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.     

Quantitative relationship between microstructure and optical properties of Er3+-Yb3+ co-doped glass-ceramics containing pyrochlore-type crystalline phases

Er³⁺-Yb³⁺ co-doped transparent glass-ceramics (GCs) containing Y₂Ti₂O₇ crystalline phase were prepared by the melting crystallization method. The qualitative relationship between light transmittance and three-dimensional structure parameters of glass-ceramic was studied according to the principle of stereology under different heat treatment conditions. As indicated by the research results, the light transmittance decreased with the increase in the equivalent spherical diameter (D_3S) and specific surface area per unit volume of the grains (S_V). However, the light transmittance increased linearly with the increase in the discrete grains (S_VP) and the mean free distance (λ). The up-conversion luminescence intensity was found to be most vigorous under Er³⁺ and Yb³⁺ doping concentrations of 0.5% and 0.9%, respectively, at 980 nm excitation. Considering the correlation between pump power and up-conversion emission intensity, the up-conversion luminescence mechanism was explored. As revealed by the results of color purity and chromaticity coordinates, Er³⁺-Yb³⁺ co-doped GCs containing Y₂Ti₂O₇ have promising applications in green up-conversion luminescence.     

Visible up-conversion luminescence of CaWO<Subscript>4</Subscript>: Er<Superscript>3+</Superscript>,Yb<Superscript>3+</Superscript> and emission enhancement by tri-doping of Li<Superscript>+</Superscript> ions

Er³⁺,Yb³⁺ co-doped CaWO₄ polycrystalline powders were prepared by a solid-state reaction and their up-conversion (UC) luminescence properties were investigated in detail. Under 980 nm laser excitation, CaWO₄: Er³⁺,Yb³⁺ powder exhibited green UC emission peaks at 530 and 550 nm, which were due to the transitions of Er³⁺ (²H_11/2)→Er³⁺ (⁴I_15/2) and Er³⁺ (⁴S_3/2)→Er³⁺ (⁴I_15/2), respectively. Effects of Li+ tri-doping into CaWO₄: Er³⁺,Yb³⁺ were investigated. The introduction of Li⁺ ions reduced the optimum calcinations temperature about 100 °C by a liquid-phase sintering process and the UC emission intensity was remarkably enhanced by Li⁺ ions, which could be attributed to the lowering of the symmetry of the crystal field around Er³⁺ ions.     

Low-temperature microwave synthesis and luminescence properties of far-red emission ZnGa2O4:Cr3+ phosphor for plant cultivation

In this work, microwave (MW) sintering method is employed to synthesize far-red (FR) emission ZnGa₂O₄:Cr³⁺ phosphor at a lower sintering temperature (900 °C), comparing with the conventional high temperature solid-state sintering (SS) method. Micrometer-size phosphor particles with smooth morphology are obtained due to the MW-induced small temperature gradient between the constituent particles upon the local dielectric volumetric heating. The MW sintering ZnGa₂O₄:Cr³⁺ phosphor exhibited excitable properties at the wavelengths of 260 nm, 410 nm and 550 nm, and the intensity of photoluminescence excitation (PLE) at 260 nm showed a dramatic enhancement, which is 2–3 times higher than that of SS sintering sample due to MW “non-thermal effect” in reducing the defect amount in the ZnGa₂O₄ host latices. These results together with decay time, thermal quenching and color coordinate evaluations showed that synthesis of FR emission ZnGa₂O₄:Cr³⁺ by MW-sintering method has advantages of high efficiency, energy saving and least environmental threats.     

Effect of crystalline fraction on upconversion luminescence in Er3+/Yb3+ Co-doped NaYF4 oxyfluoride glass-ceramics

The crystalline fraction were adjusted MgO concentration and the corresponding effect on upconversion (UC) luminescence in Er³⁺/Yb³⁺ co-doped NaYF₄ oxyfluorode glass-ceramics was investigated. With increase of MgO and the content of Na₂O reduced, the internal network structure of the glass became compact, which made the size of NaYF₄ nanocrystals unchanged, while the average distance between the nanocrystals increased significantly. Crystal growth is limited with the glass network, keeping the crystal size not changed. SNM-1 glass ceramics samples show a predominant red up-conversion emission under near infrared excitation at 980 nm, while a predominant green emission is observed in the SNM-3 samples. In this paper, it was indicated that it changed the effect of glass network modifier MgO in the glass structure. The possible mechanism responsible for the color variation of UC in Er³⁺/Yb³⁺ co-doped was discussed.     

Bright red upconversion luminescence from Er3+ and Yb3+ co-doped ZnO-TiO2 composite phosphor powder

ZnO-TiO₂ composites co-doped with Er³⁺ and Yb³⁺ ions were successfully synthesized by powder-solution mixing method and their upconversion (UC) luminescence was evaluated. The effect of firing temperature, ZnO/TiO₂ mixing ratio, and dopant concentration ranges on structural and UC luminescence properties was investigated. The crystal structure of the product was studied and calculated in detail by means of X-ray diffraction (XRD). Also, the site preference of Er³⁺ and Yb³⁺ ions in the host material was considered and analyzed based on XRD results and UC luminescence characteristics. Brightest UC luminescence was observed in the ZnO-TiO₂:Er³⁺,Yb³⁺ phosphor fired at 1300 °C in which the system consisted of mixed phases; Zn₂TiO₄, TiO₂, RE₂Ti₂O₇ and RE₂TiO₅ (RE = Er³⁺ and/or Yb³⁺). Under the excitation of a 980 nm laser, the two emission bands were detected in the UC emission spectrum, weak green band centered at 544 and 559 nm, and strong red band centered at 657 and 675 nm wavelengths in accordance with ²H_11/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺ ion, respectively. The simple chemical formula equations, for explaining the site preference of Er³⁺ and Yb³⁺ ions in host crystal matrix, were generated by considering the Zn₂TiO₄ crystal structure, its crystal properties, and the effect of Er³⁺ and Yb³⁺ ions to the host crystal matrix. The UC emission intensity of the products was changed by varying ZnO/TiO₂ mixing ratios, and Er³⁺ and Yb³⁺ concentrations. The best suitable condition for emitting the brightest UC emission was 1ZnO:1TiO₂ doped with 3 mol% Er³⁺, 9 mol% Yb³⁺ fired at 1300 °C for 1 h.     

Enhancement of up-conversion emission and emerging cool white light emission in co-doped Yb3+/ Er3+: Li2O-LiF-B2O3-ZnO glasses for photonic applications

A series of co-doped (Yb³⁺/Er³⁺): Li₂O-LiF-B₂O₃-ZnO glasses were prepared by standard melt quenching technique. Structural and morphological studies were carried out by XRD and FESEM. Phonon energy dynamics have been clearly elucidated by Laser Raman analysis. The pertinent absorption bands were observed in optical absorption spectra of singly doped and co-doped Yb³⁺/Er³⁺: LBZ glasses. We have been observed a strong up-conversion red emission pertaining to Er³⁺ ions at 1.0 mol% under the excitation of 980 nm. However, the up-conversion and down conversion (1.53 µm) emission intensities were remarkably enhanced with the addition of Yb³⁺ ions to Er³⁺: LBZ glasses due to energy transfer from Yb³⁺ to Er³⁺. Up-conversion emission spectra of co-doped (Yb³⁺/Er³⁺): LBZ glasses exhibits three strong emissions at 480 nm, 541 nm and 610 nm which are assigned with corresponding electronic transitions of ²H_9/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 respectively. Consequently, the green to red ratio values (G/R) also supports the strong up-conversion emission. The Commission International de E′clairage coordinates and correlated color temperatures (CCT) were calculated from their up-conversion emission spectra of co-doped (Yb³⁺/Er³⁺): LBZ glasses. The obtained chromaticity coordinates for optimized glass (0.332, 0.337) with CCT value at 5520 K are very close to the standard white colorimetric point in cool white region. These results could be suggested that the obtained co-doped (Yb³⁺/Er³⁺): LBZ glasses are promising candidates for w-LEDs applications.     

Fabrication and optical thermometry of transparent glass-ceramics containing Ag@NaGdF4: Er3+ core-shell nanocrystals

Novel transparent glass-ceramics containing Ag@NaGdF₄:Er³⁺ core-shell nanocrystals were fabricated successfully by a melt-quenching method and subsequent heating. X-ray diffraction and transmission electron microscope images show that precious metal Ag is successfully encapsulated by the NaGdF₄:Er³⁺ nanoparticles to form an Ag@NaGdF₄:Er³⁺ core-shell structure in glass matrix. Compared with the NaGdF₄:Er³⁺ glass-ceramics, Ag@NaGdF₄:Er³⁺ core-shell glass-ceramics shows the great enhancement of emission intensity. The thermometric parameters such as fluorescence emission intensity, fluorescence intensity ratios of thermally coupled levels (²H_11/2/⁴S_3/2), and temperature sensitivity can be effectively controlled by changing the Ag concentration. When 0.15 mol% Ag is co-doped, the sensitivity of S_R in Ag@NaGdF₄:Er³⁺ core-shell glass-ceramics reaches a maximum value. This work presents a new method to enhance emission intensity and optical thermometry ability of NaGdF₄:Er³⁺ through constructing Ag@NaGdF₄:Er³⁺ core-shell structure.     

Effect of glass-ceramics network intermediate Al2O3 content on up-conversion luminescence in Er3+/Yb3+ co-doped NaYF4 oxy-fluoride glass-ceramics

Effect of alumina as a glass network intermediate on the up-conversion luminescence (UCL) in NaYF₄:Er³⁺/Yb³⁺ co-doped oxy-fluoride glass-ceramics (GCs) was investigated. Combinations of smaller NaYF₄ nanocrystals (10 and 13 nm) and lower Al₂O₃ contents (5% and 10%), as well as larger NaYF₄ nanocrystals (26 and 40 nm) and higher lower Al₂O₃ contents (15% and 20%) were prepared after heat-treatment, respectively. The glass network of intermediate partial replacement of SiO₂ with Al₂O₃ was investigated, and the consequence on the response to the up-conversion of the lanthanide ions was also studied. The UCL properties of Er³⁺ ions were changed in accordance with the addition of Al₂O₃, the red UCL intensity decreased with an increased Al₂O₃ concentration, while the green emission intensity showed opposite tendency. Our results showed that adding Al₂O₃ to 20 mol% is an effective strategy to simultaneous control of the magnitude and luminescence properties of lanthanide ion doped GCs.     

Influence of Cr3+ on yellowish-green UC emission and energy transfer of Er3+/Cr3+/Yb3+ tri-doped zinc silicate glasses

In this paper, we study the influence of Cr³⁺ on yellowish-green upconversion (UC) emission and the energy transfer (ET) of Er³⁺/Cr³⁺/Yb³⁺ tri-doped in SiO₂–ZnO–Na₂O–La₂O₃ (SZNL) zinc silicate glasses under excitation of the 980 nm laser diode (LD). The influence of Cr³⁺ on enhancing the red UC emission of Er³⁺/Cr³⁺/Yb³⁺ tri-doped in SiO₂–ZnO–Na₂O–La₂O₃ zinc silicate glasses under the excitation of 980nm LD was also investigated. The ET processes between Yb³⁺, Cr^3+, and Er³⁺, together with the combination of Yb³⁺-Cr³⁺-Er³⁺, which led to the green UC emission intensity of Er³⁺/Cr³⁺/Yb³⁺ tri-doped in SiO₂–ZnO–Na₂O–La₂O₃ zinc silicate glasses bands centered at ~546 nm have been significantly enhanced. By increasing the concentration of Cr³⁺ from 0 up to 5 mol.%, we can locate the Commission Internationale de l'éclairage (CIE) 1931 (x; y) chromaticity coordinates for UC emissions of Er³⁺/Cr³⁺/Yb³⁺ tri-doped in the central position of the yellowish-green color region of CIE 1931 chromaticity diagram. Besides, the ET processes between the Yb³⁺, Cr³⁺, and Er³⁺ are also proposed and discussed.     

Color-tunable up-conversion luminescence of Zn(AlxGa1-x)2O4:Yb3+,Tm3+,Er3+ powders

Color-tunable up-conversion powder phosphors Zn(Al_xGa_1-x)₂O₄: Yb³⁺,Tm³⁺,Er³⁺ were synthesized via high temperature solid-state reaction. Also, the morphological and structural characterization, up-conversion luminescent properties were all investigated in this paper. In brief, under the excitation of a 980?nm laser, all powders have same emission peaks containing blue emission at 477?nm (attributed to ¹G₄→³H₆ transition of Tm³⁺ ions), green emission at 526?nm and 549?nm (attributed to ²H_11/2→ ⁴I_15/2 and ⁴S_3/2→⁴I_15/2 transition of Er³⁺ ions respectively), red emission at about 659?nm and 694?nm (attributed to ⁴F_9/2→⁴I_15/2 transition of Er³⁺ ions and ³F₃→³H₆ transition of Tm³⁺ ions, respectively), which are not changed after the doping of Al³⁺ ions. However, the doping of Al³⁺ ions can enhance the up-conversion luminescent intensity and efficiency, while the emission color of as-prepared powder phosphors can be tunable by controlling the doping amount of Al³⁺ ions. Taking Zn(Al_0.5Ga_0.5)₂O₄:Yb,Tm,Er as the cut-off value, the emissions have clear blue-shift firstly and then show obvious red-shift with the increasing doping of Al³⁺ ions. Stated thus, pink emission in ZnAl₂O₄:Yb,Tm,Er, purplish pink emission in ZnGa₂O₄:Yb,Tm,Er and Zn(Al_0.9Ga_0.1)₂O₄:Yb,Tm,Er, purple emission in Zn(Al_0.1Ga_0.9)₂O₄:Yb,Tm,Er and Zn(Al_0.3Ga_0.7)₂O₄:Yb,Tm,Er, purplish blue emission in Zn(Al_0.7Ga_0.3)₂O₄:Yb,Tm,Er, blue emission in Zn(Al_0.5Ga_0.5)₂O₄:Yb,Tm,Er can be observed, which confirm the potential applications of as-prepared Zn(Al_xGa_1-x)₂O₄:Yb³⁺,Tm³⁺,Er³⁺ powder phosphors in luminous paint, infrared detection and so on.     

Near infrared persistent luminescence of transparent Zn-Ga-Ge-O:Cr3+ glass ceramic for optical information storage

Cr³⁺-doped near-infrared (NIR) afterglow phosphors have received wide recognition in the optical storage field because of the high signal-to-noise ratio and broad excitation spectra. In this article, the high-temperature TL intensity of ZnGa₂O₄:Cr³⁺ afterglow glass ceramic (ZGO:Cr³⁺ GC) was enhanced via partial hetero-valence substitution of Ge for Ga, demonstrating the tunability of the trapped electron levels in ZGO:Cr³⁺ GC. The persistent luminescence phosphor ZGO:Cr³⁺ GC exhibits a zero-phonon lines emission peaking at 698 nm, attributing to the ²E→⁴A_2g transition of Cr³⁺ ions. Moreover, the trap levels in Zn-Ga-Ge-O:Cr³⁺ glass ceramic (ZGGO:Cr³⁺ GC) are deeper than those of the Ge-free one and the captured electrons in deeper levels cannot be released only by the ambient thermal energy, thus the optical storage capacity of ZGGO:Cr³⁺ GC is much larger. By means of an additional 980 nm laser photostimulation, an intense NIR emission could be obtained. In consequence, ZGGO:Cr³⁺ GC has a promising application prospect in optical information storage field.     

Photoluminescence characteristics and mechanism of SrAl2O4 co-doped with Eu3+ and Cu2+

SrAl₂O₄ co-doped with Cu²⁺ and Eu³⁺ was prepared at high temperature in a weakly oxidizing atmosphere by solid states reaction. X-ray diffraction (XRD) pattern of the sample shows that the doped sample exhibits SrAl₂O₄ crystalline phase. No characteristic peaks of dopant have been observed in XRD pattern of doped sample. The excitation and emission spectra of CuEu:SrAl₂O₄, Eu:SrAl₂O₄, Cu:SrAl₂O₄ samples consist of many sharp peaks. The excitation and emission spectra of the SrAl₂O₄ sample co-doped with Cu²⁺ and Eu³⁺ are significantly different from those of Eu:SrAl₂O₄ and Cu:SrAl₂O₄ samples. The novel photoluminescence (PL) characteristic of the co-doped sample is attributed to the composite luminescence of Cu²⁺ and Eu³⁺ ions in SrAl₂O₄ matrix.     

Multi-phase glass-ceramics containing CaF2: Er3+ and ZnAl2O4:Cr3+ nanocrystals for optical temperature sensing

Oxyfluoride transparent glass-ceramics (GC) containing CaF₂ and ZnAl₂O₄ nanocrystals have been fabricated with melt-quenching method. By carrying out the heat treatment of the precursor glass (PG), Er³⁺ and Cr³⁺ were selectively partitioned into CaF₂ and ZnAl₂O₄ nanocrystals, respectively. The obtained multi-phase GC exhibited strong upconversion (UC) fluorescence of Er³⁺ as well as intense down-conversion (DC) fluorescence of Cr³⁺. Under 980 nm excitation, the green UC fluorescence of Er³⁺ due to ²H_11/2,⁴S_3/2 → ⁴I_15/2 transition and the red DC fluorescence lifetime of Cr³⁺ due to ²E, ⁴T₂ → ⁴A₂ transition were found to be highly dependent on the temperature and makes them possibly suitable for Optical Thermometry. With least-square fitting methods, the FIR of Er³⁺ from thermally coupled energy states (²H_11/2 and ⁴S_3/2) produced maximum temperature sensing sensitivity values of 0.33% K⁻¹ at 437 K and 0.36% K⁻¹ at 267 K, respectively. Similarly, fluorescence lifetime of Cr³⁺ attributed to the parity forbidden (²E → ⁴A₂) and spin allowed (⁴T₂ → ⁴A₂) produced the maximum temperature sensor sensitivity value equal to 0.67% K⁻¹ at 535 K.     

Enhanced up-conversion luminescence and optical thermometry characteristics of Er3+/Yb3+ co-doped Sr10(PO4)6O transparent glass-ceramics

In this paper, the Yb³⁺/Er³⁺ co-doped parent glass (PG) with composition (in mol%) of 30P₂O₅-10B₂O₃-38SrO-22K₂O and transparent glass-ceramics (GCs) containing hexagonal Sr₁₀(PO₄)₆O nanocrystals (NCs) were synthesized for the first time by melt-quenching method and subsequent heating treatment in air. Under 980 nm laser prompting, the GCs samples showed intense red and green up-conversion emissions compared to those characteristics for the PG sample. The emission intensities varied with Er³⁺ concentration and heat treatment conditions. Furthermore, in Yb³⁺/Er³⁺ co-doped GCs specimens, the optical thermometry was researched by means of fluorescence intensity ratio (FIR) of ⁴S_3/2 and ²H_11/2 levels. The GC sample heated at 620°C for 5 hours possessed a high relative temperature sensitivity (S_r) of 0.769% K⁻¹ at 303 K and the maximal absolute temperature sensitivity (S_a) of 5.951 × 10⁻³ K⁻¹ at 663 K, respectively. It is expected that the as-fabricated GC materials with Sr₁₀(PO₄)₆O NCs are promising efficient up-conversion materials for optical temperature sensor.     

Characterization of Bi2O3 thin films for doping photoluminescent Er3+ ions

Undoped and Er³⁺-doped Bi₂O₃ thin films were sputter-deposited on Si(100) substrates. Sufficiently oxidized Bi₂O₃ films with refractive indices between 2.17?2.23 were obtained at a wavelength of 633 nm; these values are comparable to those of bulk Bi₂O₃ crystals. While the film composition was stable for deposition temperatures between room temperature (RT) and 450 °C, the refractive index steeply decreased above 450 °C and reached 1.4 at 600 °C. The lowering of the optical transmittance spectra indicated aggregation of metallic Bi and darkening of the film. All films exhibited X-ray diffraction patterns of α-Bi₂O₃. The direct and indirect bandgap energies derived from the Tauc plots were 3.4–3.7 eV and 1.9–2.5 eV, respectively, depending on the O₂ flow rate and deposition temperature. Upon excitation of Er³⁺-doped Bi₂O₃ films at 532 nm, Er³⁺ emissions peaking at 1537 and 1541 nm appeared, and the photoluminescence spectra included fine structures reflecting crystal-field splitting. Resonant excitation of Er³⁺ 4f levels and indirect excitation via the defect levels of Bi₂O₃ followed by energy transfer to Er³⁺ contributed to the emission. The films deposited at RT with Er concentrations of 2 at.% had the emission intensity of Er³⁺, but concentration quenching strongly suppressed the Er³⁺ emission because the doped Er³⁺ ions stayed inside the Bi₂O₃ crystals. At deposition temperatures above 400 °C, the concentration quenching was mitigated possibly because out-diffusion of Er³⁺ ions reduced the effective number of Er³⁺ ions in the Bi₂O₃ crystalline domains.     

Preparation and luminescence of Dy3+/Tm3+ co-doped Ca3NbGa3Si2O14 glass-ceramics for w-LED

In this work, a series of Dy³⁺ and Dy³⁺/Tm³⁺ ion activated Ca₃NbGa₃Si₂O₁₄ glass-ceramics were prepared by traditional melt crystallization method, and report on the structural, optical, and energy transfer (ET)-based photoluminescence (PL) properties of glass-ceramics co-doped Dy³⁺/Tm³⁺. The preparation of glass-ceramics was studied by DTA, XRD, SEM, and UV–vis photometer technology, phase composition, transmittance, optimum heat treatment conditions, and luminescence properties. The best heat treatment procedure for obtaining transparent and well-formed glass-ceramics is crystallization at 820 °C for 5 h. The spectra excited by Tm³⁺ and Dy³⁺ have intersections at 352 nm and 365 nm, which means that CNGS: Dy³⁺/Tm³⁺ can be effectively excited by 352 nm and 365 nm ultraviolet light. Under the excitation of 352 nm ultraviolet light, four main emission peaks corresponding to ¹D₂ →³F₄, ⁴F_9/2 → ⁶H_15/2, ⁴F_9/2 → ⁶H_13/2, ⁴F_9/2 → ⁶H_11/2 were found at 456 nm, 484 nm, 577 nm, and 663 nm, respectively. When the optimal concentration (4 at.%) of Dy³⁺ is Co-doped with a different amount of Tm³⁺, the luminous color can be adjusted by adjusting the doping amount of Tm³⁺ and changing the excitation wavelength. There is an overlapping region between the emission spectrum of Tm³⁺ doped glass and the excitation spectrum of Dy³⁺ doped glass, which indicates that there is energy transfer between Tm³⁺ and Dy³⁺. In addition, CNGS: Dy³⁺/Tm³⁺ CIE coordinates show that the color coordinates (0.3324, 0.3352) when y = 0.02 under 365 nm excitation are closest to the standard white light (0.333, 0.333), indicating that this glass has potential applications in WLED devices.     

Broadband near-infrared emission enhancement in K2Ga2Sn6O16:Cr3+ phosphor by electron-lattice coupling regulation

Cr³⁺-doped phosphors have recently gained attention for their application in broadband near-infrared phosphor-converted light-emitting diodes (pc-LEDs), but generally exhibit low efficiency. In this work, K₂Ga₂Sn₆O₁₆:Cr³⁺ (KGSO:Cr) phosphor was designed and synthesized. The experimental results show that the Cr³⁺-doped phosphor exhibited broadband emissivity in the range 650-1300 nm, with a full width at half maximum (FWHM) of approximately 220-230 nm excited by a wavelength of 450 nm. With the co-doping of Gd³⁺ ions, the internal quantum efficiency (IQE) of the KGSO:Cr phosphor increased from 34% to 48%. The Gd³⁺ ions acted neither as activators nor sensitizers, but to justify the crystal field environment for efficient Cr³⁺ ions broad emission. The Huang-Rhys factor decreased as the co-doping of Gd³⁺ ions increased, demonstrating that the nonradiative transitions were suppressed. An efficient strategy for enhancing the luminescence properties of Cr³⁺ ions is proposed for the first time. The Gd³⁺–co-doped KGSO:Cr phosphor is a promising candidate for broadband NIR pc-LEDs.     

Upconversion luminescence properties of NaBi(MoO4)2:Ln3+, Yb3+ (Ln = Er,Ho) nanomaterials synthesized at room temperature

Studies on lanthanide ions doped upconversion nanomaterials are increasing exponentially due to their widespread applications in various fields such as diagnosis, therapy, bio-imaging, anti-counterfeiting, photocatalysis, solar cells and sensors, etc. Here, we are reporting upconversion luminescence properties of NaBi(MoO₄)₂:Ln³⁺, Yb³⁺ (Ln = Er, Ho) nanomaterials synthesized at room temperature by simple co-precipitation method. Diffraction and spectroscopic studies revealed that these nanomaterials are effectively doped with Ln³⁺ ions in the scheelite lattice. DR UV–vis spectra of these materials exhibit two broad bands in the range of 200–350 nm correspond to MoO₄²⁻ charge transfer, s-p transition of Bi³⁺ ions and sharp peaks due to f-f transition of Ln³⁺ ions. Upconversion luminescence properties of these nanomaterials are investigated under 980 nm excitation. Doping concentration of Er³⁺ and Yb³⁺ ions is optimized to obtain best upconversion photoluminescence in NaBi(MoO₄)₂ nanomaterials and is found to be 5, 10 mol % for Er³⁺, Yb³⁺, respectively. NaBi(MoO₄)₂ nanomaterials co-doped with Er³⁺, Yb³⁺ exhibit strong green upconversion luminescence, whereas Ho³⁺, Yb³⁺ co-doped materials show strong red emission. Power dependent photoluminescence studies demonstrate that emission intensity increases with increasing pump power. Fluorescence intensity ratio (FIR) and population redistribution ability (PRA) of ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 transitions of Er³⁺ increases with increasing the Yb³⁺ concentration. Also, these values increase linearly with increasing the pump power up to 2 W. It reveal that these thermally coupled energy levels are effectively redistributed in co-doped samples due to local heating caused by Yb³⁺.     

Synthesis,Crystal Structure,and Enhanced Luminescence of Garnet‐Type Ca3Ga2Ge3O12:Cr3+ by Codoping Bi3+

Garnet‐type compound Ca₃Ga₂Ge₃O₁₂ and Cr³⁺‐doped or Cr³⁺/Bi³⁺ codped Ca₃Ga₂Ge₃O₁₂ phosphors were prepared by a solid‐state reaction. The crystal structure of Ca₃Ga₂Ge₃O₁₂ host was studied by X‐ray diffraction (XRD) analysis and further determined by the Rietveld refinement. Near‐infrared (NIR) photoluminescence (PL) and long‐lasting phosphorescence (LLP) emission can be observed from the Cr³⁺‐doped Ca₃Ga₂Ge₃O₁₂ sample, and the enhanced NIR PL emission intensity and LLP decay time can be realized in Cr³⁺/Bi³⁺ codped samples. The optimum concentration of Cr³⁺ in Ca₃Ga₂Ge₃O₁₂ phosphor was about 6 mol%, and optimum Bi³⁺ concentration induced the energy‐transfer (ET) process between Bi³⁺ and Cr³⁺ ions was about 30 mol%. Under different excitation wavelength from 280 to 453 nm, all the samples exhibit a broadband emission peaking at 739 nm and the intensity of NIR emission increases owing to the ET behavior from Bi³⁺ to Cr³⁺ ions. The critical ET distance has been calculated by the concentration‐quenching method. The thermally stable luminescence properties were also studied and the introduction of Bi³⁺ can also improve the thermal stability of the NIR emission.