www.elsevier.nl/locate/jelechem Journal of Electroanalytical Chemistry 492 (2000) 81 – 93 Review Scanning tunneling microscopy applications in electrochemistry — beyond imaging N.J. Tao *, C.Z. Li, H.X. He Department of Physics, Florida International Uniersity, Miami, FL 33199, USA Received 15 May 2000; received in revised form 27 July 2000; accepted 28 July 2000 Abstract Scanning tunneling microscopy (STM) has gradually matured into a powerful tool for imaging electrode surfaces in the electrochemical environment with atomic resolution. It has been used to elucidate numerous old puzzling structural issues and to reveal many new interesting phenomena. As an imaging tool, it will continue to contribute to the understanding of various electrochemical processes on electrode surfaces. STM is more than an imaging tool for structural characterizations, other important electrochemical applications, such as probing electron transfer processes, fabricating nanostructures and studying fast electrochemical kinetics, have also been actively pursued. These later unconventional applications are the focus of this discussion. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Scanning tunnel microscopy; Probing electron transfer; Fabricating nanostructures 1. Introduction Since the pioneering applications of scanning tunnel- ing microscopy (STM) in an aqueous environment nearly 15 years ago [1,2], the technique has become a powerful tool for studying various electrochemical phe- nomena with atomic or molecular resolution in real space. Important applications include atomic or molec- ular resolution studies of well-defined electrode surfaces [3 – 7], electrochemical deposition and dissolution [8 – 11] and molecular adsorption [12 – 16]. More complex sys- tems and phenomena, such as biological molecules [17–20] and electrochemical reactions of organic molecules [21,22] have also been studied with molecular resolution. While the progress in the last 15 years has been impressive, the full potential of STM applications in electrochemistry has yet to be reached, and this de- pends critically on several developments. One of these developments is better instrumentation. Today’s STM designs and costs are not that different from those of 10 – 15 years ago (excluding the computers). High qual- ity atomic-resolution images still need frequent ‘mas- sages’, which often challenges the patience of new users. Revolutionary designs of user-friendlier STM are highly desired and should also be possible given the rapid development in micromachining capability and elec- tronics. Another critical development is a more com- plete understanding of the STM imaging mechanism in the electrochemical environment. In principle, the tun- neling current measured by STM contains information about electron transfer through water layers, which is extremely important for nearly all electrochemical pro- cesses. It also contains valuable information about the electronic states of adsorbed molecules, which could be used not only to identify the molecules, but also to study the reactivity of the molecules. Extracting the information requires a good understanding of the STM tunneling/imaging mechanism, which is still incomplete. Increasing theoretical and experimental efforts have been devoted to understanding the STM tunneling/ imaging mechanism, and to extract molecular identity and reactivity information, which are discussed in this paper. Efforts to improve STM temporal resolution for studying fast electrochemical kinetics are also briefly described here. Another unconventional electrochemi- * Corresponding author. Tel.: +1-305-3483954; fax: +1-305- 3486700. E-mail address: taon@fiu.edu (N.J. Tao). 0022-0728/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII:S0022-0728(00)00295-3