Tailored Luminescence Output of Bi3+-Doped BaGa2O4 Phosphors with the Assistance of the Introduction of Sr2+ Ions as Secondary Cations

A novel blue emitting phosphor Ca1-ySryScBO4:Bi3+ with zero-thermal quenching for multi-scenario application

In recent years, Bi3+ activated phosphors have received a lot of attention from researchers; however, the performance and application areas of phosphors are yet to be developed. In this work, a series of CaScBO4(CSBO):xBi3+ phosphors were successfully prepared using a high-temperature solid-state method. Under UV excitation, blue light emission was achieved at 430 nm with a quantum yield of 91%, and at 423 K, the emission intensity retained 82.8% of the original intensity at 298 K. By crystal field engineering, the substitution of Sr2+ at the Ca2+ site enhances the temperature stability of the material, and at 423 K, 473 K and 573 K, the samples maintain 104%, 103% and 85% of the emission intensity at room temperature, respectively. It indicates that the cation substitution causes the increase in the oxygen vacancy concentration, and the oxygen vacancy defect compensates the energy lost in electrons at high temperature, producing resistance to anti-TQ performance. Finally, a blue-violet LED was fabricated by using the phosphor and an ultraviolet LED chip, and white LEDs (CCT = 4683 K, Ra = 89.7) were obtained by co-packaging this phosphor with commercial phosphors and a UV chip. Importantly, the great potential of this phosphor in the field of plant lighting and biocontrol can be demonstrated.

Prolonged Red Persistent Luminescence in Bi3+ Single-Doped LiGa5O8: Regulating Traps by Site Selective Occupation

Persistent luminescence (PersL) materials with impressive superiorities in optical performance have shown tremendous potential for information encryption application. Herein, a Bi3+-doped LiGa5O8 PersL material is facilely synthesized by a simple solid-state reaction. Site selective and preferential occupation of Bi3+ ions at distinct Ga3+ and Li+ polyhedra sites in LiGa5O8 endows it with a bimodal emission at 511 and 718 nm upon UV excitation at 254 nm. After removing the light source, an intense red afterglow was observed due to the generation of more and deeper traps classified as the oxygen vacancy defect (VO••) and the impurity defect (BiLi••). It is rarely reported in inorganic phosphors to achieve the Bi3+ single doping-activated red PersL. The LiGa5O8:x% Bi3+ (with x = 0.5-5.0) samples also exhibit tunable PL and time-dependent PersL properties. Combined with the Morse Code, a multiple information encryption model based on the abnormal optical properties of LiGa5O8:x% Bi3+ (x = 0, 0.5, and 5.0) samples was designed for high-level dynamic anticounterfeiting. These achievements further validate the use of the Bi3+ single doping strategy to explore excellent PersL materials and expand their multifunctional optical applications.

Highly efficient rare-earth free vanadate phosphors for WLEDs

Yellow-green emitting phosphors of vanadate Ca5Mg4(VO4)6 (CMV) doped with different concentrations of Ta5+ ions were synthesized by a solid-state reaction method. The formation of single-phase compounds with a garnet structure was verified by X-ray diffraction (XRD), Rietveld refinement calculations and energy-dispersive X-ray spectroscopy. Different luminescence properties of CMV phosphors such as spectral shift, luminescence lifetime, quantum efficiency, color coordinates and Stokes shift were measured and have been discussed in detail. PLE and PL spectra showed that CMV : xTa5+ (0 ≤ x ≤ 5%) phosphors could match well to 365 nm LED chips, and showed bright yellow-green emission in the visible range of 400-750 nm, with a peak at 544 nm, which is attributed to the charge transfer (CT) of an electron from the 2p orbital of the oxygen atom to the vacant 3d orbital of V5+ ions in the tetrahedral [VO4]3- group. Compared with the CMV host, the integrated luminescence intensity of CMV : 0.5%Ta5+ increased by 26.31%, and the quantum efficiency increased by 15.98%. The phenomenon can be ascribed to the substitution of V5+ ions by the large Ta5+ ions, which resulted in the squeezed and distorted VO4 tetrahedron. Finally, the white light emitting diode (WLED) devices prepared with UV WLED chips and the CMV : 0.5%Ta5+ phosphor exhibited excellent color temperature (4083 K) and CIE coordinates (0.3677, 0.3409). The CMV : 0.5%Ta5+ phosphor can be considered as a potential yellow-green emitting phosphor in the solid-state lighting field.

Multimode-Responsive Luminescence of Er3+ Single-Activated CaF2 Phosphor for Advanced Information Encryption

The current optical anticounterfeit strategies that rely on multimode luminescence in response to the photon or thermal stimuli have significant importance in the field of anticounterfeiting and information encryption. However, the dependence on light and heat sources might limit their flexibility in practical applications. In this work, Er3+ single-doped CaF2 phosphors that show multistimuli-responsive luminescence have been successfully prepared. The as-obtained CaF2:Er3+ phosphor exhibits green photoluminescence (PL) and color-tunable up-conversation (UC) luminescence from red to green due to the cross-relaxation of Er3+ ions. Additionally, as-obtained CaF2:Er3+ phosphors also display green mechano-luminescence behavior, which is induced by the contact electrification between the CaF2 particles and PDMS polymers, enabling the phosphor to flexibly respond to mechanical stimuli. Moreover, feasible anticounterfeiting schemes with the capability of multistimuli-responsive and flexible decryption have been constructed, further expanding the application of optical materials in the field of advanced anticounterfeiting and information encryption.

Exclusive confinement of Bi3+-activators in the triangular prism enabling efficient and thermally stable green emission in the tridymite-type phosphor CaBaGa4O8:Bi3

Recently, Bi3+-activated phosphors have been extensively studied for potential applications in phosphor-converted white light-emitting diodes (pc-WLEDs). However, Bi3+ activators usually exhibit low quantum efficiency and poor thermal stability due to the outermost 6s6p-orbitals of Bi3+ being strongly coupled with the host lattice, inhibiting potential applications. Herein, we rationally design a novel phosphor CaBaGa4O8:Bi3+, which adopts a tridymite-type structure and crystallizes in the space group of Imm2. CaBaGa4O8:Bi3+ presents a bright green light emission peaking at 530 nm with a FWHM narrower than 90 nm. Comprehensive structural and spectroscopic analyses unravelled that Bi3+ emitters were site-selectively incorporated into the triangular prism (Ca2+-site) in CaBaGa4O8:Bi3+ since there exist two distinct crystallographic sites that can accommodate the Bi3+ ions. An excellent luminescence thermal stability of 73% of the ambient temperature photoluminescence intensity can be maintained at 423 K for CaBaGa4O8:0.007Bi3+. Impressively, the quantum efficiency (QE) of CaBaGa4O8:0.007Bi3+ was remarkably improved to 47.2% for CaBaGa4O8:0.007Bi3+,0.03Zn2+via incorporating the Zn2+ compensators without sacrificing the luminescence thermal stability. The high thermal stability and QE of CaBaGa4O8:0.007Bi3+,0.03Zn2+ are superior to most of the Bi3+-activated green-emitting oxide phosphors. The perspective applications in pc-WLEDs for CaBaGa4O8:0.007Bi3+,0.03Zn2+ were also studied by fabricating LED devices.

Modulation of Bi3+ luminescence from broadband green to broadband deep red in Lu2WO6 by Gd3+ doping and its applications in high color rendering index white LED and near-infrared LED

Phosphors that exhibit tunable broadband emissions are highly desired in multi-functional LEDs, including pc-WLEDs and pc-NIR LEDs. In this work, broadband emissions were obtained and modulated in the unexpectedly wide spectral range of 517-609 nm for (Lu_0.99-xGd_xBi_0.01)₂WO₆ phosphors by tuning the Gd³⁺ content (x = 0-0.99). The effects of Gd³⁺ doping on phase constituents, particle morphology, crystal structure, and photoluminescence were systematically investigated. Broadband green emission was obtained from Gd³⁺-free (Lu_0.99Bi_0.01)₂WO₆ phosphors (x = 0), whose emission intensity was enhanced by 50% with 5 at% Gd³⁺ (x = 0.05). The phase transition happened when x > 0.50 and the broadband red-NIR emission was obtained when x = 0.75-0.99. Three luminescence centers were proved to be responsible for the broadband green emissions via crystal structure, spectral fitting and fluorescence decay analysis. A pc-WLED with a high color rendering index (R_a = 91.3), a stable emission color, and a low color temperature (3951 K) was fabricated from the (Lu_0.94Gd_0.05Bi_0.01)₂WO₆ broadband green phosphor, and an LED device that simultaneously emits high color rendering index white light and NIR light was obtained with the (Gd_0.99Bi_0.01)₂WO₆ broadband red-NIR phosphor. Night vision and noninvasive imaging were also demonstrated using the latter LED device.

Realization of an Optical Thermometer via Structural Confinement and Energy Transfer

The influence of temperature on a variety of physiological or chemical processes has generated considerable interest, and recently noninvasive lanthanide-incorporated optical thermometers have been considered as promising candidates for monitoring its changes at different scales. Herein, a novel Bi³⁺-activated Sr_3-xGd_xGaO_4+xF_1-x phosphor with tunable color has been constructed by a cooperative cation-anion substitution strategy with to the replacement of [Sr²⁺-F^-] by [Gd³⁺-O^2-]. When x = 0, the sample Sr₃GaO₄F/Bi³⁺ possesses a peak wavelength at 438 nm, and this value will shift to 470 nm if x is equal to 1 (Sr₂GdGaO₅/Bi³⁺). In addition, photoluminescence tuning from blue to red has been realized successfully by an efficient Bi³⁺ → Eu³⁺ energy migration model in Sr_2.6Gd_0.4GaO_4.4F_0.6 samples. The specific Bi³⁺ → Eu³⁺ energy transfer has been explained by dipole-dipole interactions derived from a model of the Dexter pathway. Intriguingly, the two dopants (a blue signal from Bi³⁺ and a red signal from Eu³⁺) possess different thermal responses to increasing temperature. Accordingly, the intensity ratio values are sensitive to the temperature changes. The energy level cross relaxation causes the quenching effect of Bi³⁺, and the multi-phonon de-excitation mode leads to the thermal quenching of Eu³⁺. At room temperature (298 K), the determined maximum relative sensitivity (S_r) is 1.27% K^-1. Moreover, the absolute sensitivity (S_a) is 0.067 K^-1 since the temperature is elevated to 523 K. The collected results are superior to most of the reported optical thermometry materials.