STM applications

Because AFM and STM are more recently developed microscopic technologies, they have not been employed as extensively by plant scientists as other microscopies. In fact, STM has not witnessed much application to biology. Thus, standardized protocols applicable to diverse plant systems have not been reported. In this regard, Table 3 provides a summary of some recent AFM and STM applications to plant systems. Through critical comparison of published Materials and Methods for these papers, the reader should be able to originate AFM and STM protocols suitable for his/her plant system. 

One of the most significant applications of STM to electrochemistry would involve the application of the full spectroscopic and imaging powers of the STM for electrode surfaces in contact with electrolytes. Such operation should enable the electrochemist to access, for the first time, a host of analytical techniques in a relatively simple and straightforward manner. It seems reasonable to expect at this time that atomic resolution images, I-V spectra, and work function maps should all be obtainable in aqueous and nonaqueous electrochemical environments. Moreover, the evolution of such information as a function of time will yield new knowledge about key electrochemical processes. The current state of STM applications to electrochemistry is discussed below. 

Muralt, P., Pohl, D. W., and Denk, W. (1986). Wide-range, low-operating-voltage, bimorph STM Application as potentiometer. IBM J. Res. Develop. 30, 443-450. 

This introductory book, moderate in size and sophistication, is not intended to be the ultimate STM treatise. The first part of this book, especially, is not intended to be a comprehensive review of all published STM theories. More sophisticated theoretical approaches, such as those directly based on first-principle numerical calculations, are beyond the scope of this introductory book. With its moderate scope, this book is also not intended to cover all applications of STM. Rather, the applications presented are illustrative in nature. Several excellent collections of review articles on STM applications have already been published or are in preparation. An exhaustive presentation of STM applications to various fields of science and technology needs a book series, with at least one additional volume per year. Moreover, this book does not cover the numerous ramifications of STM, except a brief chapter on AFM. The references listed at the back of the book do not represent a catalog of existing STM literature. Rather, it is a list of references that would have lasting value for the understanding of the fundamental physics in STM and AFM. Many references from related fields, essential to the understanding of the fundamental processes in STM and AFM, are also included. 

Monitoring by the STM technique of the lead deposition either in a bulk deposition process (BDP) or in a UPD process was an important early example of STM application to monitor electrochemical processes in real time. Christoph et al. studied bulk Pb deposition on a Ag(lOO) substrate. During the potential-controlled Pb phase deposition, they observed irreversible changes of the Ag(lOO) surface morphology by the Pb deposit. Furthermore, they reported a recrystallization phenomenon during repetitive Pb depo-sition/dissolution processes. Additionally, they determined the... 

Summary. The importance of flie electrode surface structure in electrochemistry is briefly described. Examples are given in which the structural information provided by scanning tunneling microscopy (STM) is of assistance in clarifying the electrochemical behavior. The importance of surface structure in the photoelectrochemical response of metals is illustrated by an STM application. Finally, the potentialities of newr scanning microprobe techniques suitable for mapping local photoelectrochemical properties of metal surfaces are briefly discussed. 

This chapter deals with the STM application to the study of different kinds of conducting surfaces. It is shown that relevant quantitative information can be obtained either on well defined and weakly disordered systems such as stepped single crystal faces or on strongly disordered systems such as surfaces grown far from equilibrium. 

STM found one of its earliest applications as a tool for probing the atomic-level structure of semiconductors. In 1983, the 7x7 reconstructed surface of Si(l 11) was observed for the first time [17] in real space all previous observations had been carried out using diffraction methods, the 7x7 structure having, in fact, only been hypothesized. By capitalizing on the spectroscopic capabilities of the technique it was also proven [18] that STM could be used to probe the electronic structure of this surface (figure B1.19.3). 

STM has not as yet proved to be easily applicable to the area of ultrafast surface phenomena. Nevertheless, some success has been achieved in the direct observation of dynamic processes with a larger timescale. Kitamura et al [23], using a high-temperature STM to scan single lines repeatedly and to display the results as a time-ver.sn.s-position pseudoimage, were able to follow the difflision of atomic-scale vacancies on a heated Si(OOl) surface in real time. They were able to show that vacancy diffusion proceeds exclusively in one dimension, along the dimer row. 

The molecular-level observation of electrochemical processes is another unique application of STM [53, 54]. There are a number of experimental difficulties involved in perfonning electrochemistry with a STM tip and substrate, although many of these have been essentially overcome in the last few years. 

One of the most important advances in electrochemistry in the last decade was tlie application of STM and AFM to structural problems at the electrified solid/liquid interface [108. 109]. Sonnenfield and Hansma [110] were the first to use STM to study a surface innnersed in a liquid, thus extending STM beyond the gas/solid interfaces without a significant loss in resolution. In situ local-probe investigations at solid/liquid interfaces can be perfomied under electrochemical conditions if both phases are electronic and ionic conducting and this... 

The very new techniques of scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) have yet to establish themselves in the field of corrosion science. These techniques are capable of revealing surface structure to atomic resolution, and are totally undamaging to the surface. They can be used in principle in any environment in situ, even under polarization within an electrolyte. Their application to date has been chiefly to clean metal surfaces and surfaces carrying single monolayers of adsorbed material, rendering examination of the adsorption of inhibitors possible. They will indubitably find use in passive film analysis. 

A very similar technique is atomic force microscope (AFM) [38] where the force between the tip and the surface is measured. The interaction is usually much less localized and the lateral resolution with polymers is mostly of the order of 0.5 nm or worse. In some cases of polymer crystals atomic resolution is reported [39], The big advantage for polymers is, however, that non-conducting surfaces can be investigated. Chemical recognition by the use of specific tips is possible and by dynamic techniques a distinction between forces of different types (van der Waals, electrostatic, magnetic etc.) can be made. The resolution of AFM does not, at this moment, reach the atomic resolution of STM and, in particular, defects and localized structures on the atomic scale are difficult to see by AFM. The technique, however, will be developed further and one can expect a large potential for polymer applications. 

Figure 2.8. STM image (unfiltered) of a Pt( 111) surface of a Pt single crystal interfaced with P"-A1203, a Na+ conductor showing different domains of Na coverage. The Pt(l 11 )-(2x2)-0 surface was initially covered by the Pt(ll l)-(2x2)-Na adlattice (domain A) and was intentionally only partly electrochemically cleaned (via positive UWR=1V potential application and Na+ removal into the P"-A1203 lattice) leading to the formation of clean domains (domain B) and of higher Na coverage domains (domain C) corresponding to a (V3 x V3 )-Na adlattice.

Most of the published promotional kinetic studies have been performed on well defined (single crystal) surfaces. In many cases atmospheric or higher pressure reactors have been combined with a separate UHV analysis chamber for promoter dosing on the catalyst surface and for application of surface sensitive spectroscopic techniques (XPS, UPS, SIMS, STM etc.) for catalyst characterization. This attempts to bridge the pressure gap between UHV and real operating conditions. 

The experiments were carried out in ambient air.78 79 STM images were obtained at 300 K following current, I, or potential, Uwr, application in ambient air at 550 K. Figure 5.49 shows an unfiltered atomic resolution image of the Pt (111) surface after assembling the solid electrolyte cell before any current or potential application. 

The obvious question then arises as to whether the effective double layer exists before current or potential application. Both XPS and STM have shown that this is indeed the case due to thermal diffusion during electrode deposition at elevated temperatures. It is important to remember that most solid electrolytes, including YSZ and (3"-Al2C)3, are non-stoichiometric compounds. The non-stoichiometry, 8, is usually small (< 10 4)85 and temperature dependent, but nevertheless sufficiently large to provide enough ions to form an effective double-layer on both electrodes without any significant change in the solid electrolyte non-stoichiometry. This open-circuit effective double layer must, however, be relatively sparse in most circumstances. The effective double layer on the catalyst-electrode becomes dense only upon anodic potential application in the case of anionic conductors and cathodic potential application in the case of cationic conductors. 

---