Exciton effective mass

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... 

Isotropic exciton effective mass and scattering by acoustic phonons... 

Fig. 36. Energy levels of excitonic states in CdS particles of various radii. Zero position of the lower edge of the conduction band in macrocrystalline CdS. Exc Energy of an exciton in macrocrystalline CdS. Effective masses of electrons and holes 0.19 m and 0.8 m respectively. The letters with a prime designate the quantum state of the hole...

EXAMPLE 4.5 From the absorption spectrum in Figure 4.14, determine (a) the energy gap of CU2O, (b) the reduced effective mass of the exciton, assuming By = 10, and (c) the Bohr radius of the exciton n = 2. 

Near k = 0 the electron and hole effective masses must be isotropic and constant. For a nanoparticle, the uncertainty in the exciton position depends upon its size. Ax 2R. If the relation between energy and momentum is independent of particle size the exciton... 

Exciton dispersion As the 0-0 exciton dispersion in anthracene is somewhat complex,101 we assumed a bidimensional parabolic dispersion (effective-mass approximation). This assumption is justified by the absence of interplane coupling for k perpendicular to plane (001) planes2 9—the case of b-polarized exciton under study—and it is aimed to give back the observed stepwise thresholds bound to the bottom of the band. The model most likely fails for the exciton band as a whole, but anyhow, we could not avoid it without lengthening computation times excessively. 

The conduction-band minima and valence-band maxima are studied in terms of the k p method, which relates the effective masses to the oscillator strengths discussed in Chapter 4. Wannier excitons and impurity states are also understandable in this context. 

Should a system contain donor and acceptormoieties, an exciton like behavior can be observed when the charges recombine. Again, effective mass theory can present us with the energy of the observed transition, as shown in equation (5) ... 

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