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Bulk La2O3-Ga2O3-ZrO2 (LGZ) glass and Tm3+/Yb3+ co-doped LGZ glasses were synthesized successfully using containerless technique. Raman spectra result reveals that the matrix sample possesses the low highest phonon energy of ∼ 642 cm–1. The glasses show good compatibility between large Abbe numbers (>31) and high refractive indices (nd > 1.93). Moreover, transmittance measurements reflect the glasses have high infrared transmittance of ∼ 81.9%, small OH– absorption coefficient and long mid-infrared cut-off wavelengths (∼7.5 μm). The surface morphology of host glass was characterized by SEM micrograph and EDS tests reflect that the doped compositions are distributed into the matrix glass homogeneously. The results of thermal analysis show that the glasses have good thermal properties (Tg > 769 °C). Excited by 980 nm laser, an intense 1810 nm fluorescence is obtained originating from the transition: 3F4 → 3H6 of Tm3+ ion, accompanied by upconversion emission. It can be observed that 1810 nm fluorescence has the highest intensity at 1 mol% Yb2O3 and owns broad full width at half-maximum (>245 nm), the luminescence intensity of 3F4 → 3H6 transition increases with rising temperature from 300 to 550 K. Furthermore, the value of energy transfer efficiency shows that Yb3+ can transfer energy to Tm3+ effectively. By fitting the attenuation curves, the lifetimes of 1810 and 474 nm emission can be acquired.

Lanthanide doped fluorescence nanoparticles have gained considerable attention in biomedical applications. However, the low uptake efficiency of nanoparticles by cells has limited their applications. In this work, we demonstrate how the uptake efficiency is affected by the size of nanoparticles under flow conditions. Using the same size NaYF4:20% Yb3+,2% Er3+,2% Ce3+ nanoparticles as core, NaYF4:20% Yb3+,2% Er3+,2% Ce3+@NaYF4 core-shell structured nanorods with different sizes of 60–220 nm were synthesized by thermal decomposition and hot injection method. Under excitation at 980 nm, a strong upconversion green emission (541 nm, 2H11/2 → 4I15/2 of Er3+) is observed for all samples. The emission intensity for each size nanorod was calibrated and is found to depend on the width of NRs. Under flow conditions, the nanorods with 96 nm show a maximum uptake efficiency by endothelial cells. This work demonstrates the importance of optimizing the size for improving the uptake efficiency of lanthanide-doped nanoparticles.

Combining treatment and diagnosis, called theranostics, which is achieved within single nanoparticle is an ultimate goal of many studies. Herein, we developed a new nanotheranostic agent − Nd3+-sensitized upconversion nanoparticles core for dual modal imaging (i.e., upconversion luminescence imaging and magnetic resonance imaging) and antimony nanoshell for photothermal therapy (PTT). The core-shell-shell upconversion nanoparticles (NaYF4:Yb,Er@NaYF4:Yb,Nd@NaGdF4:Nd, named as UCNP) were firstly synthesized using thermal decomposition method and then were coated by antimony shell over the surface of UCNP using simple cost and time effective new method. Furthermore, the surface of UCNP@Sb nanostructures was modified with DSPE-PEG in order to enhance the water solubility and biocompatibility. The final nanotheranostic agent, named as UCNP@Sb-PEG, exhibits very low toxicity, good biocompatibility, very good photothermal therapeutic effect, and efficient upconversion luminescence (UCL) imaging of HeLa cells under only one laser (808 nm) irradiation. The antimony shell is quenching the upconversion emission in pristine nanotheranostic agent, but interestingly, the UCL intensity of the agent recovers progressively under 808 nm laser irradiation due to light induced degradability of antimony shell. Besides, high longitudinal relaxivity (r1) obtained from the experiment approves excellent potential of the nanotheranostic agent for T1-weighted magnetic resonance imaging application.

Ion channels present in the plasma membrane are responsible for integration and propagation of electric signals, which transmit information in nerve cells. Malfunction of these ion channels lead to many neurological diseases. Recently, optogenetic technology has gained a lot of attention for the manipulation of neuronal circuits. Optogenetics is a neuromodulation approach that has been developed to control neuronal functions and activities using light. The lanthanide-doped upconversion nanoparticles (UCNPs) absorbed lower energy photons in near-infrared (NIR) window and emitted higher energy photons in the visible spectrum region via nonlinear processes. In the last few decades, UCNPs have gained great attention in various bio-medical applications such as bio-imaging, drug delivery, optogenetics etc. The near-infrared illumination is considered more suitable for optogenetics application, due to its lower degree of light attenuation and higher tissue penetration compared to visible light. Therefore, UCNPs have been considered as the new promising candidates for optogenetics applications. Upconversion nanoparticle-mediated optogenetic systems provide a great opportunity to manipulate the ion channel in deep tissue. Herein, we summarized the upconversion photoluminescence in lanthanide doped nanomaterials; its mechanisms and several approaches adopted to tune emission color or enhance upconversion efficiency. Recent advances of lanthanide-doped UCNPs design strategy and their mechanism are analyzed. Then, we discussed the neural circuitry modulation using upconversion nanoparticles mediated optogenetics. Moreover, the future perspectives towards optogenetics are also included.

For better use of solar energy, the development of efficient broadband photocatalyst has attracted extraordinary attention. In this study, a ternary composite consisting of Sr2LaF7:Yb3+,Er3+ upconversion (UC) nanocrystals and Bi nanoparticles loaded BiOBr nanosheets with oxygen vacancies (OVs, SLFBB) was designed and synthesized by multistep solvent-thermal method. Mechanisms of in-situ formation of Bi nanoparticles and OVs in BiOBr/Sr2LaF7:Yb3+,Er3+ composites (SFLB) are clarified. The Bi metal and OVs enhanced the light-harvesting capacity in the region of visible-near-infrared (Vis-NIR), and promoted the separation of electron–hole (e−/h+) pairs. Furthermore, the surface plasmon resonance (SPR) effect of Bi metal can improve the energy transfer from Sr2LaF7:Yb3+,Er3+ to BiOBr via nonradiative energy transfer process, resulting in enhancing the light utilization from upconverting NIR into Vis light. Due to the synergistic effects of UC function, SPR and OVs, the SFLBB exhibited obviously enhanced photocatalytic ability for the degradation of BPA with a rate of 8.9 × 10−3 min−1, which is about 2.78 times higher than 3.2 × 10−3 min−1 of BiOBr (BOB) under UV–Vis-NIR light irradiation. This work provides a novel strategy for the project of high-efficiency Bismuth-based broadband photocatalysts, which is helpful to further understand the mechanism of enhanced photocatalysis by UC function and plasmonic effect.

Upconversion nanosensitizers have been widely considered to have important applications in the treatment of major diseases such as tumors and the utilization of solar energy. Majority of the efforts so far have been focused on improving the efficiency of energy transfer (ET) between upconversion nanoparticles (UCNPs) and the anchored sensitizers with premise that high ET efficiency will lead to high acceptor efficacy. This premise is, however, proved by our current work to be invalid for commonly used load. Interaction between adjacent sensitizing molecules was found to be critical which undermines the amount of excited monomer sensitizers and thus fades the efficacy. Here NaYF4:Yb3+,Er3+ UCNPs and rose bengal (RB) photosensitizer molecules were used as the model energy donors and acceptors, respectively. Contrary to monotonous increase of the ET efficiency from UCNPs to RB species with increasing RB loading, acceptor efficacy characterized by the reactive oxygen species, as well as the RB fluorescence, exhibits bizarre dependence on the RB loading. The underlying mechanism was well studied by the steady-state and time-resolved spectroscopy of a series of samples. RB aggregates are believed to be responsible for the severe deviation between the ET efficiency and acceptor efficacy. The conclusion was validated by in vitro test where the photodynamic therapy with the most monomer RB in UCNPs-RB nanosensitizers kills 35.8% more cells than that with the highest RB loading. This understanding sheds light on construction of new ET based nanosystems for broad applications, such as medicine, solar energy utilization and optical storage.

Solar driven nitrogen (N2) fixation to synthesize ammonia is a potential alternative for the traditional Haber-Bosch approach to meeting industrial demand, but is largely hampered by the difficulties in the harvesting of solar energy and activating inert N2. In this work, hollow CeF3 nanospheres co-doped with activator Tm3+ and sensitizer Yb3+ (Yb3+:Tm3+:CeF3) were prepared by microwave hydrothermal method. The product was employed as a catalyst for photo-driven N2 fixation by adjusting the molar ratio of Ce3+:Yb3+:Tm3+. Results show that the porous hollow structure enhances the light-harvesting by physical scattering and reflection. In addition, heteroatom doping generates abundant fluorine vacancies (FV) which provide abundant active sites for adsorption and activation of N2. The sample with molar ratio of CeF3:Yb3+:Tm3+ at 178:20:2 demonstrates the highest utilization of solar energy attributed to the strongest upconversion capability of near-infrared (NIR) light to visible and ultraviolet (UV) light, and the NH4+ concentration achieves the highest value of 15.06 μmol/(gcat∙h) under simulated sunlight while nearly 6.22 μmol/(gcat∙h) under NIR light. Current study offers a promising and sustainable strategy for the fixation of atmospheric N2 using full-spectrum solar energy.

The red upconversion emission of Ho3+ ions, in the optical window of biological tissue, exhibits excellent prospects in biological applications. This study aims to enhance the red upconversion emission intensity of Ho3+ ions in NaLuF4:20%Yb3+/2%Ho3+/12%Ce3+ nanoparticles through building different core-shell structures with different excitation wavelengths. A significantly enhanced red upconversion emission with a higher red-to-green ratio was successfully obtained in NaLuF4:20%Yb3+/2%Ho3+/12%Ce3+@NaLuF4 core-shell nanoparticles by introducing the Yb3+ and Yb3+/Nd3+ ions into the NaLuF4 shell, with enhancement of the red emission occurring when Yb3+ and Nd3+ ions in the shell transfer more excitation energy to the Ho3+ ions. Investigation of the red emission enhancement mechanism is based on spectral characteristics and lifetimes. We examined the synergistic effect of dual-wavelength co-excitation NaLuF4:20%Yb3+/2%Ho3+/12% Ce3+@NaLuF4:10%Yb3+/15%Nd3+ core-shell nanoparticles to establish optimal excitation conditions. It is hoped that this method, using red upconversion emission core-shell nanoparticles with multi-mode excitation, can provide new ways to expand the applications of rare-earth luminescent materials in biomedicine and anti-counterfeiting.

The luminescent characteristics of spherical hafnia/silica (HfO2/SiO2) nanoparticles (NPʼs) co-doped with Tb3+/Yb3+ were analysed. These NPʼs were synthesized using the spray pyrolysis technique. The addition of SiO2 and Tb3+/Yb3+ was found to induce a cubic phase in HfO2. The luminescent spectra presented the characteristic emission peaks for inter-electronic energy levels transitions of the Tb3+ and Yb3+ ions, with an excitation band centred at 270 nm. Under solid-state laser excitation at 980 nm an upconversion emission related to the Tb3+ ion was observed. The maximum emission peak in the visible region was at 543 nm, associated with 5D4 → 7F5 transitions of the Tb3+ ions and an IR emission peak at 970 nm (2F5/2 → 2F7/2) pertaining to Yb3+, with irradiation at 270 nm (UV). The energy transfer mechanism from Tb3+→Yb3+ (excitation at 270 nm), is discussed based on the time decay of the luminescence intensity analysis and the energy transfer efficiency (ηET) and was determined to be in the range of 29.2%–40.8%.

To acquire efficient photocatalysts, it is necessary to make effective use of visible light/Near Infrared (NIR) light, which takes up a large percentage of sunlight. Integrating upconversion materials with visible light active photocatalysts has attracted much attention in this regard. The interface contact between upconversion material and photocatalyst has potential influence on the properties and thus the performance of the system. In this work, NaYF4:Yb,Er/CdS composites of the upconversion material NaYF4:Yb,Er nanorods and CdS nanoparticles were synthesized by ion adsorption/precipitation process and were then annealed in an argon atmosphere at different temperatures to modulate the microstructures. The annealing process endows the crystal transformation of cubic CdS with low crystallinity to hexagonal CdS with high crystallinity and, importantly, good interface contact between NaYF4:Yb,Er and CdS. Consequently, the hydrogen evolution activity greatly increases from 171 to 2539 μmol h−1 g−1 under the light irradiation of λ > 400 nm, and from 0 to 19 μmol h−1 g−1 under the light irradiation of λ > 600 nm. This work might provide a useful reference for the rational design of promising photocatalyst involving upconversion materials.

In this study, the hexagonal NaYF4:Yb3+/Ho3+/Ce3+ microcrystals were synthesized controllably, and upconversion luminescence excited at 940 nm and its application in temperature-responsive anti-counterfeiting are reported. It is clarified that the Ln3+ (Ln = Y + Yb + Ho + Ce) density ratio of bottom plane to side plane in the unit cell can be regulated by Ce3+ doping. It is also proved that the energy transfer of Yb3+ to Ho3+ is responsible for the activation of Ho3+ under 940 nm excitation, while the cross relaxation between Ho3+ and Ce3+ participates in the redistribution of electron population of 5S2/5F4 and 5F5 levels. Both theory and experiment confirm that the intensity ratio of red to green emission (IR/IG) as a function of temperature as an independent variable has good linear characteristics in the temperature range of 300–500 K. Due to the good responsiveness of multicolor luminescence to temperature, the hexagonal NaYF4:Yb3+/Ho3+/Ce3+ with tunable morphology is a promising candidate for advanced temperature-responsive upconversion anti-counterfeiting. Our results provide a new pathway for the controllable synthesis of hexagonal NaYF4 microcrystals as well as the regulation of upconversion luminescence that is excited by wavelengths other than 980 nm and its application in anti-counterfeiting.

Photothermal therapy (PTT) has presented its inherent application value in cancer treatment. Nevertheless, single-functional photothermal materials cannot meet the precise diagnosis and treatment of cancer. Therefore, it is important to design a nanocomposite that has both high therapeutic efficiency and multimodal imaging capabilities. In our work, a new nanostructure of gold nanorods (AuNRs) with silver shells decorated by BaGdF5:Yb3+,Er3+ nanoparticle was synthesized by a simple way. The biostability of AuNRs is increased by coating with silver shells, and the AuNR@Ag nanoparticles can be used as excellent surface enhanced Raman scattering (SERS) probe. Moreover, the modification of BaGdF5:Yb3+,Er3+ nanoparticles provides the possibility of real-time optical imaging of the tumor area. Under the irradiation of NIR laser, the AuNR@Ag/BaGdF5:Yb3+,Er3+ nanocomposites (NCs) have strong upconversion emission and excellent photothermal conversion efficiency. Meanwhile, the NCs show low cytotoxicity and good biocompatibility in MTT cytotoxicity test. Moreover, the NCs are also exceptional contrast agents for CT imaging. For in vitro photothermal therapy test, NCs show excellent killing efficiency on tumor cells. Therefore, the multifaceted research of AuNR@Ag/BaGdF5:Yb3+,Er3+ multifunctional nanomaterials provides a break for high-efficiency tumor photothermal therapy and multimodal imaging.

Rare earth-doped upconversion nanoparticles (UCNPs) have the advantages of a narrow emission peak, large anti-Stokes displacement, negligible autofluorescence and photobleaching, which make them popular in biomedical fields. With the development of biological technology and nanotechnology, great progress has been achieved in use of rare earth-doped UCNPs in the treatment and diagnosis of various diseases. To date, based on the light conversion characteristics of upconverted rare earth nanocrystals, their medical applications are mainly focused on the detection and treatment of tumors. In view of the above situation, this paper focuses on reviewing UCNPs applications for the treatment and detection of various diseases, including atherosclerosis, inflammation, bacterial infections, viruses, and tumors. Finally, challenges and opportunities regarding the use of UCNPs in diseases treatment and detection are discussed. Overall, our goal is to provide readers with a fresh perspective on recent developments in the treatment and detection of various diseases with rare earth-doped upconversion materials.

To better utilize the infrared (IR) region in sunlight for photovoltaic devices (PVs), upconversion nanoparticles (UCNPs) have been proposed to improve power conversion efficiency (PCE). However, researchers recently have found that the upconversion (UC) effect is negligible in PVs performance improvement for their ultra-low UC photoluminescence quantum yields of UCNPs solid film, while the real mechanism of UCNPs in PVs has not been clearly studied. Herein, based on the material inorganic perovskites γ-CsPbI3, NaYF4:20%Yb3+,2%Er3+ UCNPs were integrated into different transport layer to optimize device performance. Compared with reference device, the short-circuit current density and PCE of optimized device reached 20.87 mA/cm2 (20.39 mA/cm2) and 18.34% (17.72%), respectively, without sacrificing open-circuit voltage and filling factor. Further experimental characterizations verified that the improved performance was attributable to enhanced visible light absorption instead of IR. To theoretically explain the statement, the light field distribution in device was simulated and the absorption in different layers was calculated. The results revealed that the introduction of UCNPs with different refractive index from other layers caused light field disturbance, and improved visible light captured by γ-CsPbI3. Importantly, through experiments and theoretical calculation, the research deeply explored the potential mechanism of UCNPs in optimizing PVs performance..

Heavy glass system (HGS) with batch compositionsTeO2-Bi2O3-Na2O co-doped with rare earth ions Er2O3&Yb2O3 are synthesized by melt- quench and press method. The batch composition is taken (80-x-y)%TeO2-10%Bi2O3-10%Na2O-x%Er2O3-y%Yb2O3 where (x, y) are (0.1, 0) and (0.5, 0.5). Thermal study of the synthesized sample is done by Differential Scanning Calorimetry (DSC). Amorphous nature of samples is characterized by X-Ray diffractogram. Heaviness of the samples justifies by the calculated densities of the glass samples. For the study of optical properties, Refractive index (R.I.), Optical energy gap (Eg), Urbach’s energy (Eu) absorption spectroscopy in ultra violet –visible-Infrared (UV–VIS-IR) region is carried. For upconversion (UC) characteristics photoluminescence spectroscopy studied.

A series of BaBi2-xNb2ErxO9 ceramic compositions with different Er3+ concentration (x = 0.0–8 mol %) is synthesized by a conventional solid-state reaction method. The upconversion (UC) light emission under 980 nm excitation with different pump powers and luminescence-based temperature sensing ability of BaBi2-xNb2ErxO9 composition have been examined. The formation of a Bi-layered perovskite phase of BaBi2Nb2O9 is confirmed having an orthorhombic geometry and Fmmm space group. Shifts in the Raman modes indicate reduced interaction of Bi3+ ions with NbO6 octahedron leading to relaxation of structural distortion with increasing Er3+ content. The maximum value for remnant polarization and coercive field of doped BaBi2-xNb2ErxO9 ceramic for (x = 0.08) Erbium concentration comes out to be 2.9524 μC/cm2 and 49.8980 kV/cm. For an optimum content of x = 0.04, two strong UC green emission bands were observed at 549 nm via 4S3/2 → 4I15/2 transition and 527 nm via 2H11/2 → 4I15/2 transitions, and a weak red emission appears at 657 nm attributed to the 4F9/2 → 4I15/2 transition. Pump power dependence suggests that UC emission is a two-photon mechanism for red and green emission bands. Temperature sensing evaluated by the change in the fluorescence intensity ratio (I527/I549) indicates the highest sensitivity to be 0.00996 K−1 at 483 K for an optimum concentration of Er3+ at x = 0.04 in BaBi2-xNb2ErxO9 composition and is useful for non-contact optical thermometry.

Herein, the nanoscaled ATP-responsive upconversion metal-organic frameworks (UCMOFs) are aqueous-phase synthesized for co-delivery of therapeutic protein cytochrome c (Cyt c) and chemodrugs doxorubicin (DOX), achieving targeted combinational therapy of human cervical cancer. The UCMOFs are rationally fabricated by growing ZIF-90 on mesoporous silica-coated upconversion nanoparticles (UCNPs), in which the ZIF-90 layer attenuates the upconversion luminescence (UCL) and the rigid frameworks increase the stability of encapsulated proteins. Once the UCMOF@DOX/Cyt c are internalized into HeLa cells via specific recognition of sgc8 aptamers, the intracellular ATP triggers the dissolution of ZIF-90 into Zn2+, which facilitates not only the release of Cyt c and DOX but also the restoration of UCL for real-time monitoring of drug release. It has been demonstrated that the therapeutic efficacy is greatly improved by the combination of caspase-mediated apoptosis activated by Cyt c (protein therapeutics), DNA fragmentation induced by DOX (chemotherapy), and Zn2+-promoted generation of reactive oxygen species (ROS) (oxidative stress). Overall, our proposed multifunctional UCMOFs provide an effective platform for targeted combinational cancer therapy and in situ imaging, which hold great promise in biomedical and clinical applications.

Effective colour modulation of upconversion emissions in lanthanide-doped nanomaterials becomes even more important for fundamental and applied research. Herein, on the one hand, by raising the content of doped Yb3+ from 10 mol% to 50 mol%, a significant increase of the red/green emission ratio from 4.0 to 68.2 is observed in K2NaScF6:Yb/Er nanocrystals. This yellow to red colour change is attributed to the increased cross relaxation between Er3+ and Yb3+ caused by the increased Yb3+ amount, 4S3/2 (Er3+) + 2F7/2 (Yb3+) → 4I13/2 (Er3+) + 2F5/2 (Yb3+). On the other hand, the upconversion green and red emission of K2NaScF6:Yb/Er (20/2 mol%) nanocrystals are intensified 10.6 and 8.8 folds, respectively, after an active shell (K2NaScF6:Yb) is epitaxially grown, which are more effective than the 7.4- and 6.4-fold enhancement from an inert shell (K2NaScF6) growth. Moreover, the shell thickness from 2.85 to 9.5 nm through controlling the molar ratio of shell-precursor to core from 1:2 to 3:1 can be easily realized. This study will provide more opportunities for the application of K2NaScF6:Yb/Ln nanoparticles in varied fields such as theranostics, photovoltaics, and photocatalysis.

x at. % Er3+, 3 at. % Dy3+: CaF2 transparent ceramics (x=1-5) with good transparency were fabricated by hot-pressed sintering. The phase composition of nanoparticles and transparent ceramics, microstructure, in-line transmittance, upconversion spectra and lifetime of transparent ceramics, as well as energy transfer mechanism between Er3+ and Dy3+ were investigated. The mean grain sizes of nanoparticles decreased from 33.0 nm to 26.2 nm with the Er3+ doping concentration increasing from 1 to 5 at.%. The microstructure of ceramic samples presented nearly dense microstructure and EDS analysis indicated Er3+ and Dy3+ were uniformly incorporated into CaF2 lattice. Under 900 nm excitation, the emission intensity for 4F9/2→6H15/2 transition of Dy3+ decreased and for 4S3/2→4I15/2 transition of Er3+ increased, the lifetime for the 4F9/2 level of Dy3+ decreased while the 4F7/2 level of Er3+ increased with the raise of Er3+ doping concentration. The energy transfer mechanism was proved to be the dipole-dipole interaction. The upconversion luminescence color was tuned from orange through yellow to green by changing the Er3+/Dy3+ ratio. In addition, the Vickers hardness, fracture toughness, and the thermal conductivity of Er3+, Dy3+: CaF2 transparent ceramics were discussed. All the results showed the Dy3+ could be used as a sensitizer for Er3+: CaF2 transparent ceramic in the upconversion field.

Photon upconversion of lanthanides has been a powerful means to convert low-energy photons into high-energy ones. However, in contrast to the mostly investigated lanthanide ions, it has remained a challenge for the efficient upconversion of Nd3+ due to the deleterious concentration quenching effect. Here we report an efficient strategy to enhance the upconversion of Nd3+ through the Yb3+-mediated energy cycling in a core-shell-shell nanostructure. Both Nd3+ and Yb3+ are confined in the interlayer, and the presence of Yb3+ in the Nd-sublattice provides a more matched energy for the upconversion transitions occurring at the intermediate state of Nd3+ towards much better population at its emissive levels. Moreover, this design also minimizes the possible cross-relaxation processes at both intermediate level and the emissive levels of Nd3+ which are the primary factors limiting the upconversion performance for the Nd3+-doped materials. Such energy cycling-enhanced upconversion shows promise in temperature sensing.

For anti-counterfeiting and full-color display applications, adjusting the Yb3+ concentrations or excitation conditions is the basic and simple way to control luminescence color and intensity. However, this approach is significantly limited because the luminescence color is not very sensitive to the response of doped Yb3+ concentrations or excitation conditions. Herein, we have demonstrated that the as-synthesized NaLuF4:Yb/Er and their annealed counterpart LuOF:Yb/Er microrods exhibit upconversion/downshifting double-mode luminescence with complementary and rich colors from green, yellow to red under 980/365/488 nm excitation. The anti-counterfeiting patterns printed with these microrods inks have the ability of the color response to the excitation wavelength, excitation power and laser scanning speed, which make them be hard to duplicate. This study indicates that Yb/Er doping NaLuF4 and their complementary LuOF microrods have great potential in multilevel anti-counterfeiting application and display. In addition, heat treatment way also brings novel ideas to the synthesis of high efficient luminescence materials.

Cerium fluoride (CeF3) semiconductor with upconversion property was constructed on graphite carbonitride (g-C3N4) nanosheets by microwave hydrothermal method. The X-ray diffraction, transmission election microscopy, Fourier transform infrared, and X-ray photoelectron spectra techniques were used to characterize the CeF3/g-C3N4 nanocomposite. The study shows that CeF3 has upconversion property and can convert visible light (Vis) and near-infrared light (NIR) into ultraviolet light (UV). Moreover, CeF3 and g-C3N4 can form well-defined heterojunction and promote the effective separation of photogenerated electrons and holes. The synergistic effect of the CeF3/g-C3N4 nanocomposite was evaluated by photocatalytic degradation of dibenzothiophene (DBT). The optimum photocatalyst of CeF3/g-C3N4 (40 wt%) composites exhibit the highest photocatalytic desulfurization rate of the model oil under visible light radiation.

Optomagnetic multifunctional composite based on upconversion luminescence nanomaterial is regarded as a promising strategy for bioimaging, disease diagnosis and targeted delivery of drugs. To explore a mesoporous nanostructure with excellent water dispersibility and high drug-loading capacity, a novel nanorattle-structured Fe3O4@SiO2@NaYF4:Yb,Er magnetic upconversion nanorattle (MUCNR) was successfully designed by using Fe3O4 as core and NaYF4:Yb,Er nanocrystals as shell. The microstructures and crystal phase of the as-prepared MUCNRs were evaluated by transmission electron microscopy, X-ray powder diffraction and N2 adsorption/desorption isotherms. The Kirkendall effect was adapted to explain the formation mechanism of the MUCNRs. The loading content and encapsulation efficiency of doxorubicin hydrochloride (DOX) could reach as high as 18.2% and 60.7%, respectively. Moreover, the DOX loading MUCNR (DOX-MUCNR) system showed excellent sustained drug release and strong pH-dependent performance, which was conducive to drug release at the slightly acidic microenvironment of tumor. Microcalorimetry was used to quantify the interactions between the carrier structure and drug release rate directly. The heat release rates in the heat-flow diagrams are basically consistent with the DOX release rate, thereby showing that microcalorimetry assay not only provides a unique thermodynamic explanation for the structure–activity relationship of Fe3O4@SiO2@NaYF4:Yb,Er MUCNRs but also provides powerful guidance to avoid the blind selection or design of drug carriers. Therefore, our work firmly provided a comprehensive perspective for using Fe3O4@SiO2@NaYF4:Yb,Er MUCNRs as a remarkable magnetic targeted drug carrier.

Lanthanide-doped upconversion nanoparticles (UCNPs) have been extensively investigated due to their unique capabilities of upconverting near-infrared light (NIR) to visible/ultraviolet emission. However, use of conventional Yb-based UCNPs under 980 nm excitation for biomedical applications is limited due to the overheating caused by the strong light absorption by water at this wavelength. Although this could be improved by using Nd3+-Yb3+ codoped UCNPs and changing the excitation wavelength to 808 nm, the amount of Nd3+ doping is usually below 20 mol% due to the lattice strain in highly Nd-doped core–shell structures. In this study, we report Nd3+-sensitized NaYF4:Yb,Er@NaLuF4:Nd@NaLuF4 UCNPs, in which the NaLuF4 in the intermediate shell can accommodate more structural changes caused by the Nd3+ doping, and allow for high concentration of Nd3+ doping (up to 50 mol%). Due to such high Nd3+ doping in the nanostructure, the red and green upconversion emissions of as-synthesized UCNPs are significantly increased upon 808 nm excitation, which are used to activate two photosensitizer drugs, MC540 (merocyanine 540) and FePc (iron phthalocyanine), for the dual photodynamic and photothermal therapy. The results show that the generation of reactive oxygen species (ROS) upon 808 nm light excitation is substantially boosted due to the synergistic therapeutic effect, which significantly prohibits the growth of cancer cells. It is believed that the nanoplatform specially developed in this study can solve the overheating issue associated with the 980 nm light excitation and the combined photodynamic and photothermal therapy can significantly improve the cancer therapy efficacy.

Bismuth oxyhalides (BiOX (X = Cl, Br, I) are considered to be an important p-type semiconductors in the photocatalysis applications. In particular, tetragonal BiOBr is considered as a stable photocatalyst due to its resilient absorption in the visible region with an band gap energy of 2.8 eV. In the meantime, lanthanide ions (with 3+ oxidation state) implies as conversion catalyst gained huge impact and remain a serious topic in materials chemistry. Here we synthesized upconversion photocatalyst mainly consists of BiOBr with the Er 3+ and Tb 3+ ions along with low band gap g-C3N5 for the improved photocatalytic performances. The synthesized Er3+/Tb3+@BiOBr-g-C3N5 heterojunction was systematically characterized by XRD, and FT-IR for the confirmation of the composite and their morphology were analysed with FESEM and HR-TEM analysis which revealed that the sheets of g-C3N5 were decorated by Er3+/Tb3+ loaded BiOBr microspheres. The XPS analysis confirmed the suitable oxidation state of all the individual elements existing in the composite. As the UV-DRS analysis showed that the band gap of the Er3+/Tb3+ BiOBr-gC3N5 heterojunction was narrowed to 2.64 eV. To evaluate the photocatalytic efficiency of the synthesized g-C3N5, Er3+/Tb3+@BiOBr and Er3+/Tb3+@BiOBr-gC3N5 heterojunction under the simulated visible light irradiation source towards the aqueous sulfamethoxazole degradation. The Er3+/Tb3+@BiOBr-gC3N5 heterojunction shows maximum degradation efficiency of 94.2% after 60 min of visible light irradiation whereas the pure g-C3N5 provided about 43.8% and Er3+/Tb3+@BiOBr implies 55.2% degradation efficiency. The plausible degradation mechanism of pollutant removal was proposed.

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.

Upconversion spectral converters enable the utilization of infrared photons for dye-sensitized solar cells. However, the TiO2 photoanode photoelectron trapping loss caused by defects and ligands on upconversion material surface limits the dye-sensitized solar cells efficiency. Here, we separate spatially TiO2 particles and upconverters through crafting an uniform semiconductor BiOCl shell onto NaYF4:Yb3+,Er3+ upconverters surface by a hydrothermal method. Unlike common insulate SiO2 shells, the BiOCl shell decreases little the upconversion luminescence intensity from the upconversion core, but it can inhibit the photoelectron transfer from TiO2 to upconversion particles due to its higher conduction band position than that of TiO2, which eliminates the photoelectron trapping loss. In addition, the BiOCl shell can harvest 408 nm photons from upconversion cores to generate additional photoelectrons into TiO2 photoanode film. Consequently, the incorporation of upconversion-core/semiconductor-shell structured particles into TiO2 photoanodes of dye-sensitized solar cells achieves 29.8% increase of power conversion efficiency, whereas bare upconversion particles only get 11.9% increase as compared with dye-sensitized solar cells with pure TiO2 photoanodes. We quantify that among the relative efficiency increase by the incorporation of upconversion-core/semiconductor-shell particles: 17.2% from the reduced photoelectron trapping and the harvest of upconversion 408 nm photons by BiOCl shell, 4.9% from the green and red upconversion of near infrared photons, and 7.7% from the scattering effect of the designed spectral upconverters.

The applications of lanthanide-doped upconversion (UC) nanoparticles are limited by unsatisfactory photoluminescence intensity. Herein, we construct a simplified energy migration mediated upconversion (EMU) to improve the upconversion luminescence (UCL) of heavily doped particles with an active-core/active-shell structure. Er3+ ions in the shell have a temporary effect on energy, and then transfer energy to the Er3+ ions in the core, thus sensitizing the red emission of the Er3+ ions in the core. The EMU process of [Yb3+→Er3+] → [Er3+→ Er3+] and [Er3+→Er3+] → [Er3+→ Er3+] excited by 980 and 808 nm, respectively, were realized by the high doping concentration of Er3+ ions in NaErF4:xYb,40Mn@NaErF4:40Yb, 40Mn (x = 0–40) particles. The UCL intensity of NaErF4:20Yb40Mn@NaErF4:40Yb40Mn is significantly higher than that of α-NaErF4:40%Mn hollow nanocubes by 230 times under 980 nm diode laser excitation, also increased by 9.8 times under 808 nm laser excitation. The NaErF4:20Yb40Mn@NaErF4:40Yb40Mn particles can realize single-band red emission excited by 378, 522, 808 and 980 nm laser, respectively. This multiwavelength excited character of NaErF4:20Yb40Mn@NaErF4:40Yb40Mn makes it a potential candidate in anti-counterfeiting application potential.

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.

Summary Photosensitization under near-infrared (NIR) excitation is attractive for applications in photocatalysis and theragnostics because NIR light offers deeper penetration depth through various media, including biological tissues. Nevertheless, most photosensitization processes can only be stimulated by ultraviolet or visible light. Here, we demonstrate a new paradigm, based on the coupling of lanthanide inorganic nanoparticles and organic photosensitizers, for NIR photosensitization through lanthanide-triplet sensitization. We show that the triplet levels of a series of porphyrin and phthalocyanine sensitizers can be effectively populated by energy transfer from lanthanide nanoparticles. This approach allows to directly active lower-lying triplet states of photosensitizers without the need to populate their higher-lying excited singlet states. It enables photosensitization to generate singlet oxygen at ultralow NIR irradiance with over 100-fold improved performance compared with conventional upconversion-based NIR photosensitization, which may provide new opportunities for applications such as deep-tumor phototherapy and NIR-light driven photosynthesis.

In this study, a facile method to fabricate Janus microbeads based on photonic crystals and upconversion nanoparticles is designed. The Janus microbeads can be reversed under magnetic response and generate upconversion fluorescence under near-infrared light. Three kinds of core-shell upconversion nanoparticles (UCNPs) are prepared by the solvothermal method and are mixed with Fe3O4 nanoparticles and different sizes of colloidal spheres. The Janus microbeads are assembled according to the hydrophilic property of the mixture and the hydrophobic property of substrates. The upper parts of the Janus microbeads are photonic crystals assembled with colloidal spheres, and the other parts are Fe3O4. Meanwhile, UCNPs are distributed inside the Janus microbeads. Furthermore, the Janus microbeads are prepared into different lattice patterns using special templates. In the lattice patterns, the structural colors of Janus microbeads can be displayed and disappeared by magnetic field inversion, and under external NIR irradiation, Janus microbeads can generate upconversion fluorescence to achieve multiple color display. The Janus microbeads are also applied to both sides of the bank card, and various reading information methods are designed according to different response modes, which have important applications in pattern display, response materials, and anti-counterfeiting.

SrF2 transparent ceramic is a promising upconversion material due to the low phonon energy. The effect of different sintering temperatures on Er:SrF2 transparent ceramics was investigated. The suitable sintering temperature for Er:SrF2 transparent ceramics was 900 °C by hot-pressed sintering in this study. High quality of Er:SrF2 transparent ceramics with different doping concentrations were obtained. The upconversion luminescence spectra and decay behavior were compared between Er:SrF2 and Er:CaF2 transparent ceramics with different Er3+ doping concentration. The green emission of 5 at.% Er:SrF2 ceramic was much stronger than that of 5 at.% Er:CaF2 ceramic, while the red emission of Er:SrF2 ceramic was almost the same as that of Er:CaF2 ceramic. The upconversion luminescence lifetime of Er:SrF2 transparent ceramics was longer than that of Er:CaF2.All the results indicated Er:SrF2 transparent ceramics was a candidate for green fluorescent upconversion materials.

Upconversion (UC) technology makes it possible to harvest infrared (IR) light from the sun and has increasingly been employed in recent years to improve the efficiency of solar cells. The progress in the area concerns both research on fundamental principles and processes of UC and technologies of device fabrication. Significant increase of important solar cell parameters, like short-circuit photocurrent density and open-circuit photovoltage as well as the total photon-to-current efficiency, has been accomplished. We here review the research published during the last few years in the area, in particular we consider the two most cherished techniques, namely the incorporation of upconverting nanophosphors directly into the photoanodes of the solar cells and the introduction of plasmonic metal nanoparticles co-existing with the UC particles. Other ways to achieve strong field enhancement, and the use of the non-linear nature of UC, is to apply microlenses, with or without assisting plasmonic excitation. Further enhanced UC action has been demonstrated by broad band and effective harvesting by organic IR antennas, with subsequent mediation by an intermediate nanoshell of the energy into the upconverting core. Codoping, nanohybrid and layer-by-layer technologies involving upconverting particles as well as the use of upconverting nanoparticles in hole-transport and electrolyte layers, tested in recent works, are also reviewed. While most of these technologies employ upconverting rare earth metals for sequential photon absorption, the main alternative technique, namely triplet-triplet annihilation UC using organic materials, is also reviewed. It is our belief that all these approaches will be further much researched in the near future, with potentially great impact on solar cell technology.

Optomagnetic multifunctional composite has attracted much attention in recent years because of its promising application prospect in bioimaging, analysis, detection, disease diagnosis, and targeted drug delivery. To explore a dual-targeted therapy for cancer, a novel class of optomagnetic multifunctional composite (UCNP-Fe3O4@MSNs-FA) was successfully synthesized by using upconversion nanoparticles (UCNPs) as nucleus, embedding Fe3O4 nanoparticles into the SiO2 coating layer, and modifying the surface with folic acid (FA) to strengthen its tumor targeting performance. The properties of the composite were extensively studied. The obtained composite possesses excellent upconversion fluorescence, good dispersion, high specific surface area (229.347 m2/g), and saturation magnetization value (10.9 A·m2/g). Its drug loading content and encapsulation efficiency can reach as high as 14.2% and 47.3%, respectively, using doxorubicin hydrochloride (DOX) as model drug. The DOX-UCNP-Fe3O4@MSNs-FA system shows excellent sustained drug release and strong pH-dependent performance, in which the drug release would be accelerated at the slightly acidic microenvironment in the tumor; thus, the system can realize the targeted treatment of cancers. The viability of L929 cells demonstrates the good biocompatibility of the composite. Furthermore, DOX-UCNP-Fe3O4@MSNs-FA exhibits specific cytotoxicity to folate receptor (FR) positive tumor cells, whereas DOX has weak toxicity to FR-negative cells. Therefore, the as-prepared UCNP-Fe3O4@MSNs-FA can potentially be used as an anti-cancer targeted drug delivery system and enhance the therapeutic efficacy against FR-positive tumor cells.

Low luminescence efficiency of rare-earth ions doped upconversion (UC) nanomaterials is still a major limitation for their applications. Here, based on bismuth oxychloride nanosheets that show efficient photocarriers separation due to combining spontaneous polarization and layered semiconductor, we report a new carbon heterovalent doping strategy for efficient UC luminescence enhancement by suppressing the intermediate excited states of Er3+ ions. The first-principles calculations and photoelectrochemical characterizations provide evidences that the replacement of C ions for Cl strengthen the spontaneous polarization and inter electric field (IEF) of bismuth oxychloride nanosheets, which further improve the photocarriers separation efficiency. Under 808 or 980 nm excitation, the emission intensity of 4I13/2 energy level of Er3+ ions (1550 nm) increase slightly with C doping, but the its decay time and the visible UC emission are improved tremendously at the same time. We show that the recombination rate of intermediate excited state electrons of Er3+ ions with the ground state is inhibited by the enhanced IEF, which promotes the energy reabsorption transition to upper energy levels, thus enhancing the visible UC emission. This work not only may provide a new insight into the method for engineering of UC emissions but also deepen the understanding for layered semiconducting material to modify the transition of Lanthanide ions.

Maximum operating temperature in optical thermometry is limited due to poor thermal stability of the sensing material at high temperatures. Here, Yb, Er and Ho-doped α-SiAlON (Yb/Er/Ho-α-SiAlON) ceramic prepared by hot press method has been studied for optical thermometry via upconversion under 980 nm excitation. The phase of the sintered ceramic has been confirmed by Rietveld refinement of the X-ray diffraction data. The temperature-dependent upconversion was studied in a wide temperature range of 298–1023 K and fluorescence intensity ratio technique was used for temperature sensing behavior. Excellent spectral/thermal stability over the investigated temperature range was observed for the Yb/Er/Ho-α-SiAlON as the sensing material. The maximum values of absolute and relative sensitivity based on the thermally coupled levels of Er3+ are 59.2 × 10−4 K−1 and 1.10 %.K−1 at 298 K and that based on non-thermally coupled levels are 108 × 10−4 K−1 at 385 K and 0.345 %.K−1 at 329 K, respectively.

The design of the photocatalytic materials has made a great of remarkable progress in the area of the enhancement photocatalytic activity, but there are still lots of problems such as wide band gap, low utilization of sunlight, low quantum efficiency and poor stability, which further limit the extensive practical applications. Thus, it is a hot research topic and key scientific problem to be solved that how to design and prepare the catalysts, which can respond to visible and near-infrared light in sunlight. Inspired by efficient nonlinear optical upconversion materials, upconversion-based nanocomposites can indirectly broaden the absorption ranges of semiconductors by converting the captured long-band visible and near-infrared incident light into high-energy short-band visible or ultraviolet light, which can be adopted as the promising candidate in wide-spectral-light-activating photocatalytic materials coupling with conventional semiconductors. According to our recent works and literature reports, recent review summarizes the research progress of photocatalytic materials with upconversion effect on photolysis of water for hydrogen production, degradation of organic and inorganic pollutants, reduction of CO2 and photodynamic therapy. The prepared nanocomposites can suppress the recombination of electrons and holes, and greatly improve the photocatalytic efficiency by the synergistic effect. It maybe stimulates a great interest in rational design and preparation of efficient full-spectrum photocatalytic systems and their wide application in solar energy conversion.

The crystal structure and surface morphology of the Er3+/Yb3+/Na+:ZnWO4 phosphors synthesized by solid state reaction method were analyzed by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) analysis. The frequency upconversion (UC) emission study in the developed phosphors was investigated by using 980 nm laser diode excitation. The effect of codoping in the Er3+:ZnWO4 phosphors on the UC emission intensity was studied. The UC emission bands that are exhibited in the blue (490 nm), green (530, 552 nm), red (668 nm) and NIR (800 nm) region correspond to the 4F7/2→4I15/2, 2H11/2, 4S3/2→4I15/2, 4F9/2→4I15/2 and 4I9/2→4I15/2 transitions, respectively. The temperature sensing performance of the Er3+-Yb3+-Na+:ZnWO4 phosphors was investigated based on the 2H11/2→4I15/2 and 4S3/2→4I15/2 thermally coupled transitions of the Er3+ ions. The photometric study was also carried out for the developed phosphors.

It is highly significant to develop multi-mode optical anti-counterfeiting materials to efficiently fight against counterfeit products. In this study, we chose ferroelectric K0.5Na0.5NbO3 (KNN) with excellent photochromism properties as the host and rare-earth Er3+ and Pr3+ ions as dopants to prepare the Er3+/Pr3+-codoped KNN ceramics. The color-tunable emissions can be obtained from red-orange-yellow to green by controlling the excitation wavelength. Upon 980 nm excitation, the synthesized ceramics does not only have superior upconversion (UC) emission behaviors but also have good luminescence modulation properties based on the photochromism properties. It is found that the KNN:0.003Er3+/0.003Pr3+ sample with the optimal UC emission features shows a highest ΔRt value of 74.52% when irradiated by 390 nm light for 5 min, whereas the KNN:0.005Er3+/0.003Pr3+ ceramics also exhibit a high ΔRt value of 66.81% under 395 nm light irradiation. According to the XPS and EPR results, one knows that the mechanism of luminescence modulation is closely related to defects and traps caused by the oxygen vacancies. Furthermore, the optical information writing and erasing test is conducted, exhibiting a good reproducibility and fatigue resistance. These results reveal that the designed ceramics are appropriate for the anti-counterfeiting applications.

Yb3+/Er3+codoped La10W22O81 (LWO) nanophosphor rods have been successfully synthesized by a facile hydrothermal assisted solid state reaction method, and their upconversion photoluminescence properties were systematically studied. X-ray diffraction patterns revealed that the nanophosphors have an orthorhombic structure with space group Pbcn (60). A microflowers-like morphology with irregular hexagonal nanorods was observed using field emission scanning electron microscopy for the Yb3+(2 mol%)/Er3+(2 mol%):LWO nanophosphor. The shape and size of the nanophosphor and the elements along with their ionic states in the material were confirmed by TEM and XPS studies, respectively. A green upconversion emission was observed in the Er3+: LWO nanophosphors under 980 nm laser excitation. A significant improvement in upconversion emission has been observed in the Er3+: LWO nanophosphors by increasing the Er3+ ion concentration. A decrease in the upconversion emission occurred due to concentration quenching when the doping concentration of Er3+ ions was greater than 2 mol%. An optimized Er3+(2 mol%): LWO nanophosphor exhibited a strong near infrared emission at 1.53 μm by 980 nm excitation. The green upconversion emission of Er3+(2 mol%): LWO was remarkably enhanced by co-doping with Yb3+ ions under 980 nm excitation because of energy transfer from Yb3+ to Er3+. The naked eye observed this upconversion emission when co-doping with 2 mol% Yb3+. In order to obtain the high upconversion green emission, the optimized sensitizer concentration of Yb3+ ions was found to be 2 mol%. The upconversion emission trends were studied as a function of stimulating laser power for an optimized sample. Moreover, the NIR emission intensity has also been enhanced by co-doping with Yb3+ ions due to energy transfer from Yb3+ to Er3+. The energy transfer dynamics were systematically elucidated by energy level scheme. Colorimetric coordinates were determined for Er3+ and Yb3+/Er3+: LWO nanophosphors. The energy transfer mechanism was well explained and substantiated by several fluorescence dynamics of upconversion emission spectra and CIE coordinates. The results demonstrated that the co-doped Yb3+(2 mol%)/Er3+(2 mol%): LWO nanophosphor material is found to be a suitable candidate for the novel upconversion photonic devices.

We investigated the upconversion (UC) emissions and their ambient dependences of SrTiO3 polycrystals co-doped in Er3+ and Yb3+ at different substitution sites. i.e., the A-site and B-sites in ABO3-type perovskite, and its response to H2 and O2 ambient annealing. Under near-infrared excitation at 980 nm, the as-synthesized samples exhibited strong UC emission features in the green (525 and 550 nm) and red (660 nm) region from Er3+ ions owing to sensitization by Yb3+; the emission was much stronger for A-site doping than for B-site doping. Interestingly, annealing in the H2 atmosphere to increase the oxygen vacancies suppressed the photoluminescence and UC emission of the A-site doped samples, but enhanced the emission signals of the B-site doped samples. After subsequent annealing in the O2 atmosphere to decrease the oxygen vacancies, the emission intensities showed a tendency to return to those in the as-synthesized A-site doped and B-site doped samples. These intriguing behaviors were explained in terms of the relationship between the substitution site and charge compensation. We performed the temperature dependent UC emissions and found that the intensity ratio between two green emissions changed significantly with temperature. This strong fluorescence intensity ratio could be used for optical thermometry.

Tungstate based phosphor materials are very much promising because of their emerging application in various fields including optoelectronic devices, display devices, sensors, anti-counterfeiting, bioimaging, etc. In the present work, Er3+: MgWO4 phosphors have been synthesized via conventional solid state reaction method. The phase formation of the phosphors is confirmed by X-ray diffraction analysis. The vibrational modes and phonon frequency of the prepared phosphors have been investigated by FTIR analysis. Upon 980 nm diode laser excitation, the Er3+: MgWO4 phosphors exhibit several upconversion (UC) emission bands peaking at ∼489 nm, ∼523 nm, ∼545 nm, ∼655 nm and ∼800 nm. The pump power dependent UC emission study of the Er3+: MgWO4 phosphors has been carried out. The responsible mechanisms for the UC emission in the present phosphors have been explained by schematic energy level diagram. The reason behind the concentration quenching phenomenon has been discussed. The CIE chromaticity coordinates of the present phosphors have been calculated using GoCIE software. Thus, the synthesized phosphors can be suitably applicable in NIR to visible upconverter, temperature sensing, security ink and solid state lighting purposes.

Lanthanide doped crystalline materials has gathered an immense impression to develop highly efficient upconverting phosphors for industrial as well as biomedical applications. The interaction between the electron cloud of the lanthanides and the crystal field of different host matrices affects the transition probabilities between the energy levels of lanthanide ions. Here, we have investigated the structural and optical properties of Er3+ doped YVO4 phosphors with the irradiation of 980 nm continuous wave diode laser. A twin efficient green band peaking at ∼ 525 nm and ∼ 554 nm has been observed along with a relatively low intense red band at ∼ 660 nm. The radiative/non radiative transitions between energy levels of Er3+ ions are well understood with the help of energy level diagram. The pump photons required to populate the green and red levels have been calculated. The colour coordinate of the prepared phosphors has been calculated by CIE chromaticity diagram. The present lanthanide-based phosphors have an efficient and bright upconversion emission, with applications spanning optical sensing, biological imaging, solid state lighting and LEDs.

The Gd2O3 nanophosphors codoped with Yb3+/Tm3+ and Yb3+/Tm3+/Li+ ions have been successfully synthesized by conventional solid-state reaction method. Upon 980 nm laser diode excitation, the upconversion emission spectra of the synthesized phosphors have been recorded. The observed spectra show intense blue bands at ∼ 477 nm, ∼ 489 nm and one red band at ∼ 656 nm, which are assigned to 1G4(a) → 3H6, 1G4(b) → 3H6 and 1G4(b) →3F4 transitions of Tm3+ ion respectively. The absorption spectrum of the prepared phosphor has been recorded using UV–Vis-NIR spectrophotometer. The mechanism involved in UC emission process have been analysed with help of energy level diagram and pump power study. Moreover, the CIE colour coordinates of synthesized phosphors lies in blue region which indicates that the prepared phosphors have potential applications in NIR to Visible upconversion and in blue LEDs.

Novel hydrophilic NaYF4:Yb3+,Tm3+@NaGdF4:Ce3+,Eu3+ double-jacket microtubes (DJMTs) with upconversion/ downconversion dual-mode luminescence were designed and prepared through epitaxial growth of NaGdF4:Ce3+, Eu3+ shell onto the NaYF4:Yb3+,Tm3+ microtube via poly(acrylic acid) (PAA) mediated hydrothermal method. It is demonstrated that PAA ligand played an important role in guiding the direct growth of NaGdF4:Ce3+,Eu3+ shell onto the surface of NaYF4:Yb3+,Tm3+ parent microtubes. The growth of NaGdF4:Ce3+,Eu3+ shell experienced a crystal phase transition from β-NaGdF4 and β-NaYF4 mixture to β-NaYF4@NaGdF4 composite crystal, and morphology evolution from mixture of β-NaGdF4:Ce3+,Eu3+ nanorods and β-NaYF4:Yb3+,Tm3+ microtubes to NaYF4:Yb3+,Tm3+@NaGdF4: Ce3+,Eu3+ DJMTs. The formation mechanism of DJMTs was the dissolution−renucleation of β-NaGdF4:Ce3+,Eu3+ nanorods and the growth of β-NaGdF4:Ce3+,Eu3+ shell via the classical Ostwald ripening mechanism. The as-prepared DJMTs could exhibit blue upconversion and red downconversion luminescence, which was further made into environmentally benign luminescent inks for creating highly secured and fluorescent-based anti-counterfeiting patterns via inkjet printing.

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Utilization of up-conversion materials is an effective strategy to realize near-infrared (NIR) light driven photocatalysis. However, their wide emission range lead to the low utilization efficiency of near-infrared light. In this paper, carbon quantum dots (CQDs) and NaYF4: Yb3+, Tm3+ (NYF) are used to construct efficient photon conversion channel, which could realize effective utilization of NIR light via dual-upconversion process. Under 980 nm laser light irradiation, NYF emits discontinuous light in the range of 292–805 nm. Only lights below 519.8 nm can be absorbed by Ag3PO4 directly. The lights above 519.8 nm are also utilized by Ag3PO4 after the secondary conversion of CQDs. Such secondary light conversion significantly promotes the NIR light utilization efficiency by Ag3PO4. This work provides a new insight into designing and constructing superior photocatalyst by dual photon conversion channel for NIR light utilization.

Yb3+/Er3+ doped high-fluoride glass-ceramics were prepared by the high-temperature melting method. The crystal morphology and structure of the rare-earth doped glass-ceramics were characterized by X-ray diffraction, infrared spectra, and high-resolution transmission electron microscope. The influence of crystal size and distribution on the transmittance of glass-ceramics was analyzed. The upconversion emission intensity of the rare earth doped glass-ceramics at different heat treatment temperatures was evaluated. It is shown that the precipitation of CaF2 crystals greatly improves the upconversion emission efficiency of rare earth ions. By analyzing different high-power pump lasers and corresponding up-conversion luminous intensities, the mechanism of up-conversion luminescence of this material is further elaborated.

The multimodal [Upconversion (UC), Downshifting (DS) and Quantum cutting (QC)] behaviour of Tm3+/Yb3+ co-doped YTaO4 ceramic material is investigated extensively. The samples are prepared by solid-state reaction method at two different temperatures (at 1573 and 1723 K) and their structural and optical properties are compared with each other. The effect of synthesis temperature as well as doping, on crystal phase was analysed via X-ray diffraction measurements. Particles shape and size are investigated through scanning electron microscopy images. The structural behaviour is further verified through vibrational structural study of material via FTIR measurements. This shows similar structural changes as observed in XRD patterns of the samples prepared at two different temperatures. The optical band gap of YTaO4 is nearly 4.8 eV at both the temperatures. Intense blue (476 nm), red (648 nm) and NIR (802 nm) peaks are observed in UC emission spectra of Tm3+/Yb3+ co-doped YTaO4 ceramic on excitation with 980 nm. The blue UC emission is suitable for display devices. The UC emission intensity increases significantly when sample is prepared at higher temperature (1723 K). Addition of Bi3+ ion further enhances the overall UC emission intensity due to change in environment around the activator ion. On excitation with UV radiation (λExc = 284 nm), YTaO4: 1 Tm3+ produces intense DS emission at 454 nm (blue region) due to energy transfer from charge transfer band of [TaO4]3- to Tm3+ ion. The sample synthesized at 1723 K shows better DS emission. One interesting result is that addition of Bi3+ ion enhances the DS emission due to formation of metal-metal charge transfer band. We have also observed the quantum cutting phenomenon in which self-activated blue photon is converted into two or more NIR photons. The efficiency of quantum cutting emission increases significantly in presence of Bi3+ ion and shows strong, intense NIR emission through CET. Thus, the multimodal behaviour of Tm3+/Yb3+/Bi3+ doped YTaO4 makes them a suitable candidate for a spectral converter that can be used in display and solar cell devices.

Upconversion luminescence materials have stimulated growing attention in photovoltaic cell owning to the ability of converting a few low-energy photons into a single high-energy photon. However, there still remains a daunting challenge to achieve upconversion emission covering the entire visible spectrum in a single material. Here, an incandescent broadband white upconversion emission covering the entire visible region is realized in a single Gd2O3:Yb3+ nanoparticles by utilization of adverse multiphonon relaxation. Importantly, tunable long fluorescence lifetime can be synchronously implemented in the single material, where the lifetime range spans one order of magnitude. Furthermore, benefiting from the broadband emission, the luminescence intensity achieves 482-fold improvement as the power increase from 0.53 W to 1.51 W. These broadband emissions are enabled by engineering the pathways of nonradiative de-excitation via multiphonon relaxation. Based on the kinetic analysis and mechanism research, the photon avalanche is demonstrated to play an important role for broadband emission. The warm white emission together with its tunable long lifetime provides a new avenue for developing single emitter-based white-light-emission materials for next-generation display and lighting technologies.