Exciton temperature dependence

Here we shall assume that the temperature shift is due to the thermal expansion effect of proteins on the inter chromophore distances among P, Bl, and Bm- For simplicity we shall assume that the temperature dependence of interchromophore interactions are of the dipole-dipole interaction type. In this case, notice that the excitonic energy splitting of P is AE = Ep+ — Ep-,... 

The temperature dependence of luminescence from the sample irradiated at 1 x 1013 cm-2 with 28Si+ indicates, above —110 K, an activation energy of 90 meV for the competing nonradiative recombination process— this competing process may be the thermal dissociation of geminate pairs or bound excitons at donorlike or acceptorlike centers. The 0.09-eV value of activation energy is consistent with the results of Troxell and Watkins (1979). 

Measurements of the optical properties in this range of wavelengths can probe the fundamental electronic transitions in these nanostructures. Some of the aforementioned effects have in fact been experimentally revealed in this series of experiments (90). As mentioned above, the IF nanoparticles in this study were prepared by a careful sulfidization of oxide nanoparticles. Briefly, the reaction starts on the surface of the oxide nanoparticle and proceeds inward, and hence the number of closed (fullerene-like) sulfide layers can be controlled quite accurately during the reaction. Also, the deeper the sulfide layer in the nanoparticle, the smaller is its radius and the larger is the strain in the nanostructure. Once available in sufficient quantities, the absorption spectra of thin films of the fullerene-like particles and nanotubes were measured at various temperatures (4-300 K). The excitonic nature of the absorption of the nanoparticles was established, which is a manifestation of the semiconducting nature of the material. Furthermore, a clear red shift in the ex-citon energy, which increased with the number of sulfide layers of the nanoparticles, was also observed (see Fig. 21). The temperature dependence of the exciton... 

We used short broadband pump pulses (spectral width 200 cm 1, pulse duration 130 fs FWHM) to excite impulsively the section of the NH absorption spectrum which includes the ffec-exciton peak and the first three satellite peaks [4], The transient absorbance change signal shows pronounced oscillations that persist up to about 15ps and contain two distinct frequency components whose temperature dependence and frequencies match perfectly with two phonon bands in the non-resonant electronic Raman spectrum of ACN [3] (Fig. 2a, b). Therefore the oscillations are assigned to the excitation of phonon wavepackets in the ground state. The corresponding excitation process is only possible if the phonon modes are coupled to the NH mode. Self trapping theory says that these are the phonon modes, which induce the self localization. 

Table 3.14. Parameters of the temperature dependence of the transition energy Eq and the discrete free exciton FWHM W20 according to the 2-oscillator model for a ZnO thin filma...

The electron-phonon interactions also influence the linewidth of the interband and exciton transitions. This temperature-dependent homogeneous... 

FIGURE 1 Temperature dependence of the excitonic gaps of thin GaN epilayers on sapphire and on SiC, and of thick GaN, after [19],... 

TABLE 2 Temperature dependence of the wurtzite GaN A excitonic gap, following EQNS (3) or (4). 

FIGURE 4 Temperature dependence of the recombination lifetimes of the excitonic transitions in Ill-nitride epilayers and MQWs. 

FIGURE 3 Temperature dependent photoluminescence of a homoepitaxial GaN layer (0.4 p.m) grown by RMBE. At 4 K photoluminescence linewidths of the bound excitons are as narrow as 0.5 meV (after [31]). 

Of particular interest are the optical spectra. Chclikow.sky and Schluter calculated the Joint density of states for direct transitions (which would be proportional to C2 were the dipole matrix elements all equal). sec Section 4-A -with the result shown at the bottom of Fig. 11-12. It bears little resemblance to the experimental Cj curve (uppermost in the figure), for a number of reasons. Tlie prominent peak at 10.4 eV appears to be an cxciton peak (See Section 6-H), as had been stiggested earlier by Platzoder (1968) on the basis of observed temperature dependence. Pantelidcs and Harrison took this peak to result from interband transitions, since it lay at an enci gy above the photoconductivity threshold of 9 eV (DiStephano and Eastman, 1971b) that would rule out the possibility that the peak represents a simple exciton, but not that it represents an excitonlikc... 

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