Color-Tunable Photoluminescence via Site-Selective Occupancy of Bi³⁺/Eu³⁺ and Chemical Composition Modulation in Stable K₃YSi₂O₇ toward wLED and Optical Temperature-Sensing Applications

The design of a stably efficient and color-tunable photoluminescence (PL) phosphor, depending on the clear relationship between structure/composition and PL properties, remains quite a challenge. Herein, in pursuit of phosphor for white LED (wLED) lighting, chemical modification of the host (K3YSi2O7, KYS) and regulation of the energy transfer of dopants Bi3+/Eu3+ in multiple lattice sites (K/Y) are in detail investigated and achieved via a facile solid-phase reaction under air condition. Based on both experimentally and theoretically comprehensive analysis of structure (i.e., XRD/Rietveld refinement, density functional theory (DFT) calculations) and steady/transient-state spectroscopy (i.e., PL/PL excitation, decay lifetime), the adjustable and broad PL in the blue/cyan region of Bi3+ is originated from both the crystallographic sites of K1/Y2, while Eu3+ ions are more inclined to enter the Y2/Y1 sites and emit the characteristic high-purity red PL. Systematic PL tuning from blue/cyan to red is thus realized via efficient ET from Bi to Eu, and the corresponding PL and ET mechanisms are discussed and proposed. Intriguingly, significant enhancements of the PL intensity, quantum yield (QY), and thermal stability of the KYS: Bi/Eu are achieved via chemical composition modulation by artificially adding excessive K+ ions to the host to enhance the phase purity/crystallinity. By adopting the phosphor as a “warm” white color convertor, a wLED with high efficiency and color rendering index (CRI, 90.5) and low corrected color temperature (CCT, 4126 K) is constructed via a remote “capping” packaging strategy. Further, in view of the different thermal responses of the multisite occupancy of Bi3+, energy-level thermal coupling of Eu3+, and dual centers of Bi/Eu in KYS, a multimode and self-referenced thermometer is also designed based on the fluorescence intensity ratio (FIR) technology with high relative sensitivity.