Chemistry Reference
In-Depth Information
Fig. 7.3 Sequential images of the dissociation of two OH groups into atomic oxygen. a A
voltage pulse of 0.9 V is applied over one depression (indicated by the white cross) of the top-left
hydroxyl. The tip height is fixed at the set point of I
t
= 0.5 nA and V
s
= 24 mV. b The tunneling
current during the voltage pulse. c and d Images taken after the pulses show the successive
dissociation into round depressions which are assigned as atomic oxygen. The asterisk represent
the short-bridge site occupied by a parent OH, which indicate that the product oxygen is displaces
along [001] and located an adjacent hollow site. The image size is 42 9 42 Å
2
. All images were
acquired at V
s
= 24 mV and I
t
= 0.5 nA
The further dissociation of a hydroxyl into atomic oxygen can be induced.
Figure
7.3
shows the dissociation of OH groups. The STM tip is positioned over
the one depression (indicated by a white cross) in Fig.
7.3
a and then a 0.9 V pulse
is applied. Figure
7.3
b shows tunneling current during a voltage pulse and the
abrupt drop corresponds to the moment of the dissociation. After the pulse it
dissociated into an atomic oxygen imaged as a round depression with the apparent
height of -0.3 Å. The asterisk in Fig.
7.3
b, c indicates the original position of the
parent OH species (short-bridge site) and atomic oxygen is situated on the adjacent
fourfold hollow site. The appearance and adsorption site of the generated oxygen
are consistent with the previous result [
33
]. In Fig.
7.3
d another OH is also dis-
sociated. It was found that the position of the produced oxygen can be controlled
depending on the initial tip position before a voltage pulse. Atomic oxygen is
always formed at the hollow site under the tip. Therefore the current trace during
the dissociation always shows a drop as seen in Fig.
7.3
b when the dissociation
takes place. The dissociation of a hydroxyl occurs at both bias polarities, and the
threshold is estimated to be 0.9 (1.5) V for OH (OD) with the variation of *0.2 V
depending on the tip conditions. The hydrogen abstraction is assumed to be
induced via the overtone excitation of the O-H stretch mode. The isotope effect
