Co-doped Phosphors Research Articles

The synthesis and luminescence properties of Bi3+/Eu3+ co-doped Gd2O3 phosphors with high thermal stability and tunable emission via energy transfer

In this work, the highly monodisperse (Gd0.99-xBi0.01Eux)2O3 phosphors with spherical morphologies (∼170 nm) are successfully obtained through precursor synthesis via urea-based homogeneous precipitation (UBHP) method, followed by annealing at 1000 °C. The Bi/Eu-doped does not alter the crystal structure, and maintain the cubic structure of the Gd2O3 host. Owing to the Bi3+ occupies two different sites (C2, S6) in Gd2O3 of the cubic phase, the 1S0 → 3P1 transition of Bi3+ has two significant excitation bands (335 nm and 375 nm). By monitoring the excitation at 335 nm and 375 nm, the (Gd0.99-xBi0.01Eux)2O3 phosphors show both Bi3+ (420 nm and 530 nm) and Eu3+ (611 nm) emissions, respectively. The presences of Gd3+ and Bi3+ excitation bands on the PLE spectra by monitoring the Eu3+ emission indicate the Bi3+ → Eu3+ and Gd3+ → Eu3+ energy transfer, respectively, and the energy transfer efficiency is up to 99 %. Meanwhile, the energy transfer mechanism of the (Gd0.99-xBi0.01Eux)2O3 samples is dominated by the dipole-dipole interaction. Under two different excitation wavelengths, the emission intensity varies with the Eu3+ doping content and the quenching concentration is about 3 at%. The calculated value of the critical distance Rc is 12.12 Å, which indicates that the quenching mechanism is mainly caused by the multipole interaction. Meanwhile, the emission color can be tuned though adjusting the Eu3+ content. The temperature-dependent (in the range of 100–500 K) analysis has been obtained, the results indicate the (Gd0.99-xBi0.01Eux)2O3 samples have good thermal stability. The (Gd0.99-xBi0.01Eux)2O3 phosphors developed in this work is expected to be widely used in lighting and optical display applications.

A dual-mode optical thermometer based on dual-excitation Bi3+,Sm3+ co-doped Y4GeO8 phosphors

Dual-excitation and dual-emission Y4GeO8:Bi3+,Sm3+ phosphors were manufactured by traditional solid-phase sintering technique. The X-ray diffraction, morphology, photoluminescence, energy transfer process and temperature sensing properties of Y4GeO8:Bi3+,Sm3+ samples were comprehensively evaluated. The Y4GeO8:Bi3+,Sm3+ phosphors exhibit characteristic emissions of Bi3+ (3P1→1S0) and Sm3+ (4G5/2→6H) under both 290 and 347 nm excitations. In fluorescence intensity ratio and Commission International de L'Eclairage coordinates modes, Y4GeO8:Bi3+,Sm3+ samples present excellent temperature measurement performance. The maximum relative sensitivity (Sr-max) values of the former are 1.55%/K (460 K, 290 nm excitation) and 0.82%/K (506 K, 347 nm excitation). The Sr-max(x) values of the latter are 0.21%/K (437 K, 290 nm excitation) and 0.15%/K (513 K, 347 nm excitation). These results illustrate that Y4GeO8:Bi3+,Sm3+ phosphors can be used as a candidate material for a dual-mode optical thermometer under dual-excitation.

Emission color tuning and dual-mode luminescence thermometry design in Dy3+/Eu3+ co-doped SrMoO4 phosphors

The challenge of building a highly reliable contactless temperature probe with high sensitivity, good temperature-induced color discriminability, and economical synthesis has prompted the research community to work in the field of rare-earth-based luminescence thermometry. Moreover, the fast-growing market for optoelectronic devices has increased the demand for tunable color-emitting phosphors. In this study, Dy3+/Eu3+ co-doped SrMoO4 phosphors were developed as tunable color-emitting source and dual-mode luminescence thermometer. A facile and cost-effective auto-combustion method was used to synthesize the phosphors. Our work demonstrates a viable scheme for tailoring the emission of single-phase phosphors by precisely controlling the dopant concentrations and by modulating excitation wavelength. The overall emission is tuned from greenish-yellow to white and greenish-yellow to reddish-orange. A detailed energy transfer process from the host to the Ln3+ ions and between the Ln3+ ions is discussed. Further, anti-thermal quenching in the emission of Dy3+ ion is observed when excited with 297 nm. The dual-mode luminescence thermometry has been studied by analyzing the fluorescence intensity ratio of Dy3+ and Eu3+ ions upon excitation at 297 nm. The maximum relative sensitivity value for 4% Eu3+ co-doped SrMoO4:4%Dy3+ phosphor is 1.46% K−1 at 300 K. Furthermore, the configurational coordinate diagram is presented to elucidate the nature of temperature-dependent emission. Therefore, our research opens up new avenues for the development of color-tunable luminescent materials for various optoelectronic and temperature-sensing applications.

Photoluminescence and structural properties of Ce3+ and Eu3+ Co-doped Gd2SiO5 phosphor powders

Photoluminescence and structural properties of Ce3+ and Eu3+ Co-doped Gd2SiO5 phosphor powders

Multifunctional Li2MgTiO4:Cr3+, Mn4+ phosphor for potential plant cultivation, wavelength detection and anti-counterfeiting applications

Li2MgTiO4:Cr3+, Mn4+ phosphors were successfully synthesized by the high-temperature solid-state reaction method. Cr3+, Mn4+ co-doped Li2MgTiO4 phosphors have two emission peaks at 674 nm and 719 nm, corresponding to 2E→4A2 transitions of Mn4+ and 2E→4A2 transitions of Cr3+, respectively. The two emission peaks are matched with the absorption of PR and PFR, and the intensity ratio of the two emission peaks is adjusted by changing the doping concentration of Mn4+, indicating that the phosphors have potential applications in plant cultivation. In addition, it can be observed that the emission peak corresponding to Mn4+ can decrease sharply with the increase of excitation wavelength, while the emission peak corresponding to Cr3+ can change little, which indicates that the phosphors can be used for wavelength detection. The security ink prepared by Li2MgTiO4:Cr3+, Mn4+ phosphors shows that it can be applied in anti-counterfeiting labels. These results suggest that Li2MgTiO4:Cr3+, Mn4+ have the great potentiality to be used for plant cultivation, wavelength detection and anti-counterfeiting.

Simultaneous realization of FIR-based multimode optical thermometry and photonic molecular logic gates in Er3+ and Yb3+ co-doped SrTiO3 phosphor

Er3+ and Yb3+ co-doped SrTiO3 phosphors were synthesized and characterized by using the x-ray powder diffraction method, field emission scanning electron microscopy, and photoluminescence spectroscopy. The samples were subjected to thorough luminescence analysis. Under 980 nm infrared (IR) excitation, the (2H11/2,4S3/2) → 4I15/2 and 4F9/2 → 4I15/2 transitions of Er3+ gave green and red emissions, respectively. The up-conversion mechanism of the synthesized phosphor was studied using the power-dependent emission spectra of the SrTiO3:Yb3+, Er3+ phosphor. A rate equation model for the up-conversion mechanism of SrTiO3:Yb3+, Er3+ has been proposed. The temperature-dependent responses of various thermal and non-thermally coupled emission lines of Er3+ ions were used to elaborate the thermal sensing capabilities of the synthesized phosphor. The fluorescence intensity ratio technique was used for the temperature sensing measurements. Sensing measurements were performed in the temperature range of 303 to 618 K. Relative sensitivities of 1.28% K−1, 0.15% K−1, 0.98% K−1, and 0.15% K−1 were observed using various thermally and non-thermally coupled energy levels. Furthermore, using heat and incident IR excitation as physical inputs, we have shown that the synthesized phosphor can also be used to design various elementary logic gates such as AND, INHIBIT, and DEMULTIPLEX photonic molecular logic gates. A high switching ratio of ∼143% for the AND gate and ∼44.8% for the INHIBIT gate was observed using the scheme presented in this manuscript. The synthesized phosphor has the potential to be used as a bifunctional material for optical thermometry and molecular logic devices.

Energy transfer in Bi3+-Sm3+ co-doped phosphors for temperature sensing and imaging

Optical thermometry has the distinct superiority of non-invasive, non-contact, and high spatial resolution. However, the simultaneous improvement of temperature sensitivity and resolution remains challenging. Herein, the tunable emission colors can be realized via energy transfer in Bi3+-Sm3+ co-doped La3Ta0.8Sb0.2O7, which further applied for fluorescence temperature sensing and imaging. With the partial substitution of Sb5+ with Ta5+, the emission peak of La3SbO7:0.04Bi3+ shiftes from 530 nm to 460 nm, together with a 2.1-fold enhancement of emission intensity. The emission shifted from bright blue to purple under 365 nm excitation based on the energy transfer. The energy transfer efficiency from Bi3+ to Sm3+ can reach 41.6 % via dipole–dipole interaction. The La3Ta0.8Sb0.2O7:(0.04Bi3+,0.005Sm3+) phosphors exhibit maximum relative sensitivity (Sr) of 1.32 %K−1 (@498 K) and absolute sensitivity (Sa) of 0.059 K−1 (@417 K), which have excellent temperature resolution and repeatability. Thus, the energy transfer in co-coped phosphors can simultaneously improve the temperature sensitivity and resolution for fluorescence temperature sensing. Besides, the phosphor/PDMS films can exhibit obvious thermochromic imaging.

Impact of varying Li+ concentration on the upconversion emission and temperature sensing characteristics of YVO4: Tm3+/Yb3+ phosphor

A series of Li+ codoped YVO4: Tm3+/Yb3+ phosphors were prepared by Pechini sol–gel reaction technique. Structural study of the material confirms the crystal structure of the prepared samples and also gives information about the crystallite size and phase purity with the variation of Li+ ion concentrations. Emission peaks around ∼ 476, and ∼ 799 nm corresponds to the 1G4→3H6, and 3H4→3H6 transitions, which could be seen under irradiation of NIR (980 nm) laser light. Li+ (10 mol%) codoped sample shows maximum upconversion. Li+ doping significantly increases the up-conversion emission intensities by more than 5 times for emission peaks around 476 nm and 799 nm.The possible explanation for the increase in UC (Upconversion) emission intensity is attributed to both improved crystallinity and crystal field modification around Tm3+ ions. The variation of UC emission spectra with respect to pump power was observed to investigate the underlying UC luminescent mechanism in YVO4: Tm3+/Yb3+ codoped with Li+ (10 mol%). The multiphoton involvement is observed in the power-dependent study of upconverted blue and NIR emission bands. Widely used fluorescence intensity ratio (FIR) technique was used to evaluate the temperature sensing capability of the optimized Li+ codoped phosphor. The FIR study of Stark components 3F2, and 3H4 levels indicates that this material is an efficient material for temperature sensing. The optimized sample, doped with Li+, exhibits relative sensitivity of 0.78% K−1 at a temperature of 548 K. This material have utility in both light-emitting diodes and as a temperature-sensing probe.

Upconverting ultraviolet light source with extended emission wavelengths based on Bi3+-activated phosphor

Upconverted ultraviolet luminescence technology is attracting extensive interest lately due to its potential applications. To date, visible-to-ultraviolet upconversion has been achieved in Pr3+, Gd3+, Er3+, Ho3+ or Tm3+-activated phosphors. We herein report an upconverted ultraviolet emission of Bi3+ in a co-doped phosphor, YBO3:Pr3+,Bi3+, utilizing a 447 nm laser for excitation. Through spectroscopic investigations, we have determined that the blue laser excites the Pr3+ 4f5d state, followed by an energy transfer to the excited state of the Bi3+ ion in the phosphor. Consequently, the upconverted emission spectra do not show the Pr3+ (4f5d→4f2) emission, but rather the ultraviolet emission is completely dominated by the Bi3+ (3P1→1S0) emission. Taking advantage of the ultraviolet emission performance and blue light excitability of the phosphor, we have successfully built and demonstrated a prototype upconverting ultraviolet light source based on phosphor conversion. The introduction of Bi3+-activated upconversion luminescence enriches the bank of upconverting ultraviolet phosphors and provides a novel upconversion strategy for achieving Bi3+-activated ultraviolet light source.

Synthesis, structure, and luminescence properties of melilite Dy3+:LuxCaLa0.98–xGa3O7 phosphors for use in yellow light-emitting devices

Dy3+ and Lu3+ co-doped CaLaGa3O7 phosphors were prepared via high-temperature solid-phase reaction. The electronic structures of LuxCaLa0.98–xGa3O7 were investigated by the first-principles calculations. The influence of Lu3+ on yellow light emission was studied using X-ray diffraction (XRD) and photoluminescence (PL) measurements. The XRD results indicate that compared to Dy3+:CaLaGa3O7, the cell parameters of Dy3+:LuCaLaGa3O7 tend to decrease, which is due to the smaller radius of Lu3+. When pumped by blue GaN laser diode (LD), the emission peaks of phosphors with different Lu3+ doping concentrations in the visible region are similar, with the strongest peak at 574 nm in the yellow light region, which is sensitive to human eyes. The optimal doping concentration of Lu3+ is confirmed to be 1 at%, when all emission spectra and measured fluorescence lifetimes are taken into account. Moreover, the optimal phosphor composition Dy3+:LuxCaLa0.98–xGa3O7 (x = 0.01) has an internal quantum efficiency (IQE) of 46.94% and an external quantum efficiency (EQE) of 15.19%. Most notably, the prepared phosphor demonstrates excellent thermal stability and a high activation energy (0.203 eV). In addition, the International Illumination Commission color coordinates of the Dy3+:LuxCaLa0.98–xGa3O7 phosphors are in the yellow light area. The above analysis indicates that the Dy3+:LuxCaLa0.98–xGa3O7 (x = 0.01) phosphor has promising application prospects in yellow light-emitting devices.

Effect of boron incorporation on the structural, morphological, and spectral properties of CdNb2O6:Dy3+ phosphor synthesized by molten salt process

The effect of boron addition on the structural, morphologic and spectral properties was investigated by using xDy3+ (x = 0.5, 3, 6 mol%) doped, and xDy3+, yB3+ (y = 0, 5, 15, 25 mol%) co-doped CdNb2O6 powders produced by molten salt method. X-ray diffractions (XRD) of all Dy3+ doped, and B3+, Dy3+ co-doped samples showed that they retained the single-phase structure. The homogeneous dissolution of the Dy3+ doping without forming an impurity phase in the structure can be attributed to the flux effect of the molten salt process and boron doping. Scanning electron microscopy (SEM) micrographs of the B3+, Dy3+ co-doped grains showed that the effect of boron promotes grain growth and prismatic-like grain shape. Photoluminescence (PL) emissions of the phosphors with the excitation of 351.9 nm were monitored by the 4F9/2→6H15/2, 4F9/2→6H13/2, and 4F9/2→6H11/2 transitions. PL excitations and a broad peak at 275 nm originating from the O2–→Nb5+ band transition were observed with the emission of 577.9 nm. The PL emissions of Dy3+, B3+ co-doped phosphors increased about two fold compared to Dy3+ doped samples. The increase in PL may be related to the increased presence of luminescence centers at or near grain surface due to the decreasing surface area (SA)/volume (V) ratio as the growth of B3+ co-doped grains and the transformation from rod-like to prismatic-like shape. The increased Dy3+ presence led to a decrease in observed lifetime, while the B3+ increase did not cause any change. The study may provide a different perspective for the understanding and controlling of grain morphology and the luminescence relation for RE-doped phosphors.

Coordination-assembled phosphorescent microstructure from RTP HOF and Eu3+-doping ZGO:Mn phosphors for cancer biomarker amplification detection and information encryption

The ultra-long room temperature phosphorescent hydrogen-bonded organic framework (RTP HOF) materials can achieve long afterglow via ligand hydrogen bond interaction and water implement to suppress the non-radiative decays by matrices rigidification, and its electron donor conjugated structure is first developed as a phosphorescent quencher. The Eu3+/Mn2+ co-doped Zn2GeO4 phosphors (ZGO:Mn, Eu) with abundant metal sites and enhanced phosphorescence were synthesized as response factors and electron acceptors, combined with RTP HOFs to form microstructures featuring multi-color modulation, as an high-level anti-counterfeiting platform and lysophosphatidic acid (LPA) detection unit. LPA is an ideal plasma biomarker for early diagnosis of ovarian and other gynecologic cancers. This detection strategy relies on the differential coordination substitution to restore ZGO:Mn, Eu phosphorescence through synergistic coordination of LPA and the hydrophobic assistance of LPA, and dual functional groups identification of LPA achieve specific detection at the nanomolar level. The anti-counterfeiting platform can fetch specific information by controlling the afterglow distinction and excited light from ZGO:Mn, Eu and RTP HOF. This study not only provides a typical case of the preparation of two phosphors with heterogeneous optical properties, but also expands the application field of combined phosphors as intelligent luminescent materials.

Color tunability and NIR luminescence in Cr3+/Li+ co-doped ZnGa2O4 phosphor for solid state lighting and NIR LEDs

In this work, we report the color tunable photoluminescence in the Cr3+/Li+ doped and co-doped ZnGa2O4 (ZGO) phosphors synthesized by solid state reaction method. The phase and surface morphology of ZGO phosphor were examined by XRD and SEM measurements, respectively. The EDS spectra revealed the presence of Zn, Ga, Cr and O elements in the phosphor. The vibrational structure of the phosphor materials has been analyzed by FTIR measurements. The Cr3+ doped phosphor shows electronic absorption bands at 302, 408 and 544 nm alongwith the host CTB at 260 nm. The optical band gap of the phosphor decreases via doping of Cr3+ and Cr3+/Li+ ions. The ZGO is a self-activated host and emits broad blue emission with maximum at 434 nm upon excitation at 260 nm. The Cr3+ doped phosphors show emission bands due to spin-allowed as well as spin-forbidden transitions due to Cr3+ ion in the blue, green and NIR regions on excitation with 260, 302, 408 and 544 nm wavelengths. The emission intensity is optimum for 0.5 mol% concentration of Cr3+ ion. The emission intensity of Cr3+ doped ZGO phosphor increases via doping of Li+ ion. The color tunability has been observed in the phosphor upon 260 and 302 nm excitations. The lifetimes of 4T2 (4F) and 2E states of Cr3+ ion have been measured and it is found that it decreases with increasing concentrations of Cr3+ and Cr3+/Li+ ions. The Cr3+/Li+ doped and co-doped ZGO phosphors may be suitable candidates for display devices, solid state lighting and NIR LEDs.

Investigations on photoluminescence and energy transfer studies of Sm3+ and Eu3+ ions doped Sr9Y2W4O24 red emitting phosphors with high color purity for w-LEDs

Sm3+/Eu3+ co-doped Sr9Y2W4O24 phosphors were prepared using the solid-state reaction process. The structure and micro-morphological analysis of the as-prepared phosphors were explored via x-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared (FT-IR) spectroscopy. Optical & spectroscopic characteristics were investigated by the mean of diffuse reflectance spectroscopy (DRS), and photoluminescence (PL) techniques. The Sr9Y2W4O24 phosphor is tetragonal in structure and belongs to the I41/a space group. The excitation spectra recorded demonstrates that near-ultraviolet and blue LED chips can effectively excite the titled Sm3+-Eu3+ co-doped phosphors. The emission intensity pertaining to Eu3+ ions increases at the expense of decrease in intensity of Sm3+ ions with increase in the concentration of Eu3+ ion in titled Sm-Eu co-doped Sr9Y2W4O24 phosphors. The Reisfeld's approximation applied to the emission spectra of the titled Sm3+-Eu3+ co-doped Sr9Y2W4O24 phosphor reveal quadrupole-quadrupole as the mechanism responsible for energy transfer (ET) from Sm3+ to Eu3+ ions. The PL decay lifetimes values recorded for 603 nm emission of Sm3+ ions under 407 nm excitation wavelength were found to be decreasing with increase in Eu3+ ions concentration. This clearly reveal the ET from Sm3+ to Eu3+ ions in the titled phosphors. With increase in Eu3+ ion concentration the calculated CIE parameters are gradually shifting towards the deep red region of the visible spectrum under UV excitation. The temperature dependent PL (TD-PL) spectra demonstrate that the Sr9Y2W4O24: Sm3+/Eu3+ phosphor has relatively better thermally stablity with its thermal activation energy at around 0.256 eV. The results obtained in the present investigation allows us to explore the utility of Sm3+/Eu3+ co-doped Sr9Y2W4O24 phosphors as an effective deep red emitter needed to fabricate white LEDs.

Luminescence and temperature-sensing properties of Li+, Na+, or K+, Tm3+, and Yb3+ co-doped Bi2WO6 phosphors

Abstract A series of Li+, Na+, or K+, Tm3+, and Yb3+ co-doped Bi2WO6 upconversion phosphors were prepared by a high-temperature solid-phase method at 800°C for 3 h. X-ray diffraction showed that Li+, Na+, K+, Tm3+, and Yb3+ doping did not affect the orthorhombic structure of the Bi2WO6 matrix. Scanning electron microscopy images of the Bi2WO6:1% Tm3+, 6% Yb3+ and 1% Li+, 1% Na+, or 1% K+-doped Bi2WO6:1% Tm3+, 6% Yb3+ samples reveal irregular particles with a 0.5–5 µm particle size range; upon Na+ or K+ doping, the particle size increases and the particle surface becomes smooth. EDS analysis shows that the above ions are well incorporated into the powder particles. At 298 K, the relative temperature sensitivities are 0.00144, 0.0016, 0.0024, and 0.0018 K−1 for the 1% Tm3+, 6% Yb3+:Bi2WO6 samples without alkali metal ions and doped with 1% Li+, 1% Na+, or 1% K+ based on the thermally coupled energy level 3F3/3F2 characterization temperature. However, under the same conditions, when using the nonthermally coupled level 3F3/1G4 characterization temperature, the relative temperature sensitivities of these four samples are 0.0378, 0.0166, 0.046, and 0.0257 K−1, increasing by 26.3, 10.3, 19.1, and 13.9 times, respectively. The relative temperature sensitivities of the 1% Na+, 1% Tm3+, and 6% Yb3+:Bi2WO6 samples are the highest at 298 K.

Tunable luminescence and energy transfer in Tb3+,Eu3+ co-doped Gd2Zr2O7 phosphors with high thermal stability for WLEDs

Red phosphors with good thermal stability, high color temperature and color purity are critical in high power warm white LED devices. Herein, a series of single and co-doped Tb3+,Eu3+ polychromatic phosphors with pyrochlore structure based on Gd2Zr2O7 have been prepared for the first time by a conventional solid phase reaction method. Compared to single doped Eu3+ phosphors, Tb3+,Eu3+ co-doped phosphors possess following advantages: improved UV absorption, increased photoluminescence intensity, extended luminescence range and improved thermal stability. At room temperature, the Gd2Zr2O7:Tb3+,Eu3+ phosphors exhibited a gradual shift from yellow-green emission at 544 nm to orange-red emission at 613 nm with increasing doping of Eu3+ ions. Energy transfer efficiency has been evaluated to be 71% by analyzing the fluorescence lifetime of Tb3+ ions. The concentration quenching mechanism under different conditions was analyzed in detail and it was concluded that the quenching of Tb3+ is caused by dipole-dipole interaction. In addition, the activation energy (ΔEa) of the Gd1.92Zr2O7:0.03Tb3+,0.05Eu3+ phosphor was calculated to be as high as 0.2674 eV compared to other Tb3+, Eu3+ co-doped phosphors, demonstrating its high thermal stability. WLEDs devices based on Gd1.92Zr2O7:0.03Tb3+,0.05Eu3+ phosphors have been prepared, which have low correlated color temperature (CCT) (5283 K), high color rendering index Ra = 85.5 K) and CIE coordinates chromaticity coordinate (0.3371,0.3333). All of the results show that Gd2Zr2O7:Tb3+,Eu3+ phosphors provide a new avenue to develop high quality red emission and high thermal stability phosphors for WLEDs.

NIR activated Er3+/Yb3+ doped LiYF4 upconverting nanocrystals for photothermal treatment, optical thermometry, latent fingerprint detection and security ink applications

This work reports the upconversion (UC) emission studies on Er3+/Yb3+ doped LiYF4 nanoparticles for radiation-to-heat conversion, optical thermometry using thermally coupled and uncoupled levels, latent fingerprint detection and security ink applications. LiYF4:Er3+/Yb3+ nanoparticles have been synthesized using the thermal decomposition reaction method using oleic acid and 1-octadecane as reaction medium. Crystal phase, crystallinity and size have been investigated using XRD and TEM techniques. UC emission ability of the prepared phosphor was tested using a CW laser source having 980 nm excitation. Under 980 nm excitation, synthesized nanoparticles have shown emission bands around 522 nm, 550 nm and 668 nm corresponding to transitions from 2H11/2, 4S3/2, and 4F9/2 levels to 4I15/2 level of Er3+ ions, respectively. Further, the number of photons involved in the UC process has been calculated using a pump power UC study. Decay time analysis of the most intense green (550 nm) and red (668 nm) emission bands is performed to study the emission dynamics. A study on the radiation-to-heat conversion phenomenon has revealed photothermal conversion efficiency of 29.5% which is important for photothermal therapy. The green and red UC emissions from the sample under stimulation at very low pump power (35 mW) are used to study the optical temperature sensing characteristics. The findings of this study suggest that the Er3+/Yb3+ co-doped LiYF4 phosphor may be particularly useful for creating NIR to visible upconversion and temperature sensing. Moreover, latent fingerprint detection and security ink for anti-counterfeiting applications are also demonstrated.

Photoluminescence of Eu3+ in silicate-based phosphors by near-infrared and near-ultraviolet excitation for multifunctional applications

In this work, a series of Eu3+/Yb3+ doped La1.55SiO4.33 (LSO) phosphors were prepared by solid state reaction method for multifunctional applications. The phase purity and particle morphology were characterized with XRD and SEM technique, respectively. Upon ultraviolet light excitation, the downshifting luminescence of Eu3+ was observed in the red region, and the strongest emission peak located at 612 nm is attributed to the forced electric dipole 5D0→7F2 transition. The optimal Eu3+ doping concentration was determined to be 20 mol%, for which the internal quantum efficiency of the phosphor reaches 75.1%. The temperature-dependent emission spectra of LSO:20%Eu3+ revealed that the quenching temperature T0.5 is 483 K. For the upconversion (UC) luminescence, the Yb3+-Eu3+ codoped LSO phosphors were designed, where the Yb3+ ion was used as a sensitizer. The UC mechanism for a cooperative energy transfer was learnt from the pump-power-dependent emission spectra. By excitation at various temperatures, the intensities of emissions from different excited states showed different changes with temperature. All the above spectral characteristics indicate that the LSO:Eu3+/Yb3+ phosphors can show potential applications in LEDs, anticounterfeiting and optical thermometer.

ZnS:Er3+/Yb3+ phosphor for optical temperature sensing and dual-mode anti-counterfeiting

Optical temperature sensors have become a research hotspot in recent years due to their non-contact feature. In this study, Er3+/Yb3+ co-doped ZnS phosphors (1:3, 1:6, 1:9, 1:12, 1:15, 1:19) were prepared by co-precipitation method. The up-conversion and down-conversion luminescence performance of the phosphors was studied. It was found that the luminescence performance is optimized when the doping molar ratio is 1:9. Based on the fluorescence intensity ratio technology of non-thermal coupling energy level, the temperature sensing characteristics of ZnS:Er3+/Yb3+ (1:9) phosphors were studied in the temperature range of 313–533 K. The results show a maximum absolute sensitivity of 1.9 % K−1 and a maximum relative sensitivity of 2.7 % K−1. In terms of multifunctionality, this work also designed a flexible film based on thermoplastic polyurethane (TPU) to explore its anti-counterfeiting properties. The results of this work demonstrate the great potential of ZnS:Er3+/Yb3+ phosphors for optical temperature measurements and anti-counterfeiting.

Near-infrared photo-responsive Er3+-K+ co-doped Y2O3 phosphors with enhanced UC luminescence

Y2O3: x% Er3+ (x=5, 7, 10, 12, 15) and Y2O3: 10% Er3+，x% K+ (x=0, 1, 3, 5, 7, 10, 15) phosphors were successfully prepared by a low-temperature combustion method. The structure as well as the absorption/emission spectra of phosphors were investigated. The effect of doping concentration of K+ ions on the upconversion (UC) luminescence of Y2O3: 10% Er3+ phosphor was examined and the possible optical transitions were discussed. The results showed that K+ ion doping not only changed the microstructure and crystallinity of the phosphors, but also enhanced its UC luminescence intensity. The Y2O3: 10% Er3+, 7% K+ phosphor exhibit the strongest UC emission intensity. Compared with the Y2O3: 10% Er3+ phosphor, the UC luminescence intensity at 563 nm and 661 nm was enhanced by 67.8 and 27.3 times for the K-codoped samples, respectively. The phosphor with the optimal doping concentration was mixed with a polymer to form a composite film, which was employed for the fabrication of near-infrared (NIR) photo-responsive detection devices. The device exhibited strong photo-current response to NIR light at 980 nm, implying that our work could inspire new design strategy for the development of NIR photo-detection devices.