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For anti-counterfeiting and full-color display applications, adjusting the Yb3+ concentrations or excitation conditions is the basic and simple way to control luminescence color and intensity. However, this approach is significantly limited because the luminescence color is not very sensitive to the response of doped Yb3+ concentrations or excitation conditions. Herein, we have demonstrated that the as-synthesized NaLuF4:Yb/Er and their annealed counterpart LuOF:Yb/Er microrods exhibit upconversion/downshifting double-mode luminescence with complementary and rich colors from green, yellow to red under 980/365/488 nm excitation. The anti-counterfeiting patterns printed with these microrods inks have the ability of the color response to the excitation wavelength, excitation power and laser scanning speed, which make them be hard to duplicate. This study indicates that Yb/Er doping NaLuF4 and their complementary LuOF microrods have great potential in multilevel anti-counterfeiting application and display. In addition, heat treatment way also brings novel ideas to the synthesis of high efficient luminescence materials.

The dependencies of EPR g-factors on the local structural parameter and intrinsic crystal-field parameters for a 4f3 configuration ion Nd3+ in Bi4Ge3O12 crystal have been studied by diagonalising 364 × 364 complete energy matrices. The results indicate that the local structure of (NdO6)9− cluster exhibits an oblate distortion in Bi4Ge3O12 crystal, where the ligand ions O2− undergo an outward departure (Δθ1 ≈ 0.92°, Δθ2 ≈ 1.27°) away from the C3 axis. An inflexion with six-order intrinsic parameter Ā6 ≈ 10 cm−1 was found that may well account for previous abnormal g-factors evaluation. Furthermore, our studies show that a stronger covalence between the ligand ions O2− and the central Nd3+ ions may exist in the (NdO6)9− cluster. Also, the high-order mixing effects on EPR parameters have been evaluated. It is shown that the dominant high-order contribution to EPR g-factors is from the J-J mixture between the terms 4I9/2 and 2H9/2, which accounts for about 3.3%.

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.

Pr3+/Yb3+ co-doped hexagonal NaYF4(beta-NaYF4) microprisms were synthesized by the hydrothermal method, and ethylenediaminetetraacetic acid (EDTA) was introduced to control the size of the microcrystal samples. Bright upconverted fluorescence emission was observed when the samples were excited with an infrared (IR) laser at 976.4 nm. The emission was found to originate from the transitions of 3P0-3F2, 3P0-3H6 or 1G4-3H4, 3P1-3H6, 3P0-3H5, 3P1-3H5, and 3P0-3H4 of Pr3+ ions. Possible mechanisms for upconversion fluorescence and concentration dependence as well as the crystal structure and its formation of NaYF4:Yb3+/Pr3+ microprisms were explored and discussed based on the experimental observations.

In this work, the uniform bundle-shaped microtubes composed of six half-pipes are synthesized for the first time in hydrothermal solutions via an intentional delayed phase transition pathway induced by Mn(2+) doping. The structural and kinetic factors that govern the phase and shape evolution of NaYF4 microcrystals have been carefully studied, and the influences of Mn(2+) to RE(3+) ratio, the amount of trisodium citrate, and the pH value in conjunction with the intrinsic character of RE(3+) ions on the phase and shape evolution are systematically discussed. It is found that the proper Mn(2+) to RE(3+) ratio is mainly responsible for delayed phase transition process and induces interior density gradient of solid aggregate for creating hollow bundle-shaped microtubes. While the amount of trisodium citrate and the pH value are the keys for the shape control of the NaYF4 microcrystals such as prismatic microtubes, prismatic short rods, thin plates, and particles. The up and downconversion emissions were obtained independent of whether α- or β-NaYF4:Er(3+)/Yb(3+) samples doped with Mn(2+), but the significant tuning of output color was only obtained in cube NaYF4 nanoparticles rather than in hexagonal microtubes via adjusting the amount of Mn(2+) ions. These spectral measurements and EDX analyses indicate that the distribution or concentration of Mn(2+) in hexagonal phase solid solution has changed, which supports Ostwald ripening growth mechanism and rules out agglomeration or oriented attachment growth mechanism. We designed crystal growth mode by simply addition of dopant may provide a versatile approach for fabricating a wide range of hollow nano/microcrystals and thus bring us a clearer understanding on the interaction between the dopant reagents and the nano/microcrystals.

A series of β-NaYF 4 microrods doped with Yb 3+ /Er 3+ have been successfully synthesized through a facile hydrothermal method. The fluoride microcrystals have been characterized by XRD, SEM, and the photoluminescence spectra method. The upconversion spectra show that the output light color have been tuned from green to red under near infrared excitation at 977 nm when replacing Y 3+ ions with a different amount of Yb 3+ in NaYF 4 :Yb/Er microcrystals. The effect of the intrinsic structural feature of fluoride compounds and four external factors, namely the concentration of Yb 3+ , the concentration of Er 3+ , the particle size and the use of an organic additive (EDTA), on the ultimate output light color of samples are investigated detailedly. The results show that the output light color is more sensitive to the concentration of Yb 3+ than that of Er 3+ . When elevated the concentration of dopants (Er 3+ or Yb 3+ ) in the NaYF 4 host lattice, the cross-relaxation process derived from Er 3+ promoted the red emission and quenched the green emission.

Europium doped lanthanide fluoride (LaF3) nanoparticles were prepared through a hydrothermal method with different PH values of precursor complex solution. The influence of PH value on the luminescence properties is investigated. It was found that the local symmetry of doped ions reduced with the increase of PH value, leading to the increase of the inversion symmetry ratio. The fluorescence quenching was observed for small nanoparticles, which was attributed to the large amount of OH groups absorbed to the surface of the nanoparticles.

An innovative route to tune upconversion (UC) emission in β-NaYF(4):Yb(3+)/Tm(3+) nanorods through codoping a third rare-earth ion upon continuous wave excitation near 976 nm is reported. The dependence of UC emission on codopant concentration and environment temperature shows that tailored local environment and readjustable depopulation of excited-state ions are responsible for the tuning of UC luminescence. Codopant ions introduce a new distribution of active ions and a modified distance between Tm(3+) and Yb(3+) ions, making UC systems more sensitive to impurity ions than downconversion systems.

A novel and facile citrate-assisted hydrothermal approach has been developed for the shape-selective synthesis of fluoride microcrystals, including NaYF4 and YF3, by tuning the pH of the mother liquid. A series of controlled experiments demonstrate that three important factors, including the pH values of the mother solution and the amount of trisodium citrate in conjunction with the intrinsic character of rare-earth ions, are responsible for varied distinctive morphologies such as the truncated octahedrons, prismatic microtubes with unusual zigzag top ends, prismatic short rods with regular hexagonal concave centers, and thin plates. The possible formation mechanisms for products with various morphologies have been explored, and the mechanism based on selective adsorption and the cation exchange are proposed. The upconversion spectra of Yb/Er-doped fluoride samples with different shapes have been investigated. The shape effect on luminescence is attributed to the different crystal facet components of an anisotropic orientational microcrystal in which the crystal planes composed of the a-axis and c-axis have the difference in ion-space distance and crystallization quality.

Rod-shaped and plate-shaped NaYF4:Yb3+/Er3+ nanocrystals (NCs) have been synthesized via a facile oleic acid-assisted hydrothermal route. The intensity and color tunability by shape controlling of NCs is demonstrated by up and down frequency conversions in NaYF4:Yb3+/Er3+ NCs. The results obviously show that rod-shaped NCs present much stronger upconversion luminescence intensity and smaller ratio of green to red emission intensity than that of plate-shaped NCs. The underlying reason of luminescence enhancement by shape controlling is explored, and a mechanism of the enhancement based on the decrease of nonradiation relaxation probability and the increase of energy transfer efficiency from Yb3+ to Er3+ is proposed for nanorods.

Efficient spectral conversion of 325–550 nm light to 570–710 nm light has been demonstrated in LaOF:Eu3+ nanocrystals. When levels above the D50 level of Eu3+ are optically excited, strong emission arising from the D50 level is obtained in the range of 570–710 nm, a highly efficient working range for organic solar cells. The influences of ambient temperature, particle size, dopant concentration, and codoped ions on the fluorescence intensity of Eu3+ are discussed in detail. The photon conversion efficiency can reach 3.91% in LaOF:Eu3+ codoped with Tm3+, if light reflection and scattering effects are ignored

Strong blue and green visible lights are obtained in Tm3+/Ho3+-codoped LaOF tetragonal nanocrystals (NCs) through down and up fluorescence conversions when the sample is excited selectively with ultraviolet and infrared photons, respectively. The excitation spectrum by detecting at 545 nm (Ho3+:S52→I58) reveals two peaks at 348 nm (Ho3+:I58→G53) and 355 nm (Tm3+:H36→D12), which indicates that the energy transfer process from Tm3+ to Ho3+ has occurred when Tm3+ is excited with 355 nm photons in Tm3+/Ho3+:LaOF. There is no energy transfer from Ho3+ to Tm3+ when Ho3+ is excited with 348 nm photons in codoped samples because the near resonant Ho3+ levels are significantly quenched by multiphonon relaxation. But enhanced green fluorescence of Ho3+ is obtained due to the ion situated at the low symmetry site in Ho3+/Tm3+:LaOF NCs. The intensity patterns of the up-conversion fluorescence would be fine-tuned by adjusting Tm3+ concentration and excitation wavelength. Spectroscopic analyses in down- and up-conversi...