Semiconductors SECM studies

Related to the corrosion problems was a recent SECM study, which demonstrated the possibility of eliminating typical experimental problems encountered in the measurements of heterogeneous electron transfer at semiconductor electrodes (27). In this experiment, the redox reaction of interest (e.g., reduction of Ru(NH3)s+) is driven at a diffusion-controlled rate at the tip. The rate of reaction at the semiconductor substrate is probed by measuring the feedback current as a function of substrate potential. By holding the substrate at a potential where no other species than the tip-generated one would react at the substrate, most irreversible parasitic processes, such as corrosion, did not contribute to the tip current. Thus, separation of the redox reaction of interest from parallel processes at the semiconductor electrode was achieved. 

So far, in reported SECM studies the rate constant and the transfer coefficient have been sufficient to describe the experimental results. However, the ohmic resistance of solids may lead to potential gradients inside bulk materials, which have not been dealt with up to now. This effect has to be taken into account, especially at poorly conducting semiconductors and thin films on insulators. 

SECM has been used to probe heterogeneous electron transfer reaction kinetics on semiconductor electrodes, such as WSe2 (29). In these studies, as... 

A variety of studies have now been done that demonstrate that the SECM can carry out metal deposition, metal and semiconductor etching, polymer formation, and other surface modifications with high resolution. Such processes are discussed in Chapter 13. These SECM approaches have the advantage over analogous STM procedures in that the conditions of deposition or etching are usually known and well defined, based on electrochemical studies at larger electrodes. 

SECM has been applied to the investigation of various technologically important materials and interfaces, for example, metallic corrosion [91-96], fuel cell electrocatalysts [97], semiconductor photocatalysts [12, 60-63, 98], conducting polymers [49, 50, 85, 86, 99-103], liquid-liquid and liquid-gas interfaces [29, 30, 68]. The SECM may be used to image the substrate topography and/or reactivity, or with the tip at a fixed location, to study the local kinetics of the interfacial reactions of interest. 

Chapters 8 and 13 reviewed the application of SECM to the study of charge and electron transfer at the liquid-liquid and solid-liquid interfaces, respectively. Most studies have used molecular mediators (Chapter 1). However, there is a continued interest in nanoparticles, for example, of metals and semiconductors. These can also be studied by SECM. Eor example, SECM was used to determine the rate of electron transfer for gold monolayer-protected clusters (MPCs) as a function of the surface thiol group chain length. Such studies are more difficult than with molecular mediators because... 

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