Atomic imaging

MARKS AND SMITH Atomic Imaging of Particle Surfaces... 

Experimentally, the EMD function p(q) can be reconstructed from a set of Compton profiles J qz ) s, and B( r) from the EMD. However, A Air) is not a direct experimental product. By combining the experimental B(r) with theoretical B aik (r), we need to derive a semiexperimental AB(r). Since the atomic image is very weak, many problems must be cleared in experimental resolution, in reconstruction (for example, selection of a set of directions and range of qzs), in various deconvolution procedures and so on. First of all, high resolution experiments are desirable. 

The late 1980s saw the introduction into electrochemistry of a major new technique, scanning tunnelling microscopy (STM), which allows real-space (atomic) imaging of the structural and electronic properties of both bare and adsorbate-covered surfaces. The technique had originally been exploited at the gas/so id interface, but it was later realised that it could be employed in liquids. As a result, it has rapidly found application in electrochemistry. 

ETEM is thus used as a nanolaboratory with multi-probe measurements. Design of novel reactions and nanosynthesis are possible. The structure and chemistry of dynamic catalysts are revealed by atomic imaging, ED, and chemical analysis (via PEELS/GIF), while the sample is immersed in controlled gas atmospheres at the operating temperature. The analysis of oxidation state in intermediate phases of the reaction and, in principle, EXELFS studies are possible. In many applications, the size and subsurface location of particles require the use of the dynamic STEM system (integrated with ETEM), with complementary methods for chemical and crystallographic analyses. 

Atom resolution images can be obtained in particles with diameter down to 100 A. This technique can be extremely important in particle characterization. However in order to obtain usefull information from atomic images, computer calculations are required for proper image interpretation. 

Fig. 1.32. Schematics of a field-ion microscope (FIM). The sample, a tip of radius 100 A is at a high positive voltage relative to a fluorescent screen, placed in a chamber filled with a few millitorrs of He. The He ions generated at the tip surface projects an atomic image of the tip. (Reproduced from Tsong, 1990, with permission.)...

Lang, N. D. (1986). Theory of single atom imaging in scanning tunneling microscopy. Phys. Rev. Lett. 56, 1164-1167. 

Underpotential and overpotential deposition of Bi on Au(lll) have been studied applying in situ STM [453]. It has been found that the adsorbed bismuth lifts the reconstruction of the Au(lll) surface, leading to the formation of Au islands at potentials more cathodic than 0.170 V versus SCE. Atomic images of UPD Bi layer have shown the formation of a nearly rectangular unit cell of dimension 0.39 0.02 X 0.43 0.02 nm. 

Figure 6.3. High Tq cuprate superconductors (HTSC) as catalysts (a) structural schematic diagram of YBa2Cu307 ( 123 ) HTSC with Cu02 sheets (b) HRTEM atomic image of the 123 phase in [010] projection with ED pattern. The image is recorded near the Scherzer defocus. The positions of the Y, Ba and Cu atom columns are indicated. The layer separation is "- 1.18 nm and the unit cell is outlined. (After Gai and Thomas 1991.)...

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