Internal defect structure of calcium doped magnesium oxide nanoparticles studied by positron annihilation spectroscopy

Abstract

Calcium-ion substitution to different concentrations in the normally insulating but wide band gap semiconductor magnesium oxide nanocrystallites was successfully achieved through a sol-gel route and the synthesized samples were characterized by X-ray diffraction and transmission electron microscopy. The substitution of Mg2+ by Ca2+ gave rise to distinct variation in the nanocrystallite sizes and lattice constants. The band gap energies (Eg = 5.04 to 5.50 eV) obtained from ultra-violet and visible absorption spectroscopy hinted to the formation of Positronium Hydrogen (Ps-H) complex within nanocrystallites. Excitonic energy levels reduced the band gap after doping. In the samples of larger nanocrystallites, strong evidences to the presence of very large vacancy clusters were observed. The coincidence Doppler broadening measurements also indirectly supported this interpretation. The lineshape and wings parameters also supported the type of defects as evidenced from lifetime measurements. The results appeared significant from the context of choosing doping as an efficient way of molding the properties of semiconductor oxide nanocrystals as concomitant lattice parameter variations and the evolutions of vacancy type defects can significantly alter the expected physical changes towards other directions.