Radiative lifetime of excitons in ZnO nanocrystals: The dead-layer effect

We theoretically investigate exciton states of colloidal nearly spherical ZnO nanocrystals with diameters from 20.3em0exnm0.5em0exto0.5em0ex60.3em0exnm. The sizes of considered ZnO nanocrystals are chosen to be slightly larger than the exciton Bohr radius of bulk ZnO. A number of characteristic features of excitons are revealed in this intermediate quantum confinement regime. The exciton center of mass is found to be prolate along the c axis of wurtzite ZnO and squeezed to the center of the ZnO nanocrystal, thus forming a dead layer near the nanocrystal surface. The thickness of the exciton dead layer is found to increase with the nanocrystal size reaching the value of about 1.60.3em0exnm for the nanocrystal with diameter of 60.3em0exnm. Based on our calculations we proposed an analytical approximation for the exciton radiative-lifetime dependence on radius R in ZnO nanocrystal written as τ₀∕[1+(R∕R₀)³] with τ₀=73.40.3em0exps and R₀=2.550.3em0exnm. Presented results and proposed analytical approximation can be used for interpretation of experimental data, and optimization of ZnO quantum dot structures for optoelectronic applications.