Band-gap absorption

Based on the preceding results, we can say that linearly polarized light from the band-gap absorption region can induce either transitory changes or crystallization transformation in amorphous Sbj Sei alloys. These two phenomena critically depend on exposure, show threshold behavior, and seem to arise from apparently different mechanisms defect states or some kind of structural units given preferential orientation under the action of linearly polarized light. 

Figure 2. UV-visible reflectance spectra of (8-2n)Na,2nAg,2Br-SOD with varying silver concentrations. Ag+/uc (a) 0 (b) 0.05 (c) 0.28 (d) 2.0 (e) 3.1 (f) 8.0 (g) bulk AgBr (h) (8-2n)Na,2nAg,2Cl-SOD, 0.1 Ag+/uc (i) (8-2n)Na,2nAg,2Br-sodalite, 0.1 Ag+/uc (j) (8-2n)Na,2nAg,21-SOD, 0.1 Ag+/uc. Note that band-gap absorptions for 8Na,2X-SOD expanded insulators peak at 192 (Cl), 208 (Br), and 214 nm (I) (Figure 2h,i,j) and parallel the order for the bulk fee NaX materials, 138 (Cl), 165 (Br), and 211nm (I), respectively, (k) Schematic band diagram for expanded AgX semiconductors (Reprinted from ref. 3. Copyright 1990 American Chemical Society.)...

Composition Unit cell dimensions No. of H2O of hydration Estimated band-gap/ absorption edge Estimated conduction band edge/V vs. NHE... 

In indirect-gap semiconductors, this phonon-assisted electronic absorption is revealed by kinks in the vicinity of the electronic absorption edge. They are due to the different energies of the momentum-conserving phonons involved as well as to the above-discussed different phonon processes. The evolution of this near band gap absorption with temperature can be seen in Fig. 3.7... 

Fig. 3.9. Variation with hydrostatic pressure of the direct band-gap absorption threshold of InP at RT (1 GPa is taken for 10 kbar) (after [69])...

Somewhat larger siuface defect densities of iVj = 10 cm were found by Knights et al. (1977) and by Jackson et al. (1983a), who measured the thickness dependence of the spin density and of the sub-band-gap absorption hv <1.5 eV). It is not clear whether the defects measured in these experiments are near the substrate or at the free surface. The measured value is probably the sum of both contributions, so that we are back at defect concentrations near 5 X 10 cm. ... 

Sion. Furthermore, only Eu " " emissions without the background of the broad emission of ZnO were observed under band-gap excitation at 385 nm. However it is somewhat curious that the observed band-gap absorption was red shifted compared to the bulk ( 370 nm). This is rarely observed since quantum size effect is expected to blue-shift the band gap excitation. 

Absorption. Photons interact with electronic and vibrational transitions in the glass in the UV and IR regions, respectively. In the UV, absorption is due to electronic transitions across the band gap. Absorption occurs at shorter wavelengths for a larger band gap. In the IR, absorption is due to a coupling of the electromagnetic field to lattice vibrations. 

High absorption cross section for photon harvesting. Ideally, the electronic structure of conjugated polymers is that of a one-dimensional semiconductor in which the absorption coefficient increases steeply above the band gap absorption. The absorption coefficient of the frequently used poly(p-phenylene vinylene) polymer MDMO-PPV (Figure 10.2.) reaches 10 cm just... 

Calculated assuming only radiative recombination and that the absorptivity approaches 1 for wavelengths shorter than the band gap absorption. 

As mentioned above, it is an ongoing quest to effectively dope lanthanide ions into semiconductors and utilize the band gap absorption to sensitize efficient... 

---