Size-dependent intrinsic radiative decay rates of silicon nanocrystals at large confinement energies

We study ultrafast photoluminescence (PL) dynamics of Si nanocrystals (NCs). The early-time PL spectra (< 1 ns), which show strong dependence on NC size, are attributed to emission involving NC quantized states. The PL spectra recorded for long delays (> 10 ns) are almost independent of NC size and are likely due to surface-related recombination. Based on instantaneous PL intensities measured 2 ps after excitation, we determine intrinsic radiative rate constants for NCs of different sizes. These constants sharply increase for confinement energies greater than similar to 1 eV indicating a fast, exponential growth of the oscillator strength of zero-phonon, pseudodirect transitions.