Chemistry Reference
In-Depth Information
strong coupling of vibration with molecular switching eventually influences the
electron transport property.
Electron transport through single molecule junctions has been receiving enthu-
siastic interest in last few decades because next generation electronics may rely on
single molecules performing as the smallest unit of functions in electronic devices
[
9
]. The determination of molecular conductivity at the single molecule level is a
challenging subject. Electron transportation properties of nano-scale junctions
including molecules can be affected by internal molecular configurations and
environmental factors. In order to investigate electron transport within a single-
molecule junction several experimental methods have been established, including
break junction [
10
], mercury column method [
11
-
14
], nanowires method [
15
],
nanolithographcally defined pores [
16
], capillary molecular junction [
17
],
STM-based techniques [
18
-
26
], cross-wire method [
27
], metallic nanoparticle-
based contact [
28
], and junctions prepared by the electromigration effect [
29
]. It is
known that vibrational excitations in molecular junction results in characteristic
features in the I-V curve. For instance, non-linear I-V characteristics associated
with the vibrationally mediated configuration change of a molecule have been
observed for a pyrrolidine on a Cu(001) [
30
], H
2
on Cu [
31
], CO bridging a Pt
contact [
32
], and H
2
in Au contacts [
33
]. In these systems dI/dV spectra show
anomalous spikes (not like steps usually observed in inelastic electron tunneling
spectroscopy [
34
]) at the bias voltage related to the vibration energies. The iden-
tification of molecular vibrations makes it possible to discuss the structure of the
junction. At the same time, it might be used to control the charge transport properties
in molecular junctions, which is a key to the development of molecular devices.
8.2 Results and Discussions
8.2.1 Production of a Hydroxyl Dimer on Cu(110)
A hydroxyl dimer can be produced by the reaction a water molecule with atomic
oxygen on Cu(110); H
2
O ? O ? (OH)
2
. Figure
8.1
a, b show sequential images
and schematic illustrations before and after the reaction. A water molecule is
located on the adjacent atomic row of an oxygen atom located at a fourfold hollow
site (the geometry is illustrated in Fig.
8.1
c). The water molecule is dragged along
the atomic row by the STM manipulation and reacted with the oxygen atom and
then yielded a hydroxyl dimer. The reaction occurs spontaneously when the
reactants come close to each other. A hydroxyl dimer is observed as a semi-
circular depression. On the other hand, when a water molecule is located on the
row away from by 2.5b
0
, the reaction does not occur and it is weakly interacted
with oxygen atom (Fig.
8.2
). Although a spontaneous switching cannot be
observed for a dimer, it can be switched between two orientations with a voltage
pulse of the STM (Fig.
8.3
).
