Multimodal stimulus responsive luminescence in MgGa2O4:Bi3+ phosphors via thermal-assisted energy transfer

Abstract

Multimodal stimulus-responsive luminescent materials have shown great promise in advanced anti-counterfeiting due to their dynamic and reversible optical changes under multiple external stimuli. Here, a Bi3+-activated trap depth engineering strategy in MgGa2O4: Bi3+ was proposed to achieve excitation-wavelength-dependent dual-mode luminescence: thermochromic and anti-thermal quenching. The selective occupation of Bi3+ ions on octahedral [GaO6] and tetrahedral [GaO4] sites in MgGa2O4 results in bimodal emission at 408 and 705 nm. Crucially, Bi3+ doping reduces trap depth, effectively regulating the intrinsic energy level luminescence at 502 nm of the traps. Based on the thermal assisted energy transfer from traps to the intrinsic energy levels and Bi3+ in tetrahedral sites, high-sensitivity temperature sensing and anti-thermal quenching luminescence dependent on excitation wavelength have been achieved. Upon the ultraviolet (UV) irradiation of 250 nm, the fluorescence intensity ratios (FIR) of I502 nm/I408 nm for MgGa2O4: 1.5% Bi3+ showed excellent temperature-dependent enhancement behaviors in the temperature range of 283 to 393 K, and the maximum relative temperature sensitivities (Sr) for FIR is 4.15% K-1 at 283 K. Under 280 nm UV excitation, the PL intensities (I502 nm and I705 nm) of MgGa2O4: 1.5% Bi3+ at 393 K remained the initial intensities of 90.5% and 95.9%, respectively, demonstrating remarkable anti-thermal quenching performance. More importantly, by utilizing the dynamic changes in luminescence color and intensity under different excitation wavelengths and temperatures, the application of dynamic anti-counterfeiting has been successfully achieved.