Scanning local electronic states

Fein, A.P. J. Vac. Sci. Technol. A. in press)(3). Electronic structure measurements of occupied states are typically made with UPS, while unoccupied states are probed by IPS (49). EELS probes both filled and unfilled states simultaneously, and is therefore used in conjunction with either UPS or IPS to complete a band structure determination (44,49). A new electronic spectroscopy technique, Field Emission Scanning Auger Microscopy (50), utilizes STM-like technology to effect highly localized (c.a. 1 /im) Auger electron spectroscopy. The local electronic information afforded by STM is a valuable complement to these other techniques, and STM is the only one of these methods that may be applied to in situ investigations in condensed media. 

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... 

Several authors have reported that the simultaneously acquired tunneling spectra with an STM experiment contain information about the electronic states of the tip. For example, Klitsner, Becker, and Vickers (1990) observed a case in which two microtips on one tip body produced two different tunneling spectra at the same spot on the same sample surface. The two independently acquired tunneling spectra contained information about the electronic structures of the two microtips. Pelz (1991) reported in detail a case where the tip electronic state changed during a single scan, which dramatically altered the local tunneling spectra, which we will discuss later on. 

The measurement of tunnelling spectra in a scanning tunnelling microscope offers the potential of measuring the local density of states at spatially defined sites whose topography can be established at an atomic scale by STM. This information is only available however at the price of losing the information about the k-dependence of electronic states that is available in photoemission and inverse photoemission. In particular STS offers the prospect of measuring local densities of states at defect sites whose real space atomic structure can be established by STM. 

In the two-state approximation (TSA), ET kinetics for the DBA —> D" BA process may be modeled in terms of initial- ( ,) and final-state (T /) wave-functions, in which the transferring charge is localized primarily on the D and A sites, respectively. In the case of electrodes (e.g., metal or semiconductor), where multiple electronic states are involved, the D and A sites may still be taken to be localized and to involve atomic sites of the electrode near the site/or sites of attachment or contact with the bridge) SAM = self assembled monolayer STM = scanning tunneling microsccopy. 

Lennie AR, Condon NG, Leibsle FM, Murray PW, Thornton G, Vaughan DJ (1996) Structures of Fc304 (111) surfaces observed by scanning tunneling microscopy. Phys Rev B Cond Mat 53 10244-10253 Li EK, Johnson KH, Eastman DE, Freeouf JL (1974) Localized and bandlike valence electron states in... 

This is the basis for scanning tunneling spectroscopy (STS) where the variation of current with bias is used to measure the localized surface electronic states, particularly in semiconductors... 

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