Correlation Between Temperature Dependent Photoluminescence and Origin of Room Temperature Ferromagnetism in Li Doped ZnO Nano-Flowers

Nanostructured ZnO is one of the most important dilute magnetic semiconductor materials (DMS) where room temperature ferromagnetism has been achieved by the doping of transition metals. However, transition metal doped ZnO are predominantly n-type materials. ZnO nanomaterial based DMS with p-type majority charge carrier are of great research interest as one can make use of both the magnetic as well as the optical proprieties for opto-electronic and spintronics applications. Recently, it has been reported that nonmagnetic Li doping in ZnO thin films show stable p-type characteristics with room temperature ferromagnetism. Thus there is a great need to understand both the optical and the magnetic properties in Li doped ZnO nanomaterials and a correlation between them.  Here we report on the synthesis and studies of room temperature ferromagnetism and optical properties of Li doped ZnO nano-flowers (NFs).  The room and low temperature SQUID measurement confirms the ferromagnetism in Li doped ZnO NFs. The temperature dependent photoluminescence measurements shed light on the origin of ferromagnetism. The comparison of the low temperature PL spectra with those of Li doped ZnO NFs shows appearance of distinct new peaks related to acceptor bound exciton and Zn vacancy in Li doped ZnO.  Based on these findings from the optical studies we have correlated the origin of room temperature ferromagnetism with stable Zn vacancy in Li doped ZnO NFs. We also discuss the role of Li in stabilizing the Zn vacancy in particular lowering of the formation energy of the latter.