Upconversion Research Articles

Upconversion Luminescence Properties and Temperature Sensing Characteristics of Tm3+, Yb3+-codoped SrAl2Si2O8 Phosphors with High Thermometric Sensitivities

In this paper, a SrAl2Si2O8:Tm3+,Yb3+ upconversion (UC) phosphor was prepared with a high-temperature solid-phase method, and proved that the strontium feldspar structure was an excellent UC matrix. The Tm3+ were shown to have a maximum relative thermometric sensitivity (Sr) of 1.67% K−1 at 298 K and a maximum absolute thermometric sensitivity (Sa) of 1.34% K−1 at 573 K. The non-thermally coupled energy level corresponded to the maximum relative thermometry sensitivity (Sr) of 2.4% K−1 at 298 K and a maximum absolute sensitivity (Sa) of 13% K−1 at 423 K. The use of the fluorescence intensity ratio of the nonthermally coupled energy levels of Tm3+ for temperature measurements improved the temperature measurement sensitivity of the same material. These results indicated that the phosphor has good temperature sensing performance with a wide sensing range and good stability and has potential thermometry applications in the first biological window.

Multi-mode excited upconversion and downshifting of Ho3+/Yb3+ doped lead-free double perovskite Cs2NaBiCl6 for anti-counterfeiting and thermometry

Rear earth(RE)-doped halide perovskite has generated as an amazing optical material for multicolor luminescence. Free-lead halide double perovskites have attracted interesting because of their non-toxicity and higher instability. Here, RE3+ doped chloride-based double perovskite phosphors are reported. A series of heavy concentration Ho3+/Yb3+ doped Cs2NaBiCl6(CNBC) phosphors are prepared with a solvothermal route. The band structure of CNBC is calculated through density functional theory (DFT). The luminescence properties of CNBC:Ho3+ and CNBC:Ho3+/Yb3+ are investigated through upconversion(UC) and downshifting(DS) processes. From CNBC:Ho3+, the brightness orange, deep red and pale green lights are achieved under excitation at 365, 452 and 980 nm. The multicolor emissions of CNBC:Ho3+ are used to multimode fluorescent anti-counterfeiting. Comparing with CNBC:Ho3+ samples, the UC emission intensity of CNBC:Ho3+/Yb3+ are dramatically enhanced. The brightness green light can be seen by naked eyes under super lower power pumping(∼10 mW). The temperature sensing properties of CNBC:Ho3+/Yb3+ phosphor are investigated systematically based on the luminescence intensity ratio(LIR) of thermally coupled levels(TCLs)(5F1/5G6,5F2,3/3K8) and non-TCLs(5F4/5S2,5F5). The maximum relative sensitivity(SR) reaches 5.089% K−1 at 293 K according to the LIR of non-TCLs. The work will broaden the application field of RE3+-doped free-lead perovskite such as anti-counterfeiting and thermometry.

Fluorinated Hafnium and Zirconium Coenable the Tunable Biodegradability of Core-Multishell Heterogeneous Nanocrystals for Bioimaging.

Upconversion (UC)/downconversion (DC)-luminescent lanthanide-doped nanocrystals (LDNCs) with near-infrared (NIR, 650-1700 nm) excitation have been gaining increasing popularity in bioimaging. However, conventional NIR-excited LDNCs cannot be degraded and eliminated eventually in vivo owing to intrinsic "rigid" lattices, thus constraining clinical applications. A biodegradability-tunable heterogeneous core-shell-shell luminescent LDNC of Na3HfF7:Yb,Er@Na3ZrF7:Yb,Er@CaF2:Yb,Zr (abbreviated as HZC) was developed and modified with oxidized sodium alginate (OSA) for multimode bioimaging. The dynamic "soft" lattice-Na3Hf(Zr)F7 host and the varying Zr4+ doping content in the outmoster CaF2 shell endowed HZC with tunable degradability. Through elaborated core-shell-shell coating, Yb3+/Er3+-coupled UC red and green and DC second near-infrared (NIR-II) emissions were, respectively, enhanced by 31.23-, 150.60-, and 19.42-fold when compared with core nanocrystals. HZC generated computed tomography (CT) imaging contrast effects, thus enabling NIR-II/CT/UC trimodal imaging. OSA modification not only ensured the exemplary biocompatibility of HZC but also enabled tumor-specific diagnosis. The findings would benefit the clinical imaging translation of LDNCs.

Strong green up-conversion luminescence and optical thermometry of Ho3+/Yb3+ Co-doped AlN submicron towers

Rare-earth (RE) doped aluminum nitride (AlN) holds great promise for integrated optoelectronic devices. Here, the Ho and Yb co-doped AlN (AlN:Ho3+/Yb3+) submicron towers were prepared by a direct nitridation route. XRD, Raman, XPS and EDS studies showed successful doping of Ho and Yb ions into AlN. SEM images show that the submicron towers have an interesting layered structure by layer-by-layer stacking of hexagonal AlN nanosheets. Under excitation at 980 nm, AlN:Ho3+/Yb3+ submicron towers exhibit up-conversion (UC) luminescence at 540, 550, 659, and 762 nm, which are due to Ho3+/Yb3+ system from 5F4, 5S2→5I8, 5F5→5I8, and 5F4, 5S2→5I7 respectively. Based on the intensity ratio and decay lifetime of UC luminescence, the optical temperature sensing characteristics are investigated from 298 K to 548 K. The maximum relative sensitivities associated with the intensity ratio of I540, 550 and I659 and decay lifetime of 659 nm are as high as 2.73% K−1 and 0.43% K−1, respectively. This work broadens the optoelectronic properties of RE doped AlN.

Giant thermal enhancement of upconversion photoluminescence in Y2W3O12:Nd phosphor under 808 nm excitation

Luminescent materials usually suffer from thermal quenching at high temperature due to the aggravation of nonradiation relaxations. Herein, a giant enhancement of up conversion (UC) luminescence is observed in negative thermal expansion (NTE) Y2W3O12:Nd phosphor upon increasing the temperature from 310 to 790 K. In addition, the temperature-dependent UC emission is reversible. To penetrate into the mechanisms, the temperature-dependent luminescence intensity and lifetime are investigated for Nd doped negative and positive thermal expansion (PTE) phosphors. Based these investigation, we infer that the underlying mechanism of the giant enhancement for Y2W3O12:Nd phosphor are due to combined effects of phonon assisted absorptions and contraction of NTE crystals. Moreover, the observation of temperature-dependent near-infrared spectra (NIR) can support the mechanism from the other side. The present findings pave the way to overcome thermal quenching of Nd3+ ions and explore new thermal enhancement materials sensitized by Nd3+ ions.

Improvement in the upconversion efficiency of an alternating multilayer structure based on 9,10-bis(4-methylphenyl) anthracene and platinum octaethylporphyrin vacuum deposited on an in-plane oriented polythiophene film

Alternating layers of 9,10-bis(4-methylphenyl) anthracene (diMe-DPA) and platinum octaethylporphyrin were fabricated via vacuum deposition on an in-plane-oriented polythiophene (PT) film. The multilayer film showed higher upconversion (UC) quantum efficiency than bilayer films with similar composition and total thickness. The UC quantum efficiency of the multilayer structure deposited on the oriented PT film was three times higher than those deposited on glass substrates, suggesting that the molecular alignment of diMe-DPA increases the UC quantum efficiency.

Expanding the Anti-Stokes Shift of TTA-UC to 1.04 eV by Chemical Tuning of BODIPY-Based Organic Photosensitizers.

A series of BODIPY-derivatives B-1 ∼ B-5 were synthesized as metal-free photosensitizers in triplet-triplet annihilation (TTA) upconversion (UC) with the purpose of finely tuning the excited state properties to reduce the energy loss during intersystem crossing (ISC), and hence to expand the anti-Stokes shift of TTA-UC. It was revealed that B-5 showed a ΔEST of as small as 0.28 eV, thus, providing a large anti-Stokes shift of 1.04 eV in TTA-UC with organic sensitizers. A deep red-blue upconversion emission with UC quantum yields of up to 10.7% (out of a 50% maximum) was observed.

Site Preference Induced Dual‐Wavelength Mn2+ Upconversion in K2NaScF6:Yb3+, Mn2+ and Its Application in Temperature Sensing

AbstractThe realization of dual‐wavelength Mn2+ upconversion (UC) luminescence that exhibits different thermal responsive behaviors is promising for non‐invasive optical temperature sensing applications. However, it remains challenging to achieve such luminescence of Mn2+ ions. Herein, due to the site preference characteristics of Mn2+, color tunable and dual‐wavelength Mn2+ UC luminescence is achieved in K2NaScF6:Yb3+, Mn2+ by simply adjusting the Mn2+ doping concentration. Structural and spectral analysis as well as theoretical calculations reveal that the green and red UC emissions originate from two kinds of face‐sharing Yb3+‐Mn2+ dimers, which exhibit different luminescence properties, including different emission wavelength, excited state lifetime, full width at half maximum (FWHM), and luminescence thermal stability. This unique dual‐wavelength UC luminescence of Mn2+ provides a new type of fluorescence intensity ratio thermometer with absolute and relative sensitivities of up to 0.018 K−1 and 0.525% K−1, respectively. This work not only demonstrates the potential application of K2NaScF6:Yb3+, Mn2+ material in the field of optical thermometry but also provides new perspectives for designing multi‐band Mn2+ UC luminescence.

Tricolor Upconversion Phosphors of LiYO2:RE3+/Yb3+ (RE = Tm, Ho, Eu) for Metamerism Anti‐Counterfeiting and 3D Volumetric Display

AbstractThe development of blue/green/red tricolor upconversion (UC) phosphors under invisible light irradiation has attracted significant interest in emerging applications such as anti‐counterfeiting, three‐dimensional (3D) optical data storage, and solid‐state 3D displays. In this study, a series of LiYO2:RE3+,Yb3+ (RE = Tm, Ho, Eu) phosphors are successfully prepared via first precipitation process followed by calcination, and completely optimized for efficient tricolor UC luminescence with 980 nm laser excitation. Mechanisms involving two‐ or three‐photon absorption processes and low‐phonon‐energy‐suppressed multi‐phonon relaxation have been proposed for Ho3+(Eu3+)/Yb3+ and Tm3+/Yb3+ codoping, respectively. In addition, the RE3+ (Yb3+) concentration‐induced phase transition of the LiYO2 host is discussed for interesting possibilities in spectral regulation. Based on the broad color gamut achieved by the tricolor UC phosphor mixture, homochromatic and polychromatic metamerisms are experimentally fabricated via screen printing for high‐level anti‐counterfeiting and information security. Moreover, for a proof‐of‐concept demonstration, prototypes of 3D optical data storage and volumetric displays are constructed by embedding tricolor UC mixtures and their 3D printed patterns in polydimethylsiloxane. It is believed that the continuous exploration of tricolor UC under invisible light excitation for metamerism anti‐counterfeiting and 3D displays can promote the development of high‐level information security and advanced volumetric display technologies.

Blue and green upconverted emission in fluoroindate glasses under 808 nm excitation

In this study, the upconversion (UC) luminescence of fluoroindate glasses (InF3G) co-doped with Yb-Nd-Er (Tm) ions excited in the NIR-I biological window was analyzed. In the first system, UC emission of singly Er3+ doped InF3G under 808 nm excitation occurred to be more efficient than those observed in InF3G co-doped with Yb–Er and Yb-Nd-Er. The intensity of UC emission (InF3G-Er) was 22 % higher than InF3G-Yb-Er, and even more than two times higher than in the InF3G-Yb-Nd-Er sample. The effective energy transfer (η) between Nd3+- Yb3+ (η = 45 %) and Yb3+-Er3+ (η = 68 %), was reflected in the emission observed in the NIR region mainly, ascribed to Yb3+:2F5/2 → 2F7/2 (978 and 999 nm) and Nd3+:4F3/2 → 4I9/2 (1053 nm) transitions. Meanwhile, the samples co-doped with Tm3+ exhibited a strong dependence on Yb-Nd-Tm co-doping. Samples without Yb and Nd did not exhibit any UC emission due to the absence of pumping channel. This contrasts with the downshifting NIR emission, which was at least two times higher than those exhibited in samples co-doped Yb-Nd-Er. These results suggest that Tm3+ plays a crucial role in populating the lower-lying exited levels of Yb and Nd, influencing the overall emission behavior.

Enhancing the upconversion of Er3+ incorporated BaTiO3 by introducing oxygen vacancies.

Lanthanide-incorporated crystals display the phenomenon of upconversion (UC), wherein near-infrared (NIR) light is converted into ultraviolet-visible (UV-Vis) emission with a narrow bandwidth. This unique photophysical property renders lanthanide UC materials highly promising for diverse applications. However, the limited quantum efficiency (∼3%) hinders the broader utilization of UC materials. Consequently, numerous studies have focused on overcoming this low efficiency. Notably, it has been observed that manipulation of the site symmetry in UC materials significantly enhances their UC efficiency. In this study, we investigate the UC enhancement of Er3+ incorporated BaTiO3 (E-BT) crystals through the introduction of oxygen vacancies (OV). The OV were created using a post-heat treatment method, and the annealing time was varied to control the quantity of OV. An optimal annealing time of 6 hours was determined for efficient OV generation, beyond which the OV content decreased. Remarkably, E-BT crystals with OV exhibited up to three-fold greater UC compared to those without OV. This outcome suggests that OV induce symmetry changes in the E-BT crystal structure. Furthermore, the degree of UC enhancement in E-BT was found to be proportional to the amount of OV present.

Tailoring sensitization properties and improving near-infrared photon upconversion performance through alloying in superatomic molecular Au25 nanoclusters.

Noble-metal nanoclusters (NCs) protected by organic ligands have recently come to the forefront as potent triplet sensitizers for photon upconversion (UC) via triplet-triplet annihilation (TTA), owing to their capacity for atomic-level photophysical property customization. Among these, the rod-shaped bi-icosahedral [Au25(PPh3)10(S-C2H4Ph)5Cl2]2+ (Au-rod) NC is a particularly iconic superatomic molecular NC, recently identified as a near-infrared (NIR)-absorbing sensitizer for TTA-UC. In this study, we synthesized Cu-doped NCs, [Au25-xCux(PPh3)10(S-C2H4Ph)5Cl2]2+ (AuCu-rod), and paired them with 9,10-bis(phenylethynyl)anthracene (BPEA) annihilator/emitter to explore the impact of Cu-doping on the triplet sensitization and NIR-UC performance. The triplet state of AuCu-rod, with lifetime of 3 μs, exhibited a modest blue shift compared to the Au-rod, resulting in the increment in the driving force for triplet energy transfer (TET) to the BPEA acceptor. The TET rate constant was determined to be 5.0 × 107 M-1 s-1, which is an order of magnitude higher than the rate constant for the Au-rod/BPEA pair. This improvement has led to a remarkable increase in the TET efficiency. Notably, the AuCu-rod/BPEA pair facilitated the efficient UC of 805 nm NIR light into 510 nm visible light, realizing a large anti-Stokes shift close to 0.9 eV. The UC internal quantum yield of this combination was determined to be 2.33 ± 0.05%, marking a fivefold enhancement over the Au-rod sensitizer (0.49%). Thus, alloying NC sensitizers offers a promising route to enhance UC performance by tuning the triplet state energy and optimizing the compatibility between the sensitizer and annihilator. Additionally, in this series of experiments, the formation of small amounts of BPEA microaggregates was observed. These aggregates did not undergo singlet fission and could retain multiple long-lived triplet excitons. This characteristic facilitated TTA among triplet excitons, resulting in efficient NIR-to-visible UC emission.

Triplet-triplet annihilation-based photon upconversion using nanoparticles and nanoclusters.

The phenomenon of photon upconversion (UC), generating high-energy photons from low-energy photons, has attracted significant attention. In particular, triplet-triplet annihilation-based UC (TTA-UC) has been achieved by combining the excitation states of two types of molecules, called the sensitizer and emitter (or annihilator). With TTA-UC, it is possible to convert weak, incoherent near-infrared (NIR) light, which constitutes half of the solar radiation intensity, into ultraviolet and visible light that are suitable for the operation of light-responsive functional materials or devices such as solar cells and photocatalysts. Research on TTA-UC is being conducted worldwide, often employing materials with high intersystem crossing rates, such as metal porphyrins, as sensitizers. This review summarizes recent research and trends in triplet energy transfer and TTA-UC for semiconductor nanoparticles or nanocrystals with diameters in the nanometer range, also known as quantum dots, and for ligand-protected metal nanoclusters, which have even smaller well-defined sub-nanostructures. Concerning nanoparticles, transmitter ligands have been applied on the surface of the nanoparticles to efficiently transfer triplet excitons formed inside the nanoparticles to emitters. Applications are expanding to solid-state UC devices that convert NIR light to visible light. Additionally, there is active research in the development of sensitizers using more cost-effective and environmentally friendly elements. Regarding metal nanoclusters, methods have been established for the evaluation of excited states, deepening the understanding of luminescent properties and excited relaxation processes.

Exploring the potential of lanthanide-doped oxyfluoride materials for bright green upconversion and their promising applications towards temperature sensing and drug delivery.

The most efficient upconversion (UC) materials reported to date are based on fluoride hosts with low phonon energies, which reduce the amount of nonradiative transitions. In particular, NaYF4 doped with Yb3+ and Er3+ at appropriate ratios is known as one of the most efficient UC phosphors. However, its low thermal stability limits its use for certain applications. On the other hand, oxide hosts exhibit better thermal stability, yet they have higher phonon energies and are thus prone to lower UC efficiencies. As a result, developing host nanomaterials that combine the robustness of oxides with the high upconversion efficiencies of fluorides remains an intriguing prospect. Herein, we demonstrate the formation of ytrrium doped oxyfluoride (YOF:Yb3+,Er3+) particles, which are prepared by growing a NaYF4:Yb3+,Er3+ layer around SiO2 spherical particles and consecutively applying a high-temperature annealing step followed by the removal of SiO2 template. Our interest lies in employing these materials as Boltzmann type physiological range luminescence thermometers, but their weak green emission is a drawback. To overcome this issue, and engineer materials suitable for Boltzmann type thermometry, we have studied the effect of introducing different metal ion co-dopants (Gd3+, Li+ or Mn2+) into the YOF:Yb3+,Er3+ particles, focusing on the overall emission intensity, as well as the green to red ratio, upon 975 nm laser excitation. These materials are explored for their use as ratiometric thermometers, and further also as drug carriers, including their simultaneous use for these two applications. The investigation also includes examining their level of toxicity towards specific human cells - normal human dermal fibroblasts (NHDFs) - to evaluate their potential use for biological applications.

Advanced temperature sensing with Er3+/Yb3+ co-doped Ba2GdV3O11 phosphors through upconversion luminescence.

Optical thermometry is a non-contact temperature sensing technique with widespread applications. It offers precise measurements without physical contact, making it ideal for situations where contact-based methods are impractical. However, improving the accuracy of optical thermometry remains an ongoing challenge. Herein, enhancing the thermometric properties of luminescent thermometers through novel materials or strategies is crucial for developing more precise sensors. Hence, the present study focuses on the application of four-mode luminescence thermometric techniques in sol-gel synthesized Er3+/Yb3+ co-doped Ba2GdV3O11 phosphors for optical temperature sensing in the temperature range of 298-573 K. The upconversion (UC) luminescence is achieved under excitations of 980 nm or 1550 nm, resulting in bright yellow-green emission in the visible spectral range. Temperature sensing is realized by exploiting the UC emissions of 4S3/2, 2H11/2 and 4F7/2 bands, which represent intensity ratios of thermally coupled levels (TCELs) and non-thermally coupled levels (NTCELs) of Er3+/Yb3+, along with the emission lifetimes at 4S3/2. The relative sensitivity (Sr) values for TCELs exhibit a gradual decrease with rising temperature, reaching a maximum of 1.1% K-1 for 980 nm excitation and 0.86% K-1 for 1550 nm excitation at 298 K. Conversely, for NTCELs, the highest Sr value observed is 0.9% K-1 at 298 K for 1550 nm excitation. Moreover, the emission lifetimes at 4S3/2 yield notably high Sr values of up to 5.0% μs K-1 (at 425 K). Furthermore, the studied phosphors have a sub-degree thermal resolution, making them excellent materials for accurate temperature sensing. Overall, this study provides a promising new direction for the development of more precise and reliable optical thermometry techniques, which could have important implications for a range of scientific and industrial optical temperature sensing applications.

A rationally designed singlet sink for glassy polymeric photon upconverting films

We synthesized a singlet sink to enhance upconversion (UC) quantum yield while maintaining a low intensity threshold in glassy polymer blends. This strategy enables higher sensitizer loadings and thus increases the scope of UC.

Thermal sensitivity of Ho3+ fluorescence in oxyfluoride nanocrystals and derivative embedded fibers

(Y1-x-yHoxYby)OF/polyacrylonitrile (YOF-HuYv/PAN) composite fibers prepared by electrostatic spinning have been demonstrated to possess effective upconversion (UC) luminescence and sensitive temperature feedback properties for laser display and real-time temperature monitoring in complex environments. When the excitation power density is increased to 73 mW/mm2, the total luminous flux reaches 3.35 mlm, demonstrating that the Ho3+/Yb3+ co-doped YOF nanocrystals embedded in the composite fibers are efficient luminous materials. The quantum yields (QYs) of green and red emissions from Ho3+ are derived to be 2.97 × 10−5 and 1.40 × 10−5 respectively when the pump power density arrives at 73 mW/mm2 under 977 nm laser, and the high photon generation efficiency ensures sufficient fluorescence intensity for tracing temperature feedback. The maximum relative sensitivities appear at 303 K, while the absolute sensitivity (SA) and relative sensitivity (SR) of the nanofibers retain 13.77% K−1 and 0.34% K−1 at 433 K respectively, demonstrating that the nanofibers have superior optical temperature sensing properties. The composite fibers with effective luminescence and high temperature sensitivity as luminescent materials provide a potential option for the field of laser display and temperature sensing.

Excitation wavelength and pumping power dependent temperature sensing property of Nd3+/Yb3+/Er3+ triply-doped LaNbO4 phosphor

Optical thermometry based on the fluorescence intensity ratio technology between the thermally coupled energy levels of Er3+ has become a research hotspot, but it is often limited by excitation conditions, which reduces its universality and reliability. In this work, Nd3+/Yb3+/Er3+ triply-doped LaNbO4 phosphor with excellent up-conversion (UC) luminescence characteristic and reliable temperature sensing sensitivity has been prepared by solid-state reaction method. Intense green and weak red UC emissions were gained under 808 nm and 980 nm excitations. The temperature dependent UC luminescence property and temperature sensing behavior of the sample has been investigated. The results demonstrated that the intensity ratio of the two green emissions was insensitive to the excitation wavelength and pumping power of the lasers, and the maximum absolute sensitivity was derived to be 0.91 % K−1 at 481 K. This work provides a new material with reliable and high temperature sensing sensitivity for optical thermometers.

Multi-dimensional tailoring of the thermometric behaviors of Er3+/Yb3+-codoped Y2W3O12 polychromatic upconverting microparticles for multi-mode visual optical thermometry

To combat the thermal quenching and inadequate sensitivities of luminous materials, the Y1.98–2xW3O12:0.02Er3+/2xYb3+ (YWO:Er3+/2xYb3+; 0.01 ≤ x ≤ 0.11) microparticles with splendid negative thermal expansion (NTE) effect were designed. Excited at 980 nm, the produced samples emit polychromatic upconversion (UC) emissions triggered by the energy back transfer from Er3+ to Yb3+ and it has been systematically discussed via steady-state rate theory. The thermally enhanced UC emissions are accomplished in YWO:Er3+/2xYb3+ microparticles due to the distinctive NTE properties. Moreover, according to the temperature-associated UC emissions of Er3+ arising from its thermally coupled levels (TCLs) and non-thermally coupled levels (non-TCLs), the thermometric properties of synthesized microparticles are investigated. When non-TCLs are adopted, the absolute and relative sensitivities of resultant microparticles are 0.0129% and 1.7875% K−1, respectively. Furthermore, through manipulating doping content and spatial mode, the temperature sensing capacities of YWO:Er3+/2xYb3+ microparticles are multi-dimensionally regulated. In addition, contactless temperature recognition can also be realized through analyzing the thermochromic features of final products and its relative sensitivity is 2.1604% K−1. Ultimately, visual optical temperature detection is obtained in designed microparticles via its temperature-related emitting colour. Our findings may provide an important insight into the design of luminescent materials with high sensitivities for multi-mode visual optical thermometry.

Intense green up/down conversion emission and high-sensitivity optical thermometry of NaYb(WO4)2:Er3+ stoichiometric phosphors

Optical thermometry based on fluorescence intensity ratio (FIR) has garnered significant interest among researchers due to its rapid, non-contact, and high sensitivity characteristics. In this study, stoichiometric NaYb(WO4)2 phosphors, doped with varying concentrations of Er3+, were successfully synthesized using the high-temperature solid-state method. Under both 980 nm and 266 nm excitation, the samples exhibited strong green and very weak red upconversion (UC) and downconversion (DC) luminescence. The structural and morphological properties of the prepared samples were thoroughly characterized using techniques such as X-ray diffraction (XRD), XRD Rietveld refinement, scanning electron microscopy (SEM), and Fourier transform infrared (FT-IR) spectroscopy. The phosphors demonstrated high-purity UC and DC emission, with maximum green-to-red ratios of 60.9 and 77.0, respectively. This discovery opens up vast possibilities for their application as pure green color converters. The UC emission of Er3+ ions in the stoichiometric NaYb(WO4)2 phosphor was identified as a two-photon UC process, as evidenced by the relationship between the luminescence intensity and the excitation power of the sample. Moreover, the temperature-dependent emission spectra of UC and DC were monitored, and investigated the fluorescence intensity ratios (FIRs) of 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions of Er3+ ions to assess the optical temperature sensing behavior of NaYb(WO4)2:Er3+. The experimental results revealed that the maximum absolute sensitivities (SA) of NaYb(WO4)2 phosphors were 0.0120 K-1 (UC) and 0.0077 K-1 (DC), and the maximum relative sensitivities (SR) were 0.0089 K-1 (UC) and 0.0092 K-1 (DC), respectively. These findings highlight the promising of Er3+-doped Yb3+ stoichiometric NaYb(WO4)2 phosphors in optical temperature sensors.