Direct biexciton generation in Si nanocrystal by a single photon.

It has been shown theoretically that a strong quantum confinement regime in Si nanocrystals promotes highly efficient simultaneous excitation of two electron-hole pairs (biexciton) by a single photon. The rate (inverse lifetime) of biexciton generation has been calculated analytically as a function of the nanocrystal radius. The size-dependence of the rate in Si nanocrystal turns out to be sharp enough-in fact, it is inversely proportional to the sixth power of the radius.

Universality of the Förster's model for resonant exciton transfer in ensembles of nanocrystals.

For nanocrystals in a strong quantum confinement regime, it has been confirmed analytically that resonant exciton transfer proceeds in full accordance with the Förster mechanism. This means that the virtual exciton transitions between the nanocrystals of close sizes are governed only by the dipole-dipole interaction of nanocrystals even in very dense ensembles, while the contributions of all other higher-order multipoles are negligibly small. Based on a simple isotropic model of the envelope function approximation and neglecting the electron-hole interaction inside each nanocrystal, we have computed the rate of the resonant exciton transfer between two nanocrystals.

Multi-carrier processes in halogenated Si nanocrystals.

We study theoretically an effect of passivation with Cl and Br on Auger recombination and multiple exciton generation in silicon nanocrystal SiX, with X being the passivating element. The nanocrystal electronic structure and rates of these processes are calculated using time-dependent density functional theory. Comparison with the H-passivated Si nanocrystal shows that the bromine coating, despite having less electronegativity, affects the electronic structure and transition rates more than the chlorine one due to the stronger structural perturbations caused by the greater surface atoms.

Effects of surface halogenation on exciton relaxation in Si crystallites: prospects for photovoltaics.

Ab initio study of Si crystallites (diameter: 1.1-2.4 nm) with fully Cl- or Br-passivated surfaces was performed.

Optical gap of silicon crystallites embedded in various wide-band amorphous matrices: role of environment.

Within the framework of the envelope-function approximation the single-particle and the optical gaps of silicon nanocrystals embedded in amorphous SiO(2), Si(3)N(4), Al(2)O(3) and ZrO(2) dielectric matrices were calculated. We employ the model of an Si quantum dot surrounded by a spherical thin intermediate layer with a radially varying permittivity, separating the nanocrystal and the host dielectric matrix. The latter was modelled by the finite-height potential barriers.