Electron reference level

At a surface, not only can the atomic structure differ from the bulk, but electronic energy levels are present that do not exist in the bulk band structure. These are referred to as surface states . If the states are occupied, they can easily be measured with photoelectron spectroscopy (described in section A 1.7.5.1 and section Bl.25.2). If the states are unoccupied, a teclmique such as inverse photoemission or x-ray absorption is required [22, 23]. Also, note that STM has been used to measure surface states by monitoring the tunnelling current as a fiinction of the bias voltage [24] (see section BT20). This is sometimes called scamiing tuimelling spectroscopy (STS). 

Basic properties of semiconductors and phenomena occurring at the semiconductor/electrolyte interface in the dark have already been discussed in Sections 2.4.1 and 4.5.1. The crucial effect after immersing the semiconductor electrode into an electrolyte solution is the equilibration of electrochemical potentials of electrons in both phases. In order to quantify the dark- and photoeffects at the semiconductor/electrolyte interface, a common reference level of electron energies in both phases has to be defined. 

Let us choose, as an arbitrary reference level, the energy of an electron at rest in vacuum, e ) (cf. Section 3.1.2). This reference energy is obvious in studies of the solid phase, but for the liquid phase, the Trasatti s conception of absolute electrode potentials (Section 3.1.5) has to be adopted. The formal energy levels of the electrolyte redox systems, REDox, referred to o, are given by the relationship ... 

In general, the chemical potential of electrons, t., is characteristic of individual electron ensembles, but the electrostatic energy of-e< > varies with the choice of zero electrostatic potential. In electrochemistry, as is described in Sec. 1.5, the reference level of electrostatic potential is set at the outer potential of the electron ensemble. 

In electrochemistry, we deal with the energy level of charged particles such as electrons and ions in condensed phases. The electrochemical potential, Pi,of a charged particle i in a condensed phase is defined by the differential work done for the charged particle to transfer from the standard reference level (e.g. the standard gaseous state) at infinity = 0) to the interior of the condensed phase. The electrochemical potential may be conventionally divided into two terms the chemical potential Pi and the electrostatic energy Zi e as shown in Eqn. 1-21 ... 

Fig. 4-24. Electron energy levels for electrode potential relative to a reference electrode E = electrode potential (absolute) E = relative electrode potential Ps = outer potential of electrolyte solution of test electrode = outer potential of electrolyte solution of reference...

Such an interfacial degeneracy of electron energy levels (quasi-metallization) at semiconductor electrodes also takes place when the Fermi level at the interface is polarized into either the conduction band or the valence band as shown in Fig. 5-42 (Refer to Sec. 2.7.3.) namely, quasi-metallization of the electrode interface results when semiconductor electrodes are polarized to a great extent in either the anodic or the cathodic direction. This quasi-metallization of electrode interfaces is important in dealing with semiconductor electrode kinetics, as is discussed in Chap. 8. It is worth noting that the interfacial quasi-metallization requires the electron transfer to be in the state of equilibrimn between the interface and the interior of semiconductors this may not be realized with wide band gap semiconductors. 

Fig. 10-26. Energy diagram for a cell of photoelectrolytic decomposition of water consisting of a platinum cathode and an n-type semiconductor anode of strontium titanate of which the Fermi level at the flat band potential is higher than the Fermi level of hydrogen redox reaction (snao > epM+zHj) ) he = electron energy level referred to the normal hydrogen electrode ri = anodic overvoltage (positive) of hole transfer across an n-type anode interface t = cathodic overvoltage (negative) of electron transfer across a metallic cathode interface.

The occurrence of TSDC during a thermal scan of a previously excited ( perturbed ) material is probably the most direct evidence we have for the existence of electronic trap levels in the band gap of these materials. The main attraction of TSDC and related techniques as experimental methods for the study of the trapping levels in high-resistance semiconductors was their apparent simplicity. A TSDC spectrum (for historical reasons, frequently referred to as a glow curve ) usually consists of a number of more or less resolved peaks in current versus temperature dependence. The latter, in most cases, may be attributed to a species of traps. 

Work Function (WF) plays a key role in the physics and chemistry of materials. Phenomena such as the semiconductor field effect, photo- and thermionic electron emission (Allen and Gobelli, 1962), catalysis (Vayenas et al 1996), and the like are dominated by the WF. This fundamental property of electronic materials is defined as the minimum work required to extract an electron from the Fermi level Ep of a conducting phase, through the surface and place it in vacuum just outside the reach of the electrostatic forces of that phase (Trasatti and Parsons, 1986). The reference level for this transfer is thus called the vacuum reference level. Because even a clean surface is a physical discontinuity, a surface dipole t] with its associated electric field always appears at the surface of the condensed phase. Thus, the work of extracting the electron can be conceptually divided between the work required to... 

Considerably more elaborate treatments (1, 40,105) for calculating the electronic energy levels and wave functions for the excess electron have since been developed and attempt to introduce certain microscopic features of the local molecular environment. Such calculations for electrons in ammonia were first reported by Copeland, Kestner, and Jortner (40). The reader is referred to the paper by Baneijee and Simons (1) and the recent review by Brodsky and Tkarevsky (9) for comprehensive discussions of the current theoretical descriptions for solvated electrons in disordered condensed media (see also Ref. 171). 

Scheme 5.1 Electronic energy levels of selected IH-V and II-VI semiconductors using the valence band offsets from Reference 32 (VB valence band, CB conduction band).

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