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.
