Chemistry Reference
In-Depth Information
to transnational or rotational energy along the reaction coordinate, in competition
with the fast vibrational relaxation through EHP excitation, gives
v
3
=
2
e
2a
1
s
m
;
RC
1
s
m
h dx
E
B
ð
2
:
15
Þ
dx represents the anharmonic coupling between the HF and RC modes, and
v = E
B
/hx. This theory agrees in the order of magnitude with the experimental
result of a CO hopping rate on Pd(110). It is noted that this theory could explain
why CO hopping is not observed on Cu(110) on contrast to Pd(110), which follows
from the small dx and large v [
68
-
70
].
In short STM-AS provides elementary processes of adsorbate motions and
reactions as well as fruitful information about vibrational excitations. Two dif-
ferent mechanisms have been characterized: i) direct process in which the reaction
coordinate mode is directly excited by tunneling electrons, ii) indirect process that
involves energy transfer between vibrational modes via anharmonic coupling.
I focused on the vibrational mediated reactions of adsorbates. However, the
reactions can be also induced by electronic excitation or electron (hole) attachment
with STM, which takes place at the bias voltage higher than *1eV[
71
-
74
]. The
electronic excitation occurs when tunneling electrons couple with electronic states
of the atoms or molecule.
STM can induce and image a variety of fascinating phenomena at the single
molecule level. Proper understanding of current-driven events in STM junctions is
particular interest in molecular-scale electronics because it is directly related to the
conductivity, heating and current-induced failure. From a theoretical perspective,
current-driven dynamics in STM junctions includes the challenging subjects to
describe non-equilibrium and non-adiabatic phenomena under the bias voltage and
the dissipative effects of the electrodes.
2.2.6 Single Atom and Molecule Manipulation
An important application of tip-adsorbate interaction is the manipulation of single
atoms and molecules on surfaces, which offers a precise positioning of individual
adsorbates and assembly of nano-scale structures at the spatial limit. The inter-
action between the adsorbate and the surface is also important in the STM
manipulation as well as the force working between the tip and the adsorbate. While
a strong adsorbate-surface interaction makes the manipulation impossible, a weak
interaction results in an uncontrollable situation.
The first example of the atom manipulation was reported by Eigler and
Schweizer in 1990 [
75
]. They transferred Xe atoms back and forth reversibly
between the tip and the surface and positioned them in fully controlled fashion on
a Ni(110) surface. Figure
2.13
a shows a schematic illustration of this manipulation
that is called ''vertical manipulation''. It is also found that single atoms can be
