Simultaneous photocharging and luminescence intermittency in silicon nanocrystals

Contemporaneous measurements of the time dependence of photoluminescence and concomitant electrical conduction in films of alkylated silicon nanocrystals (NCs) during and between periods of continuous-wave laser irradiation of arbitrary duration establish the role played by photoionization to a conducting state in the intermittent light emission from silicon nanocrystals. The luminescence and current generated by electron photoejection both decay to non-zero, steady-state values during 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 is substantially regained. These observations are consistent with a model in which the decay of photoluminescence is ascribed to autoionization of the silicon NCs with a most probable lifetime ⟨T(a)⟩, depending on particle size, and recovery of luminescence to electron-hole recombination characterized by a most probable lifetime ⟨T(eh)⟩. Values of ⟨T(a)⟩ = 1.08 ± 0.03 s and ⟨T(eh)⟩ = 770 ± 300 s are extracted from nonlinear least-squares fitting to the time dependence of the photoluminescence intensity. The temporal behavior of the transient photocurrent is found to be quantitatively consistent with a one-dimensional model of diffusion of charge carriers between NCs. Integration of the time dependence of the photocurrent response coupled with an estimate of the volume irradiated with the laser light suggests ionization of one electron per NC during photon irradiation.