Intrinsic surface states

Surface states can be divided into those that are intrinsic to a well ordered crystal surface with two-dimensional periodicity, and those that are extrinsic [25]. Intrinsic states include those that are associated with relaxation and reconstruction. Note, however, that even in a bulk-tenuinated surface, the outemiost atoms are in a different electronic enviromuent than the substrate atoms, which can also lead to intrinsic surface states. Extrinsic surface states are associated with imperfections in the perfect order of the surface region. Extrinsic states can also be fomied by an adsorbate, as discussed below. 

Note that in core-level photoelectron spectroscopy, it is often found that the surface atoms have a different binding energy than the bulk atoms. These are called surface core-level shifts (SCLS), and should not be confiised with intrinsic surface states. Au SCLS is observed because the atom is in a chemically different enviromuent than the bulk atoms, but the core-level state that is being monitored is one that is present in all of the atoms in the material. A surface state, on the other hand, exists only at the particular surface. 

The dangling and the surface ion-induced states are intrinsic surface states that are characteristic of individual semiconductors. In addition, there are extrinsic surface states produced by adsorbed particles and siuface films that depend on the enviromnent in which the siuface is exposed. In general, adsorbed particles in the covalently bonded state on the semiconductor surface introduce the danglinglike surface states and those in the ionically bonded state introduce the adsorption ion-induced surface states. In electrochemistiy, the adsorption-induced surface states are important. 

As discussed previously, the surface states responsible for the reduction peak could be intrinsic surface states or states associated with a surface-attached intermediate in the series of reactions leading to O-evolution. The latter possibility was deemed to be more likely since no change in voltage across the Helmholtz layer (no change in capacitance) was observed when these states are in the oxidized form. 

Generally it is assumed, that TiIV cations at the surface of the titanium dioxide particle are reduced by the light induced electrons forming Tiin cations [11] which can be considered to be intrinsic surface states localised about 0.1 eV below the conduction band edge, i.e., within the bandgap [12]. An equilibrium between these trapped electrons and free electrons is assumed, but in an acidic medium nearly all electrons are trapped in surface states [11a]. On the basis of their laser flash photolysis measurements Hoffmann and co-workers have extended this mechanistic picture [13]. These authors assume that the CB electrons are trapped in two different Tim sites (reactions (7.4) and (7.5))... 

Similar effects can also occur in surface electronic structure when a moiety is weakly physisorbed onto the surface. The surface core-level shifts measured at the vacuum interface are reduced when atoms or molecules are physisorbed onto the surface. Changes may also occur in the valence electronic structure upon physisorption, such as the disappearance of intrinsic surface states on metals and semiconductors. 

With the electron and hole confined to the QD core, strong electron-hole interaction leads to efficient, fast relaxation via the Auger mechanism, and in QDs where the hole is localised at the surface, the increased spatial separation inhibits the Anger process and results in slower relaxation. The data imply that hole trapping at the intrinsic surface state occurs in less than 75 fs (Blackburn et al, 2003). 

Surface electron states, which exist on atomically pure (ideal) crystal surfaces, are usually called intrinsic. In recent years, considerable progress has been made both in theoretical and experimental methods of studying intrinsic surface states (see, e.g.. Refs. 32-34). 

Under ordinary conditions, in particular when the electrode material is in contact with an electrolyte solution, adsorbed atoms or even layers are present on the surface moreover, real surfaces may contain structural defects. They all can exchange electrons with the semiconductor bulk to give rise to surface electron states of kinds and properties other than those inherent to intrinsic surface states. The former play an important role in adsorption and catalysis processes. 

Finally in this section, Richter et a/.studied NO desorption from Si(lll) for pulsed laser excitation at wavelengths between 355 and 1 907 nm.81 The wavelength dependence was consistent with desorption resulting from excitation of intrinsic surface states of the semiconductor. This was supported by the dependence of... 

The photophysical processes of semiconductor nanoclusters are discussed in this section. The absorption of a photon by a semiconductor cluster creates an electron-hole pair bounded by Coulomb interaction, generally referred to as an exciton. The peak of the exciton emission band should overlap with the peak of the absorption band, that is, the Franck-Condon shift should be small or absent. The exciton can decay either nonradiatively or radiative-ly. The excitation can also be trapped by various impurities states (Figure 10). If the impurity atom replaces one of the constituent atoms of the crystal and provides the crystal with additional electrons, then the impurity is a donor. If the impurity atom provides less electrons than the atom it replaces, it is an acceptor. When the impurity is lodged in an interstitial position, it acts as a donor. A missing atom in the crystal results in a vacancy which deprives the crystal of electrons and makes the vacancy an acceptor. In a nanocluster, there may be intrinsic surface states which can act as either donors or acceptors. Radiative transitions can occur from these impurity states, as shown in Figure 10. The spectral position of the defect-related emission band usually shows significant red-shift from the exciton absorption band. 

The original version of the model assumes a semiconductor adsorbent with no intrinsic surface states, so that before adsorption occurs the bands are flat to the surface. Wolkenstein [6] even refers to weak and strong chemisorption with the former an adsorbed molecule is bound only by covalent forces, whilst, with the latter, charge exchange with the semiconductor takes place. The important point is that the model does not stipulate that the chemisorption bond must be completely ionic. 

The approach most in keeping with current ideas was developed by Krusemeyer and Thomas [11], who considered intrinsic surface states, which produced band bending, and showed that following adsorption they are replaced by states characteristic of the adsorbed material. 

Fig. 12. Photoelectron energy distribution for Si lll 2 X 1. The filled surface state curve represents the difference between clean and oxidized surface curves and depicts the optical density of intrinsic surface states (after Eastman and Grobman [154]).

Among surface states, there are some that originate simply from the sudden discontinuity in the ciystal lattice these are intrinsic surface states. They are sorted, depending on their source, into two categories Tamm states, which are caused by lattice deformation, and Schockley states, caused by the unsaturated bonds on the surface. There also appears on the real surfaces extrinsic surface states due to the presence of foreign species on the surface of the solid, namely adsorbed atoms or molecules originating from a gaseous phase. 

The strategy to modify surface states, and thus the Vg and SRV, is based on interaction of chemically grafted molecules with these states. The key is to find molecules that will modify the semiconductor surface chemistry in a way that involves the surface states. In this respect, the origin of the surface states should be considered. Intrinsic surface states originate from the termination of the crystal bulk and the breaking of chemical bonds at the surface, whereas extrinsic surface states originate from crystal imperfections, such as missing surface atoms, line defects, or... 

Intrinsic surface states arise due to dangling bonds for the discontinuity of the surface. 

Concerning surface states, other comparisons may be of interest. In a simple picture of an NaCl crystal, for instance, the 3 s orbitals of the Na particles overlap to form the conduction band in the bulk of the material. On the surface they form intrinsic surface states presumably located at a lower level. Within the framework of this simple scheme, forming a neutral sodium particle on the surface is just entrapping an electron in a relevant surface state. Entrapping lots of electrons, i.e. forming lots of neutral sodium particles, certainly implies a shift of the Fermi level close to the corresponding surface-state-level. Therefore it is reasonable to assume that the Fermi level is located very close to the relevant surface-state-level when a pertinent massive metal, sodium in our case, is in equilibrium with the electrolyte. This would allow us to compare experimental results typically obtained in solid state electrochemistry with theoretical calculations performed by the physicists, According... 

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