Optical and photoelectric properties of gamma-irradiated GaS:Er<Superscript>3+</Superscript> layered crystals

Erbium doping (0.1, 0.15, 0.20 at % Er) markedly increases the photosensitivity and photoluminescence (PL) intensity of GaS crystals. Most likely, erbium substitutes on the gallium site. Irradiation with low gamma doses increases the photosensitivity and PL intensity of the crystals, broadens their PL spectrum, and shifts the PL peaks to longer wavelengths. It seems likely that the radiation-induced recombination centers consist of gallium vacancies and erbium atoms. The concentration of such defect complexes depends on the gamma dose.     

Luminescence of BaSiO<Subscript>3</Subscript> crystals doped with Er<Superscript>3+</Superscript> and Yb<Superscript>3+</Superscript>

The Stokes and anti-Stokes luminescence of undoped and rare-earth-doped (Er³⁺ and Yb³⁺) BaSiO₃ has been studied in the temperature range 78–450 K under excitation at 10–1000 mV. The results indicate that the emission mechanism in BaSiO₃ crystals is hole recombination and that the anti-Stokes luminescence is due to consecutive sensitization; that is, the Yb³⁺ ions in the BaSiO₃ compound act as luminescence sensitizers, and the Er³⁺ ions, as activators.     

Influence of Mn<Superscript>2+</Superscript> on the up-conversion emission performance of Mn<Superscript>2+</Superscript>, Yb<Superscript>3+</Superscript>, Er<Superscript>3+</Superscript>: ZnWO<Subscript>4</Subscript> green phosphors

The Mn²⁺, Yb³⁺, Er³⁺: ZnWO₄ green phosphors are synthesized successfully through the high temperature solid state reaction method. The micro-structure and morphology have been investigated by means of XRD and EDS. The doped concentrations of Mn²⁺, Yb³⁺, Er³⁺ are measured by ICP. The absorption spectra and emission spectra with different doped concentrations of Mn²⁺ are presented to reveal the influence of Mn²⁺ on the green up-conversion performance. Excited with 970 nm LED, the up-conversion emission peak at 547 nm is obtained and the CIE spectra as well as the green light photo are also presented. The results indicate that the Mn²⁺ ions play the role of the luminescence adjustment in the up-conversion process, which can improve the up-conversion green emission intensity effectively. The luminescence adjustment mechanism of Mn²⁺ ions in Mn²⁺, Yb³⁺, Er³⁺: ZnWO₄ green phosphors has been discussed. The crystal parameters of Dq, B and C are calculated to evaluate the energy level split effect.     

Synthesis of Er<Superscript>3+</Superscript> and Er<Superscript>3+</Superscript> : Yb<Subscript>3+</Subscript> doped sol-gel derived silica glass and studies on their optical properties

Er₃₊ and Er₃₊ : Yb₃₊ doped optical quality, crack and bubble free glasses for possible use in making laser material have been prepared successfully through sol-gel route. The thermal and optical, including UV-visible absorption, FTIR etc characterizations were undertaken on the samples. The absorption characteristics of Er₃₊ doped samples clearly revealed the absorption due to Er₃₊ ions. On the other hand Yb₃₊ : Er₃₊ doped samples showed enhanced absorption due to² F _7/2 →² F _5/2 transition. The absorption and emission cross-section for² F _7/2 ↔² F _5/2 of Yb³⁺ were estimated. FTIR absorption spectra have clearly shown the reduction of the absorption peak intensity with heat treatment in the range 3700–2900 cm⁻¹. The 960 cm^-1 band also showed progressive decrease in the absorption band peak intensity with heat treatment. The result of the investigations with essential discussions and conclusions have been reported in this paper.     

Spectral parameters of Er<Superscript>3+</Superscript> ion in Yb<Superscript>3+</Superscript>/Er<Superscript>3+</Superscript>:KY(WO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystal

The spectral parameters of Er³⁺ in Yb³⁺/Er³⁺:KY(WO₄)₂ crystal with space group C2/c have been investigated based on Judd-Ofelt theory. The spectral parameters were obtained: the intensity parameters are: ₂ = 6.33 × 10^–20 cm², ₄ = 1.35 × 10^–20 cm², ₆ = 1.90 × 10^–20 cm². The radiative lifetime and the fluorescence branch ratios were calculated. The emission cross section _e (at 1536 nm) is 2.0 × 10^–21 cm².     

Pulsed cathodoluminescence of single-crystalline and pressed CaF<Subscript>2</Subscript>:Yb<Superscript>2+</Superscript>,Yb<Superscript>3+</Superscript>

We have studied the pulsed cathodoluminescence spectra and kinetics of CaF₂:Yb²⁺,Yb³⁺ (3 mol % YbF₃) single crystals and pressed samples under excitation with nanosecond electron pulses and determined the characteristic times and intensities of nanosecond and microsecond emission decay components at temperatures from 15 to 300 K. The results demonstrate that deformation pressing in vacuum at 1150°C followed by annealing in a CF₄ atmosphere at 1180°C has an insignificant effect on the emissive properties of CaF₂:Yb²⁺,Yb³⁺.     

Upconversion Luminescence of Gd<Subscript>2</Subscript>O<Subscript>3</Subscript> Nanocrystals Doped with Er<Superscript>3+</Superscript> and Yb<Superscript>3+</Superscript> Ions

The upconversion luminescence (UCL) of nanocrystalline gadolinium oxide (Gd₂O₃) doped with Er³⁺ and Yb³⁺ ions has been studied in the temperature range of 90–400 K. The nanocrystals were synthesized by chemical vapor deposition and possessed a cubic crystalline structure with an average particle size within 48–57 nm. It is established that the USL intensity in the red (⁴F_9/2 → ⁴I_15/2 transition in Er3+ ion) and green (⁴S_3/2 → ⁴I_15/2 transition) spectral regions depends on the sample temperature and concentration of dopant ions, as well as on the additional structural defects (anion vacancies) created in the crystal lattice by the introduction of Zn²⁺ ions or irradiation with high-energy (10 MeV) electrons. The luminescence efficiency and spectrum of the upconversion phosphor are determined by energy transfer processes.     

NIR-excited NIR and visible luminescent properties of amphipathic YVO<Subscript>4</Subscript>: Er<Superscript>3+</Superscript>/Yb<Superscript>3+</Superscript> nanoparticles

This article reports the luminescence properties of amphipathic YVO₄:Er³⁺/Yb³⁺ nanoparticles (average grain size ca. 20 nm) obtained by an oleate-aided hydrothermal process. Depending on the upconversion (UPC) and downconversion (DWC) processes, they show luminescence in the visible and near-infrared (NIR) regions, respectively, by 980-nm excitation. The sample doped with Er³⁺:2.5 mol% and Yb³⁺:10 mol% showed the highest luminescence intensity in both the visible and NIR regions as a result of efficient energy transfer from Yb³⁺ to Er³⁺ ions. The hydrothermal treatment greatly enhanced both the DWC and UPC luminescence efficiencies. This is due to the reduction in the concentration of surface defects and ligands, accompanied by grain growth. NIR Fluorescence microscopy revealed for the first time that DWC luminescence is sufficiently intense for application of these nanocrystals as a NIR bioprobe.     

X-ray diffraction,optical absorption and emission studies on Er<Superscript>3+</Superscript>- and Er<Superscript>3+</Superscript>/Yb<Superscript>3+</Superscript>-doped Li<Subscript>3</Subscript>NbO<Subscript>4</Subscript> powder

Er³⁺(/Yb³⁺)-doped Li₃NbO₄ powder were prepared by thermally sintering mixtures of Er₂O₃ (0.5, 1.0 mol%), Yb₂O₃ (0, 0.5, 1.0 mol%), Li₂CO₃ (48–49 mol%) and Nb₂O₅ (50 mol%) at 1125, 1150 and 1450 °C over the durations of 8–22 h. The crystalline phases contained in these samples were determined by using X-ray diffraction and discussed in comparison with a vapor-transport-equilibration-treated (VTE-treated) Er(2.0 mol%):LiNbO₃ single crystal and ErNbO₄ powder previously reported. The results show that the X-ray patterns of the rare-earth-doped samples reveal little difference each other, but large differences with those of the VTE crystal and ErNbO₄ powder. The doped rare-earth ions Er³⁺ (and Yb³⁺) present in the powder as the ErNbO₄ (and YbNbO₄) phase(s). The possibility that the highly Er-doped LiNbO₃ crystal contains Li₃NbO₄ precipitates is small. Optical absorption and emission studies show that the only Er-doped Li₃NbO₄ powder shows similar absorption and emission characteristics with the pure ErNbO₄. The codopant Yb³⁺ ion enhances the 980-nm-upconversion emissions of Er³⁺ ions, results in remarkable spectral alterations at 0.98 μm region, and causes the alterations of relative absorbance and relative emission intensity of individual peaks or bands at 1.5 μm region. On the other hand, the Yb-codoping hardly affects the Er³⁺ energy structure and the lifetime of Er³⁺ ion at 1.5 μm. The measured lifetimes at 1.5 μm of Er³⁺ ions in the singly Er³⁺- and doubly Er³⁺/Yb³⁺-doped mixtures have a nearly same value of ∼ 1.5 ms. For the pure ErNbO₄ powder, the lifetime is prolonged to ∼2 ms perhaps due to radiation trapping effect.     

Green and red upconversion emissions of Er<Superscript>3+</Superscript>/Yb<Superscript>3+</Superscript>-codoped SrTiO<Subscript>3</Subscript> powder prepared by a polymeric precursor method

Ultrafine Er³⁺/Yb³⁺-codoped SrTiO₃ (SEYT) powders in cubic form have been prepared by a poly-meric precursor method. The single phase perovskite for the obtained material was observed at low temperature. An efficient infrared-to-visible conversion in SEYT perovskite will be reported. Visible emissions at 550 and 663 nm corresponding to the ²S_3/2 − ⁴I_15/2 and ⁴F_9/2 − ⁴I_15/2 transitions, respectively, were observed under continuous wave excitation at 980 nm. An enhancement of the visible upconversion luminescence in Er³⁺/Yb³⁺ codoped samples was confirmed due to efficient energy transfer from Yb³⁺ to Er³⁺ ions. The quadratic pump power dependence of these emissions indicated the contribution of two photons to the upconversion process. The text was submitted by the authors in English.     

One-pot synthesis and strong near-infrared upconversion luminescence of poly(acrylic acid)-functionalized YF<Subscript>3</Subscript>:Yb<Superscript>3+</Superscript>/Er<Superscript>3+</Superscript> nanocrystals

This paper describes the synthesis of new upconverting luminescent nanoparticles that consist of YF₃:Yb³⁺/Er³⁺ functionalized with poly(acrylic acid) (PAA). Unlike the upconverting nanocrystals previously reported in the literature that emit visible (blue-green-red) upconversion fluorescence, these as-prepared nanoparticles emit strong near-infrared (NIR, 831 nm) upconversion luminescence under 980 nm excitation. Scanning electron microscopy, transmission electron microscopy, and powder X-ray diffraction were used to characterize the size and composition of the luminescent nanocrystals. Their average diameter was about 50 nm. The presence of the PAA coating was confirmed by infrared spectroscopy. The particles are highly dispersible in aqueous solution due to the presence of carboxylate groups in the PAA coating. By carrying out the synthesis in the absence of PAA, YF₃:Yb³⁺/Er³⁺ nanorice materials were obtained. These nanorice particles are larger (∼700 nm in length) than the PAA-functionalized nanoparticles and show strong typical visible red (668 nm), rather than NIR (831 nm), upconversion fluorescence. The new PAA-coated luminescent nanoparticles have the pottential be used in a variety of bioanalytical and medical assays involving luminescence detection and fluorescence imaging, especially in vivo fluorescence imaging, due to the deep penetration of NIR radiation.      

Effects of Yb<Superscript>3+</Superscript> and Er<Superscript>3+</Superscript> concentrations and doping procedure on excitation transfer efficiency in Er-Yb doped phosphosilicate fibers

We have studied the effects of preform fabrication procedure and ytterbium and erbium concentrations on the efficiency of excitation transfer from Yb³⁺ to Er³⁺ and lasing on erbium transitions in the range 1.53–1.61 μm in phosphosilicate glass fibers. The results indicate that the fabrication of fiber preforms by different MCVD-based processes has no effect on the excitation transfer efficiency. In the ranges 0.07–0.4wt % Er³⁺ and ∼ 0.4–8 wt % Yb³⁺ (Yb/Er ratio from ∼ 5 to 40), the excitation transfer efficiency is determined by the ytterbium concentration and is essentially independent of erbium concentration.     

Recombination luminescence of CaI<Subscript>2</Subscript> crystals activated with Eu<Superscript>2+</Superscript>, Gd<Superscript>2+</Superscript>, Tl<Superscript>+</Superscript>, Pb<Superscript>2+</Superscript>, and Mn<Superscript>2+</Superscript>

The spectral characteristics of thermostimulated luminescence, steady-state roentgenoluminescence and photostimulated luminescence (PSL) buildup and decay kinetics, and the effect of IR irradiation on the roentgenoluminescence yield and glow curves of CaI₂:Eu²⁺, CaI₂:Gd²⁺, CaI₂:Tl⁺, CaI₂:Pb²⁺, CaI₂:Mn²⁺, and CaI₂: Pb²⁺, Mn²⁺ crystals grown by the Bridgman-Stockbarger method have been studied in the temperature range 90–295 K. Coupled with earlier data, the present results on the influence of oxygen and hydrogen impurities on the spectral characteristics of CaI₂ indicate that the activation of calcium iodide with Eu²⁺, Gd²⁺, Tl⁺, Pb²⁺, and Mn²⁺ leads to the formation of cation impurity-native defect complexes, which act as carrier traps and are responsible for the thermostimulated luminescence in the range 150–295 K. IR exposure after 90-K x-ray excitation gives rise to flash PSL and influences the thermostimulated luminescence light sum. The nature of the emission and trapping centers involved and the mechanisms of recombination luminescence excitation in the crystals are discussed.     

Near-infrared downconversion in Ce<Superscript>3+</Superscript>–Yb<Superscript>3+</Superscript> co-doped YAG

The phosphors YAG co-doped with Ce³⁺–Yb³⁺ ion pair were successfully synthesized by solid state reaction method varying the concentration of Yb³⁺ ions from 1 to 15 % mol. The phosphors were characterized by powder X-ray powder diffraction and surface morphology was studied by scanning electronic microscope. The photoluminescence (PL) properties were studied by spectrophotometers in near infra red (NIR) and ultra violet visible region. The synthesized phosphors can convert a photon of blue region (469 nm) into photons of NIR region (979 and 992 nm). The co-operative energy transfer was studied by time decay curve and PL spectra. The theoretical value of quantum efficiency was calculated from steady time decay measurement and the maximum efficiency approached up to 145.19 %. Hence this phosphor could be used as a downconversion luminescent convertor in front of crystalline silicon solar cell (c-Si) panels to reduce thermalization loss due to spectral mismatch of the solar cells.     

Effect of phosphorus ion implantation on the optical properties of thin films of germanium dioxide doped with Er<Superscript>3+</Superscript> and Yb<Superscript>3+</Superscript> ions

It is shown that the ion implantation of phosphorus into thin amorphous films of germanium dioxide doped with Er³⁺ and Yb³⁺ ions can be used for enhancing luminescence from Er³⁺ ions at ∼1.53 μm.     

Ultraviolet upconversion luminescence in Y<Subscript>2</Subscript>O<Subscript>3</Subscript>:Yb<Superscript>3+</Superscript>,Tm<Superscript>3+</Superscript> nanocrystals and its application in photocatalysis

Infrared-to-ultraviolet upconversion luminescence agent Y₂O₃:Yb³⁺,Tm³⁺ was prepared by a combustion method using citrate as a fuel/reductant. The prepared sample was characterized by X-ray diffraction, SEM, and fluorescence spectrophotometer. Two unusual ¹I₆ → ³H₆ (~297 nm) and ¹D₂ → ³H₆ (~363 nm) emissions from Tm³⁺ ions were observed at room temperature under 980-nm laser excitation. The change of upconversion emission intensity depending on the Yb³⁺ concentrations was discussed. The results showed that modest Yb³⁺ doping could make the upconversion emission of Tm³⁺ intense, and high Yb³⁺ concentrations might lead to fluorescence quenching. Moreover, the influence of ultraviolet upconversion luminescence on the photodegradation of methyl orange aqueous solution under solar light irradiation in the presence of TiO₂ catalyst doped with Y₂O₃:Yb³⁺,Tm³⁺ was also investigated. It was concluded from the experiment of this study that TiO₂/Y₂O₃:Yb³⁺,Tm³⁺ composite had higher photocatalytic activity than pure TiO₂ under solar light. This study would make TiO₂ utilize sunlight more efficiently and accelerate the practical application of photocatalytic technology in water treatment region.     

Luminescence in various classes of Er<Superscript>3+</Superscript>-doped inorganic compounds under IR excitation

We have studied the luminescence of various classes of Er³⁺-doped inorganic phosphors under IR excitation. The highest IR Stokes luminescence efficiency in the range 1.5–1.6 μm is offered by Yb_0.99Er_0.01PO₄ (xenotime structure). This efficient IR phosphor has been commercialized under the name AM-1500M-1. It is of considerable practical interest for security and authentication applications and also as a gain medium for mid-IR lasers operating in the spectral range 1.5–1.6 μm, safe to the human eye.     

Absorption spectra of bulk aluminum nitride crystals doped with Er<Superscript>3+</Superscript> ions

This Letter presents results of analysis of the absorption spectra of AlN:Er³⁺ bulk crystals. In the spectral of 370–700 nm, absorption lines responsible for intraconfiguration electron transition from the ground state ⁴I_15/2 to the excited states of Er³⁺ ions are found. Transitions to the levels of the ⁴F_9/2, ²H_11/2, and ⁴G_11/2 states at 2 K are studied in detail. The number of observed lines for these transitions fully agrees with that theoretically possible for f–f electron transitions in Er³⁺ ions found in a noncubic crystal field. The small width of the observed lines and their number indicate that erbium ions displace mostly one regular crystal position. Most probably, Er³⁺ occupies the position of Al. Energy positions of excited states for the considered transitions are determined.     

Intense frequency upconversion luminescence in Yb<Superscript>3+</Superscript>/Tm<Superscript>3+</Superscript>-codoped oxychloride germanate glasses

Yb³⁺/Tm³⁺-codoped oxychloride germanate glasses for developing potential upconversion lasers have been fabricated and characterized. Structural properties were obtained based on the Raman spectra analysis, indicating that PbCl₂ plays an important role in the formation of glass network and has an important influence on the maximum phonon energies of host glasses. Intense blue and weak red emissions centered at 477 and 650 nm, corresponding to the transitions ¹G₄ → ³H₆ and ¹G₄ → ³H₄, respectively, were observed at room temperature. With increasing PbCl₂ content, the intensity of blue (477 nm) emission increases significantly, while the red (650 nm) emission increases slowly. The results indicate that PbCl₂ has more influence on the blue emissions than the red emission in oxychloride germanate glasses. The possible upconversion mechanisms are discussed and estimated. Intense blue upconversion luminescence indicates that these oxychloride germanate glasses can be used as potential host material for upconversion lasers.