Scanning Probe Microscopy STM and AFM

Atomic force microscopy (AFM) or, as it is also called, scanning force microscopy (SFM) is the most generally applicable member of the scanning probe family. It is based on the minute but detectable forces - order of magnitude nano-Newtons -between a sharp tip and atoms in the surface [39]. The tip is mounted on a flexible arm called a cantilever, and is positioned at a subnanometer distance from the surface. If the sample is scanned under the tip in the x-y plane, it feels the attractive or repulsive force from the surface atoms and hence is deflected in the z direction. Various methods exist to measure the deflection, as described by Sarid [40]. Before we describe equipment and applications to catalysts, we will briefly look at the theory behind AFM. 

A third mode, which has recently become the standard for work on surfaces that are easily damaged, is in essence a hybrid between contact and non-contact mode, 

AFMs have been built in many different versions, with at least six different ways of measuring the deflection of the cantilever [40,41], The commercially available AFM systems use the double photo detector system of Fig. 7.14 described by Meyer and Amer [42]. A lens focuses a laser beam on the end of the cantilever, which reflects 

Scanning probe images represent a combination between the morphology of the sample surface and the shape of the tip. Features on the samples that are sharper than the tip yield an image of the tip. In such cases so-called supertips improve the image greatly. Note, however, that blunt tips are perfectly suitable for imaging flat surfaces in atomic detail. 

In addition to piezo scanners, the AFM may contain a stepping motor for coarse x-y positioning of the sample. Most instruments also possess a built-in camera for selecting the desired area of the sample and for positioning the tip at a distance of few pm from the surface. The final approach of the tip towards the surface is done automatically. 

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Nevertheless, the good catalytic performances shown by these phases are not always well understood yet. Surface sensitive spectroscopies, as XPS and confocal Raman, operated in their mapping mode, and scanning probe microscopies (AFM and STM) are very promising tools to investigate structural, superficial and catalytic properties of these materials. However, they only offer decent spatial resolutions if operated on macroscopically flat samples, thus making conventional catalytic powders often not suited. 

In this chapter, we focus on molecular assemblies of functionalized molecules consisting of phthalocyanines (Pcs) and porphyrins noncovalently bounded on metal surfaces, in order to explore their potential as building blocks for the construction of nanostructures, by using scanning probe microscopy (SPM) including STM and atomic force microscopy (AFM). 

Besides SEM, modern methods of scanning probe microscopy such as STM, AFM, and SNOM are used to characterize the morphology of corrosion attacks of the surface down to subnanometer-size features, e.g., selective alloy dissolution, and open the way to in situ observation of corrosion at high temperatures and under electrolytes. 

Defects at the interface most likely play an important role in the delamination process. Unfortunately, the defects in self-assembled films are mostly nanoscopic and can be studied only with atomic force microscopy (AFM) and STM, which require very time-consuming preparation and limit the flexibility of the experiments. Other operation modes such as scanning Kelvin probe force microscopy (SKPFM) [73] will play an important role in future work [74]. 

The ability to control the position of a fine tip in order to scan surfaces with subatomic resolution has brought scanning probe microscopies to the forefront in surface imaging techniques. We discuss the two primary techniques, scanning tunneling microscopy (STM) and atomic force microscopy (AFM) the interested reader is referred to comprehensive reviews [9, 17, 18]. 

We confine ourselves here to scanning probe microscopies (see Section VIII-2B) scanning tunneling microscopy (STM) and atomic force microscopy (AFM), in which successive profiles of a surface (see Fig. VIII-1) are combined to provide a contour map of a surface. It is conventional to display a map in terms of dark to light areas, in order of increasing height above the surface ordinary contour maps would be confusing to the eye. 

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 success of STM has resulted in the development of a whole variety of related scanning probe microscopes, the most important of which is atomic force microscopy, AFM, also known as scanning force microscopy. AFM was first reported in 1986 by Binnig, Quate and Gerber. 

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