Band-gap semiconductor materials

QDs are highly photostable [177]. The photostable nature results from the shell protecting the core, which is generally a large band gap semiconductor material [178,179]. For instance, InAs cores have been covered by different shells made of ZnS, ZnSe, GaAs [178], etc. 

Table 1. Main physical properties of wide band gap semiconductor materials at 300 K.

Figure 6.1 Band-alignment in heterostmctured NCs. In Type-I heterostructures, a larger band gap semiconductor material overcoats a core with a smaller band gap. Meanwhile, the band alignment of the constituent materials in a Type-II heterostructure is in a staggered configuration.

Extensions in wavelength, into both the infrared and the ultraviolet ranges will continue, motivated by increasing interest in narrow band-gap semiconductors and wide band-gap materials. 

Yet another approach to sensitizing PCs to a broader light spectrum is to use composite materials with a heterojunction (Fig. 6) between a narrow band gap and wide band gap semiconductors. A particular... 

CdjPj and CdjAs are low band gap semiconductors (0.5 and 0.1 eV, respectively). The bulk materials are black and start to absorb in the infrared. These materials have been prepared as colloids in alkaline solution by precipitation of Cd with phosphine and arsine Depending on the conditions of preparation, particles of different sizes (between about 2 and 10 nm) were obtained, which could also be recovered in the solid state after evaporation of the solvent. The color of these materials ranged from black to colorless with decreasing particle size, with all kinds of intermediate colors in the visible. 

The direct photoelectrolysis of water requires that the v level be below the 02/H20 level and the ec level be above the H+/H2 level. This condition is satisfied, e.g. for CdS, GaP, and several large-band gap semiconductors, such as SrTi03, KTa03, Nb205 and Zr02 (cf. also Fig. 5.59). From the practical points of view, these materials show, however, other specific problems, e.g. low electrocatalytic activity, sensitivity to photocorrosion (CdS, GaP), and inconvenient absorption spectrum (oxides). 

Optical studies on various direct band-gap semiconductor NWs have demonstrated that these NW materials can also exhibit excellent... 

Chapter 4, presents details of the absorption and reflectivity spectra of pure crystals. The first part of this chapter coimects the optical magnimdes that can be measured by spectrophotometers with the dielectric constant. We then consider how the valence electrons of the solid units (atoms or ions) respond to the electromagnetic field of the optical radiation. This establishes a frequency dependence of the dielectric constant, so that the absorption and reflectivity spectrum (the transparency) of a solid can be predicted. The last part of this chapter focuses on the main features of the spectra associated with metals, insulators, and semiconductors. The absorption edge and excitonic structure of band gap (semiconductors or insulator) materials are also treated. 

In discussing deep levels in wide band-gap semiconductors, the first requirement is to define deep and wide. The latter can be done relatively easily, although arbitrarily. We list in Table I the 4°K band gaps of the various III-V semiconductors, based on the tabulation by Strehlow and Cook (1973). We shall call those with Eg> 1.5 eV the wide band-gap ones. In practice, our review will present data only on GaAs and on GaP as prototypes of direct and indirect gap materials of this class. These are also the only two materials of this class that have been extensively studied and that are in common use. Discussion of deep levels in ternary and quarternary alloys of III-V semiconductors are omitted since treating these in detail might well have doubled the size of this chapter. 

MoS2-type materials are indirect band-gap semiconductors. The energies of the indirect (momentum forbidden) and direct (momentum allowed) band-gap transitions are given in Table 1. The electronic structure of these materials may be qualitatively understood in terms of the crystal structure. 

The thermal conductivity of diamond at 300 K is higher than that of any other material, and its thermal expansion coefficient at 300 K is 0.8 x 10". lower than that of Invar (an Fe-Ni alloy). Diamond is a very widc-band gap semiconductor Eg = 5.5 eV), has a high breakdown voltage (I07V cm-1), and its saturation velocity of 2.7 x I01 cm s-1 is considerably greater than that of silicon, gallium arsenide, or indium phosphide. 

To probe the electronic structures of the materials in the solid state, band structure calculations on the crystal structure of compound 22 were carried out. The results obtained by using the linear muffin-tin orbital (LMTO) self-consistent field (SCF) method support the interpretation that compounds 22 (R1 = Me, Et R2 = H) are small-band-gap semiconductors. 

In this case, we have introduced more hydrogen than those incorporated in the previously referred paper [66], Therefore, more electrons were introduced during the H2 incorporation into the perovskite. Consequently, we can conclude that the studied material at 1023-1273 K behaves as a small band gap semiconductor, because of the increase of electrons with temperature in the conduction band, due to the shifting to the conduction band of the Fermi level and the hydrogen-induced level. Then, there will be a sufficient number of electrons in the conduction band to screen the proton and allow it to have a high coordination number, and be interstitially located in the tetrahedral and octahedral sites. 

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