Probe-Specimen Interaction

Atomic force microscopy (AFM) is a variant of STM and was introduced in 1986 by Binnig et al. (11). AFM belongs to a family of near-field microscopies and is capable of imaging a wide variety of specimens surface down to an atomic scale. The technique employs a probe (pyramidal tip) mounted at the end of a sensitive but rigid cantilever (see Fig. 2). The probe is drawn across the specimen under very light mechanical loading (1). Measurements of the probe s interaction with the sample s surface are accomplished with a laser beam reflected from the cantilever. 

Specimen preparation for SPM differs in the inherent challenges depending on whether or not STM or AFM is used. One challenge common to both is the necessity of preparing a stable, fiat sample that is not pushed along the substrate by the probe. In addition, deformation of the sample by probe-tip interactions must be considered. 

Gas sorption analysis probes the interaction between a gas or vapor (adsorptive) and the adsorbent this interaction can occur upon adsorption in micropores, mono- and multilayer adsorption at the inner surface of the specimen or via liquid/solid interaction when the adsorptive is condensing in the pores of the adsorbent. The probe most commonly used to characterize aerogels is N2 sorption at 77.3 K. When varying the gas pressure from vacuum to 0.1 MPa (1 bar) rel. gas pressures from almost 0 to 1 can be covered hereby typically information on specific surface areas down to 0.01 m /g and pore widths in the range from 0.3 to 100 nm can be derived. 

Figure 2.7. Schematic of an SPM. A probe is scanned in close vicinity to the sample surface while some signal that depends on some local interaction is measured. Although the signal can be measured as a function of the scanning position, in most practical applications the probe-specimen distance is continuously adjusted to keep this signal constant by a feedback control.

As in the SEM, where the probe is a focused beam of electrons, the resolution of the image is controlled by the region of interaction. The part of the probe that interacts with the sample has to be very small, and in several forms of SPM it is small enough to allow atomic resolution. Extremely precise control of position is required in all SPMs both for (x, y) scanning across the surface and for the z height control, and this is accomplished by use of piezoelectric drivers. Motion control is shown schematically in Fig. 2.7 by double-headed arrows on both probe and specimen, but normally one or the other is moved, not both. Some systems split the control, moving the specimen in the x and y direction and the probe in the z direction. 

In electron-optical instruments, e.g. the scanning electron microscope (SEM), the electron-probe microanalyzer (EPMA), and the transmission electron microscope there is always a wealth of signals, caused by the interaction between the primary electrons and the target, which can be used for materials characterization via imaging, diffraction, and chemical analysis. The different interaction processes for an electron-transparent crystalline specimen inside a TEM are sketched in Eig. 2.31. 

The phenomena of beam broadening as a function of specimen thickness are illustrated in Fig. 4.20 each figure represents 200 electron trajectories in silicon calculated by Monte Carlo simulations [4.91, 4.95-4.97] for 100-keV primary energy, where an infinitesimally small electron probe is assumed to enter the surface. In massive Si the electrons suffer a large number of elastic and inelastic interactions during their paths through the material, until they are finally completely stopped. The resulting penetration depth of the electrons is approximately 50 pm and in the... 

With this technique, under an especially equipped electron microscope, high-energy electrons are focused on a fine probe and directed at the point of interest in the specimen. The electrons interacting with the sample atoms cause the emission of the characteristic X-rays, which are detected and identified for qualitative analysis and used, generally through suitable standardization, to perform also a quantitative analysis. 

At the same time, one should notice that the real catalysts are applied in the gas/liquid environments at usually an increased temperature so that dynamic structural evolution of a real catalyst would not be probed in a conventional electron microscope. To bridge the gap, in situ environmental electron microscope is developed by placing a micoreactor inside the column of an electron microscope to follow catalytic reaction processes [58-62], However, the specimen in an in situ TEM may suffer from interaction with ionised gas (plasma), making the interpretation of in situ TEM study of catalytic reaction more complicated. Characterisation of static, post-reaction catalysts is still the most commonly used. Well-designed model catalysts and reasonable interpretation of the results are essential to a successful study. 

To find out the phase composition of the intermetallic compound layers formed, X-ray patterns were taken immediately from the polished surfaces of the Ni-Zn and Co-Zn cross-sections. Annealing and subsequent cooling the specimens of the type shown in Fig. 3.12b in most cases resulted in their rupture along the interface between the zinc phase and the intermetallic layers, with the latter remaining strongly adherent to nickel or cobalt plates. Therefore, preparation of the cross-sections for X-ray analysis presented no difficulties. These could readily made by successive grinding and polishing the plate surface until the Ni or Co phase was reached. In total, four layer sections parallel to the initial interface were analysed for each cross-section. Simultaneously, layer composition on each section of the interaction zone was determined by electron probe microanalysis. 

The main characteristic of the SPM is a sharp probe tip that scans a sample surface. The tip must remain in very close proximity to the surface because the SPM uses near-field interactions between the tip and a sample surface for examination. This near-field characteristic eliminates the resolution limit associated with optical and electron microscopy as discussed in the previous chapters, because their resolution is limited by the far-field interactions between light or electron waves and specimens. Diffraction of light or electron waves associated with far-field interactions limit their resolution to wavelength scales. The near-field interactions in a SPM, however, enable us to obtain a true image of surface atoms. Images of atoms can be obtained by an SPM because it can accurately measure the surface atom profiles in the vertical and lateral directions. The lateral and vertical resolutions of an SPM can be better than 0.1 nm, particularly the vertical resolution. The lateral range of an SPM measurement is up to about 100 /xm, and its vertical range is up to about 10 /xm. However, the SPM must operate in a... 

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