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.     

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.     

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.     

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.     

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.     

Luminescence of GaS:Er<Superscript>3+</Superscript> and GaS:Er<Superscript>3+</Superscript>,Yb<Superscript>3+</Superscript> layered crystals

Data are presented on the 300-K photoluminescence in GaS crystals doped with Er³⁺ or codoped with Er³⁺ and Yb³⁺. IR excitation (λ_ex = 976 nm) gives rise to anti-Stokes luminescence in GaS:Er³⁺ (0.1 at %) and GaS:Er³⁺,Yb³⁺ (0.1 + 0.1 at %) and leads to an increased intensity of the emission due to the ⁴ I _11/2 → ⁴ I _15/2 transitions. The anti-Stokes luminescence is shown to result from consecutive absorption of two photons by one Er³⁺ ion, and the increased intensity of Er³⁺ luminescence in GaS: Er³⁺,Yb³⁺ is due to energy transfer from Yb³⁺ to Er³⁺.     

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.     

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.     

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.     

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.      

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.     

Absorption and emission of ErNbO4 powder

Visible and near infrared absorption and emission (488 nm excitation) characteristics of ErNbO₄ powder, which were prepared by calcining the Er₂O₃ (50 mol%) and Nb₂O₅ (50 mol%) powder mixture at 1100 and 1600 °C for different durations, have been investigated at room temperature. The absorption and emission characteristics of these calcined ErNbO₄ powder were summarized and discussed in comparison with those of Er₂O₃. Weak emission of Er₂O₃ relative to the calcined ErNbO₄ is mainly conducted with absorption difference at the excitation wavelength 488 nm. The obvious spectral changes from Er₂O₃ to calcined ErNbO₄ samples are related to an elevated-temperature-assisted phase transformation according to the solid-state chemical reaction equation: Er₂O₃ + Nb₂O₅ 2ErNbO₄, which results in the changes of the ion environment of Er³⁺ and hence changes of the Stark levels of Er³⁺. The further spectral change as the strengthened calcination results from the improvement of ErNbO₄ purity in the calcined mixture. The borders between two green transitions and between two near infrared transitions in the emission spectra of both calcined samples and Er₂O₃ were tentatively identified by referencing earlier reported emission spectra of the precipitated Z-cut VTE Er(2.0 mol%):LiNbO₃ crystal and the match relation between absorption and emission spectra of the ErNbO₄ powder. A comparison was performed on the spectra of calcined ErNbO₄ powder and those of VTE Er(2.0 mol%):LiNbO₃ crystals. The results allow to preliminarily deem the contribution of ErNbO₄ precipitates, generated inside these crystals by the VTE procedure, to the spectra of these crystals.     

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.     

Effect of Pr<Superscript>3+</Superscript> on the stokes luminescence of YPO<Subscript>4</Subscript>:Nd<Superscript>3+</Superscript> under IR excitation

We have identified general relationships between the spectral and kinetic properties of the IR Stokes luminescence bands of Y_{1 − x − y} Nd _x Pr _y PO₄ solid solutions in the spectral range 0.86–1.40 μm under 0.810-μm laser excitation. The results have been used to formulate technical requirements for the purity of rare-earth oxides for the fabrication of efficient YPO₄:Nd³⁺ IR phosphors and to develop a fast YPO₄:Nd³⁺, Pr³⁺ IR phosphor that allows the decay time of the Nd³⁺ IR Stokes luminescence bands in the range 0.86–1.40 μm to be tuned from 50 to 170 μs by varying the ratio of the Nd³⁺ and Pr³⁺ concentrations.     

Synthesis and luminescence properties of a Li<Subscript>3</Subscript>BaCaY<Subscript>3</Subscript>(MoO<Subscript>4</Subscript>)<Subscript>8</Subscript>:Er<Superscript>3+</Superscript> phosphor with a layered scheelite-like structure

A Li₃BaCaY₃(MoO₄)₈:Er³⁺ phosphor with a scheelite-like structure (sp. gr. C2/c) has been synthesized and its luminescence properties have been studied. The phosphor has been characterized by X-ray diffraction, differential thermal analysis, and IR spectroscopy.     

Large enhancement of upconversion luminescence in Er<Superscript>3+</Superscript>/In<Superscript>3+</Superscript>:Ba<Subscript>0.85</Subscript>Ca<Subscript>0.15</Subscript>TiO<Subscript>3</Subscript> lead-free piezoelectric ceramics

A series of In³⁺-doped Ba_0.85Ca_0.15TiO₃:0.75%Er³⁺/xIn³⁺ (BCT:Er/xIn) lead-free piezoelectric ceramics with excellent upconversion luminescence were synthesized by the solid state reaction method. The effects of In³⁺ content on the crystal structure, ferroelectric, dielectric, piezoelectric, and upconversion luminescence properties were systematically studied. Under 980 nm excitation, a giant enhancement of the green emission (550 nm) by 10 times is achieved upon 2.5% mol In³⁺ doping, which is rarely observed in rare-earth ions-doped perovskite ferroelectric materials. The ultraviolet-visible-near infrared absorption measurements show that the In³⁺ doping may improve the dissolution of Er³⁺ ions and modify the isolate-/clustered-Er³⁺ ratio for x?≤?2.5%, resulting in the enhancement of the absorption cross-section, thereby contributing to the enhancement of green luminescence. Unfortunately, the In³⁺ doping suppresses the ferroelectric and piezoelectric properties of the BCT:Er/xIn ceramics. This problem can be resolved by adding a small amount (1 mol%) of Yb³⁺ to the BCT:Er/xIn ceramics to restore their good ferroelectric and piezoelectric properties. Such In³⁺ and rare-earth ions co-doped ceramics with greatly enhanced upconversion luminescence and good ferroelectricity and piezoelectricity may have potential applications in electro-optical devices.     

Upconversion emissions in Yb<Superscript>3+</Superscript>–Tm<Superscript>3+</Superscript>-doped tellurite glasses excited at 976 nm

Intense Tm³⁺ blue upconversion emission has been observed in Tm³⁺–Yb³⁺ codoped oxyfluoride tellurite glass under excitation with a diode laser at 976 nm. Three emission bands centered at 475, 650 and 796 nm corresponding to the transitions ¹G₄→ ³H₆, ¹G₄→ ³H₄ and ³F₄→ ³H₆, respectively, simultaneously occur. The dependence of upconversion intensities on Tm³⁺ ions concentration and excitation power are investigated. For fixed Yb₂O₃ concentrations of 5.0 mol%, the maximum upconversion intensity was obtained with Tm₂O₃ concentration of about 0.1 mol%. The blue upconversion luminescence lifetimes of the Tm³⁺ transitions ¹G₄→ ³H₆ are measured. The results are evaluated by the possible upconversion mechanisms.     

Microstructure and upconversion luminescence of Yb<Superscript>3+</Superscript> and Ho<Superscript>3+</Superscript> co-doped BST thick films

Ba_0.8Sr_0.2TiO₃ (BST) thick films co-doped with Yb³⁺ and Ho³⁺ were fabricated by the screen printing techniques on alumina substrates. The structure and morphology of the BST thick films were studied by XRD and SEM, respectively. After sintered at 1240 °C for 100 min the BST thick films are polycrystalline with a perovskite structure. The upconversion luminescence properties of the RE-doped BST thick films under 800 nm excitation at room temperature were investigated. The upconversion emission bands centered at 470 and 534 nm corresponding to ⁵F₁ → ⁵I₈ and ⁵F₄ → ⁵I₈ transition, respectively were observed, and the upconversion mechanisms were discussed. The dependence of the upconversion emission intensity upon the Ho³⁺ ions concentration was also examined; the emission intensity reaches a maximum value in the sample with 2 mol% Yb³⁺ and 0.250 mol% Ho³⁺ ions. All the results show that the BST thick films co-doped with Yb³⁺ and Ho³⁺ may have potential use for photoelectric devices.     

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.     

Spectroscopic studies of TeO<Subscript>2</Subscript>–ZnO–Er<Subscript>2</Subscript>O<Subscript>3</Subscript> glass system

Series of glass based on the (80 − x)TeO₂–20ZnO–(x)Er₂O₃ system (0.5 mol% ≤ x ≤ 2.5 mol%) has successfully been made by melt quenching technique. The optical properties of glass have been investigated by means of IR and Raman spectroscopy. It is observed that as the Er₂O₃ content is being increased, the sharp IR absorption peaks are consistently shifted from 650 to 672 cm⁻¹ while the Raman shift intensity around 640–670 cm⁻¹ is decreases but increases around 720–740 cm⁻¹. It is found out that both phenomenons are related to the structural changes between the stretching vibration mode of TeO₄ tbp and TeO₃ tp, and bending vibration mode of Te–O bonds in the glass linkages.