Complete report - Donostia International Physics Center - Euskal ...

2010 DIPC Highlight
Surveying molecular vibrations
during the formation of
metal-molecule nanocontacts
L. Vitali, R. Ohmann, K. Kern, A. Garcia-Lekue, T. Frederiksen, D. Sanchez-Portal, and A. Arnau
Nano Letters 10, 657-660 (2010)
Combined inelastic tunneling spectroscopy and first-principles calculations allow a precise
characterization of the metal-molecule interaction during the formation of a metal-molecule
nanocontact.
Charge transport through metal-molecule systems is a major subject of study in a rapidly growing interdisciplinary
research field. It deals with fundamental and applied aspects of science at the nanoscale aiming
to control the electron conductance at the molecular level and the uprising of nanotechnology. Within
this context, accessing the influence of the molecular contact to the metal in the transport properties as
metal-molecule systems are tuned from tunneling to contact is vital.
Experimentally, the metal-molecule coupling can be characterized through the spectroscopic signals of
the molecular vibrations as measured by inelastic electron tunneling spectroscopy (IETS). This can be
recorded as a function of the tip-molecule distance allowing understanding to which extent the contact
formation influences the molecular properties. Further comparison with first-principles based simulations
allows to achieve a deep understanding of the variation of the metal-molecule coupling as the tip-molecule
distance decreases.
A good molecular prototype to study the influence of the metal-molecule bond is carbon monoxide (CO).
For this reason, we have used the IETS signal measured at various molecule-metal distances by approaching
the tip of an STM to the CO molecule adsorbed on a Cu(111) metal surface. The vibration modes of
CO have been surveyed with unprecedented accuracy, and the following main observation has been
made: a continuous but nonlinear blue shift of the frustrated rotation mode in tunneling with decreasing
distance followed by an abrupt softening upon contact formation.
To obtain physical understanding of the experimentally observed frequency shifts, we have modeled the
vibration modes and the inelastic transport using density functional theory (DFT) combined with nonequilibrium
Green’s function methods. Our calculations yield shifts for all these vibration modes along the
transition from tunnel to contact, i.e., a clear indication of the existence of an appreciable tip-sample interaction.We
assign the observed shifts to a modification of the surface potential under the influence of
the tip proximity, as the tip-molecule distance is reduced. The interaction between them passes progressively
from the attractive to the repulsive regime. Our results further indicate that the presence of the
metal electrode sensibly alters the structural and conductive properties of the junction even without the
formation of a strong chemical bond.
In conclusion, changes in the molecular vibrations give evidence of a perturbation of the electronic and
structural properties of the junction as the STM tip approaches a single molecule. The observation of their
shift is therefore a powerful tool to monitor the formation of a metallic nanocontact. We believe that these
results have a general validity to the measurements of conductance through molecular junctions and might
therefore clarify a few of the uncertainties in the electron transport through nanometer scale junctions.
Left panel: Measured vibration spectra for individual CO molecules on a Cu(111) surface at different tip-sample
distances ranging from tunneling to point contact. The dotted black lines highlight the shift of the frustrated rotation
(FR) mode vs the tip position. Right panel: Calculated spectra taken on top of a CO molecule adsorbed
on a Cu(111) surface at different tipsample distances. A clear shift (red dotted lines) to lower energies is seen in
the FR mode where the peak positions shifts from 35 to 29 meV as point contact is approached from tunneling,
in close agreement with experiments. Upper panel: sketch showing the geometry of the junction and the relaxation
of the sample under the influence of the tip proximity.
Shifts of the molecular
vibration modes: fingerprints
of metal bond formation.
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