Role of quantum confinement and hyperfine splitting in lithium-doped ZnO nanocrystals

The role of quantum confinement on the electronic properties of Li interstitial impurities in ZnO nanocrystals was examined using a real-space pseudopotential-density-functional method. The Li impurity was found to be partially ionized resulting in a significant charge transfer around the impurity site. To calculate the hyperfine interaction for this system using pseudopotentials, we modified Van de Walle and Blochl's method to include explicitly the off-site contribution of the Li impurity wave function. Our modifications dramatically enhanced the agreement between the calculated and the measured isotropic hyperfine splitting constants. Our analysis with an effective-mass model demonstrates that the partial ionization of the impurity atom plays an important role both in the binding energy and in the shape of its wave function. Comparison between calculations using the local-density approximation (LDA) with LDA+U indicates that the local Coulomb correlation does not play a significant role in altering the impurity electronic states of interstitial Li-doped ZnO nanocrystals.