STM principles

In SFM, the probe tip is mounted on a highly sensitive, cantilever-type spring. The force of interaction between the sample and the tip can be calculated from the spring constant and the measured deflection of the cantilever. The deflection is sensed using the STM principle (Vignette 1.8) or capacitance or optical methods. The SFM can be operated in the contact regime or like the SFA. In the latter mode, one can measure van der Waals forces (see Chapter 10), ion-ion repulsion forces (see Chapter 11), and capillary forces and frictional forces, among others. In contrast to STM, the SFM can be used for both conductors and... 

Detection of cantilever displacement is another important issue in force microscope design. The first AFM instrument used an STM to monitor the movement of the cantilever—an extremely sensitive method. STM detection suffers from the disadvantage, however, that tip or cantilever contamination can affect the instrument s sensitivity, and that the topography of the cantilever may be incorporated into the data. The most coimnon methods in use today are optical, and are based either on the deflection of a laser beam [80], which has been bounced off the rear of the cantilever onto a position-sensitive detector (figme B 1.19.18), or on an interferometric principle [81]. 

Scanning tunneling spectroscopy (STS) can, in principle, probe the electronic density of states of a singlewall nanotube, or the outermost cylinder of a multi-wall tubule, or of a bundle of tubules. With this technique, it is further possible to carry out both STS and scanning tunneling microscopy (STM) measurements at the same location on the same tubule and, therefore, to measure the tubule diameter concurrently with the STS spectrum. No reports have yet been made of a determination of the chiral angle of a tubule with the STM technique. Several groups have, thus far, attempted STS studies of individual tubules. 

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 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. 

Figure 4.28 shows an example where STM recognizes the individual metal atoms in an alloy, thus revealing highly important structural information on the atomic level. The technique does not require a vacuum, and can in principle be applied under in situ conditions (even in liquids). Unfortunately, STM only works on well-defined, planar, and conducting surfaces such as metals and semiconductors, and not on oxide-supported catalysts. For the latter surfaces, atomic force microscopy offers better perspectives. 

Explain the principles of the scanning probe microscopies STM and AFM, and discuss the type of information these techniques provide. What are the major differences between the two ... 

The Genesis and Principle of Catalysis at Oxide Surfaces Surface-Mediated Dynamic Aspects of Catalytic Dehydration and Dehydrogenation on TiO2(110) by STM and DFT 317... 

Most of the work on nanostructuring electrode surfaces, which can be found in the literature, deals with the deposition of small metal clusters at predetermined positions. Over the years, we have developed a technique that is based on the jump-to-contact between tip and substrate [89] (Figure 5.15h) and that allows the formation of metal clusters in quick succession and without destroying the single crystallinity of the substrate. The principle behind this method is sketched in Figure 5.19 [90, 92] By applying an electrode potential to the STM tip that is slightly... 

The combination of state-of-the-art first-principles calculations of the electronic structure with the Tersoff-Hamann method [38] to simulate STM images provides a successful approach to interpret the STM images from oxide surfaces at the atomic scale. Typically, the local energy-resolved density of states (DOS) is evaluated and isosurfaces of constant charge density are determined. The comparison between simulated and measured high-resolution STM images at different tunneling... 

Fig. 2 (a) Schematic representation of a mechanically controlled break junction (MCBJ). The inset shows the SEM image of a nanofabricated gold bridge [40]. (b) Principle of an STM-based break junction experiment (STM-BJ)... 

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