Size-dependent Upconversion Luminescence Research Articles

The size-dependent thermal enhancement of down-shifting luminescence based on non-Yb3+-mediated energy migration

The past few years have witnessed extensive attention to the size-dependent up-conversion luminescence thermal enhancement of Ln3+ doped nanoparticles, where the involvement of Yb3+ ions seem to be essential. In this work, a size-dependent down-shifting luminescence thermal enhancement behavior of NaGdF4:Ce3+/Tb3+ has been reported. Specifically, it is the first time to realize the size-dependent luminescence thermal enhancement with a non-Yb3+-involved luminescence process. We demonstrate that the size-dependent luminescence behavior is ascribed to the establishment of energy migration channels via Gd3+ sublattice. These findings offer a feasible way to design the luminescence thermal behavior of nanophosphors with a broader range in terms of both chemical composition and luminescence mechanism.

Controlled synthesis of β-NaYF4:Yb3+/Er3+ microstructures with morphology- and size-dependent upconversion luminescence

Hexagonal NaYF4 microcrystals with controllable morphologies and tunable sizes have been successfully synthesized by a facile hydrothermal method. The influences of the molar ratios of trisodium citrate (Cit3−) to Y3+ and pH values on the phase and morphology evolution were investigated and discussed in detail. It is found that trisodium citrate and pH value as two significant external factors play key roles in the formation of the anisotropic architectures of β-NaYF4 microcrystals. Taking hexagonal β-NaYF4 microprisms as a candidate, the possible formation mechanism has been proposed on the basis of time-dependent experiments. In addition, the upconversion (UC) luminescence properties of β-NaYF4:Yb3+/Er3+ samples were systematically studied and discussed. The results reveal that the UC luminescence properties are strongly dependent on their morphologies and sizes. This study not only provides important information for shape-controlled synthesis of other complex rare earth fluoride compounds, but also gives a reference for exploration of morphology- and size-dependent UC luminescence properties.

The use of zinc ions to control the size of Yb/Er:KMnF3 nanocrystals with single band emission

A novel method to modify the size of Yb/Er:KMnF3 NCs by introducing Zn2+ in the initial solution is presented, and the results can provide direct evidence for the size-dependent upconversion luminescence.

Influence of core size on the upconversion luminescence properties of spherical Gd2O3:Yb3+/Er3+@SiO2 particles with core-shell structures

Spherical SiO2 particles with different sizes (30, 80, 120, and 180 nm) have been coated with Gd2O3:Yb3+/Er3+ layers by a heterogeneous precipitation method, leading to the formation of core-shell structural Gd2O3:Yb3+/Er3+@SiO2 particles. The samples were characterized by using X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, upconversion (UC) emission spectra, and fluorescent dynamical analysis. The obtained core-shell particles have perfect spherical shape with narrow size distribution. Under the excitation of 980 nm diode laser, the core-shell samples showed size-dependent upconversion luminescence (UCL) properties. The inner SiO2 cores in core-shell samples were proved to have limited effect on the total UCL intensities of Er3+ ions. The UCL intensities of core-shell particles were demonstrated much higher than the values obtained in pure Gd2O3:Yb3+/Er3+ with the same phosphor volume. The dependence of the specific area of a UCL shell on the size of its inner SiO2 particle was calculated and analyzed for the first time. It was confirmed that the surface effect came from the outer surfaces of emitting shells is dominant in influencing the UCL property in the core-shell samples. Three-photon UC processes for the green emissions were observed in the samples with small sizes of SiO2 cores. The results of dynamical analysis illustrated that more nonradiative relaxation occurred in the core-shell samples with smaller SiO2 core sizes.

Size-dependent upconversion luminescence and quenching mechanism of LiYF_4: Er^3+/Yb^3+ nanocrystals with oleate ligand adsorbed

Er3+/Yb3+ co-doped LiYF4 nanocrystals were prepared by a facile solvothermal method. By adjusting the LiOH concentration, LiYF4 crystals with the size range from 16 nm to 2.0 μm were synthesized. Under the excitation by a 976 nm laser, upconversion quantum efficiency of the LiYF4: Er3+/Yb3+ samples were measured. It was observed that upconversion quantum efficiency tended to decrease with the reduction of particle size from microscale to nanoscale. A model was proposed to clarify the surface quenching mechanism influencing the size-dependent upconversion luminescence.

The size-dependent upconversion luminescence properties of β-NaYF4: Yb3+,Ho3+ microprisms

β-NaYF4:Yb3+(25%),Ho3+(1%) microprisms with different grain sizes were synthesized by a hydrothermal method. The structure, composition and upconversion emission properties of the products were characterized by scanning electron microscopy, x-ray diffractometer, inductively coupled plasma atomic emission spectrometry and spectrophotometer. It is found that with the decrease of NaF/NH4HF2 ratios in solution, the lengths of the β-NaYF4:Yb3+(25%),Ho3+(1%) microprisms increase from ~5 to ~15μm. The enhancements in the UC emissions with the increase of grain sizes resulted from the raise of the Yb3+ concentrations in crystals. Applying these materials at the back of a-Si single junction and a-Si/a-Si tandem solar cells, currents of 2.34 and 1.02μA are obtained under 60mW (80mW/cm2) 980nm laser excitation, respectively.

Size-dependent upconversion luminescence in CaF2:Yb3 +,Tm3 + nanocrystals

CaF2:Yb,Tm nano- and micro-crystals with different sizes were synthesized by a simple hydrothermal method. All the nanocrystals present intense ultraviolet (UV), visible, and near infrared (NIR) upconversion (UC) luminescence. The relationship between the UC luminescence intensities and the sample sizes from 50nm to 1200nm was investigated. The UV UC luminescence intensity of the sample with the particle size around 600nm was the highest among all the samples, but the NIR UC luminescence intensity of the sample was the lowest. Our experimental results indicated that there was a size between 50nm and microscale at which the UV emission intensity was the highest for CaF2 matrix. The size-dependent UC luminescence was proposed and analyzed based on experimental data.

Direct Evidence of a Surface Quenching Effect on Size‐Dependent Luminescence of Upconversion Nanoparticles

A series of Yb/Tm co-doped NaGdF4 nanoparticles without or with a thin surface protection layer provide direct evidence of a surface quenching effect on size-dependent upconversion luminescence (see picture). The coating preserves the optical integrity of the nanoparticles (right-hand spectrum) and minimizes emission loss induced by surface quenching. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.