Metal surfaces electronic Tamm surface states

No "Jilt has so far been assumed that the semiconductor-electrolyte interphase does not contain either ions adsorbed specifically from the electrolyte or electrons corresponding to an additional system of electron levels. These surface states of electrons are formed either through adsorption (the Shockley levels) or through defects in the crystal lattice of the semiconductor (the Tamm levels). In this case—analogously as for specific adsorption on metal electrodes—three capacitors in series cannot be used to characterize the semiconductor-electrolyte interphase system and Eq. (4.5.6) must include a term describing the potential difference for surface states. 

After the first theoretical work of Tamm (1932), a series of theoretical papers on surface states were published (for example, Shockley, 1939 Goodwin, 1939 Heine, 1963). However, there has been no experimental evidence of the surface states for more than three decades. In 1966, Swanson and Grouser (1966, 1967) found a substantial deviation of the observed fie Id-emission spectroscopy on W(IOO) and Mo(lOO) from the theoretical prediction based on the Sommerfeld theory of metals. This experimental discovery has motivated a large amount of theoretical and subsequent experimental work in an attempt to explain its nature. After a few years, it became clear that the observed deviation from free-electron behavior of the W and Mo surfaces is an unambiguous exhibition of the surface states, which were predicted some three decades earlier. 

Since surface states with free-electron-like dispersion (Shockley type) have a low occupancy per surface unit cell and a low DOS at Ey, they are in general not considered to dominate the energetics of the surface, although - as discussed in Section 5.4.3 - situations exist where they can afiect the properties of the surface. On the contrary, metallic surface states derived from weakly dispersing bands (Tamm states) may have a high DOS at Ey and thus may influence the surface phase diagram considerably. As mentioned already in the case of quasi-2D states, transition metal surfaces are interesting in this respect, the question... 

Tamm states These are characteristic of more tightly bound systems such as the transition metals in which the valence electrons are d states. Tamm states are spht-off states because of the reduced atomic coordination of the surface... 

The elementary surface excited states of electrons in crystals are called surface excitons. Their existence is due solely to the presence of crystal boundaries. Surface excitons, in this sense, are quite analogous to Rayleigh surface waves in elasticity theory and to Tamm states of electrons in a bounded crystal. Increasing interest in surface excitons is provided by the new methods for the experimental investigation of excited states of the surfaces of metals, semiconductors and dielectrics, of thin films on substrates and other laminated media, and by the extensive potentialities of surface physics in scientific instrument making and technology. 

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