Densities of local electronic states

The scanning tunneling microscope uses an atomically sharp probe tip to map contours of the local density of electronic states on the surface. This is accomplished by monitoring quantum transmission of electrons between the tip and substrate while piezoelectric devices raster the tip relative to the substrate, as shown schematically in Fig. 1 [38]. The remarkable vertical resolution of the device arises from the exponential dependence of the electron tunneling process on the tip-substrate separation, d. In the simplest approximation, the tunneling current, 1, can be simply written in terms of the local density of states (LDOS), ps(z,E), at the Fermi level (E = Ep) of the sample, where V is the bias voltage between the tip and substrate... 

FIG U RE 4.39 Local density of electron states (LDES) of (a) C atoms in groups-C(0)OH (curve 1), =C-C(0) OH (curve 2) and aromatic C in 7+7 aromatic ring cluster (b) C, Si, and O atoms in cluster SigOjoHyCgHj with Si-O-C and (c) C and Si atoms in cluster SigOigHyC Hj with Si-C bond (HF/6-31G(d, p) method). (Adapted from Appl. Surf. Sci., 258, Gun ko, V.M., Zaulychnyy, Ya.V., Ilkiv, B.I. et al.. Textural and electronic characteristics of mechanochemically activated composites with nanosilica and activated carhon, 1115—1125, 2011f, Copyright 2011, with permission from Elsevier.)... 

Fig. 16 Left Structure model of ref. 171 (a) and energetically preferred structure (b) of ref. 135 for the Au55(PPh3)i2Cl6 cluster. The gold core of (b) is shown in (c) and can be split into layers different in height (d). Right Angular-momentum-projected local density of electron states (PLDOS) around the Fermi energy. Reproduced from ref. 135.

In principle the STM can also be used to derive information on the electronic structure of the surface by measuring the current through the tip as a function of the applied voltage (so-called I(V)-curves). The method thus also offers the possibility to directly determine the local density of electronic states, both above and below the Fermi edge. 

On the Be(OOOl) surface, a particularly large surface core-level shift has been observed in the Be Is spectrum [46, 47]. In addition, three subsurface layers have been resolved by their individual shifts away from the bulk peak position. These layer-dependent core-level positions have been rationabzed by the strong and layer-dependent relaxation (see Chapter 2.5.1) of the vertical layer spacing in this surface and the strong and layer-dependent surface state contribution to the local density of electronic states [48]. Figure 3.2.2.11 shows Be Is spectra measured with synchrotron radiation at four different photon energies, with an overall experimental resolution of 20 meV [49]. Instead of a single narrow line... 

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