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Phosphors that exhibit tunable broadband emissions are highly desired in multi-functional LEDs, including pc-WLEDs and pc-NIR LEDs. In this work, broadband emissions were obtained and modulated in the unexpectedly wide spectral range of 517–609 nm for (Lu0.99−xGdxBi0.01)2WO6 phosphors by tuning the Gd3+ content (x = 0–0.99). The effects of Gd3+ doping on phase constituents, particle morphology, crystal structure, and photoluminescence were systematically investigated. Broadband green emission was obtained from Gd3+-free (Lu0.99Bi0.01)2WO6 phosphors (x = 0), whose emission intensity was enhanced by 50% with 5 at% Gd3+ (x = 0.05). The phase transition happened when x > 0.50 and the broadband red-NIR emission was obtained when x = 0.75–0.99. Three luminescence centers were proved to be responsible for the broadband green emissions via crystal structure, spectral fitting and fluorescence decay analysis. A pc-WLED with a high color rendering index (Ra = 91.3), a stable emission color, and a low color temperature (3951 K) was fabricated from the (Lu0.94Gd0.05Bi0.01)2WO6 broadband green phosphor, and an LED device that simultaneously emits high color rendering index white light and NIR light was obtained with the (Gd0.99Bi0.01)2WO6 broadband red-NIR phosphor. Night vision and noninvasive imaging were also demonstrated using the latter LED device. [ABSTRACT FROM AUTHOR]

NaMnF3:Yb,Er and NaMnF3:Yb,Er@NaGdF4:xEu (x = 0, 0.3, 0.6, and 0.9) samples were synthesized by a simple hydrothermal method. The as-obtained NaMnF3:Yb,Er particles show a single red emission under the irradiation of infrared light (980 nm), due to the energy exchange transfer process between Mn2+ ions and Er3+ ions. Notably, further growth of NaGdF4:Eu on the basis of NaMnF3:Yb,Er enhanced the green up-conversion luminescence of Er3+ ions, which gradually shifted from red to green with the increase of the Gd3+ ion doping amount. In addition, NaMnF3:Yb,Er@NaGdF4:Eu exhibits red color under ultraviolet excitation (397 nm). These results provide a possible means for controlling the luminescence properties of the lanthanide doped up-conversion system. The luminescent film synthesized with NaMnF3:Yb,Er@NaGdF4:Eu can show different luminescent colors under different light irradiation (near infrared and ultraviolet), which indicates that they have potential applications in multi-mode anti-counterfeiting. [ABSTRACT FROM AUTHOR]

In the present study, the effect of heterovalent Fe3+ ions on the dielectric, pyroelectric, and ferroelectric properties of the (1 − x)AgNbO3–xFeNbO4 (x = 0.005, 0.01, 0.025, 0.05, and 0.1) system was investigated. The substitution of smaller ionic radius Fe3+ in B-sites and the formation of FeNbO4 as a secondary phase contributed to improved dielectric performance, especially the pyroelectric effect, of (1 − x)AgNbO3–xFeNbO4 ceramics by generating electron-rich ceramics. The (1 − x)AgNbO3–xFeNbO4 ceramics were prepared by conventional solid-state sintering. Pure AgNbO3 had a perovskite crystal structure with an orthorhombic crystal system, but the FeNbO4 in (1 − x)AgNbO3–xFeNbO4 ceramics was formed as a secondary phase with a monoclinic structure. In addition, the XRD and Raman spectroscopy data showed that some Fe3+ was substituted into B-sites of AgNbO3. The introduction of FeNbO4 effectively reduced the average grain size from 1.85 ± 0.09 μm to 1.22 ± 0.03 μm for pure AgNbO3 and 0.9AgNbO3–0.1FeNbO4, respectively. In addition, the relative density of the (1 − x)AgNbO3–xFeNbO4 ceramics decreased from 97.96% ± 0.01 for x = 0 to 96.75% ± 0.03 for x = 0.1. The real part of the permittivity ε′, at room temperature, increased from 186.6 for x = 0 to a value of 738.7 for x = 0.1. Additionally, the maximum pyroelectric coefficient increased fivefold, reaching values of 2270 nC cm−2 K−1 for x = 0.1. Furthermore, a harvested pyroelectric energy density (W) of 1140 μJ cm−3 for x = 0.025 was achieved, which is appreciably higher than the 840 μJ cm−3 value for x = 0. [ABSTRACT FROM AUTHOR]

Development of novel ultraviolet (UV) upconversion materials has been emerging as a hot research topic for application in tunable UV lasers, photocatalysis, sterilization, tagging, and most recently luminescence thermometry. We readily synthesized a series of Ho3+/Gd3+ co-doped LiYO2 upconversion phosphors by a traditional high-temperature reaction. Under excitation from a blue ∼445 nm laser, LiYO2:Ho3+,Gd3+ polycrystalline powders yield intense sharp ultraviolet B (UVB) upconversion luminescence from Gd3+ 6Pj (j = 7/2, 5/2, 3/2) excited states. By means of steady and dynamic photoluminescence spectra, we systematically investigated the involved two-photon absorption upconversion as well as the accompanying energy transfer processes between Ho3+ and Gd3+ ions in the LiYO2 host lattice. Interestingly, the distinguishable UVB luminescence constituents from Gd3+ 6Pj excited states exhibit sensitive temperature dependence in a 353–673 K range. Shedding light on thermal equilibria between Gd3+ 6Pj UV-emitting levels, their luminescence intensity ratios follow Boltzmann statistics for the application of new luminescence thermometry. For the scheme of 6P7/2–6P3/2 thermally coupled levels, it works over a temperature range of 373–673 K with a maximum relative sensitivity (Sr) of about 1.07% K−1 at 373 K, and its 6P7/2–6P5/2 counterpart works over 353–533 K with a maximum Sr of about 0.83% K−1 at 353 K. Overall, our study provides a new pathway to develop UV upconversion materials, and promotes the application of Gd3+-related UV luminescence constituents in sensitive temperature sensing over a wide temperature range. [ABSTRACT FROM AUTHOR]

Narrow band red-emitting phosphors are significant constituents but still a bottleneck for next-generation smart displays and high-performance lighting (solid-state lighting based white light-emitting diodes (WLEDs)) technology. This review emphasizes the fundamental understanding and comprehensive overview of the recent progress and challenges associated with inorganic phosphors or down (wavelength) convertors, providing special attention to narrowband red-emitting oxide phosphors for phosphor-converted WLEDs (pc-WLEDs). In this context, the comprehensive progress on trivalent europium (Eu3+, in scheelite and double perovskite structures) based oxide phosphors with special emphasis on structure–composition–property-correlations is briefly reviewed. Furthermore, the challenges faced in the design of new oxide red phosphors and strategies to improve their absorption, emission efficiency, and future research direction are highlighted. [ABSTRACT FROM AUTHOR]

Recently, Bi3+-activated phosphors have been extensively studied for potential applications in phosphor-converted white light-emitting diodes (pc-WLEDs). However, Bi3+ activators usually exhibit low quantum efficiency and poor thermal stability due to the outermost 6s6p-orbitals of Bi3+ being strongly coupled with the host lattice, inhibiting potential applications. Herein, we rationally design a novel phosphor CaBaGa4O8:Bi3+, which adopts a tridymite-type structure and crystallizes in the space group of Imm2. CaBaGa4O8:Bi3+ presents a bright green light emission peaking at 530 nm with a FWHM narrower than 90 nm. Comprehensive structural and spectroscopic analyses unravelled that Bi3+ emitters were site-selectively incorporated into the triangular prism (Ca2+-site) in CaBaGa4O8:Bi3+ since there exist two distinct crystallographic sites that can accommodate the Bi3+ ions. An excellent luminescence thermal stability of 73% of the ambient temperature photoluminescence intensity can be maintained at 423 K for CaBaGa4O8:0.007Bi3+. Impressively, the quantum efficiency (QE) of CaBaGa4O8:0.007Bi3+ was remarkably improved to 47.2% for CaBaGa4O8:0.007Bi3+,0.03Zn2+via incorporating the Zn2+ compensators without sacrificing the luminescence thermal stability. The high thermal stability and QE of CaBaGa4O8:0.007Bi3+,0.03Zn2+ are superior to most of the Bi3+-activated green-emitting oxide phosphors. The perspective applications in pc-WLEDs for CaBaGa4O8:0.007Bi3+,0.03Zn2+ were also studied by fabricating LED devices. [ABSTRACT FROM AUTHOR]

We present the combustion-based synthesis of BiFeO3 (BFO) and Gd:BiFeO3 perovskite nanoparticles. XRD analysis demonstrates that the undoped BFO (x = 0) perovskite sample shows a single perovskite phase with a rhombohedral structure. However, increase in the Gd3+ content from x = 0.05 and 0.15 to 0.25 led to the occurrence of a structural phase transformation from rhombohedral (BiFeO3) to orthorhombic (Bi2Fe4O9). With an increase in the Gd-dopant the average crystallite size of rhombohedral structures increased from 16 to 23 nm. The perovskite samples were examined using XPS, which confirmed the presence of Bi3+, Gd3+, Fe2+, and O2+ ions. FT-IR spectroscopy indicated the existence of elemental functional groups in the synthesized perovskite nanoparticles. Furthermore, the direct band gap measured by DRS reduced from 2.16 to 2.0 eV as the Gd concentration increased. The nanoparticles of the BFO perovskite had an uneven shape, a tendency to agglomerate, and fused grains with defined grain boundaries. At ambient temperature, both the undoped and Gd:BFO perovskite nanoparticles exhibit a ferromagnetic characteristic. It was found that the BET surface area of the undoped and Gd-doped BFO perovskite nanoparticles varied progressively from 4.38 to 33.52 m2 g−1. The catalytic oxidation studies conducted in a batch reactor under air conditions revealed that the synthesized catalysts, in particular, Gd:BFO (x = 0.25), exhibited higher conversion and selectivity efficiencies for glycerol (con. 100% and sel. 99.5%, respectively). [ABSTRACT FROM AUTHOR]

A facile chemical route is reported for synthesizing red-emitting photoluminescent/MRI multi-functional KLa(0.95−x)GdxF4:Eu3+ (x = 0 to 0.4) bio-compatible nanomaterials for targeted in vitro tumor imaging. Hexagonal phase pure nanoparticles show a significant and systematic change in morphology with enhanced photoluminescence due to the substitution of La3+ with Gd3+ ions. Single phase β-KLa(0.95−x)GdxF4:Eu3+ exhibits multifunctional properties, both intense red emission and strong paramagnetism for high-contrast bioimaging applications. These silica capped magnetic/luminescent nanoparticles show long-term colloidal stability, optical transparency in water, strong red emission, and low cytotoxicity. The cellular uptake of coated nanoparticles was investigated in liver cancer cell line Huh-7. Our findings suggest that these nanoparticles can serve as highly luminescent imaging probes for in vitro applications with potential for in vivo and live cell imaging applications. [ABSTRACT FROM AUTHOR]

The design and fabrication of rare earth upconversion nanoparticle (UCNP)-based nanomedical platforms have evoked increasing interest. However, their bio-safety is always the most worrisome problem. Most nanoparticles can accumulate in the internal organs, leading to acute toxicity, a long-term inflammatory response, or even fibrosis and cancer. In contrast, ultrasmall (sub-10 nm) nanoparticles have minimal safety risk because they can escape from macrophages, pass biological barriers, and be easily degraded or excreted from the body. In this review, we mainly introduce new progress in preparation strategies, imaging and drug delivery with regards to ultrasmall UCNPs, with an emphasis on rare earth fluorides, NaREF4. Finally, we discuss the future outlook and challenges relating to ultrasmall UCNPs. [ABSTRACT FROM AUTHOR]

Mesoporous Sm3+ doped CeO2 (Ce–Sm) with a nanocrystalline framework, a high content of Ce3+ and surface area (184 m2 g−1), have been synthesized through a facile aqueous solution-based surfactant assisted route by using inorganic precursors and sodium dodecyl sulphate as a template. The XRD results indicate that the calcined Ce–Sm and even the as-prepared material have a cubic fluorite structure of CeO2 with no crystalline impurity phase. XRD studies along with HRTEM results confirmed the formation of mesoporous nanocrystalline CeO2 at a lower temperature as low as 100 °C. A detailed analysis revealed that Sm3+ doping in CeO2 has increased the lattice volume, surface area, mesopore volume and engineered the surface defects. Higher concentrations of Ce3+ and oxygen vacancies of Ce–Sm resulted in lowering of the band gap. It is evident from the H2-TPR results that Sm3+ doping in CeO2 strongly modified the reduction behavior of CeO2 by shifting the bulk reduction at a much lower temperature, indicating increased oxygen mobility in the sample which enables enhanced oxygen diffusion at lower temperatures, thus promoting reducibility, i.e., the process of Ce4+→ Ce3+. UV-visible transmission studies revealed improved autocatalytic performance due to easier Ce4+/Ce3+ recycling in the Sm3+ doped CeO2 nanoparticles. From the in vitro cytotoxicity of both pure CeO2 and Sm3+ doped CeO2 calcined at 500 °C in a concentration as high as 100 μg mL−1 (even after 120 h) on MG-63 cells, no obvious decrease in cell viability is observed, confirming their excellent biocompatibility. The presence of an increased amount of surface hydroxyl groups, mesoporosity, and surface defects have contributed towards an improved autocatalytic activity of mesoporous Ce–Sm, which appear to be a potential candidate for biomedical (antioxidant) applications. [ABSTRACT FROM AUTHOR]

A novel nanocomposite photocatalyst NaLuF4:Gd,Yb,Tm@SiO2@Ag@TiO2 was developed for the first time. This composite material has a sandwich structure, including a NaLuF4:Gd,Yb,Tm upconversion nanocrystals (UCNCs) core, a media shell of amorphous SiO2 decorated with Ag nanoparticles, and an outer shell of anatase TiO2. The designed new structure takes advantage of the synergetic effect of UCNCs, Ag nanoparticles and TiO2. The UCNCs absorb near-infrared (NIR) light and transfer energy to TiO2, which extends the light responsive range of TiO2 to the NIR region. Ag nanoparticles not only enhance upconversion luminescence of the UCNCs but also enhance light harvesting and improve charge separation of TiO2. The results of photocatalytic applications show that the as-prepared catalyst has high photocatalytic activity. This study provides new insights into the fabrication of TiO2-based nanocomposite photocatalysts with high catalytic efficiency through effective integration of upconversion material, noble metal and TiO2 into a hetero-composite nanostructure. [ABSTRACT FROM AUTHOR]

A series of highly crystalline orthoferrite nanoparticles (type La1−xGdxFeO3, where x = 0 to 1) were prepared using the self-combustion method. Extensive studies including X-ray diffraction, Rietveld refinement and Fourier transform infrared spectroscopy as well as Raman spectroscopy confirmed the orthorhombic space group Pnma of the obtained materials. The calculated average grain size for powders is in the range of 30 to 80 nm. Magnetic characterization of the La1−xGdxFeO3 series, performed at 1.72 K, indicated an antiferromagnetic state characterized by some canting of iron magnetic moments, in good agreement with the data reported for similar fine-particle systems. [ABSTRACT FROM AUTHOR]

Cubic KLu3F10 nanocrystals have been fabricated by doping Mn2+ ions via a liquid–solid solution (LSS) hydrothermal method. With an increase in the Mn2+ level, a phase transformation from orthorhombic KLu2F7 to cubic KLu3F10 associated with evolution of the particle morphologies and sizes is evident. The phase transformation mechanism based on doping with Mn2+ ions was studied via a series of time-controlled experiments and the enhancement of the intensity of upconversion luminescence (UCL) via doping with Mn2+ ions is discussed in detail. In addition, under 980 nm excitation, the UCL properties of Er3+/Tm3+/Ho3+-doped KLu3F10:Yb3+ upconversion nanoparticles (UCNPs) have been investigated and a possible UCL mechanism has been discussed. It has been demonstrated that cubic KLu3F10 is an excellent host for upconversion luminescence. [ABSTRACT FROM AUTHOR]

The quest for new multiferroic materials is on the rise due to their potential application in spintronics and futuristic multiple state memory devices. Here we report the microwave synthesis of iron/gadolinium co-doped PbTiO3 nanorods, which display multiferroic behavior. Both the undoped and doped PbTiO3 samples have been characterized using powder X-ray diffraction, energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) techniques. The morphology of the samples has been studied using transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM), which confirmed the formation of nanorods. The substitution of Fe ions for Ti and Gd ions for Pb enhances the ferromagnetic and ferroelectric properties of this system. The reasons for this observation have been explored in detail. The ferroelectric, magnetic and magneto-capacitive measurements at room temperature substantiate the multiferroic nature of the codoped samples with significant magnetoelectric coupling observed in case of nano PbTiO3:Gd3+(0.5%):Fe3+(5%). [ABSTRACT FROM AUTHOR]

Upconversion nanocrystals have a lot of advantages over other fluorescent materials. However, their applications are still limited due to their comparatively low upconversion luminescence (UCL). In the present study, a novel nanocomposite of Ag/graphene@SiO2–NaLuF4:Yb,Gd,Er for enhancing UCL was fabricated successfully, and its morphology, crystalline phase, composition, and fluorescent property were investigated. It is interesting to find that the Ag/graphene@SiO2–NaLuF4:Yb,Gd,Er and Ag@SiO2–NaLuF4:Yb,Gd,Er nanocomposites showed high UCL enhancements of 52- and 10-fold compared to the control of Ag-free nanocomposite SiO2–NaLuF4:Yb,Gd,Er, respectively. The enhancement of 52-fold is greater than those reported in our previous studies and some papers. Moreover, the measured life times of the Ag-presented nanocrystals were longer than that of Ag-absent counterparts. These enhancements of UCL can be ascribed to the effect of metal-enhanced fluorescence, which is caused by the enhancement of the local electric field. The UCL intensity of Ag/graphene@SiO2-NaLuF4:Yb,Gd,Er was 5.2-fold higher than that of Ag@SiO2–NaLuF4:Yb,Gd,Er, indicating that graphene presented in the fabricated nanocomposite structure favors metal-enhanced UCL. The small-sized Ag nanoparticles anchored on the graphene sheet mutually enhanced each other's polarizability and surface plasmon resonance, resulting in a big metal-enhanced UCL. This study provides a new strategy for effectively enhancing the UCL of upconversion nanocrystals. The enhancement potentially increases the overall upconversion nanocrystal detectability for highly sensitive biological, medical, and optical detections. [ABSTRACT FROM AUTHOR]

Polyethylene glycol (PEG) modified BaLuF5:Gd/Yb/Er upconversion nanoparticles (UCNPs) were synthesized by a facile one-pot hydrothermal method for simultaneous synthesis and surface functionalization. The novel, excellently biocompatible and water-soluble bioprobes were used for simultaneous upconversion (UC) luminescence and X-ray bioimaging for the first time. The as-prepared BaLuF5:Gd/Yb/ Er UCNPs possess a face-centered cubic structure with an average size of 23.7 ± 2.7 nm. Under 980 nm laser excitation, these UCNPs emitted intense UC luminescence via a two-photon process. In vitro bioimaging and localized luminescence spectra detected from HeLa cells and the background reveal that these UCNPs are ideal candidates for optical bioimaging in the absence of autofluorescence. Furthermore, the synergistic in vivo UC luminescence and X-ray bioimaging reveal that these PEG-modified BaLuF5:Gd/Yb/Er UCNPs can be successfully used as ideal dual-modal bioprobes. These results demonstrate that these PEG modified UCNPs are ideal multi-modal nanoprobes for bioimaging. [ABSTRACT FROM AUTHOR]

We have studied the luminescence property of CaMoO4:Eu3+. The emission peaks at 590 (5D0→7F1) and 613 nm (5D0→7F2) for Eu3+ are observed after excitation at 266 nm (i.e. Mo-O charge transfer band). The peak intensity of the latter dominates over the former indicating an asymmetric environment of Eu3+ in EuO8 polyhedron or parity mixing. Luminescence intensity increases significantly with co-doping of Gd3+. This is ascribed to energy transfer from Mo-O/Gd3+ to Eu3+. Luminescence intensity increases with annealing up to 900 °C due to the extent of decrease of non-radiative rates. Very high asymmetric values (A21) of 12-16 are found indicating a red emitter. As-prepared samples are dispersible in polar solvents like water, ethanol, methanol, dimethyl sulfoxide (DMSO) and ethylene glycol (EG); and among them, optimum luminescence is found in methanol. Polymer film shows red emission. The quantum yields of as-prepared 2 and 10 at% Gd3+ co-doped CaMoO4:Eu3+ under 277 nm (UV excitation) are 21 and 80%, respectively. [ABSTRACT FROM AUTHOR]

A facile auto-combustion route is used for the synthesis of Gd3+ (2, 5, 7 and 10 at%) co-doped CaMoO4:Eu nanoparticles. X-ray diffraction study suggests that as-prepared samples have extra impurity phases in addition to main tetragonal phase of CaMoO4, and such extra phases decrease as the annealing temperature increases from 600 to 900 °C. The crystal structure has been analysed using Rietveld program. It has space group I41/a (88) and Z = 4 (number of CaMoO4 formula units per unit cell). Average crystallite sizes of as-prepared, 600 and 900 °C annealed samples for 2 at% Gd3+ are found to be ∼33, 48 and 61 nm, respectively. The lattice strains of 5 at% Gd3+ co-doped CaMoO4:Eu for as-prepared and 900 °C are 0.001 and 0.002, respectively. Fourier transform infrared spectroscopy gives the absorption bands at ∼815 and 427 cm-1, which are related to asymmetric stretching and bending vibrations of MoO4 2- tetrahedron. Particle morphology is studied using scanning and transmission electron microscopy (SEM and TEM), and aggregation of particles is found. X-ray photoelectron spectroscopy (XPS) is utilized to examine the oxidation states of metal ions/oxygen and oxygen ion vacancies in Gd3+ co-doped CaMoO4:Eu. With an increase in Gd3+ concentration, peaks corresponding to the Gd3+ (2p3/2 and 2p5/2) binding energy could be detected. [ABSTRACT FROM AUTHOR]