Page 62 - Analytical Electrochemistry 2d Ed - Jospeh Wang

2-3 SCANNING PROBE MICROSCOPY 47

FIGURE 2-13 STM image of an electrochemically activated glassy-carbon surface.
(Reproduced with permission from reference 46.)

achieved by monitoring the growth of such ®lms under different conditions (48). In
addition to topographic information, the high sensitivity of the tunneling current to
changes in the local work function (i.e., surface conductivity) offers a distinct
visualization of composite electrode surfaces (49).
The recent introduction of commercial STM machines incorporating a potentio-
stat and an electrochemical cell has greatly facilitated in-situ investigations of
electrochemical processes. A block diagram of such a STM=electrochemical
system is shown in Figure 2-15. Coupled with powerful software, such instruments
allow the simultaneous acquisition and display of the electrochemical and topo-
graphic data. Extremely useful insights can thus be obtained by correlating the
surface microstructures and the electrochemical reactivity. The interpretation of
STM images requires extreme caution, and the tip should be shielded properly (from
the electrolyte) to minimize the stray capacitance. Nevertheless the powerful
coupling of STM and electrochemical systems offers many exciting future oppor-
tunities.

2-3.2 Atomic Force Microscopy
Atomic force microscopy (AFM) has become a standard technique for high-
resolution imaging of the topography of surfaces. It enables one to see nanoscopic