We theoretically investigate exciton states of colloidal nearly spherical ZnO nanocrystals with diameters from 2 nm to 6 nm. 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 1.6 nm for the nanocrystal with diameter of 6 nm. Based on our calculations we proposed an analytical approximation for the exciton radiative-lifetime dependence on radius R in ZnO nanocrystal written as tau_0/[1+(R/R_0)^3] with tau_0 = 73.4 ps and R_0 = 2.55 nm. Presented results and proposed analytical approximation can be used for interpretation of experimental data, and optimization of ZnO quantum dot structures for optoelectronic applications. (C) 2004 American Physical Society
