Lateral atomic resolution

Indeed, in situ STM imaging with lateral atomic resolution of a flame-annealed Au(lOO) substrate in deaerated perchloric or sulphuric acid solutions fi ee of Ag ions shows the existence of a potential-induced surface reconstruction with an undulated quasi-hexagonal ( quasi-hex ) structure at Ew < 240 mV (cf. Figs. 2.5 and 2.7). Reconstructed domains were not observed at higher hE, which indicates that the quasi-hex reconstruction of the Au(lOO) surface is lifted by the applied positive electrode potential. In deaerated perchloric or sulphuric acid solutions containing Ag ions, Ag UPD is clearly indicated in cyclic voltammograms (Fig. 3.20a) as well as in q E) or r E) isotherms at = constant in the underpotential range 0 mV < A < 600 mV (Fig. 3.20b). Electrosorption valency measurements under ITL... 

Fig. 3.28 shows an in situ STM image with lateral atomic resolution of a nearly complete Pb monolayer at A = 15 mV on Ag(lll) [3.173-3.175]. The image shows a compressed hep Pb overlayer with d = 0.34 0.01 nm. Additionally, a superstructure with a moir6 pattern is observed. The mean distance between the moire spots is moir6 = 1-65 0.07 nm. This overlayer is rotated by a = 4.5 0.2° with respect to the Ag(lll) substrate. These results are in good agreement to those obtained from GKS and EXAFS measurements [3.137-3.139, 3.140, 3.141, 3.143, 3.145]. 

In situ STM images with lateral atomic resolution were recently observed for Pb UPD on Au(lOO) [3.187, 3.300], The bare and unreconstructed Au(lOO) substrate was imaged at low For high AE. An expanded Au(100)-2c(->/2 x 3- /2 ) R 45° Pb overlayer structure was observed at medium F or AE. Similar to the system Ag(100)/Pb an Au(100)-c(2 X 6) moire superstructure was found at high For low AE, indicating an anisotropically compressed hep Pb overlayer. Obviously, a phase transition from an expanded Pb overlayer to a compressed hep structure has to be taken into consideration. 

The atomistic theory becomes of additional significance for the transition from 2D Me phase formation in the UPD range to 3D Me phase formation in the OPD range. Experimental results obtained using modern in situ techniques with lateral atomic resolution showed that the transition phenomena can only be interpreted on the basis of atomistic approaches. The UPD surface modification turns out to be a more general phenomenon affecting not only the nucleation processes but also the growth mode and epitaxy of 3D metal phases. 

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. 

As we have discussed in Section 1.3, experimentally, atomic resolution has been observed on literally every clean surfaces of metals and semiconductors. Today, atomic resolution on rigid surfaces has become a "must" in STM operation (Rohrer, 1992). In order to resolve single atoms, a lateral resolution of 2 A is required. The importance of the STM — the feature that sets it apart from other instruments — is that it can resolve details in the vicinity of a single atom, otherwise it would not have created the excitement that now surrounds it (Quate, 1986). Here, we briefly discuss the origin of its atomic resolution. 

Fig. 14.2. The "illuminated area" of the tip current. The tunneling current of a tip with atomic resolution is concentrated in an area with a small diameter. On one hand, the diameter determines the lateral resolution. On the other hand, the diameter determines the lateral extent of the modification of the sample wavefunction due to the presence of the tip. The higher the resolution, the more severe the modifications is. (Reproduced from Garcia, 1986, with permission.)...

Fig. 16.3. Atomic-resolution STM image of reconstructed Au(lll). Size of the image, 80 X 60 A The two lines indicate the unit cell. The lateral displacement of the individual Au atoms between fee and hep stacking regions is shown. (Reproduced from Barth et al., 1990, with permission.)...

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