Genesis of temperature-driven red-shift of charge transfer band edge for Sm3+-doped vanadate self-activated phosphor

The temperature-driven redshift of the charge transfer band (CTB) exhibits promising potential for optical temperature sensing as well as for the design of anti-thermal quenching phosphors. Therefore, it is essential to investigate the displacement mechanism in detail. In this contribution, we created LiCa2.95MV3O12:5%Sm3+ (M = Mg/Zn) phosphors with considerably red-shifted CTB edges upon temperature stimulation and outstanding anti-thermal quenching behavior. To investigate this unusual behavior, gaussian fitting was performed on the excitation spectra and emission spectra at different temperatures to investigate the redshift mechanism. By averaging the peak energy of the lowest excitation and emission peaks, the zero phonon line (Ezp) indicating the electronic energy level location of the charge transfer state (CTS) shows a downward trend is obtained. As well as the energy reduction of the 1A2(1T1)-1B1(1T2) and 1E(1T1)-1B1(1T2) absorption bands in the [VO4]3- group is observed. Therefore, the drop in the CTS electronic energy level is the dominant factor in the temperature-driven CTB redshift. Based on the redshift phenomenon and anti-thermal quenching phenomenon of CTB, the phosphor exhibited exceptional optical temperature measurement performance in all three thermometry modes of excitation intensity ratio (EIR), International Committee on Illumination (CIE) color coordinates, and fluorescence intensity ratio (FIR), demonstrating its broad application prospects in the field of optical temperature sensing as well as guiding the design of anti-thermal quenching phosphors.

Structures, photoluminescence, and principles of self-activated phosphors

Self-activated phosphors without any luminescent dopants, usually display excellent optical properties, such as high oscillator strength, large Stokes shift, and strong luminescence efficiency, and thus have been widely investigated by researchers for several decades. However, their recent advancements in scintillators, white-light illumination, displays and optical sensors compel us to urgently understand the basic principles and significant technological relevance of this worthy family of materials. Herein, we review the structures, photoluminescence principles, and applications of state-of-the-art self-activated phosphors, such as borate, gallate, niobate, phosphate, titanate, vanadate, tungstate, nitrides, oxyfluoride, perovskite, metal halides, and carbon dots. The photoluminescence principles of self-activated phosphors are mainly summarized as transitions between energy levels of rare-earth and transition metal ions, charge transfer transitions of some oxide compounds, and luminescence in all-inorganic semiconductors. The different self-activated phosphors exhibit various structures and site-dependent spectra. Additionally, we discuss the application prospect and main challenges of self-activated phosphors.

Highly Efficient Cyan-Green Emission in Self-Activated Rb3RV2O8 (R = Y, Lu) Vanadate Phosphors for Full-Spectrum White Light-Emitting Diodes (LEDs)

Phosphor-converted white-light-emitting diodes (pc-WLEDs) rely on combining a near-ultraviolet (n-UV) or blue chip with trichromatic and yellow-emitting phosphors. It is challenging to discover cyan-green-emitting (480-520 nm) phosphors for compensating the spectral gap and producing full-spectrum white light. In this work, we successfully discovered two unprecedented bright cyan-green emitting Rb₃RV₂O₈ (R = Y, Lu) phosphors that gives emission bands centered at 500 nm upon 362 nm n-UV light excitation. Interestingly, the both self-activated compounds exhibit high internal quantum efficiencies (IQEs) of 71% for Rb₃YV₂O₈ and 85% for Rb₃LuV₂O₈, respectively. Moreover, controllable emission color can be successfully tuned from cyan-green to orange-red across the warm white light region by design strategy of VO₄^3- → Eu³⁺ energy transfer. The thermal quenching of as-prepared phosphors could be effectively mitigated by this design strategy. Finally, the as-fabricated n-UV (λ_ex = 370 nm) pumped phosphor-converted (pc) W-LED devices utilizing Rb₃RV₂O₈ (R = Y, Lu) along with commercial phosphors demonstrate well-distributed warm white light with high color-rendering index (CRI) of 91.9 and 93.5, and a low correlated color temperature (CCT) of 5095 and 4946 K. It suggests that the both vanadate phosphors have potential applications in full-spectrum pc-WLEDs.

DFT and hybrid-DFT calculations on the electronic properties of vanadate materials: theory meets experiments

Herein is presented a theoretical study of the electronic structure and optical properties of six vanadium oxides: Sr₂V₂O₇, Ba₂V₂O₇, Ca₂VO₄Cl, Sr₂VO₄Cl, Mg₃V₂O₈ and Zn₃V₂O₈.

Bluish-White Luminescence in Rare-Earth-Free Vanadate Garnet Phosphors: Structural Characterization of LiCa3MV3O12 (M = Zn and Mg)

Extensive attention has been focused toward studies on inexpensive and rare-earth-free garnet-structure vanadate phosphors, which do not have a low optical absorption due to the luminescence color being easily controlled by its high composition flexibility. However, bluish emission phosphors with a high quantum efficiency have not been found until now. In this study, we successfully discovered bluish-white emitting, garnet structure-based LiCa₃MV₃O₁₂ (M = Zn and Mg) phosphors with a high quantum efficiency, and the detailed crystal structure was refined by the Rietveld analysis technique. These phosphors exhibit a broad-band emission spectra peak at 481 nm under near UV-light excitation at 341 nm, indicating no clear difference in the emission and excitation spectra. A very compact tetrahedral [VO₄] unit is observed in the LiCa₃MV₃O₁₂ (M = Zn and Mg) phosphors, which is not seen in other conventional garnet compounds, and generates a bluish-white emission. In addition, these phosphors exhibit high quantum efficiencies of 40.1% (M = Zn) and 44.0% (M = Mg), respectively. Therefore, these vanadate garnet phosphors can provide a new blue color source for LED devices.

Morphology/phase controllable synthesis of monodisperse ScVO4 microcrystals and tunable multicolor luminescence properties in Sc(La)VO4(PO4):Bi3+,Ln3+ phosphors

A facile one-pot hydrothermal approach was employed to prepare novel chocolate-like ScVO₄ microcrystals using polyethylene glycol as an additive.

The contribution of Eu3+ doping concentration on the modulation of morphology and luminescence properties of InVO4:Eu3+

Nanostructures of InVO₄ and InVO₄:Eu³⁺ were synthesized through a hydrothermal method with a post annealing process. The morphology and luminescence properties of InVO₄:xEu³⁺ can be modulated by Eu³⁺ ion doping concentration. SEM and (HR)TEM studies indicated that different sizes of nanoparticles were obtained when the Eu³⁺ ion concentration ranged from 2 mol% to 30 mol%, and middle-concave nanodisks or nanoparticles with different sizes were obtained with the Eu³⁺ ion concentration ranging from 35 mol% to 45 mol%. Luminescence property studies indicated that the photoluminescence emission originated both from VO₄³⁻ and Eu³⁺, and the emission of VO₄³⁻ was blue shifted, affected by the doped Eu³⁺ ions. The CIE chromaticity diagram of Eu³⁺-doped indium vanadate exhibited green, blue-green, red, giving especially white light emission.

Nanomaterials of InVO₄ and InVO₄:Eu³⁺ were synthesized, and their morphology and luminescent properties could be modulated by Eu³⁺ ion doping concentration.

Crystal structure, luminescence properties and energy transfer of Eu3+/Dy3+ doped GdNbTiO6 broad band excited phosphors

GdNbTiO₆ has been used as a broad band excited host material absorbing UV radiation, and the excitation energy is transferred from the O²⁻ → Nb⁵⁺(Ti⁴⁺) charge transfer band to the excited states of Eu³⁺/Dy³⁺ ions, presenting different emission colors.

Sol-gel synthesis and luminescent properties of red-emitting Y(P,V)O4:Eu(3+) phosphors

Eu(3+)-activated Y(P,V)O4 phosphors were prepared by the EDTA sol-gel method, and the corresponding morphologies and luminescent properties were investigated. The sample particles were relatively spheroid with size of 2-3 µm and had a smooth surface. The excitation spectra for Y(P,V)O4:Eu(3+) consisted of three strong excitation bands in the 200-350 nm range, which were attributed to a Eu(3+)- O(2-) charge-transfer band and (1)A1-(1) T1/(1) T2 transitions in VO4(3-). The as-synthesized phosphors exhibited a highly efficient red luminescence at 613 nm due to the Eu(3+5) D0-(7) F2 electric dipole transition. With the increase in the V(5+)/P(5+) ratio, the luminescence intensity of the red phosphor under UV excitation was greatly improved due to enhanced VO4(3-) → Eu(3+) energy transfer.

Tunable white-light emission via energy transfer in single-phase LiGd(WO4)2:Re3+ (Re = Tm, Tb, Dy, Eu) phosphors for UV-excited WLEDs

Novel single-phase white-light-emitting phosphors have been successfully synthesized by a solid-state reaction and their photoluminescence properties and energy transfer mechanism have been carefully investigated.