Defect dynamics in Li substituted nanocrystalline ZnO: A spectroscopic analysis

Very recently, vacancy-type defects have been found to play a major role in stabilizing d⁰ ferromagnetism in various low dimensional ZnO systems. In this context, the evolution of vacancy-type defects within the ZnO nanocrystals due to the doping of ZnO by alkali metal lithium (Li) is investigated using X-ray photoelectron (XPS), photoluminescence (PL) and positron annihilation spectroscopy (PAS). Li-doping is found to have significant effects in modifying the vacancy-type defects, especially the Zn vacancy (V_Zn) defects within the ZnO lattice. XPS measurement indicated that initially the Li¹⁺ ions substitute at Zn²⁺ sites, but when Li concentration exceeds 7 at%, excess Li starts to move through the interstitial sites. The increase in positron lifetime components and the lineshape S-parameter obtained from coincident Doppler broadening spectra with Li-doping indicated an enhancement of V_Zn defect concentration within the doped ZnO lattice. The vacancy type defects, initially of the predominant configuration V_Zn+O+Zn got reduced to neutral ZnO divacancies due to the partial recombination by the doped Li¹⁺ ions but, when the doping concentration exceeded 7 at% and Li¹⁺ ions started migrating to the interstitials, positron diffusion is partly impeded and this results in reduced probability of annihilation. PL spectra have shown intense green and yellow-orange emission due to the stabilization of a large number of V_Zn defects and Li substitutional (Li_Zn) defects respectively. Hence Li can be a very useful dopant in stabilizing and modifying significant amount of Zn vacancy-defects which can play a useful role in determining the material behavior