Luminescence band-edge

Dependencies of luminescence bands (both fluorescence and phosphorescence), anisotropy of emission, and its lifetime on a frequency of excitation, when fluorescence is excited at the red edge of absorption spectrum. Panel a of Fig. 5 shows the fluorescence spectra at different excitations for the solutes with the 0-0 transitions close to vI vn, and vra frequencies. Spectral location of all shown fluorescence bands is different and stable in time of experiment and during lifetime of fluorescence (panel b)... 

We believe that the luminescence at 1.0 eV is due to a structural damage induced by ion implantation rather than to a chemical doping effect, since the spectrum does not depend on the chemical species of the ion. These centers may be similar to the vacancies induced by 3-MeV electron-beam irradiation, as reported by Troxell and Watkins (1979), who find donorlike and acceptorlike levels —0.1 eV from the band edges. 

Figure 1.7 Near-band-edge photoluminescence spectrum of 4H-SIC showing the bound exciton-related luminescence denoted and (where h is the energy in meV of the phonon involved in the transition) and free exciton-related luminescence denoted /. (Data provided by Docent Anne Henry at Linkoping University.)...

CD ZnSe has also been demonstrated to passivate surface states, 0.92 eV below the conduction band edge (measured by thermally stimulated exoelectron emission) on single crystal GaAs. This passivation resulted in bandgap luminescence from the originally non-luminescent GaAs [49a]. 

Similar behavior is observed in the potential-dependent luminescence of a ruthenium dye adsorbed to 2 [52]. Although the flat-band potential of 2 is known to shift positive in the presence of the potential-determining Li+ ion, relative to the TBA+ ion [5,68], this effect cannot explain the observed behavior. For example, in our experiments, the dye injects in both cases, but it takes a much smaller negative potential excursion to turn off the injection process in the presence of Li+ than with TBA+ [52]. This is the opposite of what would be expected if only equilibrium (i.e., dark) band edge motion were responsible for the effect. 

Measuring the variations of the intensity ratio /inp/Zyb under pressure, Takarabe (1996) was able to determine the energy difference bt and found a pressure-induced shift of 70 meV/GPa which is close to the shift of the band-edge related luminescence due to the bound e-h pairs. Furthermore, under pressure it was possible to completely recover the thermally quenched luminescence of the Yb3+ ion at temperatures of 220 K and 260 K (Takarabe et al., 1994) as well as at room temperature (Takarabe, 1996). The minimum pressure at which the luminescence could be observed again was shown to increase with increasing temperature. All these facts fitted well to the proposed back-transfer model, which was thus strongly supported by the pressure experiments. 

Analytical strategies based on the activation effect caused by the analyte on the QD luminescence emission also have been proposed. In a pioneering work, the addition of Zn and Mn ions to colloidal solutions of CdS or ZnS QDs resulted in an important enhancement of the luminescence quantum yield of the nanoparticles. This effect was attributed to the passivation of surface trap sites that are either being filled or energetically moved closer to the band edges.33 46 This behavior provided the basis for the optical sensing of such metal cations with QDs. Chen and Zhu47 proposed a method for the determination of trace levels of silver ions based... 

The band-edge luminescence of undoped, high quality a- and p-GaN is well mastered and is now a valuable characterisation tool, though the spectroscopy of P-GaN is not as mature as that of a-GaN. However, a clear understanding is still needed of the luminescence of deep levels, and of highly-doped GaN. 

Fig. 8.22. Doping dependence of the luminescence intensity of the band edge and defect transitions (Street et al. 1981).

Electroluminescence was first observed with n-GaP electrodes using hole donors such as [Fe(CN)(,] in alkaline or S2O8 in acid solutions [112]. In these two cases the corresponding standard potentials occur at or even below the valence band edge (see Table in Appendix). In the case of [Fe(CN)(s] no luminescence was found in acid solutions although the current-potential curve indicates that the redox species is reduced. The differences between alkaline and acid solutions can be explained by the pH-dependence of the position of the energy bands at the surface, as shown in Fig. 7.62. Since is far below Ep.redox at pH 1 no charge transfer between the redox couple and the valence band is possible anymore, and the cathodic current is only due to an electron transfer via the conduction band. 

Donor-acceptor absorption can also be observed in semiconductors, but this process is weak because of the small overlap of the wavefunctions (like an n -> n transition). Donor-acceptor absorption is best monitored through the emission, that is by excitation spectra. In the normal situation, the donor-acceptor absorption can be observed but the valence band-to-donor and the acceptor-to-conduction band transitions can also be seen, as they also contribute to the luminescence. All three of these transitions are weak but of similar strengths [6]. In undoped AgCl and AgBr, only a very weak excitation spectrum is seen, which consists of a relatively sharp line near the band edge. In Cd2 + doped AgBr both the sharper line, whose onset is about... 

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