Facile synthesis of β-NaYF4:Ln3+ (Ln = Eu, Tb, Yb/Er, Yb/Tm) microcrystals with down- and up-conversion luminescence

β-NaYF₄:Ln³⁺ (Ln = Eu, Tb, Yb/Er, Yb/Tm) hexagonal microrods have been successfully synthesized through a facile molten salt method without any surfactant. X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy, high-resolution transmission electron microscopy, and photoluminescence spectra were used to characterize the samples. It is found that at a preferred reaction temperature of 400 °C, the structure of β-NaYF₄ can gradually transform from microtubes to microrods as reaction time extends from 0.5 to 4 h. Furthermore, as the molar ratio of NaF:RE³⁺ (RE represents the total amount of Y³⁺ and the doped rare earth elements such as Eu³⁺, Tb³⁺, Yb³⁺/Er³⁺, or Yb³⁺/Tm³⁺) increased, the phase of sample transforms from YF₃ into NaYF₄. Under the excitation of 395 nm ultraviolet light, β-NaYF₄:5 %Eu³⁺ shows the emission lines of Eu³⁺ corresponding to ⁵D_0-3 → ⁷F _J (J = 1–4) transitions from 400 to 700 nm, resulting in red down-conversion (DC) light emission. When doped with 5 % Tb³⁺ ions, the strong DC fluorescence corresponding to ⁵D₄ → ⁷F _J (J = 6, 5, 4, 3) transitions with ⁵D₄ → ⁷F _J (green emission at 544 nm) being the most prominent group that has been observed. Moreover, upon 980 nm laser diode excitation, the Yb³⁺/Er³⁺- and Yb³⁺,Tm³⁺- co-doped β-NaYF₄ samples exhibit bright yellow and blue upconversion (UC) luminescence, respectively, by two- or three-photon UC process. The luminescence mechanisms for the doped lanthanide ions were thoroughly analyzed.     

Synthesis and luminescence of BiPO4:Ln3+ (Ln = Ce3+, Tb3+) phosphors

BiPO₄:Ce³⁺ and BiPO₄:(Ce³⁺, Tb³⁺) powders were synthesized by the method of precipitation. The X-ray diffraction patterns show that BiPO₄:Ce³⁺ and BiPO₄:(Ce³⁺, Tb³⁺) samples have pure hexagonal phases. The transmission electron microscopy results show that the synthesized samples are nanoparticles. Ethylene glycol plays an important role in the formation of nanoparticles. The excitation spectrum of BiPO₄:Ce³⁺ sample shows the transition from the ground ² F _5/2 state to the excited 5d states of the Ce³⁺ ions. The emission spectrum exhibits a strong band centered at 352 nm originating from the 5d → 4f transitions of the Ce³⁺ ions. The emission spectrum of the BiPO₄:(Ce³⁺, Tb³⁺) sample contains both a weak emission band of the Ce³⁺ ions and strong green emission bands of the Tb³⁺ ions. The excitation and emission spectra show that there are energy transfers between Ce³⁺ and Tb³⁺ ions in the BiPO₄:(Ce³⁺, Tb³⁺) sample. The energy transfers between Ce³⁺ and Tb³⁺ ions improve the emission efficiency of BiPO₄:(Ce³⁺, Tb³⁺) sample.     

Au-modified silicon nanowires for surface-enhanced fluorescence of Ln3+ (Ln = Pr, Nd, Ho, and Er)

The interaction of the surface plasmons of gold nanoparticles on silicon nanowires with fluorophores, lanthanide ions (praseodymium ions, Pr³⁺, neodymium ions Nd³⁺, holmium ions Ho³⁺, and erbium ions Er³⁺) was investigated. In the presence of Au/Si nanomaterials, the fluorescence peaks were significantly enhanced, which resulted in about 2 orders of magnitude enhancement. The photoluminescence studies revealed that the enhanced fluorescence originates from the local field enhancement around Ln³⁺ ions, caused by the electronic plasmons resonance of the gold nanoparticles. Results showed that this Au/Si nanostructure had larger enhancement factor than that caused by unsupported Au nanoparticles. These results might be explained by the local field overlap originated from the closed and fixed gold nanoparticles on silicon nanowires.     

Sol–gel auto-combustion synthesis and luminescence properties of RVO4: Ln3+ (R=La,Gd; Ln=Sm,Er, Ho,Yb/Er) microcrystals

RVO₄: Ln³⁺ (R=La, Gd; Ln=Sm, Er, Ho, Yb/Er) microcrystals were successfully synthesized by a facile and rapid sol-gel method using glycine as the chelating agent. The crystalline structure, morphology and luminescence properties of obtained products were investigated in detail. The peaks of X-ray diffraction (XRD) patterns were well-indexed to the standard RVO₄ patterns, indicating that the Ln³⁺ ions were well doped into the crystalline lattices and had not changed the crystalline structure. The luminescence properties of the samples are systematically studied. The typical peaks of the emission spectra were sharp and intense, revealing energy was transferred efficiently from VO₄ ^3- to Ln³⁺ after excitation. The schematic diagram for energy transfer between the host matrix and doped lanthanide ions were also discussed. The chromaticity coordinates of RVO₄: Ln³⁺ microcrystals were calculated. This work reveals that the rare-earth vanadates are potential candidates as excellent host matrices of phosphors.     

Synthesis and luminescence properties of novel β-Zn3BPO7:Ln (Ln = Ce3+, Eu2+, Eu3+) phosphors

This work first reports the synthesis and luminescence properties of rare earth Ce³⁺, Eu²⁺, Eu³⁺ ions doped β-Zn₃BPO₇ phosphors [β-Zn₃BPO₇:Ln (Ln = Ce³⁺, Eu²⁺, Eu³⁺)]. The phosphors were synthesized by a solid-state reaction at high temperature, and their luminescence properties were investigated by measuring the photoluminescence excitation and emission spectra. The f–d transitions of Ce³⁺ and Eu²⁺ ions in the host lattice are assigned and corroborated, which lead to the broad emission band in ultraviolet (UV) and visible region for Ce³⁺ and Eu²⁺ ions under UV excitation, respectively. Typical reddish orange emission from Eu³⁺ in the host lattice was also observed. The spectroscopic characteristics including Stokes shift, crystal field depression, electron–vibrational interaction, and charge transfer band were investigated and compared with that in other borophosphate phosphors. β-Zn₃BPO₇:Eu²⁺ and β-Zn₃BPO₇:Eu³⁺ phosphors show potential application in solid state lighting region.     

Synthesis and luminescence properties of Yb3+–Er3+ co-doped LaOCl nanostructures

LaOCl:Yb³⁺, Er³⁺ nanofibers and hollow nanofibers were prepared by electrospinning combined with a double-crucible chlorination technique using NH₄Cl as chlorinating agent. X-ray powder diffraction analysis indicated that LaOCl:Yb³⁺, Er³⁺ nanostructures were tetragonal with space group P4/nmm. Scanning electron microscope analysis and histograms revealed that diameters of LaOCl:Yb³⁺, Er³⁺ nanofibers and hollow nanofibers, respectively, were 117.87 ± 15.48 and 141.09 ± 17.10 nm under the 95 % confidence level. Up-conversion (UC) emission spectra analysis manifested that LaOCl:Yb³⁺, Er³⁺ nanostructures exhibited strong green and red UC emission centering at 526, 548, and 671 nm, respectively, attributed to ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2, and ⁴F_9/2 → ⁴I_l5/2 transitions of Er³⁺ ions under the excitation of a 980-nm diode laser. It was found that the relative intensities of green and red emissions vary obviously with the addition of Yb³⁺ ions, and the optimized molar ratio of Yb³⁺ to Er³⁺ was 10:1 in the as-prepared nanofibers. Moreover, the near-infrared characteristic emissions of LaOCl:Yb³⁺, Er³⁺ nanostructures were achieved under the excitation of a 532-nm laser. CIE analysis demonstrated that color-tuned luminescence can be obtained by changing doping concentration of Yb³⁺ (and/or Er³⁺) ions and morphologies of nanomaterials, which could be applied in the fields of optical telecommunication and optoelectronic devices. The UC luminescent mechanism and the formation mechanisms of LaOCl:Yb³⁺, Er³⁺ nanofibers and hollow nanofibers were also proposed.     

Synthesis of β–NaYF4: Yb3+, Tm3+ @ TiO2 and β–NaYF4: Yb3+, Tm3+ @ TiO2 @ Au nanocomposites and effective upconversion–driven photocatalytic properties

The β–NaYF₄: Yb³⁺, Tm³⁺ @ TiO₂ nanocomposite has been prepared by a facile hydrothermal method followed by the hydrolysis of TBOT, and then NaYF₄: Yb³⁺, Tm³⁺ @ TiO₂, HAuCl₄ and sodium citrate were put into an oil bath for reaction to obtain the β–NaYF₄: Yb³⁺, Tm³⁺ @ TiO₂ @ Au core–shell nanocomposite. XRD and HRTEM show that the samples exhibit the hexagonal phase NaYF₄, anatase TiO₂ and cubic Au, indicating that the core–shell phases of NaYF₄−TiO₂ or NaYF₄−TiO₂−Au coexist in these samples. EDS and XPS results show the presence of Na, Y, F, Ti, O and Au elements. When TiO₂ was coated on the surface of upconversion nanomaterials of NaYF₄: Yb³⁺, Tm³⁺, the photocatalytic activity was improved significantly, and the β–NaYF₄: Yb³⁺, Tm³⁺ @ TiO₂ nanocomposite gives the highest photodegradation efficiency for MB and RhB, and decomposes about 73% of MB or 80% of RhB within 4.5 h under simulated solar light irradiation respectively. When the ultraviolet light from simulated sunlight irradiation was removed by the addition of a UV filter, the β–NaYF₄: Yb³⁺, Tm³⁺ @ TiO₂ nanocomposite decomposes about 42% of MB or 48% of RhB within 4.5 h. It means that the upconversion–driven photocatalytic performance (decomposes 42% of MB or 48% of RhB) is more effective than UV light–driven photocatalytic performance (31% of MB or 32% of RhB) in the photodegradation process. In addition, the β–NaYF₄: Yb³⁺, Tm³⁺ @ TiO₂ @ Au core–shell nanocomposite does not exhibit the better photocatalytic activity, and the optimal research will be carried out in the future.     

Synthesis,structural studies,electrical conductivity and optical properties of new Sb2−xLnxTe3 (Ln: Lu3+, Er3+ and Ho3+) nanomaterials

Ln-doped Sb₂Te₃ (Ln: Lu³⁺, Er³⁺, Ho³⁺) nanomaterials were synthesised by a co-reduction method in hydrothermal condition. Powder X-ray diffraction (XRD) patterns indicate that the Ln_xSb_2?xTe₃ crystals (Ln = Lu³⁺, x = 0.00–0.06; Er³⁺ and Ho³⁺ x = 0.00–0.04) are isostructural with Sb₂Te₃. The cell parameter a decreases for Ln_xSb_2?xTe₃ compounds upon increasing the dopant content (x), while c increases. Scanning electron microscopy and transmission electron microscopy images show that doping of Lu³⁺ and Ho³⁺ ions in the lattice of Sb₂Te₃ results in spherical nanoparticles while that in Er³⁺ leads to hexagonal nanoplates, respectively. The electrical conductivity of Ln-doped Sb₂Te₃ is higher than that of pure Sb₂Te₃ and increases with temperature. By increasing the concentration of Ln³⁺ ions, the absorption spectrum of Sb₂Te₃ shows red shifts and some intensity changes. In addition to the characteristic red emission peaks of Sb₂Te₃, emission spectra of doped materials show other emission bands originating from f–f transitions of the Ho³⁺ ions.     

Preparation and luminescence properties investigation of Eu3+, Tb3+-doped LaNbO4:RE3+ (RE = Eu,Eu/Tb,Tb)

Journal of Materials Science: Materials in Electronics - In this work, doped and co-doped LaNbO4:RE3+ (RE?=?Eu, Eu/Tb, Tb) single crystals were prepared by a unique hydrothermal method...     

Synthesis and Upconversion Emission of β-Ca2SiO4: (Er3+, Yb3+)

The β-Ca₂SiO₄: (Er³⁺, Yb³⁺) powders were synthesized by the simple solid-state process. The obtained samples were given characterizations of X-ray diffraction, Fourier-transform infrared, transmission electron microscopy, and luminescence. The samples have monoclinic parawollastonite phase and irregular morphology. Under the excitation at 980 nm, the obtained β-Ca₂SiO₄: (Er³⁺, Yb³⁺) samples show the intense upconversion (UC) emission. The dosage of Yb³⁺ has obvious influence on the emission intensities of β-Ca₂SiO₄: (Er³⁺, Yb³⁺) samples. Also, the emission intensity increases gradually with the increasing pump power from 350 to 600 mW. On the basis of luminescent properties of samples, we can conclude that the UC emission originates from the biphotonic process.     

Upconversion luminescence of Ba(MoO4)h(WO4)1−h:Yb3+/Er3+ nanocrystals synthesized through hydrothermal method

A series of Yb³⁺/Er³⁺ co-doped Ba(MoO₄)_h(WO₄)_1−h upconversion nanocrystals (UCNCs) were prepared via hydrothermal method. The effects of different concentration ratios of Yb³⁺/Er³⁺ and Mo₄O²⁻/WO₄²⁻ on the upconversion luminescence were investigated, and the optimum doping concentrations of Yb³⁺ and Er³⁺ in the Ba(MoO₄)_0.5(WO₄)_0.5 host were found to be 3 mol% and 1 mol%, respectively. Structure of Ba(MoO₄)_0.5(WO₄)_0.5:0.03Yb³⁺/0.01Er³⁺ was identified as the tetragonal in the X-ray diffraction (XRD) results and the particle size observed in the scanning electron microscope (SEM) was about 40 nm. Under excitation of 980 nm semiconductor laser, three emission bands centered at 528, 550 and 660 nm, originating from ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺ ion, respectively, were observed for Ba(MoO₄)_0.5(WO₄)_0.5:0.03Yb³⁺/0.01Er³⁺. The pump power dependence research suggested that these bands arise due to two-photon absorption. The variation of CIE coordinate at different excitation powers was observed.     

Frequency up-conversion luminescence in Yb3+–Ho3+ co-doped PbxCd1−xF2 nano-crystals precipitated transparent oxyfluoride glass-ceramics

Oxyfluoride glasses were developed with composition 30SiO₂·15AlO_1.5·28PbF₂·22CdF₂·(4.9−x)GdF₃·0.1HoF₃·xYbF₃ (x=0, 0.1, 0.2, 0.5, 1, 2, 3, and 4) in mol%. Powder X-ray diffraction analysis revealed that the heat-treatments of the oxyfluoride glasses at the first crystallization temperature cause the precipitation of Yb³⁺–Ho³⁺ co-doped fluorite-type nano-crystals of about 17.8 nm in diameter in the glass matrix. These transparent glass-ceramics exhibited very strong green up-conversion luminescence due to the Ho³⁺: (⁵F₄, ⁵S₂)→⁵I₈ transition under 980 nm excitation. The intensity of the green up-conversion luminescence in the glass-ceramics was much stronger than that in the precursor oxyfluoride glass. The reasons for the highly efficient Ho³⁺ up-conversion luminescence in the oxyfluoride glass-ceramics are discussed.     

Synthesis and up-conversion luminescence of Yb3 + –Ho3 + co-doped Na(Y1·5Na0·5)F6 nanorods

We present the optical up-conversion (UC) study for Yb^3?+?–Ho^3?+? co-doped Na(Y_1·5Na_0·5)F₆ nanorods synthesized by employing a facile hydrothermal method. Numbers of Ho^3?+? ion up-conversion emissions have been observed under 980 nm infrared diode laser excitation. Three UC emissions of interest, ultraviolet, violet and blue, are specially identified at 359, 387, 418 and 483 nm, corresponding to $^{5}{{G}}^{\prime}_{5}{/}^{3}\!{{H}}_{ 6}\to\ ^{ 5}\!{ {I}}_{ 8}$ , $^{ 5}\!{ {G}}_{ 4}{/}^{ 3}\!{ {K}}_{ 7}\to\ ^{ 5}\!{ {I}}_{ 8}$ , $^{ 5}{ {G}}_{ 5}\to$ $^{ 5}\!{ {I}}_{ 8}$ and $^{ 5}\!{ {F}}_{ 3}{/}^{ 5}\!{ {F}}_{ 2}{/}^{ 3}\!{ {K}}_{ 8}\to {}^{ 5}\!{ {I}}_{ 8}$ transitions, respectively. It is also found that the centre wavelength of blue UC emission shifts to 475 nm gradually as Ho^3?+? concentration decreases. Lastly, a brief analysis about UC mechanism is demonstrated according to the experimental results.     

Diode-pumped 2 μm vibronic (Tm3+, Yb3+):KLu(WO4)2 laser

We report on laser operation in a (6 at. % Tm, 5 at. % Yb):KLu(WO4)2 codoped crystal. The vibrational frequencies of KLu(WO4)2 are coupled to the electronic transitions of Tm3+ at 1946 nm, creating virtual final laser levels at higher energy than the ground level 3H6 of Tm3+. The longest recorded laser wavelength was 2039 nm, which is longer than permitted by a pure electronic transition in Tm3+ ions in KLu(WO4)2. We show that every laser wavelength can be explained with the electron-phonon coupling effect, where the vibration frequencies were determined through Raman spectroscopy.