Simultaneous photocharging and luminescence intermittency in silicon nanocrystals

Abstract

Contemporaneous measurements of the time dependence of photoluminescenceand concomitant electrical conduction in films of alkylated silicon nanocrystals(NCs) during and between periods of continuous-wave laser irradiation of arbitraryduration establish the role played by photoionization to a conducting state in theintermittent light emission from silicon nanocrystals. The luminescence and currentgenerated by electron photoejection both decay to non-zero, steady-state valuesduring irradiation with visible laser light at incident intensities in the range 0.25–0.30 ± 0.01 kW cm−2; on cessation of irradiation, the non-conducting photoluminescent state of the NCs issubstantially regained. These observations are consistent with a model in which the decay ofphotoluminescence is ascribed to autoionization of the silicon NCs with a most probable lifetime⟨Ta⟩, depending on particle size, and recovery of luminescence toelectron–hole recombination characterized by a most probable lifetime⟨Teh⟩. Valuesof ⟨Ta⟩ = 1.08 ± 0.03 s and ⟨Teh⟩ = 770 ± 300 s are extracted from nonlinear least-squares fitting to the time dependence of thephotoluminescence intensity. The temporal behavior of the transient photocurrent is foundto be quantitatively consistent with a one-dimensional model of diffusion of charge carriersbetween NCs. Integration of the time dependence of the photocurrent response coupledwith an estimate of the volume irradiated with the laser light suggests ionization of oneelectron per NC during photon irradiation.