Electron surface states

Chiarotti G 1994 Electronic surface states investigated by optical spectroscopy Surf. Sc/. 299/300 541-50... 

Franke C, Piazza G and Kolb D M 1989 The influence of halide adsorption on the electronic surface states of silver electrodes Electrochim. Acta 34 67-73... 

Electronic surface states may exist at the interface they give rise to an additional capacity, so that the band edges at the surface change their energies with respect to the solution. 

Besides such electronic surface states which can interact either with electrons in the bulk of the semiconductor or with a redox system in the electrolyte, we have to consider another type of excess charge at the surface. This stems from adsorbed ions or from ionic groups attached to the surface of the semiconductor. This is well known from the pH dependence of the flat band potential of semiconducting oxides (8) or the dependence of the flat band potential of sulfides on the sulfide concentration in solution (9). Since surfaces of different orientation will interact differently with such ionic charge, this again will affect the photoelectrochemical processes via the different barrier heights at different surface orientation. 

Formation of Electronic Surface States in Semiconductor Band Gap as a Result of Deposition of Metal Particles on Semiconductor Surface... 

Thus, discussing the influence of the concentration of donors in a semiconductor matrix on the parameters of electronic surface states, we can only make the inference that there exists a certain tendency towards the increase of the interaction between metal nanophase and semiconductor matrix with lowering a matrix doping. 

In contrary to this, the EER response attributed to the electronic surface states induced, for instance, by the deposition of Cu nanoparticles on the nanocrystalline Ti02 appears to be very similar to that obtained in the case of the same deposition on the polycrystalline Ti02 matrix (Fig. 6.12). 

From the above reasoning one could expect that the pre-deposition of small amounts of noble metals on the Ti02 surface in a form of the intermediate sub-layer, which can induce the electroactive electronic surface states in the Ti02 band gap, may enhance the electrocatalytic effect of subsequently deposited Cu particles. Actually, the photocatalytic deposition of silver particles in amount of 5xl014 atoms/cm-2, which on its own only slightly increases the electrocatalytic activity of Ti02 electrode, leads to 2-3-fold enhancement of the electrocatalytic activity of Cu particles subsequently deposited in a relatively high concentration (1016-1017 atoms/cm 2) [52],... 

For the more accurate description of the Mott-Schottky dependences of semiconductor electrodes modified with small metal particles, it is reasonable to take into account the contribution of the capacity of electronic surface states (C ) induced by the... 

The formation of electronic surface states in a semiconductor band gap by metal nanoparticles is the major factor that determine the efficiency of electron exchange between metal particles and a semiconductor matrix. It also influences the efficiency of electro-... 

Electronic surface states are responsible for adsorptive and catalytic processes occurring at the surfaces of solids. Therefore, the catalytic behavior of the simple transition metal oxides such as NiO and CoO (Section I V.F) led to extensive experimental and theoretical investigations of the corresponding 3d surface states (115, 275-283). Whereas the role of d-n overlap is quite modest in the Ni2+ CO surface complexes, the same does not necessarily hold for the Ni2+ NO complexes formed upon interaction of NiO(OOl) with the stronger n-acceptor NO ligand. This is shown by the formation of... 

Thus, new evidence for the importance of topographically distinct parts of the surface has emerged. Yet, this must not be taken as a simplistic decision in favor of an active sites theory as a matter of distinction from, or versus, the electronic factor approach in catalysis. On the contrary, the two viewpoints have become complementary. Electronic surface states due to topographically distinct surface sites become necessary ingredients of the collective electronic theory. 

The main shortcoming of the cluster approach consists of the scission of the chemical bonds between terminal atoms of a cluster and the rest of a lattice. As a result, so-called dangling bonds occur at the terminal atoms of a cluster, artificial electron surface states appear in the partially occupied band, and the charge distribution is disturbed. A cluster in this case possesses too many surface atoms. Unfortunately, to obtain a better surface/bulk ratio, one should consider such large clusters that the approach becomes useless. 

The discussion of ionosorption centred on the equilibrium between the surface and the electrolyte. However, there are also electronic surface states... 

In addition to the models considered above, all of which have involved direction electron transfer from the surface to the redox couple in solution, it is possible for the electron transfer to be mediated by local electronic surface states on the semiconductor, which may be intrinsic or extrinsic. Transfer to these states may be classical or quantum mechanical and we will consider first the classical models. 

If there is a separate surface recombination route via electronic surface states, this may be incorporated into the above analysis. There are three possibilities. 

It is perhaps important to note here that PE exhibits a negative electron affinity (Bloor, 1976), a feature shared with only a very restricted set of materials. This means that excess electrons prefer energetically to reside outside PE rather than be in any way bound to the PE molecular structure. Nevertheless, recent calculations show that there are electronic surface states lying below the vacuum level in the forbidden band gap of PE (Righi et al., 2001). These surface states will certainly be expected to act as traps for transferred or injected electrons, and they will therefore be involved in contact charging. Their resonance interaction with negative ion states of typical PE dopants (02 and H20) may be very important too. 

Cole, M.W. (1974). Electronic surface states of liquid helium. Rev. Mod. Phys. 46, 451-464. 

Fig. 34 are presented as ij-U-y curves. Without sulfide treatment a 1.05 eV band gap is resolved, whereas the gap shrinks completely after passivation. Though results seem in line with a passivation of electronic surface states by the sulfide treatment, since no band gap is resolved on clean GaAs(llO) in vacuum [53] , one may wonder whether the results of Fig. 34 a are not related to the overlayer instead of the substrate. Recent studies have indeed shown that the density of states is not reduced after sulfur coating [87], in contrast to initial assumptions [86]. Moreover, thermal desorption of the sulfide layer opens a band gap [164], as in Fig. 34 b, which is consistent with the existence of the monolayer of oxygen at the interface between GaAs and the layer [161]. In vacuum a wide band gap is also found locally at places where oxygen is adsorbed on clean GaAs(llO) [53]. 

Figure 18. Possible orientations of 7t-orbitals with respect to the electronic surface states of the electrodes. The normal LB geometry would be the right hand case.

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