Tetragonal LiYF4 Research Articles

On the role of Gd3+ ions in enhancement of UV emission from Yb3+–Tm3+ up-converting LiYF4 nanocrystals

Materials capable of emitting ultraviolet (UV) radiation are sought for applications ranging from theranostics or photodynamic therapy to specific photocatalysis. The nanometer size of these materials, as well as excitation with near-infrared (NIR) light, is essential for many applications. Tetragonal tetrafluoride LiY(Gd)F4 nanocrystalline host for up-converting Tm3+–Yb3+ activator-sensitizer pair is a promising candidate to achieve UV–vis up-converted radiation under NIR excitation, important for numerous photo-chemical and bio-medical applications. Here, we provide insights into the structure, morphology, size and optical properties of up-converting LiYF4:25%Yb3+0.5%Tm3+ colloidal nanocrystals, where 1, 5, 10, 20, 30 and 40% of Y3+ ions were substituted with Gd3+ ions. Low gadolinium dopant concentrations modify the size and up-conversion luminescence, while the Gd3+ doping that is exceeding the structure resistance limit of the tetragonal LiYF4 results in appearance of foreign phase and significant decrease of luminescence intensity. The intensity and kinetic behavior of Gd3+ up-converted UV emission are also analyzed for various gadolinium ions concentrations. The obtained results form a background for further optimized materials and applications based on LiYF4 nanocrystals.

Excited-state absorption in thulium-doped materials in the near-infrared.

Excited-state absorption (ESA) is a key process for upconversion pumping schemes of thulium (Tm3+) doped laser materials. We have systematically studied two ESA transitions in the near-infrared spectral range, namely 3F4 → 3F2,3 (at ∼1 µm) and 3F4 → 3H4 (at ∼1.5 µm), in various Tm3+-doped fluoride (ZBLAN glass, cubic KY3F10 and CaF2, tetragonal LiYF4 and LiLuF4, monoclinic BaY2F8 crystals) and oxide (cubic Y3Al5O12, orthorhombic YAlO3 crystals) laser materials, using a pump-probe method with a polarized light. An approach to calculate the constants of energy-transfer upconversion (ETU) is also presented.

Tuning the structural and luminescence properties of LiYF4:Yb, Er upconversion phosphor: Understanding the role of K ion concentration

Influence of K+ ion concentrations on the upconversion emissions of LiYF4: Yb, Er phosphor has been investigated. Li1−xKxY0.77F4:Yb0.2, Er0.03 phosphors are synthesized by solid-state reaction method under reduced atmosphere. X-ray diffraction analysis revealed a major phase transition from tetragonal LiYF4 to hexagonal KYF4 as the concentration of K+ ion increases. Blue, green and red upconversion emissions are observed when the samples are excited at 980 nm. Green and red luminescence intensities are affected by the inclusion of K+ ion concentrations in the host lattice. Downshifting photoluminescence and the corresponding decay analysis are carried out for the Li1−xKxY0.77F4:Yb0.2, Er0.03 phosphors under the excitation wavelength of 378 nm. Significant changes in the emission intensity of the green to red ratio suggest the tuning of the emission color with the modification of the K+ ion concentrations in Li1−xKxY0.77F4:Yb0.2, Er0.03 host and such successful tuning in the color of emission can find potential applications in the field of display devices.

Efficient NIR to NIR up-conversion in LiYF4:Yb3+,Tm3+ micro-octahedrons by modified hydrothermal method

The efficient NIR-to-NIR upconverter of LiYF4:Yb3+,Tm3+ (simplified as LYF:Yb,Tm) was synthesized by a modified hydrothermal method in this work. The synthesized LYF:Yb,Tm has well controlled morphology of monodisperse micrometer sized octahedrons. XRD pattern confirms its pure tetragonal LiYF4 structure. The LYF:Yb,Tm exhibits a dominating NIR UC emission at ~793 nm along with relative weak blue and red UC emissions under 976 nm LD excitation. The optimal concentrations of Yb3+ and Tm3+ ions for NIR up-conversion (UC) emission are 20 mol% and 0.6 mol%, respectively. The internal UC quantum yield (IUCQY) of this LYF:Yb,Tm in NIR band is tested to be 5.1% at 959 nm LD excitation power density (PD) of 120 W/cm2. The UC emission mechanism is discussed based on UC fluorescent behaviors of Yb3+ and Tm3+. The results of fluorescent labeling indicate its potential applications in fluorescent markers, solar cells and 3D flat panel displays.

Highly bright and sensitive thermometric LiYF4:Yb, Er upconversion nanocrystals through Mg2+ tridoping

A new strategy is reported to enhance the upconversion (UC) luminescence emission of LiYF4:Yb, Er nanocrystals (NCs) using magnesium as a dopant. We carried out systematic experimental studies on the crystal structure, grain size, and UC emitting property of the tetragonal LiYF4:Yb, Er with varied concentrations of Mg2+. The UC luminescence properties were examined under 980 nm laser illumination with various excitation power densities. At a proper doping concentration, co-doping of Mg2+ ions into LiYF4:Yb, Er is found to result in efficient reinforcement in both the green and red upconverted emissions. Remarkably, the maximum green and red luminescence intensities were reinforced by sevenfold and fivefold, respectively, when 7 mol% Mg2+ was co-doped into tetragonal LiYF4. The possible origin and mechanism for boosting UC emission were explained according to the alteration of the cell volume and the local crystal field surrounding the Er3+ ions by co-doping of Mg2+. Moreover, the emission-optimized LiYF4 UCNCs were further investigated to understand thermal-sensing behaviors employing the fluorescence intensity ratio (FIR) approach from the two neighboring thermal coupled states (2H11/2/4S3/2). The optimization of Mg2+ co-doping in LiYF4:Yb, Er allowed the resultant UCNCs to be an excellent luminescent thermometer over a wide range of temperature. Applying the optimized UCNCs as an optical thermometer, a maximum thermal sensitivity (S) of 5.43 × 10–2 K−1 was achieved at room temperature and a low-power excited upconversion (1 W cm−2). The achieved S value is more advanced than most of the Er-based nanophosphors reported heretofore. This paper provides a perspective scheme to design and grow high-quality upconversion nanomaterials for achieving the preconditions of the pragmatic application in temperature sensing, optically heating, and color display devices.

The construction of sublattice level energy cluster for promoting UV upconversion emission in tetragonal LiYF4

The preparation of high performance upconversion luminescence materials with stronger shortwave emissions is still a challenge. In this work, the feature of hexagon circles sublattice structure of tetragonal LiYF4 was revealed, and the sublattice level energy clusters of 4 Yb3+-1Tm3+ were constructed by the substitution of 70 mol% Yb3+ for Y3+ in tetragonal LiYF4 matrix. The sensitizing ions concentration quenching was inhibited by the hexagon circles sublattice structure, and the upconversion luminescence was dramatically enhanced, especially, the UV band emission was enhanced about 20 times and the intensity ratio of UV to blue emissions was improved about 10 folds under the excitation power density of 0.230 W/cm2. Photon numbers of upconversion emissions obviously decreased with the increasing of sublattice level energy clusters that the upconversion process of three photons was changed to two steps of two photons process. The upconversion luminescence mechanism of sublattice energy cluster construction was system investigated. The material with high performance of UV and blue UC luminescence can be a well candidate for the applications of photodynamic therapy and photocatalytic, etc. The work may be an inspiration for high efficient UC luminescence materials design and preparation.

Sublattice Energy Cluster Construction for The Enhancement of NIR Photocatalytic Performance of LiYF4: Tm@TiO2

AbstractIn this work, a stratagem of improving the photocatalytic performance of TiO2 under NIR irradiation was illustrated by the combination of TiO2 with upconversion (UC) luminescent materials that present high intensity of UV band emission resulted from the sublattice energy clusters in tetragonal LiYF4: Tm UC matrix. The sublattice energy clusters were constructed by doping high concentration Yb3+ into tetragonal LiYF4: Tm. 347 nm emission intensity of Tm3+ were enhanced dramatically with the rising of Yb3+ doping concentration and reached the top while 70% Yb3+ was doped, and the intensity was enhanced about 6.15 times compared with that of the usual 20% Yb3+ doped. After being coated with TiO2, the photocatalytic performance of the complex was enhanced about 2.55 folds compared to P25. XRD, SEM, TEM, FTIR, UV‐vis and fluorescence spectrum were employed for the systematical investigation of the relationship between structure and performance. This stratagem may be helpful for the design and preparation of photocatalysts and photo sensors.

Effect of Molten Salts on Synthesis and Upconversion Luminescence of Ytterbium and Thulium-Doped Alkaline Yttrium Fluorides

Upconversion luminescence materials have recently received attentions because of theirs light conversion ability from infrared into visible and ultraviolet light. In this work, alkaline yttrium fluoride doped by ytterbium and thulium (AYF4: 20%Yb3+, 0.5%Tm3+) were synthesized by molten salt method at 400 °C for 2 hours with different eutectic molten salts, i.e. NaNO3-KNO3, NaNO3-LiNO3, KNO3-LiNO3, and NaNO3-KNO3-LiNO3. Pure hexagonal NaYF4 microrods were successfully synthesized using eutectic NaNO3-KNO3 molten salt. Under 980 nm laser diode excitation, upconversion luminescence in both visible and ultraviolet region was clearly observed. On the other hand, for the use of other eutectic molten salts containing LiNO3, the mixed phases of tetragonal LiYF4 and orthorhombic Y6O5F8 were obtained. These powders emitted only visible light with 10 times lower intensity than the hexagonal NaYF4 microrods synthesized using NaNO3-KNO3.

Dropwise addition of cation solution: An approach for growing high-quality upconversion nanoparticles

A suitable synthetic route and the associated reaction parameters play a vital role for growing high-quality nanocrystal. A design strategy of injection was introduced with the aim of enhancing monodispersity and crystallinity of upconversion nanoparticles and making them have uniform morphology. The high-quality Yb3+/Er3+-codoped LiYF4, NaYF4, KYF4 and NaGdF4 nanoparticles were synthesized by dropwise addition of cation solution into precursors mixture at high-temperature in this work. The detailed synthesis parameters of the purity tetragonal LiYF4:Yb3+/Er3+ upconversion nanoparticles in injection section were studied and ascertained. Changes of the injection temperature, reaction temperature, dropwise addition speed, mole ratio of cation (Li+) to precursors (RE(CF3COO)3), reaction time and reactants concentration have contributed much to synthesizing high-quality LiYF4:Yb3+/Er3+ in the end. Compared with traditional thermal decomposition method, a series of properties, such as high crystallinity, less agglomeration phenomenon, uniform tetragonal bipyramidal morphology and great upconversion luminescence, could be improved by this modified method.

Enhancing Upconversion Luminescence of LiYF4:Yb,Er Nanocrystals by Cd2+ Doping and Core–Shell Structure

In this work, we report a phenomenon that upconversion emission intensity of Er3+ was enhanced by cadmium. Cd2+ was taken with 0, 3, 10, 20, 30, and 40 mol % as a new dopant to codope LiY0.78Yb0.20Er0.02F4, meanwhile the doping effects on the crystal structure, morphology, and upconversion fluorescence emission are investigated by XRD, Rietveld refinement, TEM, and upconversion spectroscopy analysis methods in detail. The green upconversion emission intensity was enhanced, and the maximum emission intensities were enhanced to twice when 10 mol % Cd2+ was codoped into tetragonal LiYF4, while its decay lifetime was reduced. Additionally, we further designed the synthesis of the homogeneous LiYF4:20%Yb,2%Er,10%Cd@LiYF4 of which the emission intensity is 11 times higher than that of the Cd2+-free nanocrystals, meanwhile an intriguing morphology of core–shell nanocrystals was obtained. This investigation may be useful for the design and improvement of upconversion materials for fulfilling the prerequisite of t...

Controlled synthesis and upconversion luminescence properties of LiYF4:Yb0.2Er0.02 nanoparticles

A series of high quality tetragonal LiYF4:Yb0.2Er0.02 nanoparticles were synthesized via thermal decomposition method. The influence of reaction conditions on final products has been investigated in detail. The TEM and XRD data show that the size, shape and crystallization of the samples depend on the reaction time and concentration. The upconversion luminescence (UCL) intensity gradually enhanced along with the sizes of the samples increasing. And when the reaction solvent system is oleic acid/1-octadecence (OA/ODE), the samples show smaller size and stronger emission compared with oleic acid/oleylamine/1-octadecence (OA/OM/ODE). Under 980 nm NIR laser diode excitation, all the samples show intense green emission between 510 and 570 nm wavelength from 4S3/2 → 4I15/2 and 2H11/2 → 4I15/2 transitions of Er3+ and intense red emission between 640 and 690 nm attributed to 2F11/2 → 4I15/2 transition of Er3+, both of the processes belong to two-photon energy absorption.

Molten salt synthesis of tetragonal LiYF4:Yb3+/Ln3+ (Ln = Er, Tm, Ho) microcrystals with multicolor upconversion luminescence

Well-defined tetragonal LiYF4:Yb3+/Ln3+ (Ln = Er, Tm, Ho) microcrystals were successfully prepared by a surfactant-free molten salt method for the first time. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and photoluminescence (PL) spectra were used to characterize the samples. It is found that with prolonged time, the phase of product transforms from orthorhombic YF3 through hexagonal NaYF4 to tetragonal LiYF4, while the morphology changes from nanoparticles via microtubes to micro-sized bi-pyramids. By changing the dopant species (Ln3+), multicolor (yellow, green, and blue) upconversion (UC) emissions can be obtained in LiYF4:Yb3+/Ln3+ under 980 nm laser diode (LD) excitation. The UC mechanisms in co-doped LiYF4 samples were analyzed in detail based on the emission spectra. Importantly, this synthetic methodology may offer a new alternative in the fabrication of high quality rare earth fluorides and open the possibility to meet the increasing commercial demand.

Pr<sup>3+</sup>/Yb<sup>3+</sup>共掺氟化物微米晶体的上转换荧光研究

Pr³⁺/Yb³⁺ codoped hexagonal NaYF₄ and tetragonal LiYF4 microcrystals (MCs) were synthesized by hydrothermal method, in which the EDTA was used as a chelating agent. The influence of the crystal phase and electron-phonon interaction on the fluorescence emission of Pr³⁺ was investigated. In hexagonal phased NaYF₄: Yb³⁺/Pr³⁺ and tetragonal phased LiYF4: Yb³⁺/Pr³⁺ microcrystals, we have observed bright up conversion emission when the samples were excited with 976.4 nm laser. The fluorescence tetragonal LiYF4: Yb³⁺/Pr³⁺ microcrystals was weaker and sharper than that from hexagonal NaYF₄: Yb³⁺/Pr³⁺ microcrystals, and a blue shift was observed too. We have also studied the process of up conversion, the influence of crystal phase, and the dopant concentration.

Carbohydrate-conjugated porphyrin oligomers for two photon photodynamic therapy: Synthesis and photobiological evaluation

The preparation of high performance upconversion luminescence materials with stronger shortwave emissions is still a challenge. In this work, the feature of hexagon circles sublattice structure of tetragonal LiYF4 was revealed, and the sublattice level energy clusters of 4 Yb3+-1Tm3+ were constructed by the substitution of 70 mol% Yb3+ for Y3+ in tetragonal LiYF4 matrix. The sensitizing ions concentration quenching was inhibited by the hexagon circles sublattice structure, and the upconversion luminescence was dramatically enhanced, especially, the UV band emission was enhanced about 20 times and the intensity ratio of UV to blue emissions was improved about 10 folds under the excitation power density of 0.230 W/cm2. Photon numbers of upconversion emissions obviously decreased with the increasing of sublattice level energy clusters that the upconversion process of three photons was changed to two steps of two photons process. The upconversion luminescence mechanism of sublattice energy cluster construction was system investigated. The material with high performance of UV and blue UC luminescence can be a well candidate for the applications of photodynamic therapy and photocatalytic, etc. The work may be an inspiration for high efficient UC luminescence materials design and preparation.

Optical spectroscopy and diode-pumped laser characteristics of codoped Tm-Ho : YLF and Tm-Ho : BaYF : a comparative analysis

Single crystals of monoclinic BaY2F8 and tetragonal LiYF4 codoped with the same Tm3+ and Ho3+ concentrations were successfully grown by the Czochralski method. Here we present a comparative analysis of the two hosts including spectroscopic characterization and cw diode-pumped laser experiments in the 2-μm wavelength region at room temperature. The main differences between the two hosts are a lower slope efficiency associated with a much wider tuning range (2005–2094 nm) of BaY2F8 with respect to LiYF4.