Probing the defects and trap distribution in MgAl2O4 nanocrystals through electron spin resonance and thermoluminescence

Abstract

Trap distribution in MgAl2O4 nanocrystals has been investigated by analyzing thermoluminescence (TL) glow curves upon UV-irradiation. To probe various defects and trap distribution with the growth of spinel phase and crystallinity, three different fuels viz. monoethanolamine (MEA), glycine and urea, were employed in the combustion synthesis and annealing was performed at 700 ◦C and 900 ◦C. MEA and glycine resulted in crystalline spinel phase in annealed samples, however, urea resulted in minor secondary phase of MgO with the spinel phase in pristine as well as annealed samples. The x-ray photoelectron spectroscopy informed the presence of minor concentration of C and N in addition to Mg, Al and O, which are expected to influence the various defects of MgAl2O4 samples. The TL glow curve broadening in pristine and 700 ◦C annealed samples inferred the presence of multiple closely distributed traps. Shallow traps were more populated with urea, while higher concentrations of deep traps were observed with MEA and glycine. Computerized glow curve deconvolution of the TL glow curves revealed various trapping parameters. The observed shallow and deep traps have been attributed to various electron and hole centres such as antisite defects, O− and F+ centres. The significant high temperature shift of glow curve maxima in 700 ◦C annealed samples, synthesized using MEA and glycine, might be due to the reorganization of the local energy levels with the growth of spinel phase. Quenching of TL intensity at 900 ◦C anticipated the defect annealing accompanied by clustering of existing defects. Electron spin resonance analysis probed the existence of F+, V− centres and the spin–spin interaction of electron/hole spins trapped at these centres with the nearby cations, and estimated the formation of paramagnetic defect clusters at 900 ◦C annealing. A schematic band model is proposed to illustrate the TL behaviour in MgAl2O4 nanocrystals.