Size dependent carrier recombination in ZnO nanocrystals

Pozina, G., Yang, L. L., Zhao, Q. X., Hultman, L. and Lagoudakis, P. G. (2010) Size dependent carrier recombination in ZnO nanocrystals. Applied Physics Letters, 97 (13). (doi:10.1063/1.3494535).

Abstract

Experimental and theoretical studies of fluorescence decay were performed for colloidal ZnO nanocrystals. The fluorescence lifetime reduces from 22 ps to ∼6 ps∼6 ps with decreasing nanocrystal radius. We postulate that non-radiative surface states dominate the carrier dynamics in small ZnO nanocrystals and perform Monte Carlo simulations incorporating carrier diffusion and carrier recombination to model the experimental fluorescence decay dynamics. The percentage of excitons undergoing nonradiative decay due to surface trapping is as high as 84% for nanocrystals with 8 nm radius, which explains the ultrafast decay dynamics observed in small ZnO nanostructures even at low temperatures.
ZnO is one of the most attractive wide-band gap semiconductors for optoelectronic applications due to its huge exciton binding energy of 60 meV, which allows to design devices operating at temperatures exceeding 300 K. Reduction in physical size to nanoscale offers interesting applications for nanophotonics and nanovoltaics. Low-cost ZnO nanostructures have strong potential for fabrication of light-emitting diodes, nanolasers, nanosized sensors of high sensitivity and field emitters.1–3 Since nanocrystals (NCs) possess a relatively large surface with respect to their volume the influence of surface recombination might be significant for some important applications such as light emitters or solar-cells based on ZnO NCs. From this point of view it is necessary to understand in quantitative terms how the NC size affects the fluorescence properties of ZnO. In this paper, we report results of time-resolved fluorescence studies and model the dynamics using Monte Carlo simulations.

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Contributors

Author: L. L. Yang

Author: P. G. Lagoudakis

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