Anming Li

Facile morphology-controllable hydrothermal synthesis and color tunable luminescence properties of NaGd(MoO4)2:Eu3+,Tb3+ microcrystals

Well-crystallized and uniform NaGd(MoO4)2 microcrystals with morphologies of bipyramids, truncated bipyramids, quasi-cubes and tetragonal plates were selectively synthesized via a facile hydrothermal method without any additives under mild conditions. The effects of Na2MoO4/Gd(NO3)3 molar ratios and pH values of precursor solutions on the phase and morphology of the as-synthesized microcrystals were systematically investigated. The molar ratios and pH values played key roles in the selective synthesis of pure phase NaGd(MoO4)2 microcrystals with regular morphology. With increasing Na2MoO4/Gd(NO3)3 molar ratios in the appropriate pH range, the morphology of the products changed from bipyramids, truncated bipyramids, quasi-cubes to tetragonal plates, namely, the morphological truncation degree increased gradually. NaGd(MoO4)2 tetragonal microplates could be synthesized at the molar ratio of 9 : 1. A possible morphological formation mechanism of NaGd(MoO4)2 tetragonal microplates was proposed, i.e. nucleation-Ostwald ripening growth process. Color tunable photoluminescence properties of NaGd(MoO4)2:Eu3+,Tb3+ microcrystals were studied in detail. Among the different morphologies of NaGd(MoO4)2:5% Eu3+ microcrystals, tetragonal microplates had a broadened, red-shifted and enhanced charge transfer band in the excitation spectrum. Whats more, the full-width at half-maximum for the charge transfer band of the tetragonal plates was highest (58 nm), which favored efficient excitation and absorption in the ultraviolet region. The introduction of a small amount of Tb3+ into NaGd(MoO4)2 microplates doped with Eu3+ would enhance the 5D0 → 7FJ transition of Eu3+ at 616 nm due to the energy transfer process of cross-relaxation from Tb3+ to Eu3+. Multicolor tunable luminescence from deep red, red, orange, yellow to green under 291 nm ultraviolet excitation and from red, reddish orange to pink under 380 nm near-ultraviolet excitation could be obtained in NaGd(MoO4)2 tetragonal microplates by simply adjusting the doping concentrations of Eu3+ and Tb3+, suggesting NaGd(MoO4)2:Eu3+,Tb3+ microcrystals might have practical application in optoelectronic devices, such as light emitting diodes and color display systems. This facile morphology-controlled hydrothermal synthesis strategy was simple, low-cost and environment-friendly, and might be extended to other inorganic materials.

Morphology evolution and pure red upconversion mechanism of β-NaLuF4 crystals.

A series of β-NaLuF4 crystals were synthesized via a hydrothermal method. Hexagonal phase microdisks, microprisms, and microtubes were achieved by simply changing the amount of citric acid in the initial reaction solution. Pure red upconversion (UC) luminescence can be observed in β-NaLuF4:Yb3+, Tm3+, Er3+ and Li+ doped β-NaLuF4:20% Yb3+, 1% Tm3+, 20% Er3+. Based on the rate equations, we report the theoretical model about the pure red UC mechanism in Yb3+/Tm3+/Er3+ doped system. It is proposed that the pure red UC luminescence is mainly ascribed to the energy transfer UC from Tm3+:3F4 → 3H6 to Er3+:4I11/2 → 4F9/2 and the cross-relaxation (CR) effect [Er3+:4S3/2 + 4I15/2 → 4I9/2 + 4I13/2] rather than the long-accepted mechanism [CR process among Er3+:4F7/2 + 4I11/2 → 4F9/2 + 4F9/2]. In addition, compared to the Li+-free counterpart, the pure red UC luminescence in β-NaLuF4:20% Yb3+, 1% Tm3+, 20% Er3+ with 15 mol% Li+ doping is enhanced by 13.7 times. This study provides a general and effective approach to obtain intense pure red UC luminescence, which can be applied to other synthetic strategies.

Simultaneous realization of structure manipulation and emission enhancement in NaLuF4 upconversion crystals

Gd3+ doped NaLuF4:Yb3+,Er3+,Tm3+ nano/micro-crystals with the enhanced upconversion (UC) emission were successfully prepared through a hydrothermal method with the assistance of citric acid. As for Gd3+ doped α-NaLuF4, before the phase transformation (when Gd3+ content ≤40 mol%), UC luminescence intensities in the red region from α-NaLuF4:Yb3+,Er3+,Tm3+ nanocrystals doped with 40 mol% Gd3+ are enhanced by 13 times compared to the Gd3+-free sample. As for Gd3+ doped α/β-mixed NaLuF4, with the increase of Gd3+ content, the crystal structure transforms from the α/β-mixture to a pure hexagonal phase without a long reaction time or a high reaction temperature. Red UC emissions from NaLuF4:Yb3+,Er3+,Tm3+ crystals with 60 mol% Gd3+ doping are enhanced by 9 times. The mechanisms of the enhanced UC emission and phase transformation by introducing Gd3+ are discussed in detail. Furthermore, in order to obtain various structures of NaLuF4 nano/micro-crystals before Gd3+ doping and demonstrate the advantages of Gd3+ doping, the effects of NaF content, reaction time and temperature on the crystal structure of Gd3+-absent NaLuF4 crystals are systematically studied. The results suggest that the intense red UC luminescence from Gd3+ doped NaLuF4:Yb3+,Er3+,Tm3+ nano/micro-crystals may have potential applications in biological imaging and optoelectronic devices.

Enhanced red upconversion emission and its mechanism in Yb3+–Er3+ codoped α-NaLuF4 nanoparticles

A series of Yb3+–Er3+ codoped α-NaLuF4 nanoparticles were prepared via a hydrothermal process. Enhanced red upconversion (UC) luminescence can be achieved by adjusting the initial solution pH, and the intensity of red emission (660 nm) is enhanced 23 times. By establishing rate equations, a theoretical model about the red UC mechanism is presented. It is proposed that the highly efficient red UC emission is mainly due to the UC process from the saturated 4I13/2 (Er3+) state, which is induced by the effective energy-back-transfer process (primary): 4S3/2 (Er3+) + 2F7/2 (Yb3+) → 4I13/2 (Er3+) + 2F5/2 (Yb3+) and the cross-relaxation effect among Er3+ (minor): 4S3/2 + 4I15/2 → 4I9/2 + 4I13/2 at a high pH value of the initial solution. Moreover, the influence of solution pH on the red UC luminescence is systematically studied under different hydrothermal conditions. This paper provides an effective route to obtain nanoparticles with bright red UC luminescence, which may be suitable for biological imaging and display devices.

NaGd(MoO4)2 nanocrystals with diverse morphologies: controlled synthesis, growth mechanism, photoluminescence and thermometric properties.

Pure tetragonal phase, uniform and well-crystallized sodium gadolinium molybdate (NaGd(MoO4)2) nanocrystals with diverse morphologies, e.g. nanocylinders, nanocubes and square nanoplates have been selectively synthesized via oleic acid-mediated hydrothermal method. The phase, structure, morphology and composition of the as-synthesized products are studied. Contents of both sodium molybdate and oleic acid of the precursor solutions are found to affect the morphologies of the products significantly, and oleic acid plays a key role in the morphology-controlled synthesis of NaGd(MoO4)2 nanocrystals with diverse morphologies. Growth mechanism of NaGd(MoO4)2 nanocrystals is proposed based on time-dependent morphology evolution and X-ray diffraction analysis. Morphology-dependent down-shifting photoluminescence properties of NaGd(MoO4)2: Eu3+ nanocrystals, and upconversion photoluminescence properties of NaGd(MoO4)2: Yb3+/Er3+ and Yb3+/Tm3+ nanoplates are investigated in detail. Charge transfer band in the down-shifting excitation spectra shows a slight blue-shift, and the luminescence intensities and lifetimes of Eu3+ are decreased gradually with the morphology of the nanocrystals varying from nanocubes to thin square nanoplates. Upconversion energy transfer mechanisms of NaGd(MoO4)2: Yb3+/Er3+, Yb3+/Tm3+ nanoplates are proposed based on the energy level scheme and power dependence of upconversion emissions. Thermometric properties of NaGd(MoO4)2: Yb3+/Er3+ nanoplates are investigated, and the maximum sensitivity is determined to be 0.01333 K−1 at 285 K.

Spectral management in upconverting sesquioxide through matrix doping

Designing luminescent materials with tunable emission has always been a frontier topic of color display, photovoltaic and biodetection technologies. Despite their excellent modulation between the material composition/phase/morphology/preparation and performance, an efficient route is desired to simultaneously tune the emission color and intensity. Herein, we demonstrate the simultaneous spectral tuning in the Gd2O3 host by matrix doping using a fast and facile hydrothermal method by simply adding a fluoride source during the preparation of gadolinium-based precursors. Structural and optical microscopic characterization shows that GdOF is homogeneously incorporated into Gd2O3 micro-sized rod-like particles. Our mechanistic investigations of upconversion luminescence behaviors suggest that the spectral tunability of this structure is governed by the dominant energy transfer upconversion for the depletion of the intermediate manifolds and the reduced probabilities of multiphonon relaxation and energy back transfer processes, which results from the decreasing effective phonon energy, the refractive index and the average Ln3+ distance by doping GdOF. These findings provide useful information on designing and developing luminescent materials within simple structures for efficient spectral tuning, which may be found suitable for photovoltaic, display devices and security techniques.

Facile synthesis and emission enhancement in NaLuF 4 upconversion nano/micro-crystals via Y 3+ doping

A series of Y3+-absent/doped NaLuF4:Yb3+, Tm3+ nano/micro-crystals were prepared via a hydrothermal process with the assistance of citric acid. Cubic nanospheres, hexagonal microdisks, and hexagonal microprisms can be achieved by simply adjusting the reaction temperature. The effect of Y3+ doping on the morphology and upconversion (UC) emission of the as-prepared samples were systematically investigated. Compared to their Y3+-free counterpart, the integrated spectral intensities in the range of 445–495 nm from α-, β-, and α/β-mixed NaLuF4:Yb3+, Tm3+ crystals with 40 mol% Y3+ doping are increased by 9.7, 4.4, and 24.3 times, respectively; red UC luminescence intensities in the range of 630–725 nm are enhanced by 4.6, 2.4, and 24.9 times, respectively. It is proposed that the increased UC emission intensity is mainly ascribed to the deformation of crystal lattice, due to the electron cloud distortion in host lattice after Y3+ doping. This paper provides a facile route to achieve nano/micro-structures with intense UC luminescence, which may have potential applications in optoelectronic devices.