Exciton Absorption

In some semiconductor polymer blends the excited electron becomes a free particle in the conduction band and, similarly, the hole left in the valance band also becomes free [69]. This leaves behind a localized positively charged hole. The electron and hole attract each other by electrostatic coulombic forces, however, and may possibly form a bound state in which the two particles revolve together around their center of mass such a state is referred to as an exciton. The exciton level is in the same neighborhood as the donor level. The energy of the photon involved in exciton absorption is given by  

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Fig. 7. Calculated optical excitation spectra (left) and exciton absorption spectra (right) of a semiconducting CNT for a parallel polarisation.

The nature of the light emissions is influenced by the way in which the absorbed energy is transferred through the polymer matrix. In crystalline polymers, exciton migration is possible as all molecules lose their energetic individuality and all electronic and oscillation levels are coupled [20]. Thus, new exciton absorption and emission bands are formed and the excitation energy can move along the chain ... 

Another type of absorption is also possible, i.e., exciton absorption which enriches the crystal in free excitons if the latter annihilate then on the lattice defects, causing a change in the charged state of the defects and leading to the appearance of free carriers in the crystal. In this case photoconduction arises as a secondary effect. 

We assume that the chains were oriented parallel to the film plane but the chain direction were distributed randomly in the film plane. In this case, the correction factor for the chain orientation, , is 3/8. Consequently, the xSx value of the PTV cast film was calculated to be 1.2 x 10 9 esu, which is comparable to that of the highly oriented vacuum-deposited polydiacetylene film at the resonance with the exciton absorption. 

While f determines the radiative lifetime of an exciton at low temperature limit, fN determines the light absorption coefficient, hence the exciton absorption band will get stronger with decreasing nanoparticle size in the R< ae size regime. 

Transient bleaching and recovery rates of CdS excitonic absorption, determined by picosecond pump-probe spectroscopy, depended on [H20]/[A0T] ratio and micellar surface. Fluorescence spectra and lifetimes depended on [Cd2+]/[S2 ] ratios... 

The explanation of this peak is as follows. Suppose that the number of conduction electrons is small, so that the Coulomb field is not screened out and that a hole in the X-ray level creates an exciton level below the bottom of the conduction band. The levels are shown in Fig. 2.13. Then an exciton absorption line should be possible. But the sudden change in field will produce excitations of electrons at the Fermi level, so that the exciton line is broadened as shown in Fig. 2.14(a). Also, we do not expect a sharp increase in absorption when the electron jumps to the Fermi level, leaving the exciton level A in Fig. 2.13 unoccupied, because of the very large Auger broadening due to transitions from the Fermi level into this unoccupied state. 

A final point worth mentioning is the effect of local fields on the optical nonlinearities of strongly QC nanostructures. These arise from embedding QD s in a medium of different dielectric constant (2). One requires to know how the field intensity inside the particle varies at saturation in excitonic absorption. This has been approached theoretically by defining a local field factor f such that Em = f Eout (2). The factor f depends on the shape of the QD and the dielectric constant of the QD e = + E2 relative to that of the surrounding medium. Here... 

InSe and GaSe crystals are characterized with a weak interaction of 3D Wannier excitons with homopolar optical A -phonons [18, 19]. Therefore, when calculating the exciton absorption spectra, we took into consideration effects of broadening the exciton states using the standard convolution procedure (see in [18]) for theoretical values of a(7jco) the absorption coefficient in the Elliott s model [20] with y /io>) — 77 [n(E 2+/ 2)] the Lorentzian function in the Toyozawa s model [21], where r is the half-width at half-maximum which is usually associated with the lifetime tl/2r. 

The half-width at a half-maximum of a ground (it 1) and excited (n > 1) exciton absorption bands at fixed temperatures in general form were obtained in [22]... 

The experimental value K = 40 eVcm 1 corresponding to the integral intensity of n = 1 exciton absorption band in H0.07lnSe crystals at T 80K and 71 = 4.4 meV is practically equal to - the classical value for InSe crystal (K = 44 eVcm"1), which is in full accordance with the analytical dependence of K on half-width 71 obtained in [18] for InSe crystals ... 

Even at x = 0.07 (see Fig. 3c), the exciton absorption band n 1 is shifted to short-wave side by 0.8 meV relatively to that of pure InSe crystal. Note that the energy position A of the exciton absorption peak with = 1 for pure InSe crystal at T 80K is in full accordance with the analytical dependence ... 

Along with A shift, we observed the change of the half-width /] of the exciton absorption band n = 1 with the growing hydrogen concentration. In Fig. 3b solid triangles is il(x) dependence. It can be seen that /) increases with x in the range () 1 becomes practically constant. The experimental Fj(x) dependence can be represented with the following function ... 

At the same time, this behaviour of exciton parameters Ei(x) and 7](x) gives grounds to deem that the short-wave shift of the exciton absorption band is caused by the a, increase stemming from hydrogen presence in interlayer space, which in accord with Eq. (7) results in R0 decrease. 

Goni A., Cantarero A., Schwarz U., Syassen K., Chevy A. (1992) Low-temperature exciton absorption in InSe under pressure. Phys. Rew. B. 45(8), 4221-4226. 

Zhirko Yu.I. (1999) Investigation of the light absorption mechanisms near exciton resonance in layered crystals. N=1 state exciton absorption in InSe. 

Toyozawa Y. (1958) Theory of line-shapes of the exciton absorption bands. Progr. Theor. Phys. 20, 53-81. 

Zhirko Yu.I., and Zharkov I.P. (2003) Investigation of some mechanisms for formation of exciton absorption bands in layered semiconductor n-InSe and p-GaSe crystals. Semicond. Phys. Quantum Electronics and Optoelectronics. 6(2), 134-140. 

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