Semiconductor electrodes, scanning electrochemical microscopy

A few years ago Bard and his group developed the technique called scanning electrochemical microscopy (SECM) which makes possible a spatial analysi,s of charge transfer processes [9]. In this method an additional tip electrode of a diameter of about 2 pm is used as well as the three other electrodes (semiconductor, counter and reference electrode). Assuming that a redox system is reduced at the semiconductor, then the reduced species can be re-oxidized at the tip electrode, the latter being polarized positively with respect to the redox potential. The corresponding tip current / [ is proportional to the local concentration of the product formed at the semiconductor surface and therefore also to the corresponding local semiconductor current, provided... 

Horrocks, B. R., Mirkin, M. V., Bard, A. J. Scanning electrochemical microscopy. 25. Application to investigation of the kinetics of heterogeneous electron transfer at semiconductor (WSej and Si) electrodes, J. Phys. Chem. 1994, 98, 9106-9114. 

Section 6.2.1 offers literature data on the electrodeposition of metals and semiconductors from ionic liquids and briefly introduces basic considerations for electrochemical experiments. Section 6.2.2 describes new results from investigations of process at the electrode/ionic liquids interface. This part includes a short introduction to in situ Scanning Tunneling Microscopy. 

P. Allongue, in Scanning Tunneling Microscopy of Semiconductor Electrodes. Advances in Electrochemical Science and Engineering, H. Gerisher and C. Tobias (ed.), VCH, Weinheim, New York (1995), vol 4, p. 62. 

Electrochemical measurements of the type described earlier give indirect evidence about dissolution processes. More direct chemical information can be obtained from in-situ spectroscopies, in particular from IR and Raman methods. Chazalviel and coworkers have showed the power of this approach in studies on silicon and GaAs [73,98,99]. Electrochemical and spectroscopic techniques are macroscopic methods giving a view of the whole electrode surface. To study semiconductor dissolution at the microscopic (atomic) level, one needs techniques such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM). The anodic and chemical dissolution of sihcon has been studied in very elegant work by Allongue and coworkers [100-102]. 

Another impedance-based imaging technique for laterally resolved characterization of thin films or electrochemical systems is Scanning Photo-induced Impedance Microscopy (SPIM) [44]. It is based on photocurrent measurements at field-effect structures. In their simplest arrangement, field-effect structures consist of a semiconductor substrate with a thin insulator, and a gate electrode. This gate electrode can be a metal film resulting in the structure Metal Insulator Semiconductor (MIS) or, alternatively. Electrolyte Insulator Semiconductor structures are used, in which the electrolyte is in direct contact with the insulator, and a reference electrode is required to fulfill the function of the gate electrode. 

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