 

roughly corresponds to the reactivity. The main feature of the reactivity is that it increases 

with temperature as expected. At around 500°C, however, there is a local maximum in the 

reactivity, which is very strong during heating and weaker during cooling. It has previously 

been noticed that the surface, during these reactive low-temperature regimes, is oxidized 

while the metallic surface appears when the reactivity goes down again. Hence it has been 

concluded that PdO is more reactive towards methane oxidation than the metallic surface [1]. 

112

Figure 1: a) Mass spec, b) SXRD and c) temperature data from a methane oxidation experiment 

over Pd(100). d) Sem image of the single crystal surface after the experiment. 

On the other hand, we find that the PdO(101) surface oxide film [2] is inactive and our DFT 

calculations strongly indicate that metallic Pd is much more reactive towards methane 

oxidation than PdO. Furthermore, the SXRD signal from PdO, shown in figure 1b, does not 

drop until the reactivity has already past the local minimum during heating. Hence, the 

explanation of the data is not as simple as the oxide being more reactive than the metal. 

Instead we propose roughening as a new explanation of the low-temperature activity. During 

the oxide formation the crystal gets rough and exposes a significantly larger surface area. 

Therefore the activity goes up when the oxide is formed at low temperatures, although each 

unit area is less reactive than the same area of a metallic surface. When the temperature is 

increased above 500°C, the mobility is high enough in order for the oxide to get more well 

ordered and the surface gets smoother. This leads to a decrease of the exposed surface area 

and a drop in the activity. When the oxide is finally burned off at about 550°C, the activity 

suddenly jumps up to a higher level due to the higher reactivity of the metallic surface. 

Figure 1d shows a SEM image of the single crystal after the experiment, showing that the 

usually very flat surface is not so flat anymore, in agreement with the above interpretation. 

Finally, the difference between single crystals, nanoparticles and powder pellet samples, as 

well as between Pd and Pt will be discussed in the presentation. 

Support by the Swedish Research Council, the Crafoord Foundation, the Knut and Alice Wallenberg Foundation 

the Swedish Foundation for Strategic Research and the Anna and Edwin Berger foundation is gratefully 

acknowledged. 

[1] H. Gabasch et al., J. Chem. Phys C 111, 7957 (2007). 

[2] M. Todorova et al., Surf. Sci. 541, 101 (2003). 

113

Atomic scale friction: Physically nontrivial problems 

S. Yu. Krylov 1,2 and J. W. M. Frenken 1 

1 Kamerlingh Onnes Laboratory, Leiden University, 2300 RA Leiden, The Netherlands 

2 Institute of Physical Chemistry, Russian Academy of Sciences, 119991 Moscow, Russia 

(corresponding author: S.Yu. Krylov, e-mail: krylov@physics.leidenuniv.nl) 

The problem of friction is usually associated with its obvious practical importance, rather than 

with the possible nontrivial physics behind. Only recently, the atomic scale access to the 

phenomenon of friction (in particular, using AFM based technique) allowed and forced one to 

move from an engineering to purely physical approach. Although nanotribology seems a 

physical science by definition, and many results obtained are far from being trivial, this aspect 

is often shadowed in the literature by technical and calculational routine. In this paper we 

concentrate on physically interesting and counterintuitive results concerned with atomic scale 

friction, as well as some aspects that are familiar for tribologists but actually look surprising 

or unexpected from physical point of view. 

Physicist comes upon a surprise already at a very first look into tribology. Sizable friction is 

usually concerned with stick-slip (SS) motion, the fact which is routine for a tribologist. What 

is the reason for the discrete but not continuous sliding as one could expect? Why is SS a 

universal phenomenon which takes place both on macro (remember the noise of a creaking 

door) and atomic scales? Formal explanation of atomic SS is readily given by a mechanistic 

model proposed by Prandtl and Tomlinon as early as in 1920s. However, the physics behind 

this phenomenon is not that simple. Moreover, there is a well known but actually strange fact, 

the origin of which is shrouded in mystery. The appearance of the regular, periodic stick-slip 

(as observed in experiments) needs quite certain values of the inherent dissipation rate 

(''nearly critical damping'') which depend not on the contact details, as one would certainly 

expect, but—surprisingly―on the stiffness and mass of the measuring system. 

The alternative for stick-slip is the regime of continuous low-dissipative sliding (CLDS), 

which is a general prediction of the theory for the cases when the contact potential 

corrugation is low enough and/or stiffness of the measuring system is high enough. 

Surprisingly, by some reasons Nature does not like this regime to be in play. Only recently 

(2004) transition from SS to CLDS was observed in special, elegant physical experiments, in 

which the contact corrugation was controllably reduced by changing the normal contact 

pressure or by turning the surfaces in contact out of registry (so called superlubricity). 

A familiar fact is that friction (in SS regime) increases linearly with increasing ratio of the 

contact potential corrugation to the measuring system stiffness. It comes as no surprise that 

114

friction depends on the contact conditions, but the fact that it equally depends on properties of 

the driving system (its stiffness) is counterintuitive. This "dependence of friction on the 

driver" reflects nontrivial, complex scenario of energy dissipation. It includes not only how 

the energy stored in the system is being transferred to heat but also how one (the driver) 

invests energy to the system. A stiff driver wins over softer one. If this feature of atomic scale 

friction reproduced itself on macroscopic scale (by some reasons it does not) muscular men 

would experience lower friction forces than gentle women. 

Some deeper questions are whether and how the atomic scale friction depends on velocity and 

temperature. Both the velocity and temperature dependencies could be expected intuitively, 

but the real mechanisms turn out to be very different from initial, naive expectations. Instead 

of a linear increase of friction with velocity (like for viscous friction in liquids), the simplest 

mechanistic analysis of dry friction leads to velocity (and temperature) independent result. In 

experiments the velocity dependence in SS regime is found close to logarithmic. The 

nontrivial physics behind it is concerned with the role of thermal motion of the slider. 

Thermal activation initiates slips to start somewhat prior to the points of mechanical 

instability. The effect of such precritical slips on the mean friction force depends on velocity 

― this is the way how the logarithmic velocity dependence comes in. Thermally activated 

motion is also at the origin of temperature dependence of atomic scale friction, but here the 

effect can be really dramatic. Under certain conditions, thermally induced suppression of 

friction can be as large as of orders of magnitude, the effect called thermolubricity. 

Although the role of thermal activation in atomic scale friction has been proven 

experimentally (e.g. by the observation of the logarithmic velocity dependence, and not only), 

a physicist can still look on all this story sceptically. How can thermal motion be in a serious 

play if mass of the slider (although it is atomically sharp) is macroscopically large? The 

answer to this disgusting question have been found only recently and it goes far beyond the 

usual one-effective-spring (Prandtl-Tomlinson) modelling of atomic scale friction. 

The apex of a tip is flexible, the fact directly seen in experiments but usually underestimated. 

(The same should be true for the apexes of nano-asperities at real surfaces.) Not only this 

brings an additional spring into the problem, more importantly, this causes dramatic effects in 

the system dynamics. The effective mass of the contact turns out to be ultra-low, and this 

makes thermal effects very pronounced indeed. Moreover, under certain conditions the 

contact can be partially or even completely delocalized by the rapid thermally activated jumps 

of the tip apex between the surface potential wells. As a consequence, besides the familiar 

stick-slip and continuous low-dissipative sliding, there can be a variety of other, physically 

different regimes of friction. Interesting examples are given by "true thermolubricity" and 

low-dissipative stick-slip which is strongly counterintuitive and can be seen rather as "stackin-slipperiness". 

Importantly, all these regimes correspond to physically different scenarios of 

energy dissipation. 

115

Contribution of surface excitations to secondary-electron 

emission observed by secondary-electron-energy-loss coincidence 

spectroscopy 

F. Salvat-Pujol, W.S.M. Werner, W. Smekal, R. Khalid 

Institut für Angewandte Physik, Technische Universität Wien, Wiedner Hauptstraße 8-10/134, 

A-1040 Wien, Austria 

(Corresponding author: F. Salvat-Pujol, e-mail: salvat-pujol@iap.tuwien.ac.at) 

The mechanism of secondary-electron (SE) emission from a polycrystalline Al sample has 

been investigated by measuring the SE spectrum in coincidence with backscattered electrons that 

have suffered an energy loss corresponding to the excitation of one surface plasmon (1s), one 

bulk plasmon (1b), and combined multiples of these energy losses (see Fig. 1). The measured SE 

coincidence spectra arise from three contributions: bulk-plasmon decay, surface-plasmon decay, 

and direct transfer of energy and momentum from the backscattered electron to the emitted SE. 

The latter contribution, much weaker than the former two, extends up to the energy loss of the 

backscattered electron. 

It has been found that the SE spectra measured in coincidence with multiple bulk and surface 

losses are a linear combination of the 1b and 1s coincidence spectra. This fact constitutes a 

direct experimental evidence for the otherwise reasonable assumption (which has never been 

experimentally proven) that multiple energy-loss processes of the primary electron follow a 

Markov process. Moreover, fitting a superposition of the 1b and 1s coincidence spectra to the 

observed SE coincidence-spectra allows us to evaluate the relative contribution of surface- and 

bulk-plasmon decay to the SE yield, differential with respect to the energy loss. Significant 

oscillations are observed in the latter quantity, while the average value of the surface-plasmondecay 

contribution is found to amount to ∼30% (see Fig. 2). This implies that the depth range 

from which SEs originate is essentially smaller than believed up to date. 

Support by the Fonds zur Förderung der Wissenschaftlichen Forschung (project P20891-N20) and 

by the Becas de postgrado de la Fundación Caja Madrid is gratefully acknowledged. We acknowledge the 

initiative of Hannspeter Winter to conduct these experiments. 

116

Coincidences 

800 

700 

600 

500 

400 

300 

200 

100 

Kinetic energy (eV, vacuum level) 

0.5 1 2 3 5 10 100 200 

0 

500 

400 

Secondary-electron coincidence spectra 

Coinc. 1st bulk (1b) 

Coinc. 1st surf. (1s) 

300 

200 

Time of flight (ns) 

100 

0 

250 

200 

150 

100 

50 

Kinetic energy (eV, vacuum level) 

0.5 1 2 3 5 10 100 200 

Coinc. with 195 eV loss 

α×(1b) 

β×(1s) 

0 

500 

400 

300 

200 

Time of flight (ns) 

Figure 1: SE spectra measured in coincidence with an energy loss of the primary electron 

corresponding to one bulk-plasmon excitation (left panel, circles, 1b), one surface-plasmon 

excitation (left panel, squares, 1s), and 195 eV (right panel, dots). A linear combination 

of the 1b and 1s coincidence spectra can be fitted to the spectrum of the right-hand-side 

panel. The lower curves in this panel show the contribution of the 1b coincidence spectra 

(solid blue curve) and the 1s coincidence spectra (dashed red curve) to the full spectrum 

(solid curve through the data). We attribute the additional contribution, which extends up 

to the primary-electron energy loss (indicated by an arrow), to the direct transfer of energy 

and momentum from the primary electron to the emitted SE. 

Isurf /Ibulk (%) 

100 

80 

60 

40 

20 

0 

Excitation 

0 10 20 30 40 50 60 70 80 

Energy loss (eV) 

Emission 

Figure 2: Relative contribution of surface- and bulk-plasmon decay to the emission of 

secondary electrons from an amorphous Al sample under bombardment of 500 eV electrons 

as a function of the energy loss of the primary electrons (dots). The solid curve represents 

the relative probability of surface and bulk energy-losses for 500 eV electrons in Al according 

to multiple-scattering theory. 

117 

100 

0

Fast and with atomic precision – 

real-space investigation of hydrogen diffusion on Si(001) 

using nanosecond laser heating and STM 

M. Dürr 1,2 , C. H. Schwalb 1 , M. Lawrenz 1 , and U. Höfer 1 

1 Fachbereich Physik und Zentrum für Materialwissenschaften,Philipps-Universität, 

35032 Marburg, Germany 

2 Fakultät Angew. Naturwissenschaften, Hochschule Esslingen, 73728 Esslingen, Germany 

(corresponding author: M. Dürr, e-mail: Michael.Duerr@hs-esslingen.de) 

Diffusion on surfaces is one of the fundamental processes in surface chemistry and often the 

rate-limiting step for surface reactions. On semiconductor surfaces, the localized bonds are 

expected to lead to a more complex behavior when compared to diffusion on metal surfaces. 

E.g., even on Si(001) with its comparably simple dimer-row reconstruction, three diffusion 

pathways with very different diffusion barriers exist on the clean surface (Fig. 1). 

Scanning tunneling microscopy (STM) is the method of choice for the investigation of such 

diffusion channels with atomic precision in real space. However, high barrier diffusion 

pathways are difficult to be observed by means of STM since once the diffusion rate across 

the higher barrier is high enough for observation on the timescale of a typical experiment, the 

adsorbates move already so fast along the low barrier pathways that they cannot be imaged by 

means of STM any more. In general, STM-investigations are restricted to rather low diffusion 

rates and thus low surface temperatures [1-3]. 

In order to investigate all diffusion channels 

of hydrogen on the Si(001) surface, we thus 

initiated hydrogen diffusion at high surface 

temperatures by means of nanosecond laser 

heating. STM images taken after different 

numbers of heating pulses then show 

snapshots of the surface configurations 

frozen at various stages of the diffusion 

process. In this way, we were able to 

investigate diffusion rates of 10 8 to 10 9 s -1 

at temperatures as high as 1400 K with 

atomic precision. At these temperatures, the 

diffusion rates of the different processes are 

much closer to each other and high barrier 

processes can be observed. 

118 

Fig. 1: Diffusion processes on flat Si(001). 

Low barrier diffusion pathways (a) on one 

dimer and (b) along the dimer rows as well as 

the high barrier pathway (c) across the dimer 

rows are shown.

Fig. 2: Left: STM image of a Si(001) monohydride surface after laser-induced thermal 

heating. White circles label dbs configurations of two dbs restricted to one dimer row. 

Interrow diffusion leads to configurations labeled with red/grey circles. The sketch on the 

right illustrates the different configurations and respective diffusion pathways. 

As a starting configuration, we used a pair of dangling bond states (dbs) on two neighbored 

silicon dimers as they are created by means of laser induced thermal desorption from the 

monohydride surface [4]. This configuration represents an excited state of the surface and 

hydrogen diffusion causes this arrangement to change into the equilibrium configuration with 

most of the dbs being paired at one single Si dimer. STM images taken after different 

numbers of heating pulses then allow us to monitor the evolution of such dbs configurations. 

Using this method, all three diffusion pathways of hydrogen on flat Si(001) were observed 

with atomic resolution, including for the first time hopping across the dimer rows (Fig. 2), the 

channel associated with the highest diffusion barrier (compare Fig. 1(c)). Comparison of the 

experimentally observed distributions of dbs configurations with Monte-Carlo simulations 

allows us to determine the diffusion rates for the three different diffusion processes at 

Ts = 1400 K [5-7]. Interestingly, we observe that at high temperature diffusion across the 

dimer rows is almost as effective as diffusion along the dimer rows, despite the large distance 

between the adsorption sites in the former case. Along with this observation, comparison with 

low temperature results [2,3] lead to a surprisingly low diffusion barrier in the case of 

interrow diffusion. For intrarow diffusion (Fig. 1(b)), the high temperature data are in very 

good agreement with the extrapolation of data measured below 700 K [2,5], on the other 

hand, for intradimer diffusion (Fig. 1(a)) we find a much higher diffusion rate than expected 

from the extrapolation of the low temperature data [2,7]. 

[1] J. Wintterlin, Adv. Catal. 45, 131 (2000). 

[2] E. Hill, B. Freelon, and E. Ganz, Phys. Rev. B 60, 15896 (1999) 

[3] D. R. Bowler et al., J. Phys.: Condens. Matter 12, 7655 (2000). 

[4] M. Dürr, A. Biedermann, Z. Hu, U. Höfer, and T. F. Heinz, Science 296, 1838 (2002). 

[5] C. H. Schwalb, M. Lawrenz, M. Dürr, and U. Höfer, Phys. Rev. B 75, 085439 (2007). 

[6] C. H. Schwalb, M. Dürr, and U. Höfer, Phys. Rev. B 80, 085317 (2009). 

[7] C. H. Schwalb, M. Dürr, and U. Höfer, Phys. Rev. B 82, 193412 (2010). 

119

Fast scanning with piezo/counter-piezo elements and 

MEMS scanners: a comparison 

F.C. Tabak, P.C. van der Tuijn, H. Borsboom, G.J.C. van Baarle 1 , J.W.M. Frenken and W.M. van 

Spengen 2 

Leiden Institute of Physics, Leiden University, Niels Bohrweg 2, 2333 CA Leiden 

1. Leiden Probe Microscopy, Niels Bohrweg 2, 2333 CA Leiden 

2. Delft University, Mekelweg 2, 2628 CD Delft 

We have built a modular setup to test various STM scanning configurations. This scanner (as shown in 

figure 1) can be modified to carry a tube, stack or conical piezo element, optionally with their 

respective counter piezo elements to prevent the scanner motion exciting vibrations in the rest of the 

system. On the scanning piezo element, a MEMS (micro-electro mechanical systems) STM z-scanner 

can be mounted. This type of scanners (as shown in figure 2) can perform the z-motion in an STM 

setup with a very high speed: these scanners can be designed with a resonance frequency of up to1 

MHz. Their small mass ensures that no resonance frequencies in the mechanical loop of the scanner 

will be excited by the MEMS scanning motion. Other research groups have worked on MEMS STM 

before [among others: 1,2], but to date there is no easy to use high-speed MEMS STM scanner. In 

AFM setups, there is a wider spread use of MEMS scanners [among many others: 3,4]. 

We are currently testing various STM scanner configurations with ‘real life’ STM measurements, and 

in a laser Doppler vibrometer setup to characterize the desired motion and parasitic resonances. 

Implementing MEMS scanners in an STM setup leads to four main challenges: 

• Tip deposition; 

• Alignment MEMS scanner / sample; 

• Contamination of the MEMS; 

• Electronic interference / noise pickup 

Due to the small size of the MEMS we cannot mount a tip on the MEMS. Instead, we grow a tip by 

EBID (Electron Beam Induced Deposition) of platinum. In this way, we deposit tips with a length of a 

few µm and end radii of a few nm. 

Figure 1: Scanner design for the testing of various scan configurations. Left: scanner with stack piezo element 

carrying a MEMS z-scanner. Right: scanner with conical piezo element, along with counter piezo. 

120

Figure 2: MEMS z-scanner. The scanner 

consists of a small membrane, 40 µm in size, 

supported by 4 springs. This membrane can 

perform STM z-motion with a resonance 

frequency of 250 kHz up to 1 MHz depending 

on the specific geometry. 

Alignment of the MEMS scanner and the sample is 

complicated because of the small length of the EBID tip. 

Even a tilt of a few degrees of the sample or scanner will 

inhibit a flawless approach, and lead to a crash of the scanner 

on the sample with a corner of the chip first. This problem 

can partly be circumvented by clever MEMS design: with 

the MEMS in the far corner of the chip the alignment 

problem is less severe. However, this should be optimized 

further for instance with a MEMS holder to place the scanner 

on a fixed spot on top of the piezo element. 

Because of their small size, MEMS scanners are very 

sensitive to contamination such as dust particles: these can 

stick on the scanner and thereby inhibit its functionality. At 

this stage of the project, this problem does not inhibit 

testing. 

In MEMS scanners, there is a high capacitive coupling 

between the various on-chip electrodes and the wirebonds vs 

the sample. This capacitive coupling gets more severe as the scan speed increases. We have decreased 

the crosstalk between scanner and sample to 50 pA ptp (in stationary condition with the tip above the 

sample), which is an acceptable value for the first tunneling experiments. 

In addition to the MEMS scanner experiments, we are testing a cone-based scanner 5 with optional 

counter piezo. This can be implemented in exactly the same scanner due to the modular design of our 

test setup. From numerical simulations we know that a counter piezo element should improve the 

behavior of the scanner until near the resonance frequency of the piezo elements. No two piezo 

elements have exactly the same resonance frequency; therefore they will respond slightly different 

when driven close to the resonance frequencies; therefore, their motions will not cancel perfectly at 

resonance. 

We are currently optimizing the instrument for MEMS-based STM by fine-tuning the sample/scanner 

alignment possibilities and taking measures to prevent electrical cross-talk. The intended and parasitic 

motion of the scanners and the mechanical scanner-environment interactions are studied with a laser 

Doppler vibrometer to experimentally characterize the improvement that a MEMS scanner and a 

piezo/counter-piezo geometry have over a normal piezo setup. In this way we can assess the benefits 

of these more advanced systems, and whether it is more advantageous to use a piezo/counter-piezo or 

a MEMS scanner configuration. With the last issues solved, we will be able to perform exceedingly 

fast STM experiments with both technologies in the same basic setup. These experiments are expected 

to be of high importance in e.g. the field of real-time nanoscale catalysis research. 

1. T.R. Albrecht, S. Akamine, M.J. Zdeblick, C.F. Quate, Microfabrication of integrated scanning 

tunnelling microscope, J. Vac. Scie. Technol. A. 8 (1) (1990) 

2. Y. Xu, N.C. MacDonald, S.A. Miller, Integrated micro-scanning tunneling microscope, Appl. Phys. 

Lett. 67 (16) (1995) 

3. T. Sulchek, S.C. Minne, J.C. Adams, D.A. Fletcher, A. Atalar, C.F. Quate, Dual integrated actuators 

for extended range high speed atomic force microscopy Appl. Phys. Lett. 75 (11) (1999) 

4. C. J. Chen, A. Schwarz, R. Wiesendanger, O. Horn, W. Müller, Three-electrode self-actuating-selfsensing 

quartz cantilever: design, analysis and experimental verification, Rev. Sci. Instr. 81, 053702 

(2010) 

5. M.J. Rost, G.J.C. van Baarle, A.J. Katan, W.M. van Spengen, P. Schakel, W.A. van Loo, T.H. 

Oosterkamp and J.W.M. Frenken, Video-rate scanning probe control challenges: setting the stage for a 

microscopy revolution, Asian journal of control 11, 110-129 (2009) 

121

Sensing Atomic Forces 

Franz J. Giessibl 

Institute of Experimental and Applied Physics, University of Regensburg, D-93053 Regensburg, 

Germany, franz.giessibl@physik.uni-regensburg.de 

Atomic Force Microscopy (AFM) has been introduced in 1986 by Gerd Binnig, Christoph 

Gerber and Calvin F. Quate [1]. This article is now one of the most highly cited publications 

that have appeared in Physical Review Letters, showing that AFM is an important scientific 

tool with fruitful applications in various fields of science. The key element of AFM is the 

force sensor that probes the small forces that act between a sharp tip and a sample. 

Simplifying the force sensor and increasing its force resolution and imaging speed are 

therefore important tasks. 

The figure on the left (Fig. 1 from 

[1]) shows a schematic AFM tip 

close to a sample. The force between 

tip and sample is now measured 

using the frequency modulation 

technique [2-5], where the tip is 

mounted onto a cantilever that 

oscillates at amplitude A. The 

average force gradient that acts 

between tip and sample leads to a frequency shift ∆f=f0/(2k) where f0 and k are the 

eigenfrequency and stiffness of the cantilever. The determination of small force gradients 

requires precise frequency measurements. For forces that vary exponentially with distance 

according to Fts=F0exp(-κz), one can show that the frequency shift is given by 

∆f = f0F0exp(-κ(z+A))I1(κA)/(kA), where I1 is the modified Bessel function of the first kind. 

There are essentially four noise sources in frequency measurement: 1. sensor noise, 2. thermal 

noise, 3. oscillator noise and 4. thermal drift of the eigenfrequency. These noise sources and 

their dependence from sensor type 

will be discussed in the talk. Noise 

sources 1. to 3. are inversely 

proportional to amplitude, therefore, 

the signal-to-noise-ratio SNR 

is proportional to exp(-κA)I1(κA). 

This function is plotted in the 

figure to the left, showing a 

pronounced maximum at 

κA=1.545. Atomic forces have a 

similar decay rate as the tunnelling current with typical values of κ≈0.1 nm -1 [16], thus the 

122

optimal amplitude is on the order of 150 pm. A cantilever with such a small amplitude in the 

presence of rather strong tip-sample forces requires stiff cantilevers – much stiffer than k=17 

N/m as used in the first successful atomic imaging of a reactive surface by AFM. Two types 

of stiff force sensors have been introduced that are able to maintain oscillation amplitudes on 

the order of 100 pm in the presence of strong chemical interaction: the needle sensor [6-8] 

with an effective stiffness of 1 MN/m and the qPlus sensor [9] with k=1.8 kN/m. These 

sensors differ substantially in their operating principle. The needle sensor is based on a quartz 

length extensional resonator, while the qPlus sensor is essentially a quartz cantilever that can 

be built from a quartz tuning fork. Here, we discuss the operating principles of needle- and 

qPlus sensor as well as some 

recent successful applications of 

the qPlus sensor in probing 

small forces [10-17]. We also 

show a qPlus sensor that is no 

longer built from a quartz tuning 

fork, but is designed as a single 

quartz cantilever (see figure left) 

and discuss its performance 

benefits. 

[1] G. Binnig, C.F. Quate, Ch. Gerber, Phys. Rev. Lett. 56, 930 (1986). 

[2] T.R. Albrecht, P. Grütter, H.K. Horne, D. Rugar, J. Appl. Phys. 69, 668 (1991). 

[3] S. Morita, R. Wiesendanger, E. Meyer (eds.) Noncontact Atomic Force Microscopy, Springer, Berlin (2002). 

[4] F.J. Giessibl, Rev. Mod. Phys. 75, 949 (2003). 

[5] S. Morita, F.J. Giessibl, R. Wiesendanger (eds.) NCAFM II, Springer, Berlin (2009). 

[6] K. Bartzke, T. Antrack, K. Schmidt, E. Dammann, Ch. Schatterny, Int. J. of Optoelectr. 8, 669 (1993). 

[7] T. An, T. Eguchi, K. Akiyama, Y. Hasegawa, Appl. Phys. Lett. 87, 133114 (2005). 

[8] S. Torbrügge, O. Schaff, J. Rychen, J. Vac. Sci. Technol. B28, C4E12 (2010). 

[9] F.J. Giessibl, Appl. Phys. Lett. 73, 3956 (1998). 

[10] M. Ternes, C.P. Lutz, C.F. Hirjibehedin, F.J. Giessibl, A..J. Heinrich, Science 319, 1066 (2008). 

[11] L. Gross, F. Mohn, P. Liljeroth, J. Repp, F. J. Giessibl, G. Meyer, Science 324, 1428 (2009). 

[12] L. Gross, F. Mohn, N. Moll, P. Liljeroth, G. Meyer, Science 325, 1110 (2009). 

[13] A. Sweetman, S. Gangopadhyay, R. Danza, N. Berdunov, P. Moriarty, Appl. Phys. Lett. 95, 063112 (2009). 

[14] A. Bettac, J. Koeble, K. Winkler, B. Uder, M. Maier, A. Feltz, Nanotechnology 20, 264009 (2009). 

[15] L. Gross, F. Mohn, N. Moll, et al. Nature Chemistry 2, 821 (2010). 

[16] M. Ternes, C. Gonzalez, C.P. Lutz et al., Phys. Rev. Lett. 106, 016802 (2011). 

[17] Z. Sun, M.P. Boneschanscher, I. Swart et al., Phys. Rev. Lett. 106, 046104 (2011). 

123

Atomic-scale engineering of electrodes 

for single-molecule contacts 

T. Frederiksen1 

, G. Schull2 , R. Berndt3 , A. Arnau1,4,5 , D. Sánchez-Portal1,4 1 Donostia International Physics Center – DIPC, 20018 Donostia-San Sebastián, Spain 

2 Institut de Physique et Chimie des Materiaux de Strasbourg, 

UMR 7504 (CNRS – Universite de Strasbourg), 67034 Strasbourg, France 

3 Institut fur Experimentelle und Angewandte Physik, 

Christian-Albrechts-Universitat zu Kiel, 24098 Kiel, Germany 

4 Centro de Fisica de Materiales CSIC-UPV/EHU, Materials Physics Center MPC, 

20080 Donostia-San Sebastián, Spain, 

5 Depto. Fisica de Materiales UPV/EHU, Facultad de Quimica, 

20080 Donostia-San Sebastián, Spain. 

(e-mail: thomas_frederiksen@ehu.es) 

The transport of charge through a conducting material depends on the intrinsic ability of the 

material to conduct current and on the charge injection efficiency at the contacts between the 

conductor and the electrodes carrying current to and from the material. To explore if this 

remains valid down to the limit of single-molecule junctions, experiments with atomic-scale 

control of the junction geometry is required. 

Here we present a method for probing the current through a single C 60 molecule while 

changing, one by one, the number of atoms in the electrode that are in contact with the 

molecule [1]. We show quantitatively 

that the contact geometry has a strong 

influence on the conductance (Fig. 1). 

We also find a crossover from a regime 

in which the conductance is limited by 

charge injection at the contact to a 

regime in which the conductance is 

limited by scattering at the molecule. 

Thus, the concepts of ‘good’ and ‘bad’ 

contacts, commonly used in macro- and 

mesoscopic physics, can also be applied 

at the molecular scale. 

To interpret the experimental observations we performed electronic structure 

calculations for different contacting configurations to a C60 molecule [2] sandwiched between 

Cu(111) electrodes. The calculations, carried out with the SIESTA [3] and TranSIESTA [4] 

124

codes, reproduce the experimental behavior. The observation of a maximum conductance per 

contacting atom around N = 5 (Fig. 1b), separating cluster-size limited and molecule limited 

transport regimes, is explained in terms of projected density of states and detailed position of 

the molecular resonances. 

Figure 1. Conductances at contact between a single C60 molecule and clusters of copper 

atoms. (a) Normalized conductance at contact versus cluster size N. Experimental data from 

six different molecular tips are shown, as well as theoretical data for two electrode separations 

around the point of contact. Conductance is normalized to the value for a single C60 molecule 

in contact with a 16-atom cluster. (b) Normalized conductance at contact per atom in the 

cluster versus cluster size N. From Ref. [1]. 

This work was supported by the Deutsche Forschungsgemeinschaft (SFB 677), the Schleswig–Holstein Fonds, 

the Ministerio de Ciencia e Innovacion (FIS2010-19609-C2-00) and the Basque Department of Education (IT- 

366-07). 

References: 

[1] G. Schull, T. Frederiksen, A. Arnau, D. Sánchez-Portal & R. Berndt. Atomic-scale engineering of electrodes 

for single-molecule contacts . Nature Nanotech. 6, 23-27 (2011). 

[2] G. Schull, T. Frederiksen, M. Brandbyge & R. Berndt, Passing current through touching molecules. Phys. 

Rev. Lett. 103, 206803 (2009) . 

[3] J. M. Soler, E. Artacho, J. D. Gale, A. García, J. Junquera, P. Ordejón & D. Sánchez-Portal. The SIESTA 

method for ab initio order-n materials simulation. J. Phys.: Condens. Matter 14, 2745 (2002). 

[4] M. Brandbyge, J. L. Mozos, P. Ordejón, J. Taylor & J. M. Soler, Density-functional method for 

nonequilibrium electron transport. Phys. Rev. B 65, 165401 (2002). 

125

20:30 Dinner 20:20 Dinner 20:20 Dinner 20:30 Dinner 20:30 Conference Banquet 20:20 Dinner 

19:40-20:00 Giant Slalom Ceremony Award 19:40-19:50 Closing 

Wöll Teobaldi Lundgren Frederiksen 

18:20-19:40 

chair: Diebold chair: Scheffler chair: Brune chair: Barth chair: Muller 

Ambrosch-Draxl Stradi 18:00-18:30 Poster presentations Schneider Dürr 

Gölzhäuser Ceballos Morgenstern Tabak 

18:20-19:40 

18:20-19:40 

18:20-19:40 

Buck Dong Daimon Giessibl 

18:30-20:30 Poster sesión 

17:50-18:20 Coffee break 17:50-18:20 Coffee break 17:30-18:00 Coffee break 17:50-18:20 Coffee break 17:50-18:20 Coffee break 

16:30-20:30 Registration Menzel Varga Mugarza Himpsel Koshikawa 

Müller Yanson 16:30-17: 30 Heinzmann Cerdá Gustafson 

16:30-17:50 

16:30-17:50 

16:30-17:50 

16:30-17:50 

17:30 Bus departure from Toulouse Höfer Willmot Sanchez Portal Chulkov Krylov 

Widdra Cheynis Ortega Salvat-Pujol 

chair: Echenique chair: Lundgren chair: Himpsel chair: Menzel chair: Aumayr 

Free afternoon 





13:00 Lunch 13:00 Lunch 13:00 Lunch 13:00 Lunch 13:00 Lunch 

10:30-13:00 Outdoors activities 10:00-12:30 Outdoors activities 10:00-12:30 Outdoors activities 10:00-12:30 Giant Slalom Race 10:00-12:30 Outdoors activities 

Brune Scheffler Diebold Aumayr Stempel 

9:05-9:45 8:30-9:10 8:30-9:10 8:30-9:10 8:30-9:10 

Altman Thornton Netzer Bauer Maier 

chair: Schneider chair: Chulkov chair: Thornton chair: Taglauer chair: Morgenstern 

7:45-8:30 Breakfast 7:45-8:30 Breakfast 7:45-8:30 Breakfast 7:45-8:30 Breakfast 7:45-8:30 Breakfast 

8:00-8:50 Registration 

8:50-9:05 Opening 

Sunday, 6 March 2011 Monday, 7 March 2011 Tuesday, 8 March 2011 Wednesday, 9 March 2011 Thursday, 10 March 2011 Friday, 11 March 2011 


Baqueira Beret, Lleida, Sapin 

March 6 – March 12, 2011 

organizers 

Andrés Arnau and Pedro M. Echenique

SYMPOSIUM ON SURFACE SCIENCE 2011 Baqueira Beret, Lleida ...

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