Band-gap semiconductor

The band gap of the LED varies with composition for both these solid solutions, as shown in the figure below. The cause of the variation is different for the two substances. Semiconductor band gaps increase when orbital overlap decreases. A decrease in orbital overlap can arise from increased spacing between atoms or increased ionic character of the bonds. 

A reaction mechanism in which the precursor polymer undergoes a redox reaction followed by loss of the bridge hydrogens is proposed. The resulting conjugated aromatic/quinonoid polymers generally have very small semiconductor band gaps in accord with predictions of recent theoretical calculations. 

It is important to realize that each of the electronic-structure methods discussed above displays certain shortcomings in reproducing the correct band structure of the host crystal and consequently the positions of defect levels. Hartree-Fock methods severely overestimate the semiconductor band gap, sometimes by several electron volts (Estreicher, 1988). In semi-empirical methods, the situation is usually even worse, and the band structure may not be reliably represented (Deak and Snyder, 1987 Besson et al., 1988). Density-functional theory, on the other hand, provides a quite accurate description of the band structure, except for an underestimation of the band gap (by up to 50%). Indeed, density-functional theory predicts conduction bands and hence conduction-band-derived energy levels to be too low. This problem has been studied in great detail, and its origins are well understood (see, e.g., Hybertsen and Louie, 1986). To solve it, however, requires techniques of many-body theory and carrying out a quasi-particle calculation. Such calculational schemes are presently prohibitively complex and too computationally demanding to apply to defect calculations. 

Semiconductor Band Gap (eV) Equivalent Wavelength (nm) Semiconductor Band Gap (eV) Equivalent Wavelength (nm)... 

Fig. 3-11. Energy for decomposing ionization of compound AB to form gaseous ions A(giD) and via electron-hole pair formation and via cation-anion vacancy pair formation r = reaction coordinate of decomposing ionization e, s semiconductor band gap . vmb) = cation-anion vacancy pair formation energy (Va- Vb-) Lab = decomposing ionization energy of compound AB.

Semiconductors Band gap (eV) Sacrificial reagent Rate of gas evolution (pmol/h) Ref... 

The last section revealed that semiconductor band gaps for nanostructures vary with the size of the structure. The wavelength of light emitted when an electron in the conductance band returns to the valence band will therefore also vary. Thus, different colour fluorescence emission can be obtained from different-sized particles of the same substance (e.g., different sized quantum dots of CdSe irradiated with UV emit different colours of light). To produce fluorescence, light of greater photon energy than the band gap is shone onto the nanocrystal. An electron is excited to a... 

Fig. 96. Schematic illustration of a colloidal semiconductor. Band-gap excitation promotes electrons from the valence band (VB) to the conduction band (CB). In the absence of electron donors and/or acceptors of appropriate potential at the semiconductor surface or close to it, most of the charge-separated, conduction-band electrons (e CB) and valence-band holes (h+VB) non-pro-ductively recombine. Notice the band bending at the semiconductor interface [500]...

Semiconductor band-gap luminescence results from excited electrons recombining with electron vacancies, holes, across the band gap of the semiconductor material. Electrons can be excited across the band gap of a semiconductor by absorption of light, as in photoluminescence (PL), or injected by electrical bias, as in electroluminescence (EL). Both types of luminescence have been used in chemical sensing applications [1,3]. 

In addition to energy, the semiconductor band gap is characterized by whether or not transfer of an electron from the valence band to the conduction band involves changing the angular momentum of the electron. Since photons do not have angular momentum, they can only carry out transitions in which the electron angular momentum is conserved. These are known as direct transitions. Momentum-changing transitions are quantum-mechanically forbidden and are termed indirect (see Table 28.1). These transitions come about by coupling... 

This phenomenon arises from charge-transfer reactions between the semiconductor and excited dye molecules adsorbed on its surface. If the semiconductor band gap is large compared to the dye s excitation energy, electron transfer between the dye and the electrode may involve the highest normally filled level or the excited level of the dye, but usually not both. One of the dye levels will not be electroactive because it will match some energy in the electrode s band-gap region. 

Anodic Ni oxides catalyze H2 evolution if they are not formed at too high potentials, otherwise they may depress the activity [93, 385, 449, 456]. Insulating layers are normally inefficient for hydrogen evolution [457, 458]. It is interesting to note that semiconductors can reduce the overpotential for hydrogen evolution on Hg and the effect increases as the semiconductor band gap decreases [459]. This is in line with the observation that a passivated Nb electrode is not an efficient electrocatalyst... 

As other semiconductors, QDs are characterized by a certain band gap between their valence and conduction electron bands.20 When a photon having an excitation energy exceeding the semiconductor band gap is absorbed by a QD, electron-hole pairs are generated (electrons are excited from the valence to the conduction band) and the recombination of electron-hole pairs (the relaxation of the excited state) results in the emission of the measured fluorescence light. 

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

In the EER method, the relative variation of the optical reflection coefficient (AR/R) of the electrode surface caused by the low-frequency harmonic modulation of the electrode potential is used as the informational signal [96-98]. The high sensitivity of EER technique (it is possible to detect the AR/R values of about 10 5-10 7) allow us to identify the impurity electronic levels in a semiconductor band gap at their very small concentrations and, therefore, to control the actual surface energy states rather than those distributed in the sub-surface region [49-57]. 

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-... 

PRIMARY PROCESSES ON COLLOIDAL SEMICONDUCTORS 9.2.1 Light harvesting by semiconductor band gap excitation... 

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