ZnO quantum dots: Physical properties and optoelectronic applications

We present a review of the recent theoretical and experimental investigation of excitonic and phonon states in ZnO quantum dots. A small dielectric constant in ZnO leads to very large exciton binding energies, while wurtzite crystal structure results in unique phonon spectra different from those in cubic crystals. The exciton energies and radiative lifetimes are determined in the intermediate quantum confinement regime, which is pertinent to a variety of realistic ZnO quantum dots produced by wet chemistry methods. An analytical model for the interface and confined polar optical phonons is presented for spheroidal quantum dots of different size and barrier materials. The experimental part of the review covers results of the nonresonant and resonant Raman spectroscopy and photo-luminescence study of ZnO quantum dots with sizes comparable to or larger than the exciton diameter in ZnO. The origins of the Raman phonon shifts and the mechanisms of the carrier recombination in ZnO quantum dots are discussed in detail. The reviewed properties of ZnO quantum dots are important for the proposed optoelectronic applications of these nanostructures.