Chemistry Reference
In-Depth Information
Fig. 3.7 Schematic illustration of open feedback measurement. a An STM tip is fixed over a
molecule and then a feedback loop is opened. b A molecular motion under the tip results in a
change in tunneling current. c The tunneling current is recorded during the measurement. A
dynamical behavior can be observed as an abrupt change in the current trace
stay at the maximum or minimum position. The repetition rate of this loop is
determined by the accumulating time for the data acquisition at each point, which is
usually set to be 10-20 ms. The time resolution of this method is expected to be a
few tenth of a second (practically *0.3 s).
3.4.2 Open Feedback Loop Measurement at a Single-Point
The poor time resolution of STM results from the feedback loop controlling the tip
z position. If the feedback loop is opened with keeping the STM tip at a specific
single-point in x-y, we can reach a higher time resolution. The open feedback loop
measurement was first demonstrated by Lozano and Tringides in 1995 [
16
]. They
studied surface diffusion by investigating the tunneling current as a function of
time with opened the feedback loop. In the late 1990 s, Stipe et al. demonstrated
the direct monitoring of a single molecule motion with open feedback loop
measurement [
17
,
18
]. Since an STM tip is fixed at a specific height in the open
feedback loop measurement, dynamical phenomena under the tip can be directly
observed as a change in tunneling current (Fig.
3.7
). Molecular motions or
