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.