Spectral diffusion of quasi localized excitons in single silicon nanocrystals

Evolution in time of photoluminescence spectra of SiO_x capped single silicon nanocrystals has been investigated by means of confocal optical spectroscopy at room temperature. Large spectral jumps between subsequent spectra of up to 40 meV have been detected leading to noticeable line broadening and variation in the electron–phonon coupling. Further, a correlation between emission energy and emission intensity has been found and discussed in terms of an intrinsic Stark effect. Anti-correlated variations of the electron–phonon coupling to Si and SiO₂ phonons as a function of photoluminescence energy indicate that the nearly localized excition is to some extent coupled to phonons in the shell covering the silicon nanocrystal. However, coupling is reduced upon increasing Stark effect, while at the same time coupling to phonons of the Si core increases.