Chemistry Reference
In-Depth Information
Fig. 4.3 a Typical trajectory of the STM tip when it tracks a water molecule on a Cu(110)
surface at V
s
= 24 mV and I
t
= 0.5 nA. The solid (red) and dashed (blue) lines indicate the
displacements along the
½
110
and [001] directions, respectively. b Distribution of the time
intervals between the hopping events, where the tunneling conditions are the same as (a). Fitting
to an exponential function (solid red line) provides the diffusion rate of 0.13 s
-1
. The inset shows
the rate as a function of tunneling current at V
s
= 24 mV measured for H
2
O
pre-exponential factor of m
0
= 10
13
s
-1
, the barrier of the hopping is estimated to
be 15 meV according to R
0
= m
0
exp(-E
b
/k
B
T), where E
b
is the barrier height, k
B
is the Boltzmann constant, and T is the temperature (6 K). This barrier is com-
parable with the value expected for a water molecule on Cu(100) (*20 meV)
[
19
]. On the other hand, the diffusion rate shows an increase above I
t
* 0.5 nA,
indicating the tip-molecule interaction affects the diffusion process due to the
reduced gap (tip-molecule) distance. The diffusion rate also increases even when
the bias voltage is reduced to *5 mV (the reduction of the bias voltage also
reduces the gap distance). At present, the underlying mechanism is not clear, but it
could be ascribed to the proximity between the tip and molecule, which pre-
sumably perturbs the potential energy surface of the hopping.
The STM-induced diffusion is also observed at higher bias voltages. Since the
diffusion rate becomes too fast to follow up with the atom tracking technique at a
higher bias region, I employed the open feedback measurement to observe the
hopping event. Before opening the feedback loop the tip was fixed over a water
molecule at low bias conditions, and then a voltage pulse for few seconds was
applied while the current signal was recorded. A typical current-time plot is shown
in the inset of Fig.
4.4
a (measured at V
s
= 54 mV). The abrupt drops in the
current trace correspond to the moments of the hopping event. The first and second
drops indicate the moments of hopping to the first- and second-neighbor site,
respectively. The hopping rate is measured as a function of the applied bias voltage
(Fig.
4.4
a). The tip height is fixed to give I
t
= 0.012 ± 0.003 nA at V
s
= 24 mV
in which the tip proximity effect is negligible (inset of Fig.
4.3
b). Figure
4.4
a
shows the voltage dependence of the hopping rate for an H
2
O and D
2
O monomer.
