Enhanced Upconversion Luminescence Research Articles

Upconversion luminescence characteristics of Ho<sup>3+</sup> ion doped single-particle fluoride micron core-chell structure

Constructing core-shell structures can effectively reduce the surface quenching effect of luminescent materials, which becomes an effective method to enhance upconversion luminescence. In this work, a series of NaLnF<sub>4</sub>@NaLnF<sub>4</sub> (Ln = Y<sup>3+</sup>, Yb<sup>3+</sup>, Ho<sup>3+</sup>) core-shell microcrystals is successfully synthesized based on epitaxial growth technology, thereby enhancing and regulating the upconversion emission of Ho<sup>3+</sup> ions. The results of the XRD and SEM indicate that the NaLnF<sub>4</sub>@NaLnF<sub>4</sub> core-shell microcrystal possesses a pure hexagonal-phase crystal structure with a rod-like shape. Meanwhile, it is found that the epitaxial growth direction of the micro-shell is not affected by the crystal characteristics in the core, but determined by the crystal characteristics of the shell. Under 980 nm near-infrared laser excitation, the upconversion luminescence properties of single microrods with different core-shell structures are investigated via a confocal microscope spectroscopy. It is found that in the NaLnF<sub>4</sub> micro-crystal, the coated NaYF<sub>4</sub> inert shell can also effectively reduce the quenching effect on the surface of the micro-crystal for enhancing upconversion emission. When the Yb<sup>3+</sup> ions are introduced into NaYF<sub>4</sub> or NaYbF<sub>4</sub> active shell that is coated, the Yb<sup>3+</sup> ions in the shell can effectively transfer excitation energy to Yb<sup>3+</sup> in the core through energy migration, and then establish new energy transfer channels, thereby realizing the Ho<sup>3+</sup> ion luminescence enhancement. For NaHoF<sub>4</sub>@NaYbF<sub>4</sub> core-shell microrods, the Yb<sup>3+</sup> in the shell can transfer more excitation energy to Ho<sup>3+</sup> ions at the adjacent interface for enhancing the overall luminescence intensity, and its higher red-green ratio is mainly due to the cross-relaxation process occurring between the Ho<sup>3+</sup> ions at high doping concentration of Ho<sup>3+</sup> in the NaHoF<sub>4</sub> core. Meanwhile, the luminescence process of the micron core-shell system is further confirmed based on the luminescence characteristics of different structures and the dynamic luminescence process. It can be seen that constructing different micron core-shell structures and introducing sensitizing ions, can not only effectively enhance the luminous intensity of the micron materials, but also adjust the output color. Therefore, this research is an important experimental reference for enhancing the luminous intensity of the micron system and the precise adjustment of luminescence, and can effectively expand the applications of micron crystals in the fields of displays, micron lasers and anti-counterfeiting.

Inflection in size-dependence of thermally enhanced up-conversion luminescence of UCNPs

The investigation on the size-dependence of UCL thermal behavior of UCNPs with the size down to 3 nm reveals an inflection in the size-dependence of the thermo-enhanced luminescence of UCNPs.

Highly precise FIR thermometer based on the thermally enhanced upconversion luminescence for temperature feedback photothermal therapy.

A highly precise temperature-feedback photothermal therapy platform in deep tissue is proposed based on all-fiber fluorescence intensity ratio (FIR) thermometry, which provides a promising route to realize real-time temperature monitoring in the minimally invasive treatment of tumors. Highly disordered double perovskite Li2Zn2Mo3O12 (LZMO) phosphors doped with rare earth ions were prepared and intense green upconversion emissions were observed with an ultra-low excitation power. The thermal enhancement of the upconversion luminescence was achieved up to 423 K, which is very beneficial to achieve a good signal-to-noise performance during the temperature-rise period. Superior temperature sensing performance was demonstrated with the maximum absolute sensitivity of 89.9 × 10−4 at 423 K. The strong upconversion emissions and high temperature sensitivity result in a small temperature error (±0.4 K). The integrated bifunctional needle could simultaneously realize temperature measurement and laser heating, which was exhibited in the denaturation of egg white and laser ablation of the porcine liver in vitro.

Tumor‐Microenvironment‐Activated Reactive Oxygen Species Amplifier for Enzymatic Cascade Cancer Starvation/Chemodynamic /Immunotherapy

AbstractAt present, some progress has been made in the field of cancer theranostics based on nanocatalysts (NCs), but achieving precise theranostics in response to the specific tumor microenvironment (TME) remains a major challenge. Herein, a TME‐responsive upconversion nanoparticles (UCNPs)‐based smart UCNPs@Cu–Cys–GOx (UCCG) nanosystem is engineered, which combines natural enzymes and nanozymes so as to amplify reactive oxygen species (ROS) generation in situ for cancer starvation/chemodynamic/immunotherapy. One of the biggest merits of this material is that it can be preserved inert (off) in normal tissues, and only in the TME can it be specifically activated (on) through a series of enzymatic cascades to boost ROS production via a strategy of open source (H2O2 self‐supplying ability) and reduce expenditure (glutathione (GSH) consuming ability). More importantly, the enhanced oxidative stress by UCCG NCs reverses the immunosuppressive TME, and facilitates antitumor immune responses. Meanwhile, the starvation/chemodynamic synergistic therapy triggered by UCCG combined with PD‐L1 antibody effectively inhibits the growth of primary tumors and cancer metastasis. In addition, the UCNPs in UCCG present upconversion luminescence enhancement, which can be exploited to visualize the reinforced ROS generation in real time. Collectively, this work provides an original method for the devising and exploitation of UCNPs‐based catalytic immunotherapy.

Ultrasensitive critical-state of upconversion nanoparticles assembled on plasmonic antenna arrays for rapid and accurate determination of NH3

Plasmon enhanced upconversion luminescence has drawn extensive attention due to its advances in the fields of optical sensors, bio-detection and medical diagnosis. However, the coupling state between plasmon and upconversion emission is never clearly revealed. Here, we report a novel critical state in plasmon coupled upconversion nanoparticles (UCNPs) that has a long lifetime and is highly stimuli-responsive. The critical state is generated through the precise control over the coupling between plasmonic local field and optical near field. Platinum (Pt) film with thickness of ~7 nm is deposited on photonic crystals to form a periodic plasmonic antenna arrays. UCNPs with inert shells are embedded in plasmonic antenna arrays with a tunable plasmon coupling distance via the thickness control of inert shells. An ultrasensitive critical state is observed for the coupling distance of ~10 nm between Pt film and NaGdF4:Yb/Er luminescent core, accompanied with light confinement by NaYF4/NaGdF4 heterojunction shells. This critical-state can be employed as an ultrasensitive route for fluorescent detection of NH3 and the lowest detection of 0.47 ppm can be achieved that is about 34 times lower than ever reported.

Enhanced upconversion luminescence and temperature sensing feature in NaBi(MoO4)2: Er3+, Yb3+ transparent glass ceramics

Molybdate glasses doped with Er3+/Yb3+ ions were fabricated using the high-temperature melting method, and transparent glass ceramics were obtained after heat treatment. XRD and TEM analysis results confirmed the formation of tetragonal nanocrystalline NaBi(MoO4)2. The structural characterization of glass and glass ceramics were investigated by Infrared and Raman spectroscopy. The transmittance of glass-ceramics in the visible range is over 60%. Compared to the glass, the up-conversion (UC) luminescence intensity of glass ceramics is enormously increased. And the UC luminescence intensity is correlated with temperature. Especially, in the aspect of temperature detection, the glass heated at 640 °C possesses a high absolute sensitivity of up to1.04% K−1, which can be applied in the field of optical temperature measurement.

Enhancement of green upconversion luminescence and temperature sensitivity of Zr2(WO4)(PO4)2:Er3+,Yb3+ phosphors by co-doping Li+ ions

A serious of Zr2(WO4)(PO4)2 (ZWP):Er3+,Yb3+ phosphors by co-doping Li+ ions were prepared by solid state reaction method. Based on the X-ray diffraction (XRD) and scanning electron microscope (SEM), the lattice structure and morphology of prepared samples were performed. Meanwhile, the up-conversion luminescence (UCL) spectra were measured from room temperature to 693 K under 980 nm laser excitation. With the intervention of Li+ (0.04, 0.06, 0.08, 0.10), the green UCL intensities of ZWP:0.02Er3+, xYb3+ (x = 0.02, 0.04, 0.06, 0.08) phosphors was remarkably strengthened by 4.2, 2.6, 3.3, 5.5 times at room temperature, respectively, and the thermal enhancement of 2H11/2→4I15/2 transition UCL of Er3+ was observed due to the negative thermal expansion (NTE) of ZWP host. In Addition, the co-doping Li+ enhanced the absolute temperature sensitivities, and the maximum absolute sensitivity and relative sensitivity of ZWP:Er3+,Yb3+,Li+ phosphors were derived to be 0.01345 K−1 and 0.01198 K−1, respectively. The mechanism of the enhanced UCL intensity and temperature sensing by co-doping Li+ was discussed. The obtained results indicate that the NTE Er3+/Yb3+/Li+ tri-doped ZWP phosphors with a high temperature sensitivity can be applied as optical thermometer.

The Effects of Li+ Doping on Structure and Upconversion Luminescent Properties for Bi3.46Ho0.04Yb0.5Ti3O12: xLi Phosphors

A series of novel Li+ doped Bi3.46Ho0.04Yb0.5Ti3O12 (BHYTO: xLi, 0 ≤ x ≤ 0.15) upconversion phosphors were prepared through a sol-gel-sintering method. There exist three emission bands centered at 545 nm, 658 nm, and 756 nm in the upconversion emission spectra at 980 nm excitation, corresponding to energy transitions of 5F4/5S2 → 5I8, 5F5 → 5I8 and 5F4/5S2 → 5I7 of Ho3+, and the upconversion emission intensity of BHYTO: 0.05Li is about 2.2 times stronger than that of BHYTO samples. The luminescent lifetime of the strongest emission (545 nm) is in the range of 45.25 to 65.99 μs for the different BHYTO: xLi phosphors. The energy transfers during the upconversion pumping process from Yb3+ to Ho3+ are mainly responsible for all the emissions, each belonging to a double-photon process. Li+ mainly entered into the interspace sites or occupied Bi3+ sites in Bi4Ti3O12 host during the fabrication process according to its dosage, and the possibility is very low for Li+ to take part in the energy transfer process directly due to its lack of matching levels with 4f of Ho3+ and Yb3+. However, Li+ doping can not only increase the size of crystal grains to improve crystallinity through XRD analysis, but also reduced oxygen vacancies to decrease the number of quenching centers through XPS analysis. The improved crystallinity and reduced quenching centers are proposed to be the main causes for the enhanced upconversion luminescence of the Li+ doped BHYTO phosphor.

Enhancing the Up-conversion luminescence using All dielectric Three-Dimensional multiscale anodized aluminum oxide nanowire structure

Light management at nanometric dimension for boosting the enhancement of photo luminescence from Lanthanide doped up-conversion nanoparticles (UCNPs) has evidenced epochal advancement attributed to their conspicuous emission properties and plethora of potential applications. In this work, a multiscale funnel shape three dimensional (3D) anodized aluminum oxide (AAO) nanowire topography is explored to improve up-conversion luminescence (UCL) of synthesized core shell (C/S) NaYF4:Yb3+,Tm3+/NaYF4 UCNPs dispersed in chloroform. The different hexagonally arranged AAO topographies, based on cost-effective and modified self-aggregation fabrication method, are presented. These unique topographies of nanowire structures provide archetypal for enhanced UCL and have been investigated to study the effect of light management in detail. The systematic studies indicate that UCL enhancement is substantiated by the prolonged light path due to multiple scattering, and guided modes in the length of nanowire structures with the modified surface topography hoisted by capillary force inside the nanowires. The length of the nanowire and structure’s topographies are endorsed as structural parameters to tune the reflection as well as resonance mode for tuning the enhancement. The overall UCL enhancement for an optimized wire length and surface topography of AAO substrate is found to be ∼ 115 fold in ultra-violate regime at low excitation power density.

Enhancing up-conversion luminescence of Er3+ with copper sulfide nanostructures

A ternary composite structure containing hotspots (a small area with strong electromagnetic field) was obtained by Er2O3@SiO2 particles and copper sulfide nanoparticles for up-conversion luminescence, and the enhancement mechanism of up-conversion luminescence in the above-mentioned composites have been systematically studied. By adjusting the optical field in the vicinity of the binary composite structure and the ternary composite structure via hotspots, the enhancement of up-conversion luminescence in the composites were realized, and the maximum enhancement factor can reach to 16.2 times. The results showed that under the synergistic effect of the absorption of incident photons by the copper sulfide film, the scattering of incident photons by Er2O3@SiO2 particles, and the strong absorption of incident photons by copper sulfide nanoparticles, the up-conversion luminous intensity of the ternary composite structure was greatly improved.

Multiple logic gates system based on dual‐wavelength triggered enhancing upconversion luminescence of Gd2(MoO4)3:Yb3+/Er3+

AbstractA complex UC logic system using 1470 and 1550 nm dual‐wavelength as triggering signals has been developed to implement multilevel logic operations. Gd2(MoO4)3:Yb3+/Er3+ upconversion luminescence material can achieve significantly emission enhancement upon simultaneous excitation of 1470 and 1550 nm dual‐wavelength due to the synergistic effect of two kinds of photons realizing better match with energy level. The influences of excitation power and low temperature on dual‐wavelength enhancement effect are also in‐depth researched. Multiple logic combination operations including two‐output (YES + OR, AND + XOR, AND + INHIBIT, YES + INHIBIT) and three‐output (AND + YES + OR) are developed in this Gd2(MoO4)3:Yb3+/Er3+ UC logic system by employing 1470 and 1550 nm excitation resources as input signals and detecting the change of relative intensity of green, red emission and red/green intensity ratio. It is notable that two outputs and three outputs signals in this UC emission system can simultaneously execute different logic operations, which could greatly enlarge the capacity of logic information base and enhance the ability of information processing. These results not only provide a good enlightenment for the design of complex logic systems but also are indispensable for development of various functional devices.

Multiphoton Upconversion Materials for Photocatalysis and Environmental Remediation.

Solar-driven photocatalysis holds great potential for energy conversion, environmental remediation, and sustainable chemistry. However, practical applications of conventional photocatalytic systems have been constrained by their insufficient ability to harvest solar radiation in the infrared spectrum. Lanthanide-doped upconversion materials possess high photostability, tunable absorption, and the ability to convert low-energy infrared radiation into high-energy emission, making them attractive for infrared-driven photocatalysis. This review highlights essential principles for rational design of efficient photocatalysts. Particular emphasis is placed on current state-of-the-arts that offer enhanced upconversion luminescence efficiency. We also summarize recent advances in lanthanide-doped upconversion materials for photocatalysis. We conclude with new challenges and prospects for future developments of infrared-driven photocatalysts.

Enhanced up-conversion luminescence and temperature sensing property of Ba3-xSrxLu4O9:Tm3+/Yb3+ phosphors

Improved up-conversion (UC) luminescence and applications in optical temperature sensors of up-converting materials have attracted growing interest in recent years. Here, we reported a new series of Ba3-xSrxLu4O9 (B3-xSxLO):Tm3+/Yb3+ phosphors. By designing Yb3+→Tm3+ energy transfer and modifying the local lattice environment of activator ions via cationic substitution, highly efficient enhancement of UC luminescence of Tm3+ is realized when excited by a 980 nm laser. Moreover, the representative B2.2S0.8LO:Tm3+/Yb3+ sample displays superior temperature sensing behaviors with highly relative sensitivity of 0.88 % at 428 K, absolute sensitivity of 1.50 % at 573 K, wide sensing range and good stability. All of findings reveal that B3-xSxLO:Tm3+/Yb3+ are promising candidates for optical thermometry.

Up‐Conversion Luminescence Enhancement and Temperature Sensitivity Properties of La2O3: Yb3+/Er3+ Nanoparticles Induced via Tri‐Doping Li+ Ions

AbstractDue to the lower up‐conversion (UC) luminescence intensity and efficiency, up‐conversion nanoparticles (UCNPs) still have many obstacles when they are applied, especially in biology. In this paper, La2O3: Yb3+/Er3+/Li+ nanoparticles were prepared by hydrothermal method firstly and were tested by multiple methods. The green emission and red emission of La2O3: Yb3+/Er3+/Li+ (7/1/10 mol%) nanoparticles were enhanced 63 and 56 folds respectively than the Li+‐free counterpart under 980 nm excitation. The reason for the UC luminescence enhancement has been investigated in detail, which can be illustrated by the increase of the crystallinity of La2O3, the increase of the local asymmetry near the Er3+ ions, the increase of the surface charge, and the decrease of the quenching centers by doping Li+ ions. Additionally, the possible mechanisms of UC luminescence and UC enhancement were discussed briefly. The absolute sensitivity SA of La2O3: Yb3+/Er3+/Li+ (7/1/10 mol%) with temperature from 303 K to 543 K was 1561.58/T2, which indicated the material was an excellent candidate for temperature detection.

Solid solution Na(Gd/La)(MoO4)2:Yb3+/Er3+ upconversion nanocrystals with simultaneously enhanced photothermal conversion efficiency and luminescence intensity

Lanthanide doped upconversion nanocrystals with remarkable optical properties have been utilized for many emerging applications. Here, solid solution Na(GdxLa1-x)(MoO4)2 nanocrystals are synthesized hydrothermally, and their upconversion photoluminescent and photothermal properties as luminescent host matrix for Yb3+/Er3+ activators are systematically studied. The effect of La3+ doping content on the phase, crystal structure, upconversion luminescence and photothermal conversion are investigated. Rietveld refinements based on XRD data indicate that La3+ occupy Gd3+ sites substitutionally, and pure phase solid solution Na(GdxLa1-x)(MoO4)2 is formed. Due to lattice distortion, lower lanthanide site symmetry and augmented odd-parity crystal field interactions after La3+ doping, the 4f-4f transition probabilities of the lanthanide ions are effectively increased. Na(Gd/La)(MoO4)2 solid solution hosts exhibit enhanced upconversion luminescence from Er3+/Yb3+, compared with single-component NaGd(MoO4)2 counterparts. What's more, based on the calculated temperatures of the samples derived from the luminescence intensity ratio of states 2H11/2 and 4S3/2 for Er3+, enhanced photothermal conversion efficiency is speculated in solid solution Na(GdxLa1-x)(MoO4)2:Yb3+/Er3+ nanocrystals, which is further confirmed by infrared thermographic images. The underling mechanisms of simultaneously enhanced photothermal effects and luminescence intensity in Na(GdxLa1-x)(MoO4)2 solid solution nanocrystals are elucidated. Modulation of composition in solid solution hosts may be an effective method for combating luminescent thermal quenching and/or regulating photothermal conversion and photoluminescence properties simultaneously. Multifunctional solid solution upconversion nanocrystals could be potential for photoluminescent and photothermal applications.

The LSPR regulation of TiO2: W nanocrystals and its application in enhanced upconversion luminescence

The localized surface plasmon resonance (LSPR) absorption peaks of semiconductor nanocrystals are mainly concentrated in the infrared band, and the absorption characteristics can be controlled by the amount of element doping. The coupling of upconversion nanocrystals (UCNPs) and semiconductor nanocrystals can improve the upconversion luminescence (UCL) of rare-earth ions. Here, the LSPR absorption and morphology of the semiconductor nanocrystalline TiO2: W were adjusted by using ammonium fluoride during synthesis. Significant absorption enhancement of TiO2: W in the near-infrared region was obtained to enhance the UCL of NaYF4: Yb3+, Er3+. The Glass/NaYF4: Yb3+, Er3+/TiO2: W@SiO2 layered structure films were fabricated through spin coating. Compared with Glass/NaYF4: Yb3+, Er3+, the green and red lights of the Glass/NaYF4: Yb3+, Er3+/TiO2: W@SiO2 films were enhanced by 15.9 and 17.8 times, respectively. The UCL enhancement of Glass/NaYF4: Yb3+, Er3+/TiO2: W@SiO2 was derived from the LSPR property of TiO2: W through the enhancement of the excitation. The present work is important for possible applications of these layered structures as biomarkers, photocatalysts, flexible materials, and photoluminescence display panels.

Er3+:NaYb2F7 imbedded fluorotellurite transparent glass ceramic for enhanced upconversion luminescence and related thermometric behavior

Fluorotellurite glass ceramics (GCs) are promising photonic materials owing to their unique characteristic of integration of low cut-off phonon energy of fluorides and the high refractive index of tellurite glasses. Here, appropriate heat treatment to obtain the transparent GC from the parent glass (PG) of TeO2–Al2O3–NaF–CaF2-YbF3-ErF3 was established. The XRD, TEM, and spectroscopy techniques were employed to characterize the structure and properties of the as-prepared samples. The single fluoride phase NaYb2F7 crystallized from the PG was confirmed by XRD. TEM images displayed that in the transparent GC the nanocrystals are uniformly distributed in glassy matrix with an average size of about 6 nm. Under the 980 nm NIR excitation, the transparent GC exhibited bright upconversion luminescence in the visible region with intensity enhancement of ~73% comparing with the PG. The upconversion fluorescence kinetics were studied, along with the upconversion mechanism analyzed in details. Furthermore, the dependence of upconverted fluorescence intensity ratio (FIR) of the 2H11/2 → 4I15/2 emission to 4S3/2 → 4I15/2 emission from Er3+ ions on temperature was studied over a temperature range of 323–548 K. The results have shown that the Er3+:NaYb2F7 imbedded transparent fluorotellurite GC can be favorably employed for temperature sensing.

Structural design and synthesis of new MOO3-x interlayer bi-functional nanomaterials for enhanced up-conversion luminescence properties

A novel imaging materials based on bi-functional Fe3O4@MOO3-x@YF3:Yb/Er nanoparticles (NPs) with strong up-conversion luminescence and magnetic properties was designed and synthesized by inlaying MOO3-x with localized surface plasmon resonance (LSPR) and ferromagnetic property in the bi-functional Fe3O4@YF3:Yb/Er NPs. The morphology, structure and properties of Fe3O4@MOO3-x@YF3:Yb/Er NPs are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray (EDX), photoluminescence (PL) spectra, UV–Vis-NIR spectrophotometer and superconducting quantum interference devices (SQUID). It was found that the experimental conditions (reaction temperature, reaction time, the mass ratio of core:shell and oxygen pressure in heat treatment) have a certain effect on the oxygen defect concentration of the MOO3-x interlayer. XRD also showed the crystal structure of Fe3O4, Fe3O4@MOO3-x and Fe3O4@MOO3-x@YF3:Yb/Er NPs. The SEM and TEM results showed that the average grain size of the prepared NPs were 200 nm. The SQUID and PL results showed that the Fe3O4@MOO3-x@YF3:Yb/Er NPs have stronger magnetism (14.3 emu/g) and excellent up-conversion luminescence performance (the emission peak at 525 nm is nearly 20 times higher than the corresponding emission intensity of the Fe3O4@YF3:Yb/Er). The Fe3O4@MOO3-x@YF3:Yb/Er NPs can be easily used to magnetic resonance and fluorescence dual-mode image-guided visual delivery drug and also increase the accurate diagnosis and treatment effect of malignant tumor.

Thermal enhancement of up-conversion luminescence in Lu2W2.5Mo0.5O12: Er3+, Yb3+ phosphors

Lu2W2.5Mo0.5O12: Er3+/Yb3+ phosphors were synthesized through high temperature solid state method. Under 980 nm laser excitation, the Lu2W2.5Mo0.5O12: Er3+/Yb3+ compounds show thermal enhancement of up-conversion luminescence (UCL), which is attributed to the lattice contraction and distortion from negative thermal expansion (NTE) of Lu2W2.5Mo0.5O12 host enhancing the energy transfer of Yb3+ to Er3+, eliminating the energy transfer of Er3+ to Er3+ through Er3+ single-doped Lu2W2.5Mo0.5O12 phosphors without thermal enhancement of UCL. The green luminescence intensities at 693 K of the Lu1.98-xW2.5Mo0.5O12: 0.02Er3+, xYb3+ (x = 0.2, 0.3, 0.4) samples are 4.6, 4.3 and 7.0 times as that of 302 K, respectively. And through fluorescence intensity ratio (FIR) technique, the corresponding maximum absolute sensitivities are 0.00741, 0.00744 and 0.00723, respectively. The green monochromaticity of UCL spectra in Er3+/Yb3+ co-doped samples increase with the increasing of temperature, and the possible UCL mechanism with temperature was discussed. The results indicate that the Lu2W2.5Mo0.5O12: Er3+/Yb3+ phosphors can be applied at a high temperature as optical thermometer with a good green monochromaticity.

Morphology modulation and near infrared to visible upconversion luminescence properties of Er3+/Yb3+-doped α-NiMoO4 nanoparticles

In this work, for the first time, the near infrared (NIR) to visible upconversion luminescence (UCL) properties of α-NiMoO4 were explored by doping with Er3+/Yb3+ ions. A microwave hydrothermal method was used to fabricate Er3+ (2 mol%)/Yb3+ (12–20 mol%)-doped α-NiMoO4 phosphors. X-ray diffraction patterns suggest the formation of the Yb2(MoO4)3 phase, which is beneficial for UCL enhancement. Nanorods (α-NiMoO4), bundles of rods (α-NiMoO4: Er3+), and cactus (α-NiMoO4: Er3+/Yb3+)-like morphologies were obtained, and a morphology modulation mechanism has been proposed. TEM elemental mapping, XPS, and UV–Vis DRS measurements confirm the existence of Ni, Mo, O, Er, and Yb elements in the samples. The synthesized samples demonstrate intense green (522 nm: 2H11/2 → 4I15/2 and 546: 4S3/2 → 4I15/2), weak red (660 nm: 4F9/2 → 4I15/2), and NIR (803 nm: 4S3/2 → 4I13/2) emissions under 980 nm excitation. Downshifting emission was also obtained at 1538 nm (Er3+: 4I13/2 → 4I15/2). The highest emission intensity was achieved in the Er3+ (2 mol%)/Yb3+ (15 mol%)-doped α-NiMoO4 sample. An intense green color and strong UCL were observed in the MN/Er2/Yb15 sample based on CIE chromaticity and fluorescence decay curves. A graph of pump power versus intensity for the phosphor samples indicates that the upconversion process occurs by two photonic processes. The proposed energy transfer process is explained based on ground state absorption and excited state absorption. The particles were found to be safe at doses up to 0.125 mg/mL, as assessed by cytotoxicity investigation in bovine skeletal muscle cells. The obtained results open the possibilities of potential applications of Er3+/Yb3+-doped α-NiMoO4 as an efficient luminescent material for bioimaging applications.