Elaboration,Structure, and Intense Upconversion in Transparent KYb2F7:Ho3+ Glass‐Ceramics

Novel transparent oxyfluoride glass‐ceramics containing KYb₂F₇:Ho³⁺ nanocrystals were successfully elaborated by melt‐quenching technique with further thermal treatment for the first time. Their structural and luminescent properties were investigated systemically by XRD, HRTEM, absorption spectra, upconversion spectra, and lifetime measurements. Under 980 nm laser excitation, all samples exhibited characteristic emissions of Ho³⁺. Attractively, the upconversion emissions of Ho³⁺, especially green and near‐infrared emissions, were enormously enhanced 190 times after crystallization. The incorporation of Ho³⁺ into KYb₂F₇ nanocrystals with lower phonon energy was responsible for this phenomenon. Our research may enrich the understanding of fluoride‐nanocrystals‐based transparent oxyfluoride glass‐ceramics.     

Effect of lanthanide doping on crystal phase and near-infrared to near-infrared upconversion emission of Tm doped KF–YbF3 nanocrystals

Tm³⁺ doped KF–YbF₃ nanocrystals were synthesized by a hydrothermal method using oleic acid as a stabilizing agent at 190 °C. The influence of Gd³⁺ and Sm³⁺ content on the phase structure and upconversion (UC) emission of the final products was investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and UC spectra. XRD analyses and TEM observations evidence that the phase and size of the as prepared Tm³⁺ doped KF–YbF₃ nanocrystals are closely related to the Gd³⁺ doping content. Without Gd³⁺ impurity, the undoped nanocrystals crystallize in orthorhombic KYb₂F₇ with an average diameter of 42 nm. When the Gd³⁺ doping is below 10 mol%, the orthorhombic KYb₂F₇ nanocrystals grow up. However, with Gd³⁺ addition beyond about 30 mol%, the complete phase transformation from orthorhombic KYb₂F₇ to cubic KGdF₄ occurs in the final products. Under the excitation of a 980 nm laser diode, the as prepared Tm³⁺ doped nanocrystals exhibit strong near-infrared UC emission at 800 nm. Particularly, the intensity of high energy UV and blue UC emissions of Tm³⁺ ions in Tm³⁺ doped KYb₂F₇ nanocrystals are selectively reduced compared to the NIR emission at 800 nm by co-doping a small amount of Sm³⁺ ions into the host matrix. Possible dynamic processes for UC emissions in Tm³⁺ doped nanocrystals are discussed in detail.     

Energy transfer mechanism and new ratiometric thermometry strategy by the blue and yellow emissions of Dy

In this paper, we report the energy transfer mechanism between Dy³⁺ ions as well as a new optical ratiometric thermometry strategy in Dy³⁺ doped CaWO₄ (CaWO₄:Dy) phosphors. The optimal Dy³⁺ doping concentration was determined and the electric dipole-dipole interaction among Dy³⁺ was investigated based on the analysis of the concentration-dependence luminescent properties in CaWO₄:Dy phosphors. The photoluminescence (PL) of two blue and two yellow emissions produced by Dy³⁺ transitions from the thermally coupled energy levels (TCLs) of ⁴I_15/2/⁴F_9/2 to the ⁶H_15/2 ground level and ⁴I_13/2 metastable level, respectively, exhibited opposite PL behavior with increasing temperature. The luminescence intensity ratio (LIR) between blue and yellow emissions (B/Y) was discovered to have a quantitative relationship with temperature based on the classic thermalized population distribution of TCLs. In comparison to traditional thermometry based on the LIR of two blue emissions, the proposed new ratiometric thermometry based on the LIR of B/Y demonstrated good optical thermometry behavior with extended temperature detection range to cryogenic region, excellent absolute sensitivity, good repeatability, and high accuracy, which can also be applied to other Dy³⁺ doped materials such as Y₃Al₅O₁₂ and NaYF₄ phosphors using either down- or up-conversion PL of blue and yellow emissions.     

Elaboration,Structure, and Luminescence of Eu3+‐Doped BaLuF5‐Based Transparent Glass‐Ceramics

Novel Eu³⁺‐doped transparent oxyfluoride glass‐ceramics containing BaLuF₅ nanocrystals were successfully fabricated by melt‐quenching technique for the first time. Analyses of XRD patterns prove that the new precipitated glass‐ceramics are crystallized in cubic BaLuF₅ based on isostructural BaGdF₅. Intense red emissions observed in glass ceramics are attributed to the enrichment of Eu³⁺ ions into BaLuF₅ nanocrystals. Besides, obvious stark splitting emissions, low forced electric dipole ⁵D₀→⁷F₂ transition, and long decay lifetimes of Eu³⁺ ions also evidence the partition of Eu³⁺ ions into BaLuF₅ nanocrystals with low phonon energy. Such transparent material may find applications in photonics.     

Multifunctional α-NaYbF4:Tm3+ nanocrystals with intense ultraviolet self-sensitized upconversion luminescence and highly efficient optical heating

Lanthanide-doped upconversion photoluminescent nanoparticles with unique anti-Stokes spectroscopic properties excel in many fields of application. Ytterbium-based self-sensitized fluorides with rich Yb³⁺ possess higher absorption efficiency of incident near infrared laser, and are more favorable for photoluminescence or optical heating applications. In this work, α-NaYbF₄:Tm³⁺ crystalline nanoparticles are synthesized, which exhibit intense ultraviolet self-sensitized upconversion photoluminescence and highly efficient optical heating capability under 980 nm laser excitation. NaYbF₄:Tm³⁺ nanocrystals emit multi-band luminescence with emission peaks located in the ultraviolet, blue and red spectral regions. The energy transfer mechanism and electronic transition pathways for the Upconversion luminescence are investigated based on the energy level scheme, and are further confirmed by luminescent dynamic analysis. Due to cross-relaxations between Tm³⁺ and energy back transfer from Tm³⁺ to Yb³⁺ processes, the NaYbF₄: 1 mol% Tm³⁺ nanoparticles possess the highest luminescence intensity. The luminescent dynamic characteristics, such as decay time and rise time, vary with Tm³⁺ doping concentrations. Highly efficient optical heating effect is observed in the NaYbF₄:Tm³⁺ nanoparticles with slope efficiency of photothermal conversion for 10 s laser irradiation is as high as 100.48 °C/W.     

Color-tunable and white emission of Tm3+ doped transparent zinc silicate glass-ceramics embedding ZnO nanocrystals

Tm³⁺ doped zinc silicate glass-ceramics composed of SiO₂-Al₂O₃-ZnO-K₂O-Tm₂O₃ embedded with ZnO nanocrystals were successfully fabricated by melt-quenching method with subsequent heat treatment. Tm³⁺ ions and ZnO nanocrystals were introduced as blue and yellow luminescence centers, respectively. The effects of heat treatment, excitation wavelength and Tm³⁺ doping concentration on the photoluminescence behaviors of these glass-ceramics were studied. Short-time (5 minutes) heat treatment was considered as the optimal heat treatment time, which facilitates simultaneously emitting narrow blue peak located at 453 nm and a broad yellow band centered at 580 nm. Blue and yellow emissions could be attributed to the ¹D₂ → ³F₄ transition of Tm³⁺ and Zn_i/O_i-related defect emission of ZnO nanocrystals, respectively. The combination of these two emissions allows the realization of white light emitting in the glass-ceramic samples. Furthermore, tunable luminescent color and chromaticity coordinates, including yellow, white and blue, can be realized by varying the pumping wavelengths as well as the content of Tm³⁺ dopant in the glass matrix. Nearly perfect white light emission with Commission Internationale de l'Eclairage coordinate (x = 0.33, y = 0.32) was achieved for the 0.05 mol% Tm³⁺ doped glass-ceramic embedding ZnO nanocrystals by heat treatment at 750°C for 5 minutes under the excitation of 360 nm. These luminescent glass-ceramics doped with Tm³⁺ ion and ZnO nanocrystals could be a promising candidate for white light emitting devices under near-ultraviolet excitation.     

Effect of silica coating on the structural and luminescent properties of Sm3+/Yb3+ or Tm3+/Yb3+ co-doped TiO2 nanoparticles

The luminescent characteristics of spherical titanium dioxide (TiO₂) nanoparticles (NP's) doped with Sm³⁺/Yb³⁺ and Tm³⁺/Yb³⁺ with and without a silica coating were analyzed. These nanoparticles were synthesized using the spray pyrolysis technique and coated with silica through a wet chemical process. The Sm³⁺/Tm³⁺ and Yb³⁺ doping induces a triphasic poly-crystalline structure of rutile and anatase TiO₂ and a Sm₂Ti₂O₇/Tm₂Ti₂O₇ cubic phase. A Williamson-Hall analysis was used to monitor the tensions of the NP's crystallites at the various doping concentrations and with addition of the silica shell. The luminescent spectra presented the characteristic emission peaks for the electronic energy levels transitions of the Sm³⁺/Tm³⁺ and Yb³⁺ ions. The Sm³⁺/Yb³⁺ co-doped NP's showed a maximum emission peak in the visible region at 612 nm, associated with ⁴G_5/2 → ⁶H_7/2 transitions of the Sm³⁺ ions. The IR emission peak at 973 nm (²F_5/2 → ²F_7/2) pertaining to Yb³⁺. For the combination of Tm³⁺/Yb³⁺, two emissions associated with Tm³⁺ ions were observed at 440 nm (¹D₂ → ³F₄) and 806 nm (³H₄ → ³H₆). The emission at 973 nm (²F_5/2 → ²F_7/2) is correlated to the Yb³⁺ ions. Silica coating of the NP's resulted in luminescence emission intensity increase of about 4 times.     

Upconverting luminescence based dual-modal temperature sensing for Yb3+/Er3+/Tm3+: YF3 nanocrystals embedded glass ceramic

Yb³⁺/Er³⁺/Tm³⁺ doped transparent glass ceramic containing orthorhombic YF₃ nanoparticles was successfully synthesized by a melt-quenching method. After glass crystallization, tremendously enhanced (about 5000 times) upconversion luminescence, obvious Start-splitting of emission bands as well as long upconversion lifetimes of Er³⁺/Tm³⁺ confirmed the incorporation of lanthanide activators into precipitated YF₃ crystalline environment with low phonon energy. Furthermore, temperature-dependent upconversion luminescence behaviors of glass ceramic were systematically investigated to explore its possible application as optical thermometric medium. Impressively, both fluorescence intensity ratio of Er³⁺: ²H_11/2 → ⁴I_15/2 transition to Er³⁺: ⁴S_3/2 → ⁴I_15/2 one and fluorescence intensity ratio of Tm³⁺: ³F_2,3 → ³H₆ transition to the combined Tm³⁺: ¹G₄ → ³F₄/Er³⁺: ⁴F_9/2 → ⁴I_15/2 ones were demonstrated to be applicable as temperature probes, enabling dual-modal temperature sensing. Finally, the thermal effect induced by the irradiation of 980 nm laser was found to be negligible in the glass ceramic sample, being beneficial to gain intense and precise probing signal and detect temperature accurately.     

Tm<Superscript>3+</Superscript>-Activated Transparent Oxyfluoride Glass Ceramics: A Study by Raman Scattering of the Nanocrystal Size Distribution

Absorption and emission spectroscopy of Tm³⁺ dopants in ultratransparent oxyfluoride glass ceramics indicates that most of the active dopants have been incorporated into the nanocrystals. The size of the nanocrystals has been estimated based on experimental studies of polarized and depolarized low-frequency Raman scattering of the Tm³⁺-doped glass ceramics in the range 0.5–100 cm⁻¹. Symmetric and quadrupolar acoustic vibrations have been observed and are ascribed to the respective modes of the nanoparticles, which are argued to be β-PbF₂ nanocrystals. The size distribution of the nanoparticles has been deduced from the shape of the acoustic bands. The Raman data show that the mean size of the PbF₂ nanocrystals increases and that the width of the size distribution decreases with the time and temperature of the heat treatment.Original English Text Copyright © 2005 by Fizika i Khimiya Stekla, Mattarelli, Montagna, Rossi, Ferrari, Tikhomirov, Seddon.This article was submitted by the authors in English.     

Nonlinear negative transmittance at a CW 980‐nm laser diodes pumping in Yb3+:CaF2 nanocrystals‐embedded glass ceramics

Cooperative upconversion luminescence (CUCL) occurs in spectral regions in which single ions do not have energy levels. However, all results reported so far are concentrated on luminescence properties from Yb³⁺ ions‐doped various hosts. Here, we report the observation of nonlinear negative transmittance (NNT) at continuous‐wavelength (CW) 980‐nm laser diodes (LDs) pumping in silicate oxyfluoride glass ceramics (GCs)‐containing CaF₂:Yb³⁺ nanocrystals. The unique optical nonlinearity is analyzed based on energy‐level transitions, dynamic evolution, rate equation, and power transmission equation, which can be explained as the cooperative optical absorption for the intense CUCL of Yb³⁺ ions. The NNT in the CaF₂:Yb³⁺ nanocrystals‐embedded GCs can be tailored with the power of a CW 980‐nm LDs, which possesses potential for the development of future optical limiters and switches.     

Yttrium-dopants-induced phase-controllable and luminescence-tunable lanthanide-doped α/β-NaYbF4 nanocrystals in glass for laser-driven upconverted lighting

In-situ glass crystallization is an effective way to integrate lanthanide-doped upconversion nanocrystals into amorphous glass, producing bulky nanocomposites for specific applications. However, the precipitation of hexagonal NaYbF₄ nanocrystals from glass is rarely reported owing to the preferred crystallization of cubic phase. Herein, phase-controllable in-situ crystallization of cubic/hexagonal NaYbF₄ nanocrystals in glass is achieved via Y³⁺ doping. A series of structural and spectroscopic characterizations evidence the formation of Na(Yb/Y)F₄ solid-solution nanocrystals in glass and the incorporation of lanthanide emitting centers (Er³⁺, Tm³⁺, Eu³⁺) into the crystalline lattice. Introducing high-content Y³⁺ dopants is beneficial to induce the crystallization of hexagonal NaYbF₄ phase in glass and enables elaborated tunability of Er³⁺ upconversion emissive colors. Finally, a prototype upconverted solid-state-lighting device is constructed by coupling 980 nm laser with the as-prepared Er/Tm doped monolithic glasses containing Na(Yb/Y)F₄ nanocrystals, producing bright upconversion white-light with luminous efficiency of 1 lm/W and energy efficiency of 0.5%.     

Energy transfer,tunable emission and optical thermometry in Tb3+/Eu3+ co-doped transparent NaCaPO4 glass ceramics

Tb³⁺/Eu³⁺ co-doped glass ceramics containing NaCaPO₄ nanocrystals were successfully synthesized via traditional melt-quenching route with further heat-treatment and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and photoluminescence spectroscopy. The energy transfer process of Tb³⁺→Eu³⁺ was confirmed by excitation and emission spectra and luminescence decay curves, and the energy transfer efficiency was also estimated. The results indicated that the efficient emission of Eu³⁺ was sensitized by Tb³⁺ under the excitation of 378 nm, realizing tunable emission in the transparent bulk glass ceramics containing NaCaPO₄ nanocrystals. Furthermore, optical thermometry was achieved by the fluorescence intensity ratio between Tb³⁺:⁵D₄→⁷F₅ (~542 nm) and Eu³⁺:⁵D₀→⁷F₂ (~612 nm). The maximum absolute sensitivity of 4.55% K⁻¹ at 293 K and the maximal relative sensitivity of 0.66% K⁻¹ at T=573 K for Tb³⁺/Eu³⁺ co-doped transparent NaCaPO₄ glass ceramic are obtained. It is expected that the investigated transparent NaCaPO₄ glass ceramics doped with Tb³⁺/Eu³⁺ have prospective applications in display technology and optical thermometry.     

Distribution of Tm3+ and Ni2+ in chalcogenide glass ceramics containing Ga2S3 nanocrystals: Influence on photoluminescence properties

The distribution of Tm³⁺ and Ni²⁺ ions is unambiguously exhibited in 80GeS₂-20Ga₂S₃ chalcogenide glass ceramics (GCs) containing Ga₂S₃ nanocrystals (NCs) by using advanced analytical transmission electron microscopy. Distinctively different distribution patterns of Tm³⁺ and Ni²⁺ ions are observed in the GCs obtained by controlled crystallization. The distribution of the dopants imposes strong influence on their optical properties which are revealed by absorption and photoluminescence (PL) spectra. Detailed discussions are given of the mechanisms of the crystallization-induced PL enhancement and quenching of the Tm³⁺ mid-infrared and Ni²⁺ near-infrared emissions, respectively.     

Optical Thermometry Based on Up‐Conversion Luminescence Behavior of Er3+ ‐Doped Transparent Sr2YbF7 Glass‐Ceramics

Transparent novel glass‐ceramics containing Sr₂YbF₇:Er³⁺ nanocrystals were successfully fabricated by melt‐quenching technique. Their structural and up‐conversion luminescent properties were systemically investigated by XRD, HRTEM, and a series of spectroscopy methods. The temperature‐dependent up‐conversion spectra prove that ²H_11/2 and ⁴S_3/2 levels of Er³⁺ are thermally coupled energy levels (TCEL). Consequently, the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 emissions of Er³⁺ in Sr₂YbF₇:Er³⁺ glass‐ceramics can be used as optical thermometry based on fluorescence intensity ratio (FIR) technique. Combined with low phonon energy and high thermal stability, Er³⁺ ions in Sr₂YbF₇ glass‐ceramics present broad operating temperature range (300–500 K), large energy gap of TCEL (786 cm^?1) and high theoretical maximum value of relative sensitivity (62.14 × 10^?4 K^?1 at 560 K), which suggests that Sr₂YbF₇:Er³⁺ glass‐ceramics may be excellent candidates for optical temperature sensors.     

Blue Upconversion Emission in Germanate Glass Co-Doped with Yb3+/Tm3+ Ions

In the paper, the upconversion luminescence of 70GeO₂–30[Ga₂O₃–BaO–Na₂O] glass system co-doped with Yb³⁺/Tm³⁺ ions was investigated. Strong blue emission at 478 nm corresponding to the transition ¹G₄ → ³H₆ in thulium ions was measured under the excitation of 976-nm diode laser. The dependence of the upconversion emission upon the thulium ion concentration was studied to determine the optimal conditions of energy transfer between energy levels of active dopants. The most effective energy transfer Yb³⁺ → Tm³⁺ was obtained in glass co-doped with molar ratio of dopant 0.7 Yb₂O₃/0.07 Tm₂O₃. The increase in thulium concentration more than 0.07 mol% results in the reverse energy transfer from Tm³⁺ → Yb³⁺, which leads to rapid quenching of the luminescence line at the wavelength 478 nm. In germanate glass co-doped with 0.7Yb₂O₃/0.07Tm₂O₃, the longest lifetime of ¹G₄ level equal 278 μs was achieved. The presented results indicate that elaborated germanate glass co-doped with Yb³⁺/Tm³⁺ ions is a promising material that can be used to produce fiber lasers and superluminescent fiber sources generating radiation in the visible spectrum.     

Luminescent properties of Sr2B2O5: Tm,Na blue phosphor

A novel blue phosphor, Sr₂B₂O₅: Tm³⁺, Na⁺ for white light-emitting diodes (W-LEDs) was prepared by solid-state synthesis and its structure and luminescence properties were investigated. This phosphor can be effectively excited within the broad near ultraviolet (NUV) wavelength region, from 340 nm to 370 nm, and exhibits a satisfactory blue performance. The emission peaks are observed at 457 nm (blue) and 475 nm (blue), due to the respective transitions of ¹D₂→³F₄ and ¹G₄→H₆. Seven mole percent of doping concentration of Tm³⁺ was shown to be optimal. Concentration quenching occurs when Tm³⁺ concentration is beyond 7 mol%, its mechanism being explained by dipole–dipole interaction of Tm³⁺ and being confirmed by decay property measurements. We have made a deep analysis on the effect of charge compensation reagent on luminescence intensity. Good blue emissions with the CIE chromaticity coordinates (0.173, 0.165) could be achieved. Our results suggest that the Sr₂B₂O₅: Tm³⁺, Na⁺ phosphor is a potential blue-emitting material.     

Ce3+ induced broadband enhancement around 2 μm in Tm3+/Ho3+ co-doped tellurite glass

Tellurite glasses doped with Tm³⁺, Ho³⁺ and Ce³⁺ ions were prepared via melt-quenching to realise broadband and fluorescence enhancement in near-infrared (NIR) band. Under the pumping of a commercial 808 nm laser diode (LD), the emission bands at 2.0 μm, 1.85 μm, 1.47 μm, and 705 nm were observed in the Tm³⁺/Ho³⁺ co-doping glass samples, which originated from the transitions of Ho³⁺:⁵I₇→⁵I₈ and Tm³⁺:³F₄→³H₆, ³H₄→³F₄, ³F_2,3 → ³H₆, respectively. The existence of 2.0 μm band fluorescence is due to the energy transfer from the Tm³⁺:³F₄ level to the Ho³⁺:⁵I₇ level. This band overlaps with the 1.85 μm band which forms a broadband fluorescence spectrum in the range of 1600–2200 nm. In glass samples co-doped with Tm³⁺/Ho³⁺ with 0.085 mol% Ho₂O₃ and 1 mol% Tm₂O₃, the full width at half maximum (FWHM) of this broadband spectrum (1600–2200 nm) was as high as ∼370 nm. After introducing 0.6 mol% CeO₂, the emission intensity of broadband fluorescence increased by ∼50%, which was caused by the cross-relaxations between Ce³⁺ and Tm³⁺ ions. The lifetime of fluorescence decay was determined to prove the interactions among the doped rare-earth ions, the radiative parameters such as transition probability, branching ratio and radiative lifetime were calculated from the absorption spectra based on the Judd-Ofelt theory to better understand the observed luminescence phenomena. In addition, X-ray diffraction (XRD) confirmed the amorphous state structure of the synthesised glass samples, while Raman spectrum revealed the different vibrational structural units forming the glass network.     

A Single‐Phase Phosphor NaLa9 (GeO4)6O2: Tm3+, Dy3+ for Near Ultraviolet‐White LED and Field‐Emission Display

Single‐phase white‐light‐emitting phosphors NaLa_9(1?x?y) (GeO₄)₆O₂: xTm³⁺, yDy³⁺ (NLGO: xTm³⁺, yDy³⁺) have been synthesized by a traditional solid‐state reaction method. The powder X‐ray diffraction (XRD), photoluminescence (PL), PL excitation (PLE) spectra, fluorescence decay curves, chromaticity coordinates, correlated color temperature (CCT), and the cathodoluminescence (CL) properties of the obtained phosphors are measured and discussed in detail. It is discovered that the series samples could be color‐tunable (from blue to yellow) by tuning the doping content of Dy³⁺ with a fixed Tm³⁺ content excited at 357 nm and white light (0.341, 0.324) could be obtained with the CCT of 5079 K. A NLGO: 0.01Tm³⁺, 0.02Dy³⁺ is studied carefully as representative. The main emissions of Tm³⁺ (453 nm, ¹D₂–³F₄) and Dy³⁺ (478 nm, ⁴F_9/2–⁶H_15/2; 572 nm, ⁴F_9/2–⁶H_13/2) make it emit white light with good thermal stability (67% of the initial till 523 K). The energy transfer from Tm³⁺ to Dy³⁺ is noticed and further research has been done to explain the enhancement of Dy³⁺ emission and the excellent thermal stability. It also keeps stable under continuous electron bombardment with high intensity. All of these indicate that it could be a suitable candidate for white‐emitting phosphor applied for near ultraviolet‐white light‐emitting diode (NUV‐WLED) and field‐emission display (FED).     

Highly sensitive optical thermometer of Sm3+, Mn4+ activated LaGaO3 phosphor for the regulated thermal behavior

Sm³⁺, Mn⁴⁺ co-activated LaGaO₃ phosphors, giving the characteristic emissions of orange and red emission simultaneously, were prepared by a solid-state reaction. Their luminescence properties, energy transfer behavior, thermal stability, and ratiometric temperature sensing performance were investigated. Thanks to the inhibition of energy transfer between Sm³⁺ and Mn⁴⁺ ions at high temperature and the reconstruction of the traps, the distinct optical behavior of the involved activators dependent on the ambient temperature was evaluated. Anti-thermal quenching performance of Sm³⁺ ions along with the emission declination of Mn⁴⁺ ions was observed. Hence, the optical thermometry characteristics of the resultant phosphor based on the fluorescent intensity ratio (orange/red) realize a recorded temperature sensitivity of 4.19% K⁻¹ and 2.09% K⁻¹. Moreover, the as-explored film combined with the LaGaO₃: Sm³⁺, Mn⁴⁺ phosphor is demonstrated to be a promising multi-color optical thermometer.     

The role of the Tm3+ concentration on CaMoO4 properties processed by microwave hydrothermal under stirring condition

The compounds based on calcium molybdate (CaMoO₄) are the subject of extensive research due to their excellent optical properties and a broad range of potential technological applications. In this work, we report a systematic study of CaMoO₄:Tm³⁺ phosphors synthesized by coprecipitation and processed in a microwave-hydrothermal system at low temperature (100°C) and stirring. The effect of the Tm³⁺ doping content (0%–12%) is studied in full detail to understand their role in the CaMoO₄:Tm³⁺ morphological, structural, and luminescent properties. The X-ray diffraction, Raman, and Fourier Transform Infrared spectroscopic techniques revealed that all the prepared powders have a tetragonal crystal structure with a distinct density of cation vacancies and structural disorders. The band gap remains almost constant for doping levels lower than 8%, but it narrows strongly for powders doped with 12% Tm³⁺ ions. The designed phosphors have shown two emission bands in which intensity depends on the Tm³⁺ ions doping level. For doping levels lower than 2%, the photoluminescence profile displays a broad emission band peaking at 543 nm (green). For concentrations higher than 4%, the band centered at 543 nm decreases in intensity and the near-infrared emission band at around 800 nm, assigned to ³F₃, ³H₄ → ³H₆ transitions from Tm³⁺ ion, become more intense. The outcomes of this work reveal that appropriated Tm³⁺ ions doping levels can be applied to suppress the PL emission in the visible range and improve that in the near-infrared region in CaMoO₄-based materials.