Observation of strong direct-like oscillator strength in the photoluminescence of Si nanoparticles

We have performed time-resolved photoluminescence measurements on suspensions of silicon nanoparticles using near-infrared two-photon femtosecond excitation. Our results for 1 nm particles show wide bandwidth but indicate full conversion to directlike behavior, with a few nanosecond time characteristic, corresponding to oscillator strength comparable to those in direct semiconductors. In addition to fast nanosecond decay, the photoluminescence from 2.85 nm nanoparticle suspension exhibits considerably slower decay, consistent with a transition regime to directlike behavior. The quantum yield is measured to be ~0.48, 0.82, and 0.56 for excitation at 254, 310 and 365 nm, respectively, for the blue 1 nm particles, and ~0.22, 0.36, and 0.50 for the red 2.85 nm particles. The directlike characteristics are discussed in terms of localization on radiative deep molecularlike Si-Si traps with size-dependent depth.