Bright Red Self-Trapped Exciton Emission with Highly Linear Polarization in Bi-Doped Wide Bandgap In-Plane Anisotropic GaPS4.

Self-trapped exciton (STE) emission from halides with high photoluminescence quantum yields (PLQYs) has been explored extensively in recent years. In contrast, studies on STE emission in thiophosphates are very limited, and the corresponding PLQY is rather low. Herein, we report an unexpected bright red STE emission with highly linear polarization in Bi-doped wide bandgap in-plane anisotropic van der Waals GaPS4 (Bi:GaPS4). The photoluminescence (PL) spectrum of Bi:GaPS4 exhibits two ultra-broadbands with a full width at half-maximum (FWHM) of 143/147 nm and a large Stokes shift of 1.54/1.81 eV. Although pristine GaPS4 is nonluminescent, Bi:GaPS4 displays a bright red emission with a PLQY of up to 14.1%. The excitation-power- and temperature-dependent PL reveals a typical excitonic recombination rather than defect-related emission, accompanied by a strong electron-phonon interaction, implying that this unexpected red emission can be assigned rationally to STEs. Notably, benefiting from the synergistic contributions of low-symmetry crystal structure and robust in-plane anisotropic electron-phonon interaction, Bi:GaPS4 demonstrates pronounced angle-resolved polarized Raman spectroscopy (ARPRS) and highly linear polarized emission. The degree of polarization (DOP) reaches as high as 0.54. This work offers an effective doping strategy to enhance the STE emission in GaPS4-based thiophosphates and highlights their potential applications in linear polarization-sensitive photonics and optoelectronics.