NaYF4 Crystals Research Articles

Upconversion Emission and Dual-Mode Sensing Characteristics of NaYF4:Yb3+/Er3+ Microcrystals at High and Ultralow Temperatures.

In this study, we investigate micrometer-sized NaYF4 crystals double-doped with Yb3+/Er3+ lanthanide ions, designed for temperature-sensing applications. In contrast to previous studies, which focused predominantly on the high-temperature regime, our investigation spans a comprehensive range of both high and ultralow temperatures. We explore the relationship between temperature and the upconversion luminescence (UCL) spectra in both frequency and time domains. Our findings highlight the strong dependence of these spectral characteristics of lanthanide-doped NaYF4 crystals on temperature. Furthermore, we introduce a dual-mode luminescence temperature measurement technique, leveraging the upconversion emission intensity ratio for both green and red emissions. This study also examines the correlation between temperature sensing, energy level disparities, and thermal coupling in Er3+ ions across various temperature scales. Our research contributes to advancing the understanding and application of lanthanide-doped materials, setting a foundation for future innovations in temperature sensing across diverse fields.

Deterministic relation between thermal-phonon dressings and a non-Hermitian multi-Fano interferences router in ion-doped microcrystals

We report the multi-Fano interference obtained through the simultaneous acquisition of bright and dark states in different phase transitions of Eu3+: BiPO4 (7:1, 6:1, 1:1, and 0.5:1) and Eu3+: NaYF4 (1:1/4) crystals. We employ multi-dressed spontaneous four-wave mixing and multi-dressed fluorescence (multi-order) to optimize the strong photon-phonon nested dressing effect resulting in more obvious multi-Fano interference. Firstly, the multi-Fano is produced through interference in continuous and multi-bound states. Secondly, five multi-Fano dips originate from nested five dressings (one photon and four phonons) under symmetrical splitting of 7F1 energy level. We depict that the pure H-phase (0.5:1) sample exhibits the strongest photon-phonon dressed effect (five Fano dips). Further, we investigate high-order non-Hermitian exceptional points in multi-Fano interference by adjusting the ratio of Rabi-frequency to de-phase rate through nested photon and phonon dressing. Our experimental results are validated by theoretical simulations, which may be applied to designing optoelectronic devices such as non-Hermitian multi-Fano interferences (multi-channel) router.

The influence of Al2O3 concentration on the NaYF4 crystallization in oxyfluoride glass–ceramics

AbstractThe influence of Al2O3 addition on the precipitation of NaYF4 crystals in oxyfluoride glasses has been investigated through the thermal, structural, and optical characterization of the parent glasses and corresponding glass–ceramics (GCs). The high‐resolution transmission electron microscopy analysis of the GC with 5 mol% of Al2O3 shows the presence of phase‐separated droplets about 69 nm in size containing several NaYF4 nanocrystals with the diameter of about 10–15 nm. Raman and magic angle spinning nuclear magnetic resonance (MAS‐NMR) spectroscopy were used to examine the structural changes attributed to the addition of Al2O3. The 19F MAS‐NMR analysis indicated that fluorine atoms are present in different chemical environments (Na–F, Na–Si–F, Na–Al–F, and NaYF4). The increasing amount of Al2O3 reduces the crystallization of NaYF4 phase due to the increased fraction of fluorine bound in Na–Al–F environments. The visible luminescence investigation of the glasses and GCs demonstrated that the intensity of Er3+ ions emission transitions in the GCs was higher than that of the parent glass. This difference was attributed to the presence of Er3+ ions bound in NaYF4 crystalline phase. Further evidence that the Er3+ ions were present in NaYF4 phase was provided by the fact that the excited level Er3+:4S3/2 lifetime was increased in GCs as compared to parent glass.

Ho3+ doped low-phonon single crystals and chalcogenide glasses for mid-IR source application

A comparative study was conducted to investigate the 3.9 µm mid-IR emission properties of Ho3+ doped NaYF4 and CsCdCl3 crystals as well as Ho3+ doped Ga2Ge5S13 glass. Following optical excitation at ∼890 nm, all the studied materials exhibited broad mid-IR emissions centered at ∼3.9 µm at room temperature. The mid-IR emission at 3.9 µm, originating from the 5I5 → 5I6 transition, showed long emission lifetime values of ∼16.5 ms and ∼1.61 ms for Ho3+ doped CsCdCl3 crystal and Ga2Ge5S13 glass, respectively. Conversely, the Ho3+ doped NaYF4 crystal exhibited a relatively short lifetime of ∼120 µs. Temperature dependent decay time measurements were performed for the 5I5 excited state for all three samples. The results showed that the emission lifetimes of Ho3+:CsCdCl3 and Ho3+:Ga2Ge5S13 were nearly temperature independent over the range studied, while significant emission quenching of the 5I5 level was observed in Ho3+:NaYF4. The temperature dependence of the multi-phonon relaxation rate for 3.9 µm mid-IR emission in Ho3+:NaYF4 crystal was determined. The room temperature stimulated emission cross-sections for all three samples were calculated using the Füchtbauer-Landenburg equation. Furthermore, the results of Judd-Ofelt analysis are presented and discussed.

NaYF4 upconversion crystals with red light emission by low Er3+ concentration doping

In the work, a range of Er3+-sensitized NaYF4 upconversion (UC) particles doped with Tm3+ and Ho3+ are successfully prepared by hydrothermal process. The UC crystals appear two green emission peaks at 529 and 541 nm and a remarkable red emission peak at 655 nm under 980 and 1532 nm irradiations. Here, Tm3+ and Ho3+ serve as trapping centers, which confine the excitation energy efficiently to promote the red UC emission and improve the red to green ratio. UC mechanisms in the Er3+/Tm3+ co-doped and Er3+/Ho3+ co-doped NaYF4 crystals are discussed at length based on the UC luminescence spectroscopy. Unlike the traditional sensitization with high Er3+ concentration doping to obtain red light, the red UC luminescence is acquired by low Er3+ concentration doping, which can effectively avoid concentration quenching caused by high Er3+ concentration doping. Furthermore, NaYF4:Er3+, Tm3+ crystals exhibit stronger red UC emission compared with NaYF4:Er3+, Ho3+ counterparts. Er3+-sensitized NaYF4 crystals doped with Tm3+ and Ho3+ ions generate pure red light more efficaciously under 1532 nm excitation than under 980 nm excitation. The phosphors emitting brilliant red UC emission have the potential applications in solid state lasers and biological imaging.

New Cysteine-Containing PEG-Glycerolipid Increases the Bloodstream Circulation Time of Upconverting Nanoparticles.

Upconverting nanoparticles have unique spectral and photophysical properties that make them suitable for development of theranostics for imaging and treating large and deep-seated tumors. Nanoparticles based on NaYF4 crystals doped with lanthanides Yb3+ and Er3+ were obtained by the high-temperature decomposition of trifluoroacetates in oleic acid and 1-octadecene. Such particles have pronounced hydrophobic properties. Therefore, to obtain stable dispersions in aqueous media for the study of their properties in vivo and in vitro, the polyethylene glycol (PEG)-glycerolipids of various structures were obtained. To increase the circulation time of PEG-lipid coated nanoparticles in the bloodstream, long-chain substituents are needed to be attached to the glycerol backbone using ether bonds. To prevent nanoparticle aggregation, an L-cysteine-derived negatively charged carboxy group should be included in the lipid molecule.

Fluorescence regulation derived from Eu3+ in miscible-order fluoride-phosphate blocky phosphor

Miscible-order fluoride-phosphate blocky phosphor (FBP), composed with ordered-phase of NaYF4 crystals and unordered phase of tin-fluorophosphate glass, is prepared by a two-step process and luminescent properties of FBPs embedded with different particle sizes of NaYF4 crystals are presented. High-frequency fluorescence from higher metastable 5D J (J = 1, 2 and 3) energy levels are effectively released in Eu3+ doped fluoride crystals. Taking the blue emission of Sn2+ as the framework, multi-peak emissions from metastable energy levels are controlled to adjust the color coordinates of the FBP to the white-light region, which the color rendering index (CRI) reaches 89. Tunable color FBP with high CRI retains splendid luminescence property of fluoride, providing a potential candidate for the development of white LED.

Investigating the effect of Yb3+ and Er3+ concentration on red/green luminescent ratio in β-NaYF4: Er, Yb nanocrystals using spectroscopic techniques

β-NaYF4 co-doped with Er3+ and Yb3+ ions were successfully synthesized at the reaction temperature as low as 180 °C by the two steps procedure using the rare-earth stearates as precursors. The XRD and TEM data show that the NaYF4 nanocrystals were relatively uniform with the approximate size of 50 nm and the dopants led to shrinking of NaYF4 crystals. Moreover, the EDX mapping analysis indicate that the Yb3+, Er3+ ions were distributed homogeneously in host matrix even the Yb3+ doping concentration as high as 25%. The PL spectroscopy was used to investigate the optical properties of these systems. The red/green emission ratio of Er3+ was controlled by changing the doping concentration in matrix. An increase of Yb3+ contents from 2 to 25 mol % induced a remarkably tunable emission from green to red. Addition, the highest red/green ratio was also achieved for the sample of 1% Er3+ and 20% Yb3+. Combining the results of the up-conversion emission intensity as a function of the pump power and luminescence data, the energy transfer mechanism between the Yb3+ sensitizers and Er3+ activators was demonstrated. The intense efficiency of red emission under 980 nm excitation provides the potential applications in bio-labeling and bio-imaging.

Synthesis, characterization and upconversion luminescence of core-shell nanocomposites NaYF4: Er/Yb@SiO2@Ag/Au

The improvement in the upconversion (UC) luminescence due to the surface plasmonic properties of precious metals is an effective mode to increase UC luminescence efficiency. In the present research work, pure hexagonal-phase NaYF4: Yb, Er UC nanocrystals were prepared via a reverse micro emulsion method. A single-step method was adopted to change the hydrophobic surfaces of the as-prepared NaYF4: Yb, Er nanocrystals. After the surface modification, Au/Ag nanoparticles were decorated on the surfaces of NaYF4:Yb, Er nanocrystals. The structural analysis and composition of the nanocomposites were studied in details. The XRD pattern indicated the formation of the hexagonal structure of NaYF4 crystals. The decorated UC nanocrystals exhibited a prominent UC emission in green and red bands under 980 nm laser excitation due to normal two-photon UC process. The UC intensity increased by ∼3 times than the uncoated samples. The enhancement mechanism of UC luminescence was investigated in details.

Suppressing Electron-Phonon Coupling through Laser-Induced Phase Transition.

Using first-principle calculations, we introduced a strategy of laser-induced phase transition that suppress electron-phonon couplings in crystal lattice. We explained unusual irreversible phase transitions in previous experiments on MoTe2 and NaYF4 crystals. Laser irradiations produced local heats in 2H-MoTe2 and Hex NaYF4, driving atom reorganizations toward new lattices. The reorganization with effective electron-phonon couplings continues with spontaneously generated heats, whereas a 1T'-MoTe2 and a metastable cubic NaYF4 phases were kept because of suppressed vibrational relaxations. Long time laser treatments create phases with weak electron-phonon couplings. Such irreversible transitions guarantee complete conversions, opening a new door to selective material modifications.

Relationship between Site Symmetry, Spin State, and Doping Concentration for Co(II) or Co(III) in β-NaYF4

Magnetic dopants provide versatile tunability of properties for transparent host matrix materials, semiconductors, and nanostructured materials. The relationship between doping concentration, site symmetry, crystal-field splitting, and spin state for Co(II) and Co(III) doped in hexagonal-phase NaYF4 crystals and nanocrystals was investigated in a spin-polarized density functional theory approach. With decreasing dopant concentrations, the geometry of the Co(II) fluoride nearest-neighbor coordination changes from approximately square pyramidal to square planar to cubic before converging to pentagonal bipyramidal coordination. A transition from low-spin to high-spin was observed, accompanying the geometry transformation from square planar to cubic, which is consistent with expectations from basic crystal-field theory. Magnetic ordering of adjacent ions was found to have little influence on this spin-transition process. For Co(III), decreasing concentration resulted in a geometry transformation from approxim...

Controlled synthesis and luminance properties of lanthanide-doped β-NaYF4 microcrystals

Hexagonal Yb3+,Er3+-doped NaYF4 crystals with different morphologies were synthesized by a facile hydrothermal method at 140–200 °C. Their shape and size could be controlled by varying hydrothermal temperature, time and doping effect of Ce3+ ions. Interestingly, the products displayed hexagonal sheet, plate, crown-like prism to tube changes in the temperature range from 140 to 200 °C. A combination of “diffusion-controlled growth” and “selective adsorption” was proposed to understand the formation of the NaYF4 crystals with well-defined shapes. NaYF4:Yb3+/Er3+ crystals exhibited shape-dependent up-conversion (UC) luminescence under excitation of 980 nm, and their luminescence property could be tuned by doping Ce3+ ions.

Enhanced high-order upconversion luminescence of hexagonal phase NaYF 4 :Yb 3+ ,Tm 3+ crystals coated with homogeneous shell

Abstract High-order upconversion luminescence (ultraviolet (UV), violet and blue emissions) of rare earth nanocrystals is quite desirable for photodynamic therapy and near-infrared photocatalysis applications. The hexagonal phase NaYF4:Yb3+,Tm3+/x mmol NaYF4 (x = 0, 0.1, 0.5, and 1) crystals were prepared by a hydrothermal method. With the amount of the shell precursor increasing, the crystals shape changed from hexagonal plate to hexagonal prism gradually, with the particles size increased. After coating a homogeneous inert NaYF4 shell, the high-order upconversion luminescence (1I6 and 1D2 radiative transitions) of all core–shell crystals were enhanced by several times compared with core-only crystals. Impressively, when the amount of the shell precursor was 0.1 mmol, the increased ratio of the emissions from 1I6 and 1D2 were 7.44 and 5.63 times, respectively. With further increasing the amount of the shell precursor, the increased ratio displayed a slight decline. The further photoluminescence decay results were consistent with the upconversion luminescence spectra. We demonstrated that the enhancement of high-order upconversion luminescence was attributed to the well-known surface passivation effect of the homogeneous shell.

Hexagonal NaYF4:Yb3+/Er3+ nano/micro-structures: Controlled hydrothermal synthesis and morphology-dependent upconversion luminescence

Hexagonal NaYF4 crystals with a variety of well-defined morphologies and sizes have been successfully synthesized through the precise manipulation of a series of experimental conditions, such as pH values and fluoride sources. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and photoluminescence spectra (PL) were used to characterize the final products. It is found that the intrinsic structure of NaYF4, the pH values and fluoride sources are responsible for the ultimate shape evolution of the final samples. The possible formation mechanism for β-NaYF4 samples with various morphologies has been proposed on the basis of XRD analysis and SEM investigation of the samples obtained at different reaction time periods. Additionally, the upconversion (UC) luminescence properties of Yb3+/Er3+ co-doped β-NaYF4 samples with different shapes have been systematically investigated and discussed. This study is expected to provide important information for morphology-controlled synthesis of other complex rare earth fluoride compounds.

The effects of structural characterization on the luminescence of Eu3+-doped fluoride nano/microcrystals

The pure tetragonal crystal phase LaOF:Eu3+ (2 mol%) nanocrystals (NCs) are obtained by annealing ultrasmall-sized LaF3:Eu3+ (2 mol%) NCs assisted with the surfactant oleic acid. Spectroscopic characterization of Eu3+ in various host materials, including hexagonal-phase LaF3, NaYF4 and NaLaF4, cube-phase NaYF4, tetragonal-phase LiYF4 and LaOF crystals, is performed to tune the luminescence by controlling the host matrices with different compositions or different phase structures of the same inorganic compound. The excitation and emission spectra arising from LaOF:Eu3+ (2 mol%) clearly show that the excitation and emission profiles are different from those of the other matrices. By combining the equation of the spectral conversion with the emission and excitation spectra, it is evident that the efficiency of the spectral conversion in the LaOF host material is superior to that in others, indicating a potential application in solar cells. The underlying reason for the enhanced efficiency of the spectral conversion in the LaOF host material is explored, and a mechanism based on the low local symmetry induced by the shortened bond distance and the asymmetric spatial distribution of the chemical bonds of anions coordinating the rare-earth ions is proposed.

Ultraviolet upconversion spectra of sonochemically synthesized doped NaYF4 crystals

A facile sonochemical method was developed for the production of α- and β-forms of Yb3+(20%)/Tm3+(2%) or Er3+(2%) doubly doped NaYF4 crystals. The reactions took place in aqueous solutions at low temperatures and employed oxide or salt precursors. Micrometer sized β-NaYF4 and nanometer sized α-NaYF4 were obtained with upconversion property. The required activation energy for the solid–solid phase transition was overcome with the energy provided by cavitations. Upconversion emission properties of the Yb3+(20%), Er3+(2%) or Tm3+(2%) doubly doped, α- and β-NaYF4 were investigated. The α-form with emission in the ultraviolet region had higher efficiency contradicting the literature. Several new peaks in the spectra, most likely due to crystal defects, were temporarily assigned.

Ligand dynamic effect on phase and morphology control of hexagonal NaYF4

In this work, the ligand dynamic effect was utilized to synthesize hexagonal NaYF4 (β-NaYF4) crystals with controllable morphology by a hydrothermal method.

Citric acid-assisted phase controlled synthesis of NaYF4:Yb3+,Tm3+ crystals and their intense ultraviolet upconversion emissions

The selective synthesis of cubic and hexagonal NaYF4 crystals was successfully performed by a facile citric acid assisted hydrothermal method. The crystal phase conversion was observed through tuning the added amount of fluoride. A possible growth mechanism was proposed for the formation of hexagonal NaYF4 microcrystals (MCs). Under 980nm excitation, intense ultraviolet (UV), blue, and weak violet upconversion (UC) emissions were obtained in the hexagonal NaYF4:20%Yb3+, 0.5%Tm3+ MCs. The 5-photon UC emissions from the 1I6 level of Tm3+ ions were much stronger than the 4-photon UC emissions from the 1D2 level and the 3-photon UC emissions from the 1G4 level. The enhancement of UV UC emissions was attributed to higher crystallization degree and less luminescence quenching centers.

Controllable synthesis, formation mechanism and upconversion luminescence of β-NaYF4 : Yb3+/Er3+ microcrystals by hydrothermal process

Hexagonal sodium yttrium fluoride crystals with predictable shapes, such as microtubes, microspheres, microrods, micro-bipyramids, microplates, and microprisms, have been synthesized by simply tuning the molar ratio of NaF to RE3+ (RE represents the total amount of Y3+ and the doped rare earth elements such as Yb3+, Er3+). The structural and kinetic factors that govern the phase and shape evolution of NaYF4 crystals have been carefully studied, and the influence of NaF to RE3+ ratio on phase and morphology evolution was systematically investigated and discussed. It is found that the molar ratio of NaF to RE3+ can strongly control the size and structure of as-prepared β-NaYF4 : Yb3+/Er3+ samples owing to the difference of capping effect of F− on the different crystal planes of β-NaYF4 microcrystals. By choosing bipyramidal β-NaYF4 microdisks as a candidate, the possible formation mechanism was proposed on the basis of XRD analysis and SEM observation of the products at the different reaction time periods. Most interestingly, the upconversion (UC) luminescent properties of β-NaYF4 crystals show that the optical properties of as-synthesized materials with different morphologies exhibit great distinction. This study is expected to be suggestive for the precisely controlled growth of other complex rare earth fluoride compounds and provide a reference for exploration of the morphology-dependent UC luminescence properties.

Low-Temperature Flux Growth and Upconversion Fluorescence of the Idiomorphic Hexagonal-System NaYF4 and NaYF4:Ln (Ln = Yb, Er, Tm) Crystals

Idiomorphic NaYF4 and NaYF4:Ln (Ln = Yb, Er, Tm) crystals with upconversion fluorescence were successfully grown by the NaNO3 flux cooling method at a relatively low holding temperature. The grown NaYF4 and NaYF4:Ln crystals had a hexagonal prismatic form, and their well-developed surfaces were relatively flat. TEM images indicated that the NaYF4 crystals were of good crystallinity. Their size and shape were relatively uniform, and they were poorly aggregated. The crystal phase, form, and size depended on the growth temperature and the solute concentration. In contrast, the addition of dopant ions (Yb3+, Er3+, and Tm3+) did not affect the shape, morphology, or crystallinity of the flux-grown NaYF4:Ln crystals. Additionally, the upconversion fluorescence properties of NaYF4:Ln crystals were also dependent on the type and mixture ratio (i.e., starting composition) of the dopants. The green, orange, and blue upconversion fluorescences of NaYF4:10%Yb,1%Er, NaYF4:50%Yb,1%Er, and NaYF4:10%Yb,1%Tm crystals, resp...