Seminars Archive
D. Drakova
Abstract
Monday, May 14, 2001, 14:30
Seminar Room, ground floor, Building "T"
Sincrotrone Trieste, Basovizza
Theory of STM, STS and molecule manipulations in the STM
D. Drakova
(University of Sofia, Faculty of Chemistry, Bulgaria and Ludwig-Maximilians
Universitt, Mnchen, Germany)
ABSTRACT
Three-dimensional scattering theory, which takes into account the tip-sample
interaction, is capable to explain many observations in
the STM as for instance the contact and resonance phenomena, image
contrast reversal and the change of intensity of STS features as a function
of
tip-sample interaction. However, the understanding of the tip height
corrugation amplitude on geometrically and electronically flat metal
surfaces requires a theory of tunnelling which goes beyond the single-particle
picture and takes into account the electron dynamics
in the presence of the tunnelling charge.
In my talk I shall present the results of a dynamic theory of STM,
STS and current induced molecule manipulations with the STM.
The injection of the tunnelling charge in a relatively localized region
on the sample surface and the relaxation effects in the transient negative
or positive ion resonances of the sample surface are the main features
of the dynamic theory. Within the dynamic approach the large corrugation
amplitude on sp-metal surfaces is
explained as due to the strongly corrugated charge density of the local
negative ion resonances. The observed, but unexpected,
features in the STS on metal surfaces (for instance on Al(111) and
Cu(111)) are due to the projection of tip states on the sample and, therefore,
they
depend upon the nature of the tip. The contrast on adsorbate covered
metal surfaces is also tip-dependent as it will be
illustrated with the example of imaging a single CO molecule adsorbed
on Cu(111) with a clean metal tip and with a CO-terminated tip.
Within the same theory and model the current induced desorption of
CO from Cu(111) is explained as the result of a decoherent mechanism.
The peculiar characteristics of this process, as observed experimentally,
are the existence of a voltage threshold
for the desorption and the transfer of CO in the reverse direction
of electron flow. The process is non-adiabatic and results
from coupling of the core and electron movement.
