Up-conversion luminescence and temperature-sensing properties of Li+, K+ doped Na2Zn3Si2O8: Er3+ phosphors

In this work we detail the preparation of new luminescent Li⁺ and K⁺ doped Na₂Zn₃Si₂O₈: Er³⁺ up-conversion phosphors using the high-temperature solid-phase method. We investigate the phosphors phase structure, elemental distribution, up-conversion luminescence characteristics and temperature sensing properties. Our fabricated samples were found to be homogeneous and when excited using 980 nm light, they emitted wavelengths in the green and red visible wavelength bands, which correspond to two major emission bands of Er³⁺. Doping with Li⁺ and K⁺ increased the luminescence intensity of the Na₂Zn₃Si₂O₈: Er³⁺ phosphor at 661 nm by 36 and 21 times respectively. The highest relative temperature sensitivity (Sa) of the fabricated phosphor reached a value of 19.69% K^?1 and the highest absolute temperature sensitivity (Sr) reached 1.20% K^?1. These values are superior to other materials which utilize up-conversion by Er³⁺ ions as a tool for temperature sensing. We anticipate that these new phosphors will find significant application as components in optical temperature measurement systems.     

Enhanced up-conversion luminescence of Tb3+-Yb3+ doped glass ceramics by doping Li+

Tb³⁺-Yb³⁺ co-doped transparent glass ceramics (GCs) containing Y₂Ti₂O₇ crystal phases were synthesized by the melt crystallization. The light transmittance of GCs in the visible region reached 78%, and the average grain size was 278 nm under the optimal heat treatment conditions (720 °C/2 h). The GCs exhibited greater up-conversion luminescence intensity than precursor glass, and the reason for this result was explained in accordance with the Judd-Ofelt theory. Moreover, the introduction of Li⁺ did not change the crystalline phase of GCs. The emission intensity of the green light of the 8% Li ⁺ doped GCs was significantly enhanced by nearly 4.48 times under 980 nm excitation. The XRD refinement results suggest that the enhanced luminescence intensity is correlated with the change of the Y₂Ti₂O₇ crystal lattice caused by Li⁺ doping. The relevant luminescence mechanism was elucidated. The results suggest that Li⁺ doped transparent GCs open novel avenues for green UC applications.     

Cr3+‐activated Li5Zn8Al5Ge9O36: A near‐infrared long‐afterglow phosphor

Near‐infrared long‐afterglow (LAG) materials have attracted considerable attention owing to their high potential for in vivo imaging applications. Here, we present a series of near‐infrared LAG phosphors Li₅Zn₈Al_5?xGe₉O₃₆:xCr³⁺ (LZAG:Cr³⁺), which were synthesized using a solid‐state reaction method. The pure LZAG host exhibits blue photoluminescence and LAG emission. We investigated the effect of the zinc vacancy contents on the photoluminescence and LAG performance by adjusting the zinc content and introducing Ga³⁺ ions to substitute the Zn²⁺ sites in LZAG host. When Cr³⁺ ions were introduced into the LZAG host, LZAG:Cr³⁺ produced a strong, broad blue emission band centered at 456 nm and a near‐infrared emission band at 700 nm caused by the ²E → ⁴A₂ transition of Cr³⁺. The energy transfer processes from the LZAG host to Cr³⁺ were identified in the photoluminescence and LAG process. After irradiation at 258 nm for 10 minutes, the LAG emission of LZAG:0.008Cr³⁺ can last nearly 2.5 hours. Moreover, the LAG intensity and duration of LZAG: 0.008Cr³⁺ were significantly improved by introducing a small dose of Ga³⁺ ions. Finally, the traps and mechanism of LAG in LZAG, LZAG:Ga³⁺, and LZAG:Cr³⁺ were discussed in detail.     

Long persistent properties of CaGa2O4:Bi3+ at different ambient temperature

A yellow long persistent luminescence (LPL) phosphor of CaGa₂O₄:Bi³⁺ (CGO:Bi) is developed, of which the LPL can be visually recognized (0.32 mcd m^?2) over 36 hours after the removal of the excitation source. Furthermore, an excellent LPL performance above the room temperature and unusual thermal quenching behavior are observed. The thermoluminescence (TL) curves indicate that the continuity of the traps structure with different depths is a key factor in ensuring that carriers could be released from deep traps above room temperature successively. The work verifies unambiguously that the continuous defect‐state structure is beneficial to the LPL temperature resistant abilities, which would provide a reasonable approach to design new LPL phosphors in extreme environments.     

Ultra-sensitive low-temperature thermometer regulated by the crystal field strength

Developing novel optical thermometry with ultrahigh relative sensitivity and temperature resolution has become a cutting-edge topic. For this purpose, under obeying Boltzmann distribution, a series of Li₂Zn_0.9992-xA_xGe_3-yB_yO₈:0.08% Cr³⁺ (B= Sc³⁺, In³⁺, A = Si⁴⁺) phosphors were studied, which the luminescence intensity ratio between the transition of ⁴T_2g→⁴A_2g emission and the R line based on thermally coupled energy levels constitutes a temperature sensing work with a relative sensitivity of 9.46% K^?1, 9.73% K^?1, and 10.38% K^?1, respectively. It is worth mentioning that the luminescence intensity of the R line (peak 1) increases significantly with the increase of temperature, while the transition of ⁴T_2g→⁴A_2g (peak 2) with high intensity at low temperature gradually quenching, and this opposite trend is an important advantage for the design of excellent thermometers. Compared the best relative sensitivity of Li₂Zn_0.9992-xA_xGe_3-yB_yO₈:0.08%Cr³⁺ (B= Sc³⁺, In³⁺, A = Si⁴⁺) with the crystal field Dq/B, it can be concluded that relative sensitivity increasing gradually with decreasing the intensity of crystal field. Finally, by testing the stability of the sample at 50 K, a thermal resolution of 0.082 K, 0.080 K and 0.077 K was obtained, respectively, which is one of the best thermal resolutions so far, while the repeatability of the sample stability at 50 K and 300 K cycles was higher than 99%. Our work is expected to provide guiding insights for optimizing the sensitivity of Cr³⁺-based luminescence intensity ratio thermometers.     

Local charge regulation by doping Li+ in BaGa2O4:Bi3+ to generate multimode luminescence for advanced optical Morse code

In the field of advanced anti-counterfeiting research, it is a hot issue to develop a multimodal anti-counterfeiting material with adjustable luminescence characteristics. Here, persistent luminescent materials of BaGa₂O₄:xBi³⁺ (x = 0-0.02) and BaGa₂O₄:0.005Bi³⁺,yLi⁺ (y = 0.001–0.02) were synthesized by a solid state reaction at high temperature. BaGa₂O₄: Bi³⁺ exhibited a broad blue emission at ～470 nm (transition from [GaO₄]) and a sharp NIR emission at ～710 nm (³P₁→¹S₀ transition of Bi³⁺), upon UV excitation at 250 nm. Incorporation of Li⁺ in BGO: 0.005Bi³⁺ induced the emission color shifting from blue to green. After stoppage of UV excitation, the BGO:0.005Bi³⁺ exhibited white afterglow with emission peaks at the range of 500–700 nm. However, incorporation of Li⁺ leaded to a stronger green afterglow and a weaker NIR afterglow. When the afterglow disappeared, the sample outputted afterglow again after heating processing. The prepared samples exhibited time- and temperature-dependent multimode luminescence, so they were used as components, combined with Morse code to realize multi-modal dynamic anti-counterfeiting. The outcomes in this work indicate that the prepared luminescent materials have broad prospects in advanced anti-counterfeit applications.     

Orange-red persistent luminescence in Mn2+-Doped Sr3Y2Ge3O12 films for dynamic anti-counterfeiting

The research focus in the field of anti-counterfeiting has shifted toward the secure luminescent label technology, which has gained prominence. The exceptional luminescence properties of persistent luminescence materials are important in the achievement of effective anti-counterfeiting measures. It is in this context that Sr₃Y₂Ge₃O₁₂ can serve as an excellent matrix material. In this study, it is found that Sr₃Y₂Ge₃O₁₂: 4.00% Mn²⁺ exhibits a luminescence intensity, persistence time, and stability that meet the practical requirements for applications. It is shown that the fabricated persistent luminescence film can be used in anti-counterfeiting applications, thereby, demonstrating the possibility to broaden the application scope of persistent luminescence materials. An emission band centered at 634 nm and a high-energy inflection point at 578 nm are observed in the samples excited at 260 nm. Fluorescence emission at 634 nm is also achieved through X-ray excitation. Through thermoluminescence, photoluminescence, and phosphorescence spectroscopy, a comprehensive investigation of the trap characteristics and inherent mechanisms of the long persistent phosphorescence has been conducted. The novel orange-red Sr₃Y₂Ge₃O₁₂: Mn²⁺ phosphor is extensively examined.     

A novel far-red phosphors Li2ZnTi3O8:Cr3+ for indoor plant cultivation:Synthesis and luminescence properties

A novel far-red phosphors Li₂ZnTi₃O₈:Cr³⁺ were successfully synthesized via the conventional solid-state method. The structural characteristics, luminescence properties and concentration quenching of the Li₂ZnTi₃O₈:Cr³⁺ phosphors were investigated systematically. Under the excitation at 360 nm and 468 nm, the Li₂ZnTi₃O₈:Cr³⁺ phosphors displays the emission spectra in the range from 600 nm to 850 nm. The far-red emission centered at 735 nm was attributed to the spin-forbidden ²E→⁴A₂ transition of Cr³⁺ ions. The research results of this paper indicate that the phosphors Li₂ZnTi₃O₈:Cr³⁺ has prospective applications in indoor plant cultivation.     

Fabrication and optical properties of transparent fine-grained Zn1.1Ga1.8Ge0.1O4 and Ni2+ (or Cr3+)-doped Zn1.1Ga1.8Ge0.1O4 spinel ceramics

For the first time, a Zn_1.1Ga_1.8Ge_0.1O₄ transparent spinel ceramic has been fully densified by spark plasma sintering. XRD measurements show that this ceramic is composed of a pure cubic spinel phase. SEM analysis revealed a homogeneous and dense microstructure with the average grain size being 200 ± 100 nm. The transmittance of these fine-grained ceramics reached 70 % in the visible range and is very close to 80 % at 2 µm, thus close to the T_max value deduced from the measurement of the refractive index. The ceramics exhibit excellent mechanical properties with a Young modulus of 222 GPa, a Vickers hardness of 14.25 GPa and a thermal conductivity of 7.3 W.m⁻¹. K⁻¹. By doping with Cr³⁺ ions, transparent Zn_1.1Ga_1.8Ge_0.1O₄ ceramics present both a red luminescence and a long-lasting afterglow during several minutes. Moreover, a near infrared broadband emission at 1.3 µm is also achieved with Ni²⁺ ions.     

Role of oxygen vacancies in long persistent phosphor Ca2Ga2GeO7: Zn2+

Acting as the electron trap, oxygen vacancies can strongly influence the long persistent luminescence (LPL) properties were verified in Ca₂Ga₂GeO₇: Zn²⁺ (CGGZ) phosphor. The existence of oxygen vacancies in this self‐excitation material was confirmed by a simple fluorescence probe method. The introduction of zinc ions promotes the generation of oxygen vacancies, as well as optimizes its LPL properties. Thermoluminescence (TL) analysis revealed that electrons in deep trap will effectively transfer to the shallow one via the bridge of interstitial zinc. Accordingly, a mechanism for PL and LPL in this Gallogermanates was provided.     

Competitive Cr3+ occupation in persistent phosphors toward tunable traps distribution for dynamic anti-counterfeiting

Red/near infrared (NIR) long persistent phosphors have received extensive attentions in biomedical, food inspection, iris recognition, biological imaging, etc. Herein, a new phosphor, Li₂ZnGe₃O₈:Cr³⁺, is reported with deep red persistent luminescence peaking at 708 nm. By adjusting the Cr³⁺ doping concentration, the competitive site occupation at [ZnO6] and [GeO6] polyhedral enables different traps behaviors including trap types, trap concentration and trap depth, which in turn leads to different afterglow duration time from 2 to 20 h. The persistent luminescence mechanisms originated from different trap models have been discussed, and it is found that they can cooperate or inhibit each other, enabling different luminescence depending on time. The dynamic anti-counterfeiting applications have been demonstrated, which provides a new way to rationally designing for multi-functional luminescent materials.     

Defect cluster engineering in ZnGa2−x(Mg/Ge)xO4:Cr3+ nanoparticles for remarkably improved NIR persistent luminescence

Recently, study on Cr³⁺-doped zinc gallate and zinc gallogermanate persistent phosphors has become a hot topic in persistent luminescence and bio imaging areas, because of their near infrared (NIR) emission and long afterglow. However, regulation of efficient traps and improvement of persistent luminescence through bottom-up design are the key challenge. Here, we recommend a new paradigm of chemical unit co-substitution with [Mg²⁺-Ge⁴⁺] substituting for [Ga³⁺-Ga³⁺] in ZnGa₂O₄, which contributes to the opposite charged and distorted octahedral defects of Mg_Ga′ and Ge_Ga ^· in pair around the Cr_N2 ions. The formed defect clusters of Mg_Ga′-Cr_N2-Ge_Ga ^· , which are closely related to the trap depth, can be accurately regulated through varying the doping content of Mg²⁺/Ge⁴⁺ in the resulting spinel solid solutions of ZnGa_2−x(Mg/Ge)_xO₄:Cr³⁺ (x = 0–1.25). Moreover, the defect clusters cannot only store and recharge visible and UV radiations that contributes to the long lasting NIR persistent luminescence but also can enhance the NIR emission intensity at ~695 nm. The persistent luminescence induced by UV light excitation exhibits an improvement at a deeper trap depth, but it follows an opposite law through visible light excitation. The prepared nanoparticles have the advantages of intense NIR emission, long lasting afterglow, and excellent rechargeability for visible/UV radiations, so they are the potential nanoprobes for long-term bio imaging in living animals.     

Trap engineering in ZnGa2O4:Tb3+ through Bi3+ doping for multi-modal luminescence and advanced anti-counterfeiting strategy

Optical information encryption based on luminescence materials have received much attention recently. However, the single luminescence mode of the luminescence materials greatly limits its anti-counterfeiting application with high safety level. Here, a series of luminescence materials of Tb³⁺ and Bi³⁺ co-doped ZnGa₂O₄ phosphors with great correspondence in photoluminescence (PL), persistent luminescence (PersL), and thermoluminescence (TL) modes was synthesized by the conventional solid-phase method for the application in multi-modal anti-counterfeiting fields. Under the excitation of 254 nm, ZnGa_1.99O₄:0.01 Tb³⁺, yBi³⁺ (y = 0.001,0.002) sample exhibited a broad blue emission band (the transition from [GaO₆]) at 440 nm and the characteristic emission peaks of Tb³⁺ at 495 nm, 550 nm, 591 nm and 625 nm, corresponding to the transitions of ⁵D₄-⁷F_n (n = 6, 5, 4, 3), respectively. Interestingly, the co-doping of Bi³⁺ ions improve the crystallinity and particle size of the phosphor, subsequently enhanced the PL intensity of Tb³⁺ to 6 times that of Tb³⁺ singly doped ZnGa₂O₄ phosphor. Further, the flexible films with multi-modal luminescence properties have been fabricated through the unique TL and PersL characteristics of ZnGa₂O₄: Tb³⁺, Bi³⁺ phosphors, including “Optical information storage film”, “snowflake and characters” and “QR code”. Moreover, a set of optical information encryption is obtained by combining ZnGa₂O₄:Tb³⁺, Bi³⁺ phosphor and red emitting phosphor. The results indicate that ZnGa₂O₄:Tb³⁺, Bi³⁺ phosphor with multi-modal stimulus response can be expected to be potentially used in the applications of optical information storage and anti-counterfeiting fields.     

Effect of Li+ Ions Doping on Microstructure and Upconversion Emission of Y2Ti2O7:Er3+/Yb3+ Nanophosphors Synthesized Via a Sol–Gel Method

Er³⁺/Yb³⁺/Li⁺‐tridoped Y₂Ti₂O₇ nanophosphors were synthesized via a facile sol–gel process. The samples were characterized by the inductively coupled plasma atomic emission spectrometer (ICP‐AES), X‐ray diffraction (XRD), transmission electron microscopy (TEM), and infrared‐to‐visible upconversion (UC) luminescence spectra. XRD analysis showed that the crystallization temperature of pyrochore‐type Y₂Ti₂O₇ was reduced due to the flux effect of Li⁺ ions, whereas TEM measurements confirmed that the particles size of (Y_0.815Er_0.01Yb_0.075Li_0.10)₂Ti₂O₇ was about 30–40 nm when calcining at 800°C for 1.0 h. The calcining temperature and Li⁺ ion concentration dependence on UC luminescence spectra were investigated. It was found that, when incorporating 10.0 mol% Li⁺ ion, the UC red and green emission intensity was drastically increased by a factor of 18.6 and 8.3, respectively. The enhancement of UC emission may be mainly attributed to the modification of local symmetry around Er³⁺ ions by tridoping Li⁺ ions. And also, the pump power dependence of the emission intensity was investigated to understand the fundamental UC mechanism.     

Li+ enrichment to improve the microwave dielectric properties of Li2ZnTi3O8 ceramics and the relationship between structure and properties

Herein, Li⁺-enriched Li_(1+x)2ZnTi₃O₈ ceramics are prepared via the solid-phase methods. As x increases, the unit cell volume gradually increases, while the grain size initially increases and then decreases gradually. The Li_(1+0.06)2ZnTi₃O₈ ceramics exhibit the best dielectric properties: ε_r = 25.92, Q × f = 109534 GHz (@7.37 GHz, which is a 48 % increase compared with the stoichiometric counterpart.), and τ_f = ?8.21 ppm/°C. The complex chemical bond theory and Raman spectroscopy reveal that Ti-O bonds have a significant effect on the dielectric properties. An optimal Li⁺ enrichment leads to an overall reduction in the distortion of the Li/ZnO₄ tetrahedra, resulting in a reduction in τ_f. First-principles calculations demonstrate that a suitable excess of Li⁺ leads to an increase in the band-gap as well as an enhanced electron cloud density in the internal space of the Li1/ZnO₄ tetrahedra, thereby increasing the Q × f. In summary, Li⁺-enriched Li_(1+0.06)2ZnTi₃O₈ ceramics are promising for a wide array of applications in microwave communications.     

Exploring the luminescent thermometer and optical storage applications in negative thermal quenching phosphor of BaSc2Ge3O10:Pr3+

The optical contactless thermometer demonstrates remarkable advantages over traditional contact thermometers. However, maintaining a strong optical signal output across a wide temperature range poses a challenge due to the thermal quenching effect. In this study, we aimed to overcome the limitation of low signal intensity in high-temperature applications by developing a Pr³⁺-doped BaSc₂Ge₃O₁₀ (BSGO) phosphor with anti-thermal quenching properties. This phosphor was designed to ensure a stable and sufficiently strong light signal for accurate temperature measurements. The phosphor exhibited a negative thermal quenching effect, where the integrated emission intensity spanning from 450 nm to 800 nm increased from 173 K to 253 K, and the intensity at 353 K still remained stronger than at 173 K. Multiple 4f-4f transition emissions were observed in the phosphor, and the fluorescence intensity ratio (FIR) was utilized as a standard parameter for temperature evaluation ranging from 82 K to 353 K. At 82 K, the phosphor achieved maximum relative sensitivity (S_r) of 2.3% K⁻¹. The presence of multiple traps with depths ranging from 1.09 eV to 1.50 eV not only supported luminescence with a negative thermal quenching effect but also resulted in long persistent luminescence (LPL), which was systematically investigated in this study.     

A broadband-sensitive upconverter: Garnet-type Ca3Ga2Ge3O12 codoped with Er3+, Y3+, Li+, Ni2+, and Nb5+

We have developed a new broadband-sensitive photon upconversion (UC) material that can be used for transparent ceramic plates mounted on the rear faces of crystalline silicon solar cells. We selected the host material of a cubic crystal structure codoped with Er³⁺ and Ni²⁺ so that the Ni²⁺ dopants were fully activated to sensitize the Er³⁺ emitters. In garnet-type Ca₃Ga₂Ge₃O₁₂ with additional codopants of Nb⁵⁺ and Li⁺ for charge compensation, all the Ni²⁺ dopants occupied the six-coordinated Ga³⁺ sites, leading to highly efficient energy transfer from the Ni²⁺ to the Er³⁺. Formation of four-coordinated Ni²⁺ that quenches the UC emission of the Er³⁺ was prevented, because Ni²⁺ cannot substitute the four-coordinated Ge⁴⁺ much smaller than Ni²⁺. Consequently, energy dissipation from the Er³⁺ to the Ni²⁺ was well reduced compared with the previously developed Gd₃Ga₅O₁₂:Er,Ni,Nb in which the Ni²⁺ dopants partially occupied the four-coordinated Ga³⁺ sites. Additional introduction of Y³⁺ and Li⁺ enhanced optical transitions and improved the UC performance, owing to more enhanced lattice distortion, along with eliminating different phases. The optimal composition (Ca_0.6Er_0.1Y_0.1Li_0.2)₃(Ga_0.98Ni_0.01Nb_0.01)₂Ge₃O₁₂ exhibited a broadband sensitivity ranging from 1.1 μm (the absorption edge of silicon) to 1.6 μm for the UC emission at 0.98 μm.     

Visible green upconversion luminescence of Er3+/Yb3+/Li+ co-doped CaWO4 particles

The upconversion (UC) luminescence of Li⁺/Er³⁺/Yb³⁺ co-doped CaWO₄ phosphors is investigated in detail. Single crystallized CaWO₄:Li⁺/Er³⁺/Yb³⁺ phosphor can be obtained, co-doped up to 25.0/5.0/20.0 mol% (Li⁺/Er³⁺/Yb³⁺) by solid-state reaction. Under 980 nm excitation, CaWO₄:Li⁺/Er³⁺/Yb³⁺ phosphor exhibited strong green UC emissions visible to the naked eye at 530 and 550 nm induced by the intra-4f transitions of Er³⁺ (²H_11/2,⁴S_3/2→⁴I_15/2). The optimum doping concentrations of Yb³⁺/Li⁺ for the highest UC luminescence were verified to be 10/15 mol%, and a possible UC mechanism that depends on the pumping power is discussed in detail.     

The effect of Li+ incorporation in Yb3+-Nd3+ co-doped CaF2 phosphors over the NIR photoluminescence emission excited under visible light

The structural and optical characteristics of Nd³⁺-Yb³⁺ doped CaF₂ phosphors with and without the addition of Li⁺ ions are described in this work. The phosphors synthetized by hydrothermal and co-precipitation methods showed near-infrared (NIR) luminescence emission associated with inter-electronic transition of the Yb³⁺ ion in the range of 900–1050 nm via energy transfer process from Nd³⁺ ions under visible light excitation. The addition of Li⁺ to these phosphors resulted in an improvement of the NIR luminescence intensity by a factor up to 5. The effect of the incorporation of Li⁺ ions into the CaF₂ crystallite structure, the reduction of luminescence quenching states, as well as the energy transfer mechanism involved are discussed.     

Enhanced photoluminescence of SiO2 coated CaTiO3:Dy3+,Li+ nanophosphors for white light emitting diodes

Systematic analysis of SiO₂@CaTiO₃:Dy³⁺_{(3 mol %)}:Li⁺_{(0.25–1 mol %)} core shell nanoparticles (C–S NPs) were carried out. CaTiO₃:Dy³⁺ _{(3 mol %)}, Li⁺ _{(0.25–1 mol %)} NPs were prepared using low temperature solution combustion method. C–S structured samples were synthesized using SiO₂ in equal weight quantities. All samples were characterised by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) in order to study and compare its structural properties. Photoluminescence (PL) properties of uncoated and SiO₂ coated samples were studied in detail and optimum luminescence behaviour was investigated. Higher luminescence intensity is observed for codoped CaTiO₃:Dy³⁺_{(3 mol %)}:Li⁺_{(0.5 mol %)} sample. Further enhancement was observed in SiO₂@CaTiO₃:Dy³⁺:Li⁺ structured nanoparticles. Emission spectra of SiO₂@CaTiO₃:Dy³⁺_{(3 mol %)}:Li ⁺_{(0.25–1.0 mol %)} C–S nanophosphors exhibit white emission dominated by ⁴F_9/2 → ⁶H_13/2 (570 nm) transition of Dy³⁺ ions under. All the results suggested that the present material provides a platform for fabricating new functional materials which can be applied in the field of wLEDs and solid state display applications.