Noncontact Atomic Force Microscopy - Yale School of Engineering ...

Th-0940
Intramolecular features of organic molecules characterized by force
field spectroscopy: The case of PTCDA on Cu and Ag
Gernot Langewisch 1 , Daniel-Alexander Braun 1 , Domenique Weiner 2 ,
Bartosz Such 3 , Harald Fuchs 1 , and Andre Schirmeisen 1
1CeNTech (Center for Nanotechnology) and Institute of Physics, University of Muenster, Germany
2 SPECS Zurich GmbH, Switzerland
3 Marian Smoluchowski Institute of Physics, Jagiellonian University Krakow, Poland
Thin films of π-conjugated organic molecules, like the organic semiconductor PTCDA,
are of high relevance for nanoelectronic applications. We use non-contact atomic force
microscopy in ultrahigh vacuum at room temperature to investigate the forces between
the tip and PTCDA molecules deposited on Cu(111) and Ag(111) surfaces by molecular
beam epitaxy.
Submolecular features of the PTCDA layers on both substrates are resolved in the
topography scans. In particular we find that the second layer molecules show an
intramolecular structure with a height corrugation of up to 40pm, while molecules in the
first layer above the substrate are depicted as featureless ovals [1]. To study this effect in
detail, 2-dimensional cuts of the spatial tip-sample force landscape above individual
molecules were obtained by force field spectroscopy. In the double layer these cuts, each
consisting of 40 force spectroscopy curves, reveal an enhanced tip-sample force
interaction at the molecular end groups compared to the centre of the molecule [2].
However, for the monolayer molecules this effect is not present. This is interpreted with
respect to different mechanical relaxation processes of the molecular functional end
groups as well as variations of the internal electron density distributions of the molecules.
Figure 1: Left: Topography scans of a double layer of PTCDA molecules on Cu(111) deposited
by molecular beam epitaxy, showing intramolecular contrast. Right: Force field spectroscopy
image along line in left image, visualizing the intramolecular variations of the tip-sample force.
[1] B. Such, A. Schirmeisen, D. Weiner, and H. Fuchs, Appl. Phys. Lett. 98, 093104 (2006).
[2] D.-A. Braun, D. Weiner, B. Such, H. Fuchs and A. Schirmeisen, Nanotechnology (2009) submitted.
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