Burstein shift

Bandgap measurements for Cu sulphides and selenides are complicated by the fact that these semiconductors are normally degenerate, with high free-carrier absorption in the near-infrared and possible Moss-Burstein shifts (due to saturation of the top of the valence band by holes) in the optical gap. It is quite possible that variations in bandgaps in these materials are due to differences in stoichiometry, phase, and doping rather than to any quantum size effect. Only studies where crystal size can be estimated and are possibly in the quantum size range are given here. 

Whilst this may initially appear to be in opposition to the results of the optical rotating disc electrode study on colloidal CdS (Fig. 9.9), this may be readily explained by consideration of the relatively low illumination intensities used in the ORDE experiments, and the high surface state concentrations typical of the samples employed therein. The former precludes the generation of a Burstein shift while the latter, with a quantum yield of 0.77 for (S )surf generation from S2 ions at the CdS particle surface [115, 116], provides a highly efficient mechanism for positive charge accumulation at the particle surface. 

The optical bleaching by stored electrons is the basis for the occurrence of strong optical nonlinearity observed in Q-particles [64]. The physical reason for this optical bleaching is still not discussed conclusively in literature. The most obvious explanation comes from a state filling model. The stored electrons occupy the lowest electronic levels in the conduction band and, consequently, the optical transition has to occur to higher electronic levels (i.e., at shorter wavelength). This effect is known in solid-state semiconductor physics as the Burstein shift [65]. Other theoretical models describe the optical bleaching as a consequence of the polarization of the exciton in the electric field of the stored electron, which is then... 

Fermi level of particles is a function of irradiation intensity. The photo-onset potentials were shifted toward more negative values as a function of the photon flux. These experiments were supported by a blue shift of the absorption peak in the UV-vis spectra measured immediately after a high-intensity flash (134). This phenomenon is known as the Burstein shift (135). Various potentials were applied to dispersions on an optically transparent electrode,... 

A direct method of detection of free electrons is provided by techniques such as spectroelectrochemistry and microwave conductivity. The first is based on the detection of the delocalized carrier by the specific absorption features, such as the Burstein shift, which is a spectral blueshift due to band filling, and additional intraband absorption in the infrared region of the spectrum [129, 151, 167, 168], Recently Hamann et al. developed a detailed method based on temperature dependence of the free carrier density to locate the position of the conduction band [166]. The time-resolved microwave conductivity allows for a detection of electron carriers [169, 170] although the distinction between free and trapped electrons requires a detailed analysis [151]. A similar concern arises often in the interpretation of absorption data, since there are a number of possible absorption effects and these cannot be simply linked to the concentration [168]. 

Burstein effect spect The shift of the absorption edge in the spectrum of a semiconductor to higher energies at high carrier densities in the semiconductor. bor.stTn i.fekt ... 

As the treated ITO samples show comparably high effective band gaps, we draw a conclusion as the following. A combination of Burstein-Moss-Shift and contributions of scattering [11] causes the increased Eg. It is not only a reason of Burstein-Moss-Shifl like Bender et al mentioned [13]. Eg = Eg q + AEg = Eg o + "R (37t nJ + fiZ, Where Eg... 

The optical properties were further studied by Nishino et al. [217], and Hu and Gordon [209, 210, 240], They observed an increase in the optical band gap (3.3-3.7 eV) with increasing doping, which can be explained again by the Burstein-Moss shift [128, 129]. The refractive index of ZnO films was in the range of 1.54-2.02 [234]. 

ESR of pristine ZnO nanoparticles in vacuum before and after UV illumination and subsequently after exposure to air. Change in resistance to Ohmic behavior for a ZnO/PEDOT PSS junction upon UV exposure. Burstein-Moss effect (blue shift) occurs upon UV illumination by emptying the valence band and filling the conduction band. Adapted with permission from ref. 104. Copyright 2007, AIP Publishing EEC. Adapted with permission from ref. 105. Copyright (2010) American Chemical Society. 

---