Band gap transition

Indium monoselenide, InSe, is a semiconductor with a weakly allowed direct band gap transition at 1.3 eV and an indirect at 1.2 eV, having a strongly anisotropic... 

LEDs and semiconductor, 22 174-175 in organic semiconductors, 22 201, 202 silicon, 22 485, 488 silicon carbide, 22 530 Band gap transition type, for binary compound semiconductors, 22 145, 146-147t Band structure... 

Figure 4.8 Interband transitions in solids with band-gap energy Eg-, (a) A direct band gap. Two direct transitions are indicated by arrows, (b) An indirect band gap. Two indirect band-gap transitions are indicated by arrows. The transitions at photon energies lower than Eg require absorption of phonons. The transitions at photon energies higher than Eg involve emission of phonons.

When an energetic electron scatters inelastically, an electron from the (filled) valence band can be promoted to the (empty) conduction band creating an electron/hole pair. On recombination, the excess energy is released as a photon, the wavelength of which is well defined by the band-gap transition. The technique is powerful in catalysis it is diagnostic of the electronic/chemical state and is sensitive to point defects. It can be used to probe the distribution of dopants in catalytic oxides. 

The devices which we consider involving band gap transitions are, however, concerned with emission of light instead of absorption or reflection the electrons being initially excited by electrical energy. 

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. 

Figure 6.7 Top of the vaience band of a VPO cataiyst showing the band gap transition ofV 3d.

Thompson. The theoretical curve is aligned with the experiment to coincide with the band gap transition at 1 eV. (The peak marked by the arrow corresponds to gas phase signal, while intensity in the position labeled with a circle o is partly due to phosphate orbitals.)... 

Optical transitions in semiconductors can also involve localized states in the band-gap. These become particularly important for semiconductors in nanocrystalline form (see below). Sub-band-gap transitions can be probed with photons of energy below the threshold defined by Eg. 

FIGURE 5.16. Quantum yield as a function of photon energy. The points are experimental and the full curve is basted on calculation. The fast increase of T] near hv = 0.95 eV is due to the onset of band gap transitions. After Chazalviel."" (Reproduced by permission of The Electrochemical Society, Inc.)... 

On the other hand, calcination neither at the lower temperatures in Nz nor in air produced such a band-gap transition because of their negligible amount of anatase. Only a gradual slope centered at ca.530 nm (2.3 eV) was observed as characteristic absorption of the stuffed layered composite. To ascertain the different band gap, the valence band spectra were taken by XPS measurement for Cu-300-N2 and Cu-450-Nz. The former exhibited a position of the highest filled bands ca. 0.9 eV higher than the latter. However, such an... 

Fig. 3 summarises the possible sub-band gap transitions between localised and delocalised states. 

The electronic levels of crystalline solids separate into bands of allowed and forbidden energies [53]. A solid whose highest occupied band (valence band) is completely filled and separated from the lowest unoccupied (conduction) band is an insulator. Ionic solids are typically insulators. In this one-electron band description, the lowest electronic excitation corresponds to a transition from the top of the valence to the bottom of the conduction band (a band-gap excitation). Direct band-gap transitions do not involve simultaneous emission or absorption of a phonon, whereas indirect ones do. 

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