Flame synthesis and effects of host materials on Yb/Er co-doped upconversion nanophosphors

The upconversion nanophosphors (UCNPs) of Yb³⁺/Er³⁺ co-doped into Y₂O₃, La₂O₃, and Gd₂O₃ were synthesized via the combustion method and characterized by powder X-ray diffractometer (XRD), scanning electron microscopy (SEM) and upconversion fluorescence spectroscopy. The characterization results showed that at the same flame temperature (2705 K) and precursor concentration (0.1 M), pure monoclinic and cubic-phase phosphors were achieved on Gd₂O₃ and Y₂O₃ hosted UCNPs, respectively; while the mixed phases were observed on La₂O₃ hosted UCNPs. Further annealing process at 850 °C produced pure cubic-phase La₂O₃:Yb³⁺,Er³⁺ UCNPs; while there was no phase transition observed on Gd₂O₃:Yb³⁺,Er³⁺ UCNPs. The dependence of upconversion luminescence on precursor concentrations and host materials was then examined. The La₂O₃ and Gd₂O₃ hosts were shown to be the promising alternates for the commonly used Y₂O₃ hosts for rare-earth doped phosphors.     

Monodisperse upconversion NaYF4 nanocrystals: Syntheses and bioapplications

A facile strategy using cheap and readily available precursors has been successfully developed for the synthesis of rare-earth doped hexagonal phase NaYF₄ nanocrystals with uniform shape and small particle size as well as strong photoluminescence. Due to their optical properties and good biocompatibility, these multicolor nanocrystals were successfully used as a bio-tag for cancer cell imaging. This novel synthetic method should also be capable of extension to the synthesis of other fluoride nanocrystals such as YF₃ and LaF₃.      

Study on NIR optical rewritable film based on dithienylethene and upconversion nanocrystals

Tm3+ and Yb3+ codoped NaYF4 upconversion (UC) nanoparticles (NPs) with intense ultraviolet (UV) fluorescence were synthesized using a solvothermal approach. NIR optical rewritable film incorporated with the UCNPs and dithienylethene (DTE) were performed for optical storage based on the photochromic reaction of DTE induced by the intense UV from themultiphoton UC fluorescence of NaYF4NPs. The photochromic DTE did not exhibit obvious fatigue after repetitious write/erase cycles using NIR/green irradiation.     

Solvothermal synthesis and upconversion emission of monodisperse ultrasmall SrYbF5 nanocrystals

Uniform ultrasmall monodisperse SrYbF₅ and SrErF₅ nanocrystals (NCs) were synthesized via solvothermal method using oleic acid as stabilizing agent. Transmission electron microscopy and X-ray diffraction assays reveal that the as-synthesized NCs are of face-centered cubic structure and the cell parameters are 5.608 Å (SrYbF₅) and 5.632 (SrErF₅) Å. Under the excitation of a 980 nm laser, visible upconversion (UC) emission can be observed by naked eyes in Tm³⁺ or Er³⁺ doped SrYbF₅ NCs. The integrated intensity of infrared emission is nearly two orders of magnitude stronger than that of the blue emission of SrYbF₅:Tm³⁺ NCs. The energy transfer UC emission mechanisms were also investigated.     

Monte-Carlo simulation of upconversion processes in erbium-doped materials

Nonlinear upconversion quenching in Er-doped materials has been studied with use of Monte-Carlo modeling. Structure models allowed one to consider homogeneous Er³⁺ ions space distributions as well as clustering. Two variants of the simulation algorithm have been developed which took into account an evolution of one excitation or evolutions of all excitations simultaneously. The obtained results provided quantitative data on the influence of the following factors on the upconversion efficiency: (a) erbium content, (b) migration of excitation, (c) pump power, and (d) duration of pulse pumping.     

Citric acid-assisted growth of lanthanide ions co-doped one-dimensional upconversion microcrystals and their photovoltaic applications

We report for the first time the preparation of Yb³⁺–Er³⁺ co-doped β-phase sodium gadolinium fluoride (NaGdF₄:Yb/Er) microcrystals using citric acid (CA) as a shape modifier and the application of NaGdF₄:Yb/Er to amorphous silicon solar cells. The proposed hydrothermal method was facile and eco-friendly. It was found that CA played a critical role in the shape evolution of the final products. The related synthesis mechanisms are discussed. Under 980 nm laser excitation, the optimized NaGdF₄:Yb/Er crystals exhibited strong upconversion (UC) emissions at visible wavelengths. Amorphous silicon thin film solar cells were designed and fabricated for investigating different UC configurations. In combination with a metallic back reflector, the cell with the front side UC layer achieved a six-fold improvement of the photocurrent while the cell with the rear side UC layer exhibited a ten-fold enhancement of the photocurrent, under 980 nm light, compared with the cell without upconverter.     

Nanocomposites of graphene oxide and upconversion rare-earth nanocrystals with superior optical limiting performance

Upconversion rare-earth nanomaterials (URENs) possess highly efficient near-infrared (NIR), e.g., 980 nm, laser absorption and unique energy upconversion capabilities. On the other hand, graphene and its derivatives, such as graphene oxide (GO), show excellent performance in optical limiting (OL); however, the wavelengths of currently used lasers for OL studies mainly focus on either 532 or 1064 nm. To design new-generation OL materials working at other optical regions, such as the NIR, a novel nanocomposites, GO-URENs, which combines the advantages of both its components, is synthesized by a one-step chemical reaction. Transmission electron microscopy, X-ray diffraction, infrared spectroscopy, and fluorescence studies prove that the α-phase URENs uniformly attach on the GO surface via covalent chemical bonding, which assures highly efficient energy transfer between URENs and GO, and also accounts for the significantly improved OL performance compared to either GO or URENs. The superior OL effect is also observed in the proof-of-concept thin-film product, suggesting immediate applications in making high-performance laser-protecting products and optoelectronic devices.     

Infrared to visible and ultraviolet upconversion processes in Nd-doped potassium lead chloride crystal

Infrared to visible and ultraviolet upconversion in Nd³⁺ doped KPb₂Cl₅ crystal is reported. Three intense bands have been observed at 535, 595, and 668 nm coming mainly from levels ⁴G_7/2–⁴G_9/2, together with emissions at 362, 388, 420 and 456 nm coming from level ⁴D_3/2, and a peak at 435 nm, corresponding to the ²P_1/2 → ⁴I_9/2 transition. A study of the upconversion mechanisms has been carried out that includes the upconversion emission dependence on the wavelength and intensity of the pumping light together with a comparison of the luminescence decays after infrared excitation and after direct one photon excitation of the emitting levels. The results indicate that two- and three-body energy transfer upconversion processes are responsible for emissions from levels ⁴G_7/2–⁴G_9/2 and ⁴D_3/2 respectively, while level ²P_1/2 is populated by either energy transfer upconversion or excited state absorption depending on the pumping wavelength.     

Spectroscopy and upconversion processes in YAlO3:Ho crystals

Emission from the high lying excited states, energy transfer, and upconversion processes are investigated in YAlO₃:Ho³⁺. Selectively excited emission spectra in the range from 300 to 800 nm starting from the ³D₃, ³G₅, ⁵F₃, ⁵S₂ and ⁵F₅ multiplets were measured at 15 K. This, together with the detailed absorption and excitation measurements at 15 K allowed determination of the Stark energy levels of Ho³⁺ ions in YAlO₃ up to UV energies. The ⁵S₂ fluorescence decays were recorded as a function of temperature and Ho³⁺ concentration in order to investigate the process of quenching of fluorescence due to cross relaxation among two ions. Conversion of red and infrared laser radiation to green ⁵S₂ and blue ⁵F₃ emission is reported. Under pulsed resonant excitation of the ⁵F₅ or ⁵I₅ levels the upconversion was found to be due to energy transfer process between two excited ions. The photon avalanche effect was observed under cw excitation around 585 nm.     

Infrared to visible upconversion luminescence in Er/Yb co-doped CeO2 inverse opal

Infrared to visible upconversion luminescence has been investigated in Er³⁺/Yb³⁺ co-doped CeO₂ inverse opal. Under the excitation of 980 nm diode lasers, visible emissions centered at 525, 547, 561, 660 and 680 nm are observed, which are assigned to the Er³⁺ transitions of ²H_11/2 → ⁴I_15/2 (525 nm), ⁴S_3/2 → ⁴I_15/2 (547, 561 nm), ⁴F_9/2 → ⁴I_15/2 (660 and 680 nm), respectively. The effect of photonic band gap on the upconversion luminescence intensity was also obtained. Additionally, the upconversion luminescence mechanism was studied. The dependence of Er³⁺ upconversion emission intensity on pump power reveals that it is a two-photon excitation process.     

Enhancement of upconversion luminescence of Er and Yb co-doped Y2O3 nanoparticle by Ag half-shell

Upconversion photoluminescence (PL) properties of single Y₂O₃ nanoparticles doped with Yb and Er (Y₂O₃:Yb,Er) with a Ag over-layer is studied. We traced the PL and light scattering images of individual nanoparticles by changing the thickness of a Ag over-layer. When the Ag thickness is relatively small and only the upper part of a nanoparticle is covered by Ag (Ag half-shell), the PL is strongly enhanced. On the other hand, when the Ag thickness is increased and a continuous Ag over layer is formed, the enhancement factor decreases. From the correlation between the enhancement factors of the upconversion PL and scattering intensities as well as the change of the PL lifetime, the mechanism of the PL enhancement is discussed.     

Growth and relaxation processes in Ge nanocrystals on free-standing Si(001) nanopillars

We study the growth and relaxation processes of Ge crystals selectively grown by chemical vapour deposition on free-standing 90 nm wide Si(001) nanopillars. Epi-Ge with thickness ranging from 4 to 80 nm was characterized by synchrotron based x-ray diffraction and transmission electron microscopy. We found that the strain in Ge nanostructures is plastically released by nucleation of misfit dislocations, leading to degrees of relaxation ranging from 50 to 100%. The growth of Ge nanocrystals follows the equilibrium crystal shape terminated by low surface energy (001) and {113} facets. Although the volumes of Ge nanocrystals are homogeneous, their shape is not uniform and the crystal quality is limited by volume defects on {111} planes. This is not the case for the Ge/Si nanostructures subjected to thermal treatment. Here, improved structure quality together with high levels of uniformity of the size and shape is observed.     

Influences of Er3+ content on structure and upconversion emission of oxyfluoride glass ceramics containing CaF2 nanocrystals

Transparent 45SiO₂–25Al₂O₃–5CaO–10NaF–15CaF₂ glass ceramics doped with different content of erbium ion (Er³⁺) were prepared. X-ray diffraction (XRD) and transmission electron microscope (TEM) analyses evidenced the spherical CaF₂ nanocrystals homogeneously embedded among the glassy matrix. With increasing of Er³⁺ content, the size of CaF₂ nanocrystals decreased while the number density increased. The crystallization kinetics studies revealed that CaF₂ crystallization was a diffusion-controlled growth process from small dimensions with decreasing nucleation rate. Er³⁺ could act as nucleating agent to lower down crystallization temperature, while some of them may stay at the crystal surfaces to retard the growth of crystal. Intense red and weak green upconversion emissions were recorded for glass ceramics and their intensities increased with the increasing of Er³⁺ content under 980 nm excitation. However, the concentration quenching effect appeared when Er³⁺ doping reached 2 mol%. These results could be attributed to the change of ligand field of Er³⁺ ions due to the incorporation of Er³⁺ ions into precipitated fluoride nanocrystals.     

Photoluminescence of transparent glass-ceramics based on ZnO nanocrystals and co-doped with Eu3+, Yb3+ ions

Glasses of the K₂OZnOAl₂O₃SiO₂ system co-doped with Eu₂O₃ and Yb₂O₃ were prepared by the melt-quenching technique. Transparent zincite (ZnO) glass–ceramics were obtained by secondary heat-treatments at 680–860 °C. At 860 °C, traces of Eu oxyapatite appeared in addition to ZnO nanocrystals. The average crystal size obtained from the X-ray diffraction data was found to range between 14 and 35 nm. Absorption spectra of the initial glasses are composed of an absorption edge and absorption bands due to electronic transitions of Eu³⁺ ions. With heat-treatment, the absorption edge pronouncedly shifts to the visible spectral range. The luminescence properties of the glass and glass-ceramics were studied by measuring their excitation and emission spectra at 300, 78, and 4.2 K. Strong red emission of Eu³⁺ ions dominated by the ⁵D₀–⁷F₂ (612 nm) electric dipole transition was detected. Changes in the luminescence properties of the Eu³⁺-related excitation and emission bands were observed after heat-treatments at 680 °C and 860 °C. The ZnO nanocrystals showed both broad luminescence (400–850 nm) and free-exciton emission near 3.3 eV at room temperature. The upconversion luminescence spectrum of the initial glass was obtained under excitation of the 976 nm laser source.     

Sensitized upconversion emissions of Tb3+ by Tm3+ in YF3 and NaYF4 nanocrystals

Yb3+-Tm3+-Tb3+-codoped YF3 and NaYF4 nanocrystals (NCs) were synthesized using a simple hydrothermal method. Under 980 nm excitation, violet and ultraviolet upconversion (UC) emissions of 5D3 --> 7FJ (J = 6, 5, 4) and 5D4 --> 7FJ (J = 6, 5, 4, 3) of Tb3+ ions were observed with the fluoride NCs. In the Yb-Tm-Tb codoped NCs, energy transfer (ET) processes from Tm3+ to Tb3+ were proposed to be the main mechanisms for the UC emissions of Tb3+ ions. They are more efficient than the phonon assisted cooperative sensitization of the Yb3+ couple proposed previously for similar material system. The analysis of power dependence indicated that populating the 5D4 level of the Tb3+ ions was a four photon UC process, which demonstrated the existence of the two step ET process of Yb3+ --> Tm3+ --> Tb3+. It was also found that UC luminescence properties of Tb3+ ions were sensitive to crystal structures.     

Controllable synthesis of tetragonal LiScF4:Yb3+, Er3+ nanocrystals and its upconversion photoluminescence properties: Lithium scandium fluoride nanocrystals

Tetragonal rare-earth ions codoped LiScF₄ nanocrystals have been synthesized by a modified solvothermal method. The results revealed that the phase and morphology can be tailored through varying the synthesis conditions, such as reaction temperature and time. Meanwhile, the UC fluorescence emission spectra were measured. It turned out that the UC emission intensity can be significantly influenced by reaction temperature and time. Different from the NaYF₄:20%Yb³⁺,2%Er³⁺ nanocrystals that usually emit green emission, yellow color emission can be observed in the LiScF₄:20%Yb³⁺,2%Er³⁺ samples under 980 nm excitation, which illustrates that the obtained new phase LiScF₄ is suitable as a promising host for efficient UC fluorescence generation and tunable UC emission spectra. Moreover, the UC mechanism was investigated in detail.     

Modeling of energy-transfer upconversion and thermal effects in end-pumped quasi-three-level lasers

An analytical model of cw quasi-three-level lasers that includes the influence of energy-transfer upconversion (ETU) has been developed. The results of the general output modeling were applied to a laser with Gaussian beams, and rigorous numerical calculations have been made to study the influence of ETU on threshold, output power, spatial distribution of population-inversion density, and fractional thermal loading. The model was applied to a laser operating at 946 nm in Nd:YAG, where the dependence of laser-beam size on laser performance was investigated in particular. A simple model for the degradation of laser-beam quality from a transversally varying saturated gain is proposed that is in good agreement with measurements of the laser in a plane-plane cavity.