Chemistry Reference
In-Depth Information
Fig. 8.5 a The voltage
dependence of the switching
yield for an (OH)
2
(red
circles) and (OD)
2
(blue
squares). The upper-left inset
is the current trace measured
over an (OD)
2
. The current
fluctuates between the high
and low current states
corresponding to the
orientations depicted in the
bottom-right inset. The
tunneling current was
recorded during the pulse
voltage of 303 mV. The tip
was fixed over the depression
of the STM image
(corresponding to the low
current state) at the set point
of I
t
= 0.05 nA and
V
s
= 24 mV, and then the
feedback was turned off and
the voltage was applied. The
yields were measured with
the tip fixed at the low current
state. b The current
dependence of switching rate.
The inset shows voltage
dependence of the reaction
order for an (OH)
2
characteristics in the I-V curve. To elucidate the mechanism, the voltage depen-
dence of the fractional occupations of the two states was investigated. The tun-
neling current (I
high, low
) and the fractional occupation (O
high, low
) at the high and
low states are obtained as a function of the voltage (Fig.
8.7
a). The fractional
occupation is defined as the ratio of the time spent at which the high and low
current states in the current trace. The tunneling current at the high and low states
shows a linear increase (bottom of Fig.
8.7
a). On the other hand, the fractional
occupations of each state show an abrupt change at the threshold energy of the
m(OD) excitation (upper of Fig.
8.7
a). The average current I
average
at a given bias
can be reproduced from the following equation
I
average
¼
O
high
I
high
þ
O
low
I
low
ð
8
:
4
Þ
I
average
is plotted in Fig.
8.7
b, which coincides with the averaged I-V curve. It is
now clear that the non-linear characteristics in the averaged I-V curves are
