Band gap models

Figure 19.1 Schematic representation of the band gap model . (1) Photoinduced electron-hole creation ...

Figure 1. Perspective views of the wide band gap models showing the three-dimensionally connected networks,...

Some recent studies of the electronic properties of poly silanes have suggested that the first UV transition is not a band-to-band transition, but is instead excitonic in nature [13,16,17,18].These predict that the onset of band-to-band transitions must therefore be at higher energies, with an exciton binding energy of an eV or more, as is common for Frenkel excitons in molecular systems. Since the vacuum ultraviolet (VUV) specmim has not been measured until now, the question of whetha- a one-dimensional semiconductor band gap model or a molecular orbital model is appropriate for the polysilanes has been unresolved. 

As described in the chapter on band structures, these calculations reproduce the electronic structure of inhnite solids. This is important for a number of types of studies, such as modeling compounds for use in solar cells, in which it is important to know whether the band gap is a direct or indirect gap. Band structure calculations are ideal for modeling an inhnite regular crystal, but not for modeling surface chemistry or defect sites. 

Within the Slater-Koster appro.ximation, we can easily test the validity of the approximations made in eqn (7) based on the graphene model. In Fig. 5 we depict the band gaps using the empirical tight-binding method for nanotube radii less than 1.5 nm. The non-metallic nanotubes n m) are shown in the... 

Fig. 6. Band gap as a function of nanotube radius using first-principles LDF method. Solid line shows estimates using graphene sheet model with...

Efficient photoelectrochemical decomposition of ZnSe electrodes has been observed in aqueous (indifferent) electrolytes of various pHs, despite the wide band gap of the semiconductor [119, 120]. On the other hand, ZnSe has been found to exhibit better dark electrochemical stability compared to the GdX compounds. Large dark potential ranges of stability (at least 3 V) were determined for I-doped ZnSe electrodes in aqueous media of pH 0, 6.3, and 14, by Gautron et al. [121], who presented also a detailed discussion of the flat band potential behavior on the basis of the Gartner model. Interestingly, a Nernstian pH dependence was found for... 

The first calculations of the shift of the band gap in small particles of a semiconductor were made by Efros und Efros They treated the model of two Coloumb-interacting... 

The electronic band structure of a neutral polyacetylene is characterized by an empty band gap, like in other intrinsic semiconductors. Defect sites (solitons, polarons, bipolarons) can be regarded as electronic states within the band gap. The conduction in low-doped poly acetylene is attributed mainly to the transport of solitons within and between chains, as described by the intersoliton-hopping model (IHM) . Polarons and bipolarons are important charge carriers at higher doping levels and with polymers other than polyacetylene. 

Fig. 1. Ligand-field model for the electronic structure of substitutional hydrogen in silicon in terms of the interactions between the vacancy orbitals and the atomic-hydrogen orbitals [Although the a state is shown as being not entirely passivated (still below the bottom of the conduction-band edge), it could in fact be in the conduction band, but with a host-like state pushed down slightly into the band gap.] (Reprinted with permission from the American Physical Society, DeLeo, G.G., Fowler, W.B., Watkins, G.D. (1984). Phys. Rev. B 29, 1819.)...

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