Morphology and phase evolvement of Yb/Er co-doped NaYF4 microtubes prepared with YF3 submicrospindles as precursor

Hexagonal phase NaYF₄ microtubes co-doped with Yb³⁺ and Er³⁺ were synthesized through a hydrothermal process with YF₃ submicrospindles as precursor. The X-ray powder diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) were utilized to characterize the structure and morphology of the as-prepared products. XRD results show that pure cubic NaYF₄ crystals can be obtained when reaction time is 2 h. While the product is mixture of cubic and hexagonal phase NaYF₄ when reaction time is from 7 to 20 h. Continuing to increase the reaction time to 24 h, the pure hexagonal NaYF₄ crystals were formed. The FE-SEM and TEM results show that the morphology of pure cubic NaYF₄ is spherical clusters composed of spherical nanoparticles with average diameter of about 100 nm and the pure hexagonal NaYF₄ crystals have tubular structure with out diameter of about 0.3-0.5 μm, inner diameter of about 0.5-1 μm and length ranging from 3 to 12 μm. The luminescence properties of Yb³⁺/Er³⁺ co-doped cubic and hexagonal phase NaYF₄ microcrystals were also studied. Under 980-nm excitation, the upconversion luminescence (UCL) intensity of hexagonal phase NaYF₄ microtubes is much stronger than that of cubic phase clusters. Moreover, both red and green upconversion are ascribed to the two-photon process. Therefore, hexagonal phase NaYF₄ microtubes with high UCL efficiency may have a potential application in photonic device.     

Controllable synthesis of bifunctional NaYF4:Yb/Ho@SiO2/Au nanoparticles with upconversion luminescence and high X-ray attenuation

A novel type of bifunctional water-soluble NaYF₄:Yb³⁺/Ho³⁺@SiO₂/Au nanocomposite is fabricated by a facile layer-by-layer technology in which the mercapto-silica shell is used as the functional layer coating on the central NaYF₄:Yb³⁺/Ho³⁺ nanocrystals. Then by adjusting the mole ratio of the Au nanoparticles to the NaYF₄:Yb³⁺/Ho³⁺@SiO₂ nanoparticles, control of the gold loading on the upconversion nanocrystal surface is achieved. The fabricated nanocomposites inherit the excellent physical and chemical properties from their building blocks, simultaneously exhibiting upconversion luminescence and high X-ray attenuation and as well are easily modified with various molecules. These properties render the synthesized NaYF₄:Yb³⁺/Ho³⁺@SiO₂/Au nanocomposite not only useful as a multimodality contrast agent to increase the efficiency of molecular imaging but also has the potential of in situ curing of diseases.     

White luminescence of Ho3+/Tm3+/Yb3+-codoped CaWO4 synthesized via citrate complex route assisted by microwave irradiation

The nanocrystalline Ho³⁺/Tm³⁺/Yb³⁺ co-doped CaWO₄ upconversion (UC) phosphors were successfully synthesized by a modified citrate complex method using microwave irradiation. The citrate complex precursors were heat-treated at temperature ranging from 300 to 700 °C for 3 h. Crystallization of the Ho³⁺/Tm³⁺/Yb³⁺ co-doped CaWO₄ was detected at 400 °C, and entirely completed at 600 °C. The Ho³⁺/Tm³⁺/Yb³⁺ co-doped CaWO₄ heat-treated at 600 °C showed primarily spherical and homogeneous morphology. Under the laser excitation of 980 nm, Ho³⁺/Tm³⁺/Yb³⁺ co-doped CaWO₄ shows the bright white upconversion (UC) emission visible to the naked eye, which is composed of a blue emission at 475 nm from Tm³⁺, and green and red emissions at 543 and 651 nm respectively from Ho³⁺. The coordinates of Ho³⁺/Tm³⁺/Yb³⁺ co-doped CaWO₄ in the Commission International De'eclairage (CIE) chromaticity diagram could be controlled from a cool to a warm white color depending on the Tm³⁺ and Ho³⁺ concentrations. The UC luminescent properties on Tm³⁺ and Ho³⁺ concentrations and related mechanism based on laser pump power were discussed in detail.     

Water-soluble Yb, Tm codoped NaYF4 nanoparticles: Synthesis, characteristics and bioimaging

Water soluble NaYF₄ nanocrystals codoped with 20 mol% Yb³⁺, 0.5 mol% Tm³⁺ were prepared by a facile solvothermal approach using polyvinylpyrrolidone (PVP) as a surfactant. The upconversion NaYF₄ nanocrystals were pure cubic phase with an average size of ∼40 nm. They could be well redispersed in water to form a clearly transparent solution without obvious precipitation. With the excitation of a 980-nm diode laser, the nanocrystal solution presents bright violet and blue upconversion luminescence. These upconversion nanoparticles (UCNPs) were incubated with HeLa cells at 37 °C for 24 h, and bright blue upconversion luminescence were observed from the UCNPs endocytosed into the HeLa cells on a microscope equipped with a 980-nm fiber laser. These results indicated that the UCNPs had potential applications for biological imaging as luminescent probes.     

Broadband UV excitable near-infrared downconversion luminescence in Eu/Yb:CaF2 nanocrystals embedded glass ceramics

Cooperative downconversion was realized in glass ceramics containing Eu²⁺/Yb³⁺:CaF₂ nanocrystals with Eu²⁺ greatly absorbing ultraviolet photons. Upon excitation of Eu²⁺ ions to the 5d level with an ultraviolet photon at 320 nm, emission of two near infrared photons at 976 nm of Yb³⁺ were achieved. The dependence of the visible and near-infrared emissions, decay lifetime, and quantum efficiency on the Yb³⁺ doping content has been investigated. The maximum energy transfer efficiency and the corresponding downconversion quantum efficiency were estimated to be 51% and 151%, respectively.     

Efficient first-order resonant near-infrared quantum cutting in β-NaYF4:Ho,Yb

We demonstrated an efficient two-photon near-infrared (NIR) quantum cutting (QC) in Ho³⁺-Yb³⁺ co-doped hexagonal β-NaYF₄, which could efficiently convert an incident high-energy photon in the wavelength region of 300-550 nm into two NIR photons. Underlying mechanism for the two-photon NIR-QC process is analyzed in terms of static and dynamic photoemission and monitored excitation spectra. It is found that NIR-QC can occur through two possible energy transfer (ET) approaches: (i) the excited Ho³⁺:⁵F₃ state may simultaneously excite two Yb³⁺ neighbors via a cooperative ET process, and (ii) the NIR-QC can be feasibly induced by a first Ho³⁺(⁵S₂,⁵F₄) + Yb³⁺(²F_7/2) → Ho³⁺(⁵I₆) + Yb³⁺(²F_5/2) resonant ET process and a sequential ⁵I₆ → ⁵I₈ transition of Ho³⁺. This novel NaYF₄:Ho³⁺,Yb³⁺ NIR-QC phosphor, may explore a new approach to maximize the performance of solar cells.     

Color-tunable and white-light emitting lanthanide complexes based on (CexEuyTb1−x−y)2(BDC)3(H2O)4

Ce³⁺, Eu³⁺ and Tb³⁺ complexes were synthesized through facile and mild approaches with terephthalic acid (H₂BDC) as the ligand. Their chemical compositions were determined as (Ce_xEu_yTb_1−x−y)₂(BDC)₃(H₂O)₄ by elemental analysis (EA), Fourier transform infrared (FTIR) spectra and X-ray diffraction (XRD) measurements. Fluorescent properties of the as-synthesized complexes were investigated by changing the molar ratio of Ce³⁺, Tb³⁺ and Eu³⁺ ions, and the optimized ratio of 3.0:2.0:0.15 for Ce³⁺:Tb³⁺:Eu³⁺ in the complex was determined for white-light emission. Tuning on the emitting color was realized by adjusting the ratio among lanthanide ions, indicating the energy transfer process inside the complex. It was found that Tb³⁺ could sensitize the fluorescence of Eu³⁺ while its own fluorescence was quenched by the latter ion, and concentration quenching of Ce³⁺ was also observed. Fairly good thermal stability and oxidation resistance of the as-synthesized complexes were also demonstrated.     

Enhanced mid-IR emission in Yb-Tm co-doped oxyfluoride glass ceramics

Tm³⁺-Yb³⁺ co-doped transparent oxyfluoride glass ceramics were prepared through thermal treatment of the as-prepared glasses. The precipitation of nanocrystals and the incorporation of Tm³⁺ and Yb³⁺ into the nanocrystals were confirmed by X-ray diffraction and absorption spectra. Based on the Judd-Ofelt theory, the J-O parameters Ω_λ (λ = 2, 4, 6), spontaneous radiative transition rates, radiative lifetimes and fluorescence branching ratios of Tm³⁺ in both as-prepared glasses and glass ceramics were calculated. Intense mid-IR emission and upconversion luminescence in the Tm³⁺ and Yb³⁺ co-doped glass ceramics were observed under 980 nm excitation. Especially, compared with that of the as-prepared glasses, mid-IR luminescence intensity of Tm³⁺ in the glass ceramics was greatly enhanced. Desirable spectroscopic characteristics suggest that these oxyfluoride glass ceramics may be promising mid-IR laser active medium.     

Photoluminescence and energy transfer studies on Eu and Ce co-doped SrCaSiO4 for white light-emitting-diodes

The luminescence of SrCaSiO₄:Eu²⁺, Ce³⁺ is studied as a potential ultraviolet light-emitting diodes (UV-LEDs) phosphor that is capable of converting the ultraviolet emission of a UV-LED into green light with good luminosity. There are two emissions peaks peaking at 420 and 500 nm, respectively. The two emissions come from d-f transitions of Ce³⁺ and Eu²⁺, respectively. Effective energy transfer occurs in Ce³⁺/Eu²⁺ co-doped SrCaSiO₄ due to a part of spectral overlap between the emission of Ce³⁺ and excitation of Eu²⁺. Co-doping of Ce³⁺ enhances the emission intensity of Eu²⁺ greatly by transferring its excitation energy to Eu²⁺. The Ce³⁺/Eu²⁺ energy transfer, thoroughly investigated by the diffuse reflection emission and excitation spectra, photoluminescence decay curves, is demonstrated to be in the mechanism of electric dipole-dipole interaction.     

A study on microstructure and luminescent properties of oxyfluoride silicate glass-ceramics with (Ho,Yb):NaYF4 crystallites

Glass-ceramics containing NaYF₄ nanocrystals were prepared by heat-treatment from oxyfluoride silicate-based glass doped with Ho³⁺ and Yb³⁺ ions. The formation of crystalline fluoride phase was confirmed by X-ray diffraction and transmission electron microscopy. Absorption and emission spectra revealed that a fraction of Ho³⁺ and Yb³⁺ ions is incorporated into the NaYF₄ ordered lattice influencing spectroscopic features of glass-ceramics in comparison with those of precursor glass. Green up-conversion emission (545 nm) originating in the ⁵S₂ level in glass-ceramics and up-converted red emission (650 nm) originating in the ⁵F₅ level in as-melted glass were observed under optical pumping into Yb³⁺ absorption band and analyzed. Although both emissions in both materials are achieved by two-photon excitations, the relation between green and red emission intensity in glass-ceramics and glass implies that processes relevant to up-conversion phenomena are different. Based on a careful analysis of relaxation dynamics of Ho³⁺ and Yb³⁺ excited states, the mechanisms involved in conversion of the infrared radiation into the visible emission in these materials are proposed and discussed.     

Tunable red-green-blue multicolor luminescence in Yb/Tm/Ho:Gd3Ga5O12 nano-crystals

The Yb³⁺/Tm³⁺/Ho³⁺: Gd₃Ga₅O₁₂ nano-crystals have been successfully prepared via a citric acid complex procedure. The luminescence spectra were measured and the up-conversion processes were discussed. By means of adjusting the doping concentrations of Yb³⁺/Tm³⁺/Ho³⁺, the red-green-blue up-conversion luminescence changed obviously. Results indicated that the ratio of red-green-blue up-conversion emissions enhanced heavily with the increasing concentrations of Tm³⁺ doped in the Yb³⁺/Tm³⁺/Ho³⁺:Gd₃Ga₅O₁₂ nano-crystals, which was rooted in the three-photon resonant cross relaxation processes(¹G₄ (Tm) + ⁵I₇ (Ho) → ³H₅ (Tm) + ⁵S₂ (Ho)). The tunable red-green-blue luminescence could be used in the fields of display, illumination, and photonics such as the white light generation.     

Bredigite-structure Ca14Mg2[SiO4]8:Eu2+,Mn2+: A tunable green–red-emitting phosphor with efficient energy transfer for solid-state lighting

A single-phase green–red-emitting phosphor, Ca_13.7Eu_0.3Mg_2?xMn_x[SiO₄]₈ (CMS:Eu²⁺,Mn²⁺), was prepared by a solid-state reaction, and its energy transfer from Eu²⁺ to Mn²⁺ was investigated as a function of Mn²⁺ concentration. To explore the substitution of an Mn²⁺ site for each Mg site, a determination of the number of Mg substitutional sites was carried out using the Rietveld refinement method and bond valence sums. The dipole–dipole interaction was a dominant energy transfer mechanism of the electric multipolar character of CMS:Eu²⁺,Mn²⁺. The critical distance was calculated as 7.5 Å when using critical concentrations of Eu²⁺ and Mn²⁺. When CMS:Eu²⁺,Mn²⁺ was incorporated with an encapsulant in ultraviolet (λ_max = 400 nm) light-emitting diodes (LEDs), white light with a color rendering index of 67 under a forward bias current of 20 mA was obtained. The results of this work indicate that CMS:Eu²⁺,Mn²⁺ could be applicable to a single-phase phosphor for white LEDs under a near-ultraviolet source.     

Synthesis, photoluminescence, thermoluminescence and dosimetry properties of novel phosphor KSr4(BO3)3:Ce

Polycrystalline powder sample of KSr₄(BO₃)₃ was synthesized by high-temperature solid-state reaction. The influence of different rare earth dopants, i.e. Tb³⁺, Tm³⁺ and Ce³⁺, on thermoluminescence (TL) of KSr₄(BO₃)₃ phosphor was discussed. The TL, photoluminescence (PL) and some dosimetric properties of Ce³⁺-activated KSr₄(BO₃)₃ phosphor were studied. The effect of the concentration of Ce³⁺ on TL intensity was investigated and the result showed that the optimum Ce³⁺ concentration was 0.2 mol%. The TL kinetic parameters of KSr₄(BO₃)₃:0.002 Ce³⁺ phosphor were calculated by computer glow curve deconvolution (CGCD) method. Characteristic emission peaking at about 407 and 383 nm due to the 4f⁰5d¹ → ²F_{(5/2, 7/2)} transitions of Ce³⁺ ion were observed both in PL and three-dimensional (3D) TL spectra. The dose–response of KSr₄(BO₃)₃:0.002 Ce³⁺ to γ-ray was linear in the range from 1 to 1000 mGy. In addition, the decay of the TL intensity of KSr₄(BO₃)₃:0.002 Ce³⁺ was also investigated.     

Structural and luminescent properties of nano-sized NaGdF4:Eu synthesised by wet-chemistry route

Hexagonal Eu³⁺:NaGdF₄ fluoride with average grains size of 20 nm was obtained from solution by a co-precipitation method. Morphology of the obtained powder was examined by XRD and TEM methods. Absence of the Eu³⁺-O²⁻ charge-transfer band, expected in excitation spectrum at 260 nm indicates, that oxygen ions are not incorporated into a fluoride lattice. As-received fluoride contains considerable amounts of the water molecules, adsorbed at the surface of the material, which may be relatively easily removed by heating the powder at 300 °C. Thermal treatment at 650 °C is sufficient for removing of the OH⁻ groups built into fluoride lattice. Influence of method of synthesis as well as oxygen, water molecules and OH⁻ groups content on optical properties of the obtained phosphors is investigated and discussed by comparison with optical properties of the Eu³⁺:NaGdF₄ fluoride synthesised by a solid-state reaction.     

LaPO4:Eu, LaPO4:Ce, and LaPO4:Ce,Tb nanocrystals: Oleic acid assisted solvothermal synthesis, characterization, and luminescent properties

LaPO₄:Ln³⁺ (Ln = Eu, Ce, Tb) nanocrystals were successfully synthesized via a facile solvothermal process in the presence of oleic acid. The as-prepared crystals were well characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectra (XPS), Fourier transform infrared spectroscopy (FT-IR), optical spectra as well as the kinetic decay times, respectively. In the synthesis process, oleic acid as a surfactant has played a crucial role in confining the growth and size of the LaPO₄:Ln³⁺ phosphors. All the samples are well crystallized and assigned to the monoclinic monazite-type structure of the LaPO₄ phase. The prepared LaPO₄:Ln³⁺ phosphors present a narrow distribution with an average particle size of about 15 nm. Upon excitation by ultraviolet radiation, the LaPO₄:Eu³⁺ phosphors show the characteristic ⁵D₀-⁷F_1-3 emission lines of Eu³⁺, while the LaPO₄:Ce³⁺,Tb³⁺ exhibits the characteristic ⁵D₀-⁷F_3-6 emission lines of Tb³⁺. It is believed that these rare earth ion doped (Eu³⁺ ion or Ce³⁺ and Tb³⁺ ions co-doped) monoclinic monazite-type LaPO₄ nanocrystals could find potential application as future advanced optical materials.     

Combustion synthesis of long-persistent luminescent MAl2O4: Eu2+, R3+ (M = Sr,Ba, Ca,R = Dy,Nd and La) nanoparticles and luminescence mechanism research

Eu²⁺, R³⁺ co-doped alkaline earth aluminates MAl₂O₄: Eu²⁺, R³⁺ (M = Sr, Ba and Ca; R = Dy, Nd and La) nanoparticles with high brightness and long afterglow have been prepared by solution-combustion synthesis at 600 °C without a post-annealing process for the first time. The morphologies and the phase structures of the products have been characterized by transmission electron microscopy and X-ray diffraction. The excitation and emission spectra of the products have been measured by an Edinburgh FLS920 spectrometer at room temperature. The characteristic luminescence of the as-prepared sample has been evaluated, and the reason why the wavelength changed from yellow-green to blue-green and then to blue-purple in visible range when the composition changed from SrAl₂O₄: Eu²⁺, Dy³⁺ to BaAl₂O₄: Eu²⁺, Nd³⁺ and then to CaAl₂O₄: Eu²⁺, La³⁺ has been explained. Furthermore, a new persistent luminescence mechanism was proposed in this article.     

Efficient green up-conversion emission in Yb3+/Ho3+ co-doped CaIn2O4

Intense green up-conversion (UC) emissions from ⁵F₄/⁵S₂ → ⁵I₈ transitions of Ho³⁺ in Yb³⁺/Ho³⁺ co-doped CaIn₂O₄ have been observed with excitation at 980 nm. The optimal processing parameters were determined by investigating emission intensities as a function of annealing temperature, duration time and Ho³⁺/Yb³⁺ dopant concentrations. It has been confirmed that the green UC luminescence was generated via a two-photon process from the quadratic dependence of the emission intensity on the pump power. A UC mechanism was proposed and the lifetime of the green emission was measured to be ～204 μs. The infrared-to-visible UC efficiency of the optimized CaIn₂O₄: 0.005Ho³⁺, 0.1Yb³⁺ sample increases to ～5.5% with the excitation power and saturates at 1.5 W. The chromaticity coordinates (0.281, 0.708) of the samples are located in the green region and hardly changed due to the negligible red emission. The results indicate that CaIn₂O₄: Yb³⁺, Ho³⁺ could act as an effective UC green light emitter and CaIn₂O₄ is an ideal oxide host for UC luminescence.     

Preparation and characteristics of Fe3O4@YVO4:Eu bifunctional magnetic-luminescent nanocomposites

A facile direct precipitation method has been developed for the synthesis of bifunctional magnetic-luminescent nanocomposites with Fe₃O₄ nanoparticles as the core and YVO₄:Eu³⁺ as the shell. Transmission electron microscopy (TEM) images revealed that the obtained bifunctional nanocomposites had a core-shell structure and a spherical morphology. The average size was ∼150 nm, and the thickness of the shell was ∼15 nm. The X-ray diffraction (XRD) patterns showed that a cubic spinel structure of Fe₃O₄ core and a tetragonal phase of YVO₄ shell were obtained. Fourier transform infrared (FT-IR) spectra confirmed that the YVO₄:Eu³⁺ had been successfully deposited on the surface of Fe₃O₄ nanoparticles. Photoluminescence (PL) spectra indicated that the nanocomposites displayed a strong red characteristic emission of Eu³⁺. Magnetic measurements showed that the obtained bifunctional nanocomposites exhibited superparamagnetic behavior at room temperature. Therefore, the bifunctional nanocomposites are expected to develop many potential applications in biomedical fields.     

Sensitized high-order ultraviolet upconversion emissions of Gd by Er in NaYF4 microcrystals

High-order ultraviolet (UV) upconversion (UC) emissions of Gd³⁺ and Er³⁺ ions were observed in NaYF₄:Yb³⁺/Gd³⁺/Er³⁺ microcrystals under 980 nm excitation. These UC emissions came from six- and five-photon UC processes at low pump power range, which were confirmed by the pumping power dependences of UC fluorescence intensities. In these high-order UC processes, energy transfer (ET) processes of Er³⁺ → Gd³⁺ played crucial roles in populating the excited states of Gd³⁺ ions. Experiments on concentration variation and dynamic analysis revealed the ET processes between Er³⁺ and Gd³⁺ in detail. Some of possible population routes for populating excited Gd³⁺ ions were proposed based on spectral and dynamic analysis.