Trap engineering through chemical doping for ultralong X-ray persistent luminescence and anti-thermal quenching in Zn2GeO4

Abstract

Recently, defect luminescence-based ultralong persistent luminescent (PersL) materials have been increasingly appreciated for advanced applications. However, an in-depth understanding of trap manipulation is a big challenge in controlling trap distribution. In this work, we provide a complete understanding of defect-induced photoluminescence (PL) in Zn2GeO4 and the significant role of different defects and defect complexes. Excitation-dependent tunable emissions from blue to green regions indicated different mechanisms for filling traps for different excitation energies. Time-resolved emission spectroscopy (TRES) revealed time-dependent trap distribution, shifting the PL band from blue to green. The existence of different electron–hole recombination pathways in different time windows shed light on the complex PL of Zn2GeO4. Systematic temperature-dependent PL studies imply different trapping and de-trapping processes. We demonstrate the activation energies for different trapping mechanisms and the role of negative thermal expansion (NTE) of Zn2GeO4 in achieving the negative thermal quenching (NTQ) of PL. Further, the aliovalent doping of Pr3+ was used for trap manipulation and introducing additional intermediate defect states. Density functional theory calculations as well as thermoluminescence and electron paramagnetic resonance studies revealed a reduction in defect formation energies for selective defects, four-fold increase in trap density, and trap re-shuffling to optimum trap depths (0.73 eV) on the doping of Pr3+. The rich trap distribution resulted in a two-fold increase in the quantum yield of green emissions (∼19%) due to Zn interstitial defects. Improved afterglow on UV charging and an increase in the afterglow time from a few minutes in undoped Zn2GeO4 to an intense X-ray activated afterglow for 18 hours was observed in the Pr3+ doped Zn2GeO4 phosphor. Analysis of afterglow decay kinetics revealed the prominent trap-to-trap tunnelling mechanisms for long lifetimes. Impedance studies revealed the widening of electron channels and reduced resistance to electron movement with the incorporation of Pr3+ ions that enhanced PersL. LED fabrication was carried out to demonstrate the potential of the Zn2GeO4 phosphor for solid-state lighting. We believe that such kinetic and thermodynamic interpretation of defect chemistry will be helpful in tailoring the optoelectronic properties of native defect phosphors.