Effect of Surface States

FIGURE 2.31. Platband potential W of p-Si in MeCN as a function of redox potential Fredo of added redox ), be 

FIGURE 2.32. The flatband potential obtained from the Mott-Schottky plots for n-type silicon in O.IM K4pe(CN)6 + 0.5 M KCl solution at pH 9 versus the plateau photocurrent density. After Oskam et al. (Reproduced by permission of The Electrochemical Society, Inc.) 

FIGURE 2.33. Initial value of the flatband potential for n-Si with deposited gold atoms (tO M gives about a monolayer). After Chazalviel and Stefenel.  

FIGURE 2.34. Examples of the energetic positions of the band edges determined for a number of silicon/electrolyte interfaces. The energy levels of the redox couples related to Si and H2O are also plotted. 

Au concentration in the HF electrolyte used to deposit the metals. The amount of shift is about 0.3 V for a coverage of about one monolayer of gold. 

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Although the conductivity change Aa [relation (8)] of microwave conductivity measurements and the Ac of electrochemical measurements [relation (1)] are typically not identical (owing to the theoretically accessible frequency dependence of the quantities involved), the analogy between relations (1) and (8) shows that similar parameters are addressed by (photo)electrochemical and photoinduced microwave conductivity measurements. This includes the dynamics of charge carriers and dipoles, photoeffects, flat band and capacitive behavior, and the effect of surface states. 

The capacitance Css and the resistance Rss connected parallel to Csc, which are introduced to describe the effect of surface states. 

Effect of Surface States on Semiconductor Electrode Kinetics22... 

The effect of surface states on the potential distribution across the interface can be estimated by the relation... 

One of the most important theoretical contributions of the 1970s was the work of Rudnick and Stern [26] which considered the microscopic sources of second harmonic production at metal surfaces and predicted sensitivity to surface effects. This work was a significant departure from previous theories which only considered quadrupole-type contributions from the rapid variation of the normal component of the electric field at the surface. Rudnick and Stern found that currents produced from the breaking of the inversion symmetry at the cubic metal surface were of equal magnitude and must be considered. Using a free electron model, they calculated the surface and bulk currents for second harmonic generation and introduced two phenomenological parameters, a and b , to describe the effects of the surface details on the perpendicular and parallel surface nonlinear currents. In related theoretical work, Bower [27] extended the early quantum mechanical calculation of Jha [23] to include interband transitions near their resonances as well as the effects of surface states. 

In a porous film consisting of interconnected nanometer sized semiconductor particles the effective surface area can be enhanced 1000-fold [121]. Therefore, nanostructured electrodes can be good for unravel the surface phenomena. By scrutinizing the effect of iodine on the performance of the electrode it was concluded that the effect of surface states was small in the nanostructured hematite electrode. It was stated that the bulk and grain boundary recombination remained dominant. This is in consistency with the report from Cherepy et al [43]. 

The nature of the frequency dependence of Mott-Schottky plots for semiconductor electrodes has been discussed in the electrochemical literature for more than three decades (see e.g. reviews [6, 84]). It has been speculated that it can be caused by the following factors (1) frequency dependence of dielectric relaxation of the space charge region [85], (2) roughness of the electrode surface [84], (3) slow ionization of deep donors (acceptors) in the space charge region in the semiconductor [86], and (4) effect of surface states. 

Models for the Photoelectrolytic Decomposition of Water at Semiconducting Oxide Anodes Although title is general, theoretical study focuses on the TiCL-electrolyte interface and the effect of surface states. 247... 

Defects and impurities, in general, play a comparably important role for the luminescence properties of nanodiamond like they do for the bulk material. Owing to their existence, there are electronic states situated within the bandgap, which allow for inducing luminescence in nanodiamond samples also with longer wave radiation. Upon excitation with wavelengths between 300 and 365 nm, fluorescence bands are observed at more than 400 nm. They arise from various nitrogen defects. In comparison to bulk diamond, the Ufetime of the excited states is rather short, which possibly is due to the effect of surface states and to the increased density of excitons on the surface. 

It is important to appreciate that even if a material possess desirable characteristics, as discussed in the previous sections, it may not exhibit the desired efficiency of light energy conversion into electrical energy. For example, in spite of semiconductor and redox electrolyte possessing desired Fermi levels experimentally determined contact potential might be much less than the anticipated value. Presence of surface state is one of the factors responsible to show these deficiencies. Hence, efforts are required to minimize the adverse effects of surface states. 

The Effect of Surface States upon the Distribution of Potential at the... 

Semiconductor Junctions, Solid-Solid Junctions, Fig. 3 Effect of surface states on band structure, (a) Semiconductor without any surface states and hence no Fermi-level pinning, (b) n-type semiconductor with a depletion region and upward band bending, (c) p-type semiconductor with a hole depletion layer and resulting downward band bending, (d) p-type semiconductor with a hole accumulation region and upward band bending... 

For a p-type semiconductor we assume that the surface states are all donors. Photogeneration lowers the hole Fermi level and electrons are released into the semiconductor. This increases the negative space charge there and hence band-bending. (This effect was not considered in Part ii where the effect of surface states in the junction was neglected). The positive drop (11.4) increases, favouring a bigger... 

Figure 9. The effect of surface-state density and insulator charge on conversion efficiency (Ref 23)...

Surfactants may be used to reduce the effects of surface states and to minimize surface energies. These are typically impurities with compatible molecular orbital structures to the atom they replace but with a single extra or fewer electron. An oversized impurity will segregate onto surfaces for size reasons as well. 

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