Chemistry Reference
In-Depth Information
Fig. 4.1 a Typical STM
image (53 9 53 Å
2
) of single
water molecules on a Cu(110)
surface. The image was
acquired at V
s
= 24 mV and
I
t
= 0.5 nA. A water
molecule appears as a round
protrusion with the apparent
height of 0.6 Å. The tip was
scanned horizontally from top
to bottom. The inset image
shows relative position of a
water molecule to co-
adsorbed CO molecules. The
white grid lines represent the
lattice of Cu(110). The image
was acquired at V
s
= 24 mV
and I
t
= 0.2 nA. b Side (left)
and top (right) view of the
optimized structure for a
water molecule on a Cu(110)
surface
Fig. 4.2 Schematic
illustration of the possible
high symmetry adsorption
sites on a Cu(110) surface.
a on-top site, b short-bridge
site, c long-bridge site and
d hollow site
was used, in which the STM tip traced the molecular motion in the x-y plane and
its trajectory was recorded [
17
,
18
]. Figure
4.3
a shows a typical trajectory of a
water monomer measured at V
s
= 24 mV and I
t
= 0.5 nA. The displacements
from the original position along the
½
110
(solid red line) and [001] (dashed blue
line) directions are plotted as a function of the measurement time. The trajectory
clearly indicates a water molecule diffuses exclusively along the atomic row
direction. The distribution of the time intervals between hopping events is plotted
in Fig.
4.3
b. The distribution is fitted by an exponential decay function,
N = N
0
exp(-Rt), where R is the diffusion rate. Consequently, R is determined to be
0.13 s
-1
at V
s
= 24 mV and I
t
= 0.5 nA. The inset of Fig.
4.3
b shows the current
dependence of the diffusion rate at V
s
= 24 mV. The rate is almost constant
(*0.13 s
-1
) at less than I
t
= 0.5 nA, suggesting the tip effect is negligible and the
diffusion is induced via merely thermal activation in this voltage region. Given the
