Self-activated Phosphor Research Articles

Energy Transfer of a New Self-Activating Ba<sub>3</sub>TaGa<sub>3</sub>Si<sub>2</sub>O<sub>14</sub>: ySm<sup>3+</sup>, xNb<sup>5+</sup> Phosphor Obtained by a Small Substitution of Nb<sup>5+</sup> for Ta<sup>5+</sup>

Self-activated multicolor phosphors are being used in more and more emerging fields. In order to explore the energy transfer between [NbO6]7- coordination groups and Sm3+ ions, a series of Ba3-ySmyTa1-xNbxGa3Si2O14 samples co-doped with Nb5+ and Sm3+ were prepared in this work, and the changes of luminous intensity under the condition of fixed Nb5+ concentration regulating Sm3+ concentration and fixed Sm3+ concentration regulating Nb5+ concentration were obtained respectively. By further fitting the emission decay curves of Sm3+ under two conditions, the decreasing average lifetime is obtained. Thermal stability tests of the samples also hinted the presence of such energy transfer. This regular change can provide a good reference for developing new self-activated phosphors.

Intrinsic and defect-induced luminescence of lithium antimonate LiSbO3

Studies of investigating intrinsic and defect-induced luminescence in semiconductors can yield valuable insights into the underlying mechanisms and their significance for optoelectronic applications. This research focuses on self-activated lithium antimonate LiSbO3, synthesized through a solid-state reaction method. The as-prepared samples underwent post-annealing treatment in air and a CO atmosphere, respectively. The work investigated the phase formation, structures, elemental composition, band transition characteristics, luminescence properties, and decay lifetimes of the self-activated phosphors. LiSbO3 is identified as an n-type semiconductor with a direct allowed transition and the band gap energy of 2.97 eV. The experiment also confirmed the presence of oxygen vacancy (VO) centers during the synthesis of LiSbO3. The self-activated luminescence of LiSbO3 was found to be sensitive to the post-annealing atmosphere. Notably, post-annealing in a reducing environment significantly enhances the luminescence efficiency. The luminescence characteristics and thermal stability of LiSbO3 are closely linked to oxygen defects, which play a critical role in the color emissions of this self-activated phosphor.

Nonstoichiometry-Induced Self-Activated Phosphors for Dynamic Anti-counterfeiting Applications.

Optical signals with distinctive properties, such as contactless, fast response, and high identification, are harnessed to realize advanced anti-counterfeiting. However, the simultaneous attainment of multi-color, -temporal, and -modal luminescence performance remains a compelling and imperative pursuit. In our work, a temperature/photon-responded dynamic self-activated luminescence originating from nonstoichiometric Zn2GeO4 is developed with the modulation of intrinsic defects. The increased concentration of oxygen vacancies (VO••) contributes to an enhanced recombination of ZnGe″-VO••, ultimately improving the self-activated luminescence performance. Additionally, the photoluminescence (PL) color of the representative Zn2.2GeO4 sample changes from green to blue-white with the increased ultraviolet (UV) irradiation time. Concurrently, the emission color undergoes a variation from blue to green as the ambient temperature raises from 280 to 420 K. Remarkably, green long persistent luminescence (LPL) and photostimulated luminescence (PSL) behaviors are observed. Herein, this study elucidates a sophisticated anti-counterfeiting approach grounded in the dynamic luminescent attributes of nonstoichiometric Zn2GeO4, presenting a promising frontier for the evolution of anti-counterfeiting technologies.

Structural characterization of Li2(MgxZn1-x)2W2O9 for bluish-white luminescence in rare earth-free tungstate phosphors.

Self-activated phosphors have attracted considerable attention due to their low synthesis temperature, high excitation threshold, and broad emission spectrum. And self-activated tungstate phosphors are distinguished by their low cost and stable chemical properties. Generally, it is difficult to observe luminescence from tungstate phosphors at room temperature. Furthermore, blue-emitting tungstate phosphors with high quantum efficiency are rarely reported. In this study, we succeeded in discovering high quantum-efficiency bluish-white-emitting Li2(MgxZn1-x)2W2O9 phosphors and investigating their detailed crystal structures. Upon near-ultraviolet excitation at 266 nm, these phosphors exhibit a broadband emission peak. The red shift of emission is slight with increasing Zn content in Li2(MgxZn1-x)2W2O9. A highly compact octahedral [WO6] unit is observed in the Li2(MgxZn1-x)2W2O9 phosphors. The phosphors exhibit high internal quantum efficiencies (IQEs) of 68.70% (M = Mg), 43.90% (M = Mg0.5Zn0.5), and 22.90% (M = Zn), respectively. This study provides a bluish-white-emitting tungstate phosphor with high quantum efficiency.

Multimodal dynamic luminescence of self-activated Na2CaGe2O6 phosphor via defect manipulation

A multi-mode dynamic optical output for anti-counterfeiting and information encryption with the as-explored self-activated phosphor is achieved.

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.

Broadband emission characteristics and WLED application of self-activated YMZn3AlO7 (M=Ca, Sr, Ba) phosphors

In this study, a non-activated ion-doped full-spectrum emission phosphor for white light-emitting diode (WLED) lighting is proposed using oxygen defect engineering. The YMZn3AlO7 (M = Ca, Sr, Ba) phosphors with "114″ structure were prepared via a high-temperature solid-state reaction. Under excitation with 373 nm ultraviolet (UV) light, YSrZn3AlO7 in YMZn3AlO7 (M = Ca, Sr, Ba) exhibited the highest emission intensity and formed a wide yellow emission band centred at 563 nm. The full width at half maximum of the YSrZn3AlO7 phosphor was 172.6 nm. A theoretical model based on the designed oxygen vacancies was proposed to determine the self-activated luminescence mechanism. Density functional theory calculations revealed that oxygen vacancies lead to the generation of new energy levels. Electron paramagnetic resonance experiments further confirmed that the luminescence originated from oxygen vacancies. Eventually, a warm WLED device ((x, y) = (0.4057, 0.4359)) with a correlated color temperature of 3814 K and a relative color rendering index of 83 was successfully prepared using the YSrZn3AlO7 phosphor and a 365 nm UV LED chip. These results suggest that YSrZn3AlO7 is a promising WLED phosphor and provide new insights into the application of self-activated phosphors in WLED.

Reproduction of absorption spectra and analysis of multiplet energies for [formula omitted] phosphors using first-principles calculations

In recent years, M2VO4ClM=Ca,Sr and M3VO42M=Mg,Zn have been synthesized as rare-earth-free, self-activated phosphors. It is generally accepted that charge transfer transitions between V5+ and O2− are responsible for their absorption and emission of light. In this study, we attempted to reproduce the experimental absorption spectra and analyze the multiplet energy structure of the symmetric TdVO43− cluster using a first-principles calculation method, DV-Xα. This is a type of density functional theory based on the linear combination of atomic orbitals approximation and discrete variational multi-electron method that can account for multiplet effects by including configuration interaction (CI) in the calculation. Herein, we show that although the experimental absorption spectra could not be reproduced by considering only the VO43− clusters, they were successfully reproduced by considering M8VO413+(M=Ca,Sr) clusters and M9VO415+(M=Mg,Zn) clusters. However, we discovered that Zn9VO415+ differs from the other three crystals because its molecular orbitals in the valence band are formed from Zn 3d orbitals and O 2p orbitals. This makes it extremely difficult to calculate the appropriate multi-electron wavefunctions using molecular orbitals using single-electron calculations. Accordingly, we obtained an approximately suitable multi-electron wavefunction using a CI calculation. We grouped the molecular orbitals according to the O 2p orbital population. It was found that the absorption spectra calculated using molecular orbitals with a high O 2p population reproduced the experimental absorption spectra. In addition, calculated multiplet energies and spectra of the Td symmetry VO43− cluster suggest that in this case the excitation model may differ from the conventional one.

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.

Photoluminescence behavior of rare earth doped self-activated phosphors (i.e. niobate and vanadate) and their applications.

In the present study, the photoluminescence behaviors of rare earth doped self-activated phosphors are discussed briefly. Different techniques were used to develop these phosphor samples. We prepared pure and rare earth doped phosphor samples to look for their various applications. The structural confirmations and surface morphologies were performed using X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements, respectively. The upconversion (UC) phenomenon was investigated in Tm3+/Yb3+ and Ho3+/Yb3+ co-doped niobate and vanadate based phosphors, which gave intense blue/NIR and green/red emissions with a 980 nm diode laser as an excitation source. Pure niobate and vanadate phosphor materials are self-activated hosts which give broad blue emission under UV excitation. Upon UV excitation, intense broad blue emission along with sharp emissions due to Tm3+ and Ho3+ ions are observed via energy transfer between niobate/vanadate and rare earth ions. These self-activated hosts show prominent downshifting (DS) behavior. Broad band quantum cutting (QC) was observed in these self-activated hosts, in which a blue emitting photon is converted into two NIR photons by co-doping Yb3+ ions in it. The multimodal (upconversion, downshifting and quantum cutting) behaviors of these phosphors make them very promising in various applications, such as spectral converters to enhance the efficiency of a c-Si solar cell, security ink and color tunable materials.

Oxygen vacancy substitution tuning photoluminescence of self-activated LiGaSi(1-x)GexO4 phosphors

Preparing low-cost and multifunctional phosphor is an important and challenging job. In this paper, a high-purity self-activated LiGaSiO4 phosphor has been successfully synthesized by the high-temperature solid-state method, which can emit near-infrared light peaking at 697 nm. The reversible thermochromic phenomenon can be observed by heat-treating LiGaSiO4 compounds in different atmospheres. The luminescent mechanism and thermochromic phenomenon are studied in detail by photoluminescence spectrum, electron paramagnetic resonance and the first principle calculations. The results show that oxygen vacancies in LiGaSiO4 compounds play an important role in the near-infrared luminescence process. Based on this luminescent mechanism, a series of LiGaSi(1-x)GexO4 (x = 0.2, 0.4, 0.6, 0.8, 1.0) self-activated phosphors have been fabricated and the corresponding luminescent modulation has been inferred. The results indicate that thermochromic LiGaSi(1-x)GexO4 phosphors can be used as special anti-counterfeiting and bio-imaging materials.

Prospecting the dual mode emission properties of lanthanides co-doped self-activated phosphor, Ca9Y (VO4)7: Tm3+/Yb3+

In the current work, we have examined the downconversion and upconversion luminescence behavior of Tm3+/Yb3+ co-doped.Ca9Y (VO4)7 phosphors for the first time. The phosphors sample has been successfully prepared by a solid-state reaction method. The crystallographic structure and phase purity have been certified by X-ray diffraction analysis. The upconversion (UC) spectra of Tm3+/Yb3+ co-doped Ca9Y (VO4)7 phosphor show strong blue and relatively weak red emission using excitation at 980 nm. The upconversion emission intensities of both blue and red emissions increase with an increase in pumping power. This phosphor sample also gives intense blue emission under 321 nm excitation.

Self-activated luminescence in AZn4(BO3)3 (A = K, Rb, Cs) and oxygen-defects-related photoluminescence tuning

The discovery of rare-earth free (RE-free) self-activated phosphors is of fundamental importance for the inorganic luminescence materials. Herein, we report the novel RE-free phosphors AZn4(BO3)3 (A ​= ​K, Rb, Cs) with intrinsic green (505 ​nm) or yellow emissions (585 ​nm), respectively, depending on different annealing conditions. The self-activated luminescence is mainly ascribed to the different kinds of oxygen defects, and the green emission peaking at 505 ​nm is attributed to the singly ionized oxygen vacancy (Vo+) and zinc vacancy (VZn), while the yellow emission arises from the doubly ionized oxygen vacancy (Vo++) and interstitial oxygen (Oi). The as-fabricated white light-emitting diodes (WLEDs) by using the green and yellow RE-free phosphors exhibit a high color-rendering index (Ra ​= ​90.4), indicating the potential for solid-state lighting. The controllable synthesis strategy opens a new gate for the exploration of RE-free self-activated phosphors with tunable emission.

Synthesis, structural feature and properties of rosiait structure compound BiGeSbO6

The layered oxide BiGeSbO6 has been synthesized for the first time via solid state reaction. Rietveld refinement showed that BiGeSbO6 belongs to PbSb2O6 structural type (P-31m space group) with unit cell parameters a = 5.1314(9), c = 4.9974(1) Å, α = 90°, γ = 120°, Z = 1. Using the EXAFS and XANES methods, it was shown that a structural feature of this compound is a strong disordering of the Bi–O octahedron caused by the stereochemical activity of the 6s2 lone pair of Bi3+ electrons. A model according to which one oxygen vertex in the oxygen coordinated Bi3+ octahedron is absent, and the lone pair of electrons builds this polyhedron to the octahedron, was proposed. A particularity of the structure of BiGeSbO6 is that the existing oxygen vacancy does not have a specific position. It was established that BiGeSbO6 does not exhibit any polymorphic transition up until its incongruent melting point Тm = 985°С. Isobaric heat capacity was measured and thermodynamic functions (entropy, enthalpy increment and Gibbs free energy) were calculated for BiGeSbO6 in the 10–1200 K temperature range. We found that the band gap of BiGeSbO6 is due to the allowed direct interband transition with the energy of 3.16 eV. It was shown that BiGeSbO6 has a wide transmission region in the visible and near-IR ranges. BiGeSbO6 is a self-activated phosphor, but its luminescence spectrum is atypical for Bi3+-containing oxides. The luminescent properties of Bi1-xEuxGeSbO6 solid solutions (x = 0.3, 0.5) were also studied.

The electronic structure and tunable emission of self-activated white-light-emitting Na2TiSiO5 phosphor

The exploration of rare-earth-free phosphors is desirable for white LEDs applications. In this work, Cr3+ doped Na2TiSiO5 self-activated phosphors were synthesized by conventional solid-state reaction. The electronic structure of Na2TiSiO5 was calculated using density functional theory (DFT) methods. The variations in crystal structure, luminescence properties and thermal stability of Na2Ti1-xSiO5: xCr3+ were exhaustively investigated by the analysis of XRD patterns, photoluminescence (PL) spectra, diffuse reflection spectra, PL decay curves and temperature-dependent PL spectra. Owing to the red shift of absorption edge and energy transfer between Cr3+ ions and Ti4+ ions, the emission intensity and thermal stability of Na2TiSiO5 were improved by the introduction of Cr3+ ions, and the peak position was red-shifted. The results indicate that the introduction of Cr3+ ions could be an effective method to regulate the luminescence of Ti4+-activated oxide phosphors.

K3Eu5(PO4)6: hydrothermal synthesis, crystal structure and photoluminescence properties.

An anhydrous orthophosphate, K3Eu5(PO4)6 (tripotassium pentaeuropium hexaphosphate), has been prepared by a high-temperature solid-state reaction combined with hydrothermal synthesis, and its crystal structure was determined by single-crystal X-ray diffraction analysis (SC-XRD). The results show that the compound crystallizes in the monoclinic space group C2/c and the structure features a three-dimensional framework of [Eu5(PO4)6]∞, with the tunnel filled by K+ ions. The IR spectrum, UV-Vis spectrum and luminescence properties of polycrystalline samples of K3Eu5(PO4)6, annealed at temperatures of 650, 700, 750, 800 and 850 °C, were investigated. Although with a full Eu3+ concentration (9.96× 1021 ions cm-3), the self-activated phosphor K3Eu5(PO4)6 shows s strong luminescence emission intensity with a quantum yield of 37%. Under near-UV light excitation (393 nm), the series of samples shows the characteristic emissions of Eu3+ ions in the visible region from 575 to 715 nm. The sample sintered at 800 °C gives the strongest emission and its lifetime sintered at 800 °C (1.88 ms) is also the longest of all.

Insights into anti-thermal quenching of photoluminescence from SrCaGa4O8 based on defect state and application in temperature sensing

Herein, a self-activated phosphor SrCaGa4O8 with excellent thermal stability was synthesized by high temperature solid state reaction method. It exhibited the broad emission range from 300 to 600 nm under 220 nm excitation, which may be caused by the intrinsic defects of host. Specifically, with the temperature increasing, the photoluminescence (PL) color changed from bluish to yellow-green and the maximum of PL intensity at 550 nm was obtained when reaching 493 K. From thermoluminescence curves and high temperature spectra, it was found that the anti-thermal quenching phenomenon could be dependent on the self-compensation of the carriers. Accordingly, the results of temperature-dependent intensity ratio and sensitivity show that SrCaGa4O8 may have potential in the optical temperature sensing.

Luminescence mechanism of self-activated phosphor Ca0.75Sr0.2Mg1.05Si2O6 and abnormal second concentration quenching phenomenon of Ca0.75Sr0.2Mg1.05Si2O6:Ce3+

Luminescence mechanism of self-activated phosphor Ca0.75Sr0.2Mg1.05Si2O6 (CSMS) originating from oxygen vacancy defects has been deeply investigated by regulating synthesis temperature. The results demonstrate that the luminescence comes from three different oxygen defects in the crystal structure. For Ce3+ doped CSMS (ICSD#68180), two concentration quenching phenomena can be observed at x = 0.03 and 0.12 as the result of the formation of new substance CaMgSi2O6 (ICSD#17043), which leads to the actual concentration of Ce3+in CSMS (ICSD#68180) is far less than the doping concentration since part of Ce3+ ions enter CaMgSi2O6 (ICSD#17043). The emission spectra of CSMS:xCe3+ shifts from 388 nm to 405 nm with increasing Ce3+ content, which mainly should be due to the 5d energy center of Ce3+ shifting down and the crystal field split.

Facile synthesis of self-activated oxyfluorotungstate phosphor with high QE and its thermal quenching

Self-activated phosphors could convert high energy light into visible light from the internal ion groups of host lattice before externally introducing luminescent ions. In this work, we propose a facile route to synthesize self-activated oxyfluorotungstate (Na2WO2F4) phosphor. Excited by the UV light, the emission spectrum covers nearly the entire visible light region. After careful analysis of the first principle calculation results and other sufficient investigation, it is believed that the efficient emission of Na2WO2F4 phosphor comes from the strong parity-allowed absorption of O-2p to W-5d charge transfer band. Moreover, the quantum efficiency of Na2WO2F4 phosphor reaches up to 76% at room temperature. However, the increase of temperature not only lowers the intensity of the PL emission and the quantum efficiency but gives rise to a blue shift of the emission maxima, due to the phonon interaction with the luminescence center in a vibrating crystalline environment.

Tunable whole visible region color emission, enhancing emission intensity and persistent performance of a self-activated phosphor:Na2CaSn2Ge3O12

The composition and tunable luminescence properties of garnet phosphors are flexible because of their unique A3B2C3O12 structure. Garnet phosphors with tailored luminescence properties can be designed by varying in the A, B, and C cation sublattices. In this research, the self-activated long-persistent phosphor (LPP) Na2CaSn2Ge3O12 (NCSG) was successfully synthesized by a solid-state method. A series of dopants (Mn2+, Pr3+, Tb3+, and Dy3+) were selected as single-component emitters to tailor the emission color of the NCSG phosphor. NCSG emits in yellow at ~ 560 nm and in the near-infrared at ~ 790 nm with long-persistent luminescence (PersL). By doping with these ions, the PersL of the NCSG was significantly improved, and various colors covering the entire visible region from violet to the near-infrared were obtained. The trap types and depths were examined by electron paramagnetic resonance (EPR) and the thermo-luminescence (TL) glow curves. The EPR signals revealed that oxygen vacancies are the trap centers. The TL glow curves showed that the intrinsic and extrinsic luminescence centers both act as charge carrier recombination centers. Furthermore, the PersL mechanism of the NCSG and NCSG:R (R = Mn2+, Pr3+, Tb3+ and Dy3+) phosphors was discussed in detail. The strategies employed in this work can be an effective way to fabricate new multi-wavelength, garnet-based LPPs with potential applications.