Raman and Photoluminescence Spectroscopic Detection of Surface-Bound Li+O2- Defect Sites in Li-Doped ZnO Nanocrystals Derived from Molecular Precursors

We present a detailed study of Raman spectroscopy and photoluminescence measurements on Li-doped ZnO nanocrystals with varying lithium concentrations. The samples were prepared starting from molecular precursors at low temperature. The Raman spectra revealed several sharp lines in the range of 100-200 cm(-1), which are attributed to acoustical phonons. In the high-energy range two peaks were observed at 735 cm(-1) and 1090 cm(-1). Excitation-dependent Raman spectroscopy of the 1090 cm(-1) mode revealed resonance enhancement at excitation energies around 2.2 eV. This energy coincides with an emission band in the photoluminescence spectra. The emission is attributed to the deep lithium acceptor and intrinsic point defects such as oxygen vacancies. Based on the combined Raman and PL results, we introduce a model of surface-bound LiO2 defect sites, that is, the presence of Li+O2- superoxide. Accordingly, the observed Raman peaks at 735 cm(-1) and 1090 cm(-1) are assigned to Li-O and O-O vibrations of LiO2.