The role of chromium on the structural, electronic and photoluminescence properties of alumina: theoretical and experimental study.

Abstract

Chromium (Cr)-doped alumina has been applied as bio-probes due to its excellent properties including high photoluminescence (PL), long lifetime, and unlimited photostability. Experimental and theoretical investigations were performed to elucidate the role of Cr atoms in the alumina matrix that contributes to its unique characteristics. Cr-doped alumina samples with chromium ion (Cr3+) contents ranging from 0.2 to 1.2 wt% were synthesised using a facile solution combustion method. All samples produced a sharp emission peak at 694 nm. At high concentrations of Cr3+ ions (1.0–1.2 wt%), too narrow Cr3+–Cr3+ space greatly suppressed the red emission indicating a quenching phenomenon. The 0.8 wt% of Cr3+ displays the most intense red emission and a lifetime of 3.25 ms. Aluminium (Al) atoms were replaced by Cr atoms in the Al2O3:Cr3+ crystals, thus resulting in an increase in the lattice parameters, according to Rietveld's refinement. Density Functional Theory (DFT) calculation revealed that the increase in lattice parameters is due to the replacement of Al atoms with the Cr atoms. Based on the DFT simulation, the Al-O distance increases from 1.946 Å to 1.963 Å. The O[sbnd]O distance decreases from 2.494 Å to 2.493 Å. Cr doping was found to reduce the band structure Eg from 6.701 to 2.447 eV. The experimental data was in good match with the first-principles calculation in proposing the electronic excitation and emission mechanism of Al2O3:Cr3+. These findings provide theoretical and experimental evidence that the red emission of Al2O3:Cr3+ is induced by electronic excitations of Cr 3d states, which are predominantly occupied from the top of VB to CB. The synthesised Al2O3:Cr3+ nanoparticles can be exploited for bio-probes application due to their intense and sharp red emission.