Electrochemical scanning tunneling microscope

Electrochemical nanotechnologies using ultramicroelectrodes such as the tips of electrochemical scanning tunneling microscopes and related devices [446,447] are of special interest both, for conducting local electrosynthesis and for electrochemical modification. The tip nanotechnique in electrolyte solutions ensures the optimal level of surface purity, offers additional possibilities in governing the processes by varying the potentails of the tip electrode and the substrate, and may also be used for... 

Xie, X.F., Yan, J.W., Liang, J.H. et al. (2013) Measurement of the quantum conductance of germanium by an electrochemical scanning tunneling microscope break junction based on a jump-to-contact mechanism. Chemistry - An Asian Journal, 8, 2401-2406. 

Fig. 3.8 High temperature electrochemical scanning tunnelling microscope.

A current example investigating the mechanism of this method was employed by Borguet and coworkers. Utilizing small concentrations of silver ions and an electrochemical scanning tunnelling microscope, they were able to visualize the electrodeposition of Ag utilizing a sodium dodecyl sulfate (SDS) surfactant. In this case, the SDS adsorbs in parallel lines on the Au(lll) current collector surface. The lines appear wavy due to the adsorption of silver ions. After the application of... 

Wilms, M., Kruft, M., Bermes, G., and Wandelt, K. (1999) A new and sophisticated electrochemical scanning tunneling microscope design for the investigation of potentiody-namic processes. Rev. Sci. lustrum., 70, 3641-3650. 

The development of scanning probe microscopies and x-ray reflectivity (see Chapter VIII) has allowed molecular-level characterization of the structure of the electrode surface after electrochemical reactions [145]. In particular, the important role of adsorbates in determining the state of an electrode surface is illustrated by scanning tunneling microscopic (STM) images of gold (III) surfaces in the presence and absence of chloride ions [153]. Electrodeposition of one metal on another can also be measured via x-ray diffraction [154]. 

Moffat T P, Fan FRF and Bard A 1991 Electrochemical and scanning tunneling microscopic study of dealloying of CUjAu J. Electrochem. Soc. 138 3224... 

The main technique employed for in situ electrochemical studies on the nanometer scale is the Scanning Tunneling Microscope (STM), invented in 1982 by Binnig and Rohrer [62] and combined a little later with a potentiostat to allow electrochemical experiments [63]. The principle of its operation is remarkably simple, a typical simplified circuit being shown in Figure 6.2-2. 

The experiments were performed in a combined system for UHV and electrochemical measurements. It consists of a UHV system equipped with standard facilities for surface preparation and characterization and a pocket-size scanning tunneling microscope (STM) [Kopatzki, 1994], a pre-chamber containing a flow cell for electrochemical measurements, which was attached to the main UHV system via a gate valve, and... 

While the first STM studies of electrode surfaces were performed with self-built instruments, scanning tunneling microscopes for electrochemical use are nowadays commercially available at a price that hardly justifies the effort of homemade equipment. Nevertheless, new instrumental designs are now and then discussed in the literature, which are still worthwhile to be considered for special applications. There is, however, additional equipment required for the operation of an electrochemical STM, for which homemade designs may be advantageous over commercially available ones and hence is briefly mentioned here in terms of tip preparation and isolation, the electrochemical cell, and vibration damping. 

The scanning tunneling microscope (STM) is an excellent device to obtain topographic images of an electrode surface [1], The principal part of this apparatus is a metal tip with a very fine point (see Fig. 15.1), which can be moved in all three directions of space with the aid of piezoelectric crystals. All but the very end of the tip is insulated from the solution in order to avoid tip currents due to unwanted electrochemical reactions. The tip is brought very close, up to a few Angstroms, to the electrode surface. When a potential bias AF, usually of the order... 

The Scanning Tunneling Microscope has demonstrated unique capabilities for the examination of electrode topography, the vibrational spectroscopic imaging of surface adsorbed species, and the high resolution electrochemical modification of conductive surfaces. Here we discuss recent progress in electrochemical STM. Included are a comparison of STM with other ex situ and in situ surface analytic techniques, a discussion of relevant STM design considerations, and a semi-quantitative examination of faradaic current contributions for STM at solution-covered surfaces. Applications of STM to the ex situ and in situ study of electrode surfaces are presented. 

Figure 6. Scanning electron micrograph (SEM) of n-GaAs surface electrochemically etched with a scanning electrochemical and tunneling microscope (SETM). Etching was accomplished in Aq. 5 mU NaOH, 1 mM EDTA. Photoelectric current - 0.7 /iA, Scan rate - 0.1 /tm/sec, bias voltage — 4 V. Tip was moved in an "L" pattern. Reproduced with permission of Ref. 89. Copyright 1987 The Electrochemical Society Inc.

Figure 8.17 Left Schematic of a scanning tunneling microscope (STM). Right STM image (2.7 x 2.7 nm) of the atomic structure of a copper (111) surface imaged in an aqueous medium after electrochemical cleaning [357]. The image was kindly provided by P. Broekmann and K. Wandelt.

The scanning tunneling microscope (STM) has led to several other variants (61). Particularly attractive for electrochemical studies is scanning electrochemical microscopy (SECM) (62-65). In SECM, faradaic currents at an ultramicroelectrode tip are measured while the tip is moved (by a piezoelectric controller) in close proximity to the substrate surface that is immersed in a solution containing an electroactive species (Fig. 2.17). These tip currents are a function of the conductivity and chemical nature of the substrate, as well as of the tip-substrate distance. The images thus obtained offer valuable insights into the microdistribution of the electrochemical and chemical activity, as well... 

The main contaminants in an ionic liquid will be introduced from the synthesis, absorbed from the atmosphere or produced as breakdown products through electrolysis (see above). The main contaminants for eutectic-based ionic liquids will be from the components. These will be simple amines (often trimethylamine is present which gives the liquid a fishy smell) or alkyl halides. These do not interfere significantly with the electrochemical response of the liquids due to the buffer behavior of the liquids. The contaminants can be effectively removed by recrystallization of the components used to make the ionic liquids. For ionic liquids with discrete anions the major contaminants tend to be simple anions, such as Li+, K+ and Cl-, present from the metathesis technique used. These can give significant difficulties for the deposition of reactive metals such as Al, W and Ti as is demonstrated below with the in situ scanning tunnelling microscope. 

The scanning tunneling microscope is also a tool for surface morphological studies that is widely used in situ [73], It is based on the analysis of a tunneling current between a very sharp microscopic tip and the electrode surface caused by a bias potential applied between the two. This method is well established for the study of electrochemical systems [58,59,74-76], Its advantage over AFM is that it is technically much simpler to use for in situ studies of electrochemical systems, and it obtains better resolution. However, the application of STM to nonaqueous systems may be complicated by the following factors ... 

The invention of the scanning tunneling microscope and the developmental work " that ensued to adapt the technique in the study of the electrode-electrolyte interface under reaction conditions, have led to significant advances in electrochemical surface science. The singular power of STM lies in... 

Figure 6.32 Schematic diagram depicting the mechanism of electrochemical metal deposition on graphite surfaces in the scanning tunneling microscope [6.193). Reprinted by permission of Kluwer Academic Publishers.

W. Li, J.A. Virtanen, R.M. Penner, Nanometer-scale electrochemical deposition of silver on graphite using a scanning tunneling microscope. Appl. Phys. Lett. 60, 1181-1183, 1992. 

A model P4-18-SPM scanning tunneling microscope (NT-MDT, Russia) was employed to investigate the structure, in atmosphere, of nanometer-scale thin film materials and also to measure the thickness of the film. A setup [28] combining electrochemical studies and X-ray photoelectron spectroscopic (XPS) analysis served to characterize the surface composition of alloy films. 

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