Toward Bi³⁺ red luminescence with no visible reabsorption through manageable energy interaction and crystal defect modulation in single Bi³⁺-doped ZnWO₄ crystal

Abstract

The last decades have witnessed the discovery of tens of thousands of rare earth (RE) (e.g., Eu²⁺) and non-RE (e.g., Mn²⁺) doped photonic materials for near-ultraviolet (NUV) and blue converted white light-emitting diodes (wLEDs), but the future development of wLEDs technology is limited greatly by the intrinsic problems of these traditional dopants, such as the insurmountable visible light reabsorption, the weak absorption strength in NUV or blue region, and so on. Here we report a feasible strategy guided by density functional theory (DFT) calculation to discover novel Bi³⁺ red luminescent materials, which can solve the above problems eventually. Once the untraditional ion of bismuth is doped into ZnWO₄ crystal, multiple defects can be possibly created in different charge states such as BiZn, BiW, interstitial Bi, and even defect complexes of 2 BiZnVW among others, and they, as DFT calculated results illustrate, have the potential to produce emission spanning from visible to near-infrared. As confirmed by experiment, tunable emission can be led to cover from 400 to 800 nm after controls over temperatures, defect site-selective excitation schemes, and the energy transfer between these defects and host. A novel red luminescence was observed peaking at 665 nm with a broad excitation in the range of 380-420 nm and no visible absorption, which is evidenced by the temperature-dependent excitation spectra and the diffuse reflection spectra. DFT calculation on defect formation energy shows that BiZn³⁺, the valence state of which is identified by X-ray photoelectron spectroscopy, is the most preferentially formed and stable defect inside a single Bi-doped ZnWO₄ crystal, and it produces the anomalous red luminescence as confirmed by the single-particle level calculations. Calculation based on dielectric chemical bond theory reveals that the high covalency of the lattice site which Bi³⁺ prefers to occupy in ZnWO₄ is the reason why the emission appears at longer wavelength than the previously reported compounds. On the basis of this work, we believe that future combination of DFT calculation and dielectric chemical bond theory calculation can guide us to efficiently find new phosphors where Bi³⁺ can survive and emit red light upon NUV excitation. In addition, the DFT calculation on Bi defects in different charge states will help better understand the longstanding as yet unsolved problem on the mechanism of NIR luminescence in bismuth-doped laser materials.