Exciton effect

Even in semiconductors, where it might appear that the exciton binding energies would be of interest only for low temperaPire regimes, excitonic effects can strongly alter tlie line shape of excitations away from the band gap. 

Here, n is the exciton effective mass parallel to the c axis and Lz is the (average) thickness of the WS2 nested structure (Lz=nx 0.6.2 nm, where n is the number of WS2 layers) in the nanoparticle. In a previous study of ultrathin films of 2H-WSe2, AEg of the A exciton was found to obey Eq. (1) over a limited thickness range. The parameter AEg exhibited a linear dependence on 1/Z for Lz in the range of 4-7 nm and became asymptotically constant for Lz> 8 nm (91). A similar trend is observed for IF-WS2 and MoS2, as shown in Fig. 23 (90). Therefore, the quantum size effect... 

The (4R)-absolute configuration of a new chromophore of native visual pigment (159) (negative Cotton effect at 375 nm, negative Cotton effect at 254 nm) was established by the CD exciton chirality method applied to the 4-(dimethylamino)cinnamate (160). The split negative (381 nm) and positive (338 nm) exciton effects of 160 show a counterclockwise helicity between pentaenal and a-4-(dimethylamino)cinnamate chromophores355. 

The optical gain observed in Si-NC embedded in SiC>2 formed by different techniques [24-27] has given a further impulse to these studies. Interface radiative states have been suggested to play a key role in the mechanism of population inversion at the origin of the gain [24,25,28]. However many researchers are still convinced of the pure quantum confinement model and they are focusing their efforts mainly on the self trapped excitonic effects [29,30] in order to explain the differences between their results and the experimental outcomes. 

Figure 26 Imaginary part of the dielectric function of [100] oriented Ge-NWs (left panels) and Si-NWs (right panel) with diameters of 0.8 nm. The first row shows optical spectra at the GW level, the second row shows the spectra obtained including excitonic effects. The dashed line represents the GW optical...

S. Abe, Excitonic Effects in the Linear and Nonlinear Optical Properties of Conjugated Polymers, in N. S. Sariciftci (Ed.), Primary Photoexcitations in Conjugated Polymers, World Scientific, Singapore, 1997, p. 115. 

In extreme cases a multiple-scattering, sharp resonant structure can result in which the electron is in a quasi-bound state (155). One example is the white line, which is among the most spectacular features in X-ray absorption and is seen in spectra of covalently bonded materials as sharp ( 2eV wide) peaks in absorption immediately above threshold (i.e., the near continuum). The cause of white lines has qualitatively been understood as being due to a high density of final states or due to exciton effects (56, 203). Their description depends upon the physical approach to the problem for example, the LiUii white lines of the transition metals are interpreted as a density-of-states effect in band-structure calculations but as a matrix-element effect in scattering language. 

Calculation of Wannier Functions.—In most further applications of the wave-functions obtained in HF CO studies (calculation of excitonic effects, CDW s, impurity and vacancy levels, etc.) the use of Wannier functions22 instead of the original Bloch functions seems to be very promising.23 The connection between the two basis sets is given by the transformation... 

Novel properties can be obtained with the combination of exciton effects with dynamic molecular processes such as for the coupling of the perylene derivative 7 to form the bichromophore 8. 7 forms yellow fluorescent solutions [19] as is shown in Fig. 5. The coupling of two chromophores such as in 8 induces an appreciable increase of the absorptivity by exciton interaction both... 

Crystals exhibit excitonic effects near the band edges, in which the Coulomb interaction between an electron and a valence band hole results in absorption which does not follow the one-particle joint density of states in Eq. (3.25). Excitons produce an absorption peak just below the band gap energy and modify the absorption at higher energies. There is no exciton absorption peak observable in any amorphous semiconductor, because it is broadened out by the disorder. The Coulomb interaction is present in a-Si H, but its significance in the optical absorption is unclear. 

Electron-hole correlations are an important aspect of the recombination. The recombination transition necessarily involves two particles, the electron and the hole, and so the recombination rate depends on whether they are spatially correlated or distributed randomly. The electron-hole pairing in a crystal is manifested in excitonic effects, but these are not detectable in the absorption spectra... 

Exciton effects in the crystal lead to a series of peaks in the optical spectra, as illustrated in Fig. 9.24(6). It is unlikely that these would be seen in the amorphous multilayers, as excitons are not observed in the bulk material. However, the step structure should be present, although probably smeared out by the disorder (see Fig. 9.24(6)). The quantum confinement raises the energy levels of the electron and hole and shifts the absorption edge to higher energy. 

Fig. 9.24. Illustration of the multilayer optical absorption (a) the one-particle joint density of states (b) the ideal absorption spectrum including exciton effects (solid line) and the expected broadened absorption of a disordered material (dashed line).

It is especially timely to review the subject of exciton effects because, with the advent of the X-ray structural model of the Rhodopseudomonas viridis RC [9-12], it is becoming apparent that analyses of exciton effects exhibit a dichotomy. On the one hand there are analyses based on incomplete structural information, on the other there are those based on X-ray structural models. The former generally seem theoretically straightforward and consistent with all experimental data, while the latter tend to be theoretically involuted and inconsistent with at least some of the data. Because the underlying interactions are quite important in photosynthesis, it is worthwhile exploring this situation and trying to understand what underlies it. In Section 2 basic theoretical concepts are briefly summarized. Exciton analyses based on partial structural information are discussed in Sections 3 and 4, and those based on X-ray models are considered in Sections 5-7. 

The resonance interactions that give rise to the exciton effects in spectra are interactions among the electronic transition moments of closely juxtaposed pigment... 

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