Raman scattering and interstitial Li defects induced polarization in co-doped multiferroic Zn0.96-yCo0.04LiyO (0.00 ≤ y ≤ 0.10) nanoparticles
Structural and Raman analysis confirmed a single phase wurtzite hexagonal crystalline structure of Li co-doped ZnO nanoparticles. In the Raman backscattering spectra, ELow2and EHigh2 modes corresponded to zinc and oxygen lattice vibrations. No extra vibration modes of secondary or metallic phases of Co or Li were observed. The intensity of Raman modes decreased with increasing Li content. High resolution X-ray photoelectron spectroscopy (XPS) of Zn and Co confirmed the +2 oxidation state. The deconvolution of high resolution XPS spectra of Li showed the presence of interstitial and substitutional Li defects. The room temperature polarization lay in the range of 0.155–0.225 μC cm−2 for different compositions. We observed an interesting result in the co-doped system, that even the low Li concentration sample, e.g., Zn0.94Co0.04Li0.02O, showed a P–E hysteresis loop of polarization at ∼0.20 μC cm−2. There was higher polarization for y = 0.10, due to more interstitial Li defects, and lower polarization for y = 0.06, due to more substitutional Li defects. We suggest that polarization might have appeared and been enhanced due to the net interaction of dipoles, formed by Li off-center impurities (interstitial Li defects).