Structure and up-conversion luminescence in sol–gel derived Er–Yb co-doped SiO2:PbF2 nano-glass–ceramics

Transparent oxyfluoride nano-glass–ceramics 90(SiO₂)10(PbF₂) co-doped with 0.3 Yb³⁺ and 0.1 Er³⁺ (mol%) have been prepared by thermal treatment of precursor sol–gel glasses. X-ray diffraction and high resolution transmission electron microscopy analysis pointed out a precipitation of cubic β-PbF₂ nanocrystals of certain diameter in nano-glass–ceramics varying from 10 to 20 nm depending on heat treatment conditions. The incorporation of Yb³⁺ and Er³⁺ dopants in these nanocrystals has been confirmed by signatures of luminescence spectroscopy. Up-conversion luminescence pumped at 980 nm has been detected. Colour tuneability of up-conversion luminescence varying pump power has been analyzed in terms of standard chromaticity diagram. This tuneability opens applications for up-conversion phosphors and three-dimensional optical recording.     

Synthesis and luminescence of fluorochloride glasses activated by Er3+

Fluorochloride glasses activated by Er³⁺ and glass ceramics of 57HfF₄ · 20BaCl₂ · 3LaF₃ · 3InF₃ · 17NaF composition were synthesized. For the given glasses, Er³⁺ luminescence in the region of 1.55 μm was studied in comparison with fluoride glasses and glass ceramics of similar composition. Original Russian Text ? M.N. Brekhovskikh, B.I. Galagan, L.N. Dmitruk, L.V. Moiseeva, V.A. Fedorov, 2009, published in Neorganicheskie Materialy, 2009, Vol. 45, No. 5, pp. 638–640.     

Optical and spectroscopic properties of soda lime alumino-silicate glasses doped with erbium and silver

Spectroscopic properties of Ag/Er co-doped soda lime silicate glasses have been studied with the aim of assessing the effective role of silver as a sensitizer for erbium. Changes in spectroscopic properties of Er³⁺ as a function of silver addition to the base composition have been measured. Transmission electron microscopy (TEM), absorption as well as photoluminescence measurements in the visible and infrared spectral region, particularly ⁴I_13/2 → ⁴I_15/2 transition of the Er³⁺ ion were performed; excitation wavelengths in the range from 325 to 808 nm were used. Enhancement of the Er³⁺ luminescence at 1.54 μm was observed when Ag was added.     

Up-conversion luminescence and near infrared luminescence of Er3+ in transparent oxyfluoride glass-ceramics

The up-conversion luminescence and near infrared luminescence of the Er³⁺ in transparent oxyfluoride glass-ceramics have been investigated. The formation of PbF₂ nano-crystals in the glass was confirmed by XRD. From optical absorption spectra, the oscillator strengths for several transitions of the Er³⁺ in the glass and glass-ceramic have been obtained and then the Judd–Ofelt parameters were calculated by a least squares fitting. The split near infrared emission peaks of the Er³⁺ ions in the glass-ceramics can be observed because the Er³⁺ ions have entered into crystalline environment of the β-PbF₂ nano-crystals. The up-conversion luminescence intensity of the Er³⁺ in the glass-ceramics can be observed to be much stronger than that in the glasses. The dependence of the up-conversion luminescence intensity on the current of the LD laser used as excited source indicated that the transition mechanism of the up-conversion luminescence can be ascribed to two-photon absorption process.     

Er3+/Yb3+ codoped oxyfluoride borosilicate glass ceramic containing NaYF4 nanocrystals for amorphous silicon solar cells

Transparent oxyfluoride borosilicate glass ceramic containing cubic NaYF₄ nanocrystals were successfully fabricated. The cubic NaYF₄ nanocrystals with average size of 30 nm were precipitated in the glass matrix, which was confirmed by the X-ray diffraction and TEM results. In comparison with the as-made glass, significant enhancement of upconversion luminescence is observed in the Er³⁺/Yb³⁺ codoped transparent glass ceramic, which may be due to the variation of coordination environment of Er³⁺ and Yb³⁺ ions after crystallization. The high transparency, intense upconversion luminescence and the simple, low-cost fabrication process make this material exhibiting potential applications in the fields of amorphous silicon solar cells.     

Fabrication of Er3+-doped LaOCl nanostructures with upconversion and near-infrared luminescence performances

LaOCl:Er³⁺ nanofibers and nanobelts were prepared by electrospinning combined with a double-crucible chlorination technique using NH₄Cl powders as chlorinating agent. X-ray powder diffraction analysis indicated that LaOCl:Er³⁺ nanostructures were tetragonal with space group P4/nmm. Scanning electron microscope analysis and histograms revealed that diameter of LaOCl:Er³⁺ nanofibers and the width of nanobelts respectively were 161.15 ± 18.11 nm and 6.11 ± 0.19 μm under the 95 % confidence level, and the thickness of nanobelts was 116 nm. Up-conversion (UC) emission spectra analysis manifested that LaOCl:Er³⁺ nanostructures exhibited strong green and red UC emission centering at 525, 548 and 671 nm, respectively attributed to ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_l5/2 energy levels transitions of Er³⁺ ions under the excitation of a 980-nm diode laser. It was found that the relative intensities of green and red emissions vary obviously with the changing of concentration of Er³⁺ ions, and the optimum molar percentage of Er³⁺/(La³⁺+Er³⁺) ions was 5 % in the LaOCl:Er³⁺ nanostructures. The LaOCl:x %Er³⁺ nanobelts have higher UC emission (both red and green) intensity than the counterpart nanofibers. Moreover, the near-infrared characteristic emissions of LaOCl:Er³⁺ nanostructures were achieved under the excitation of a 532-nm laser. Commission Internationale de L’Eclairage analysis demonstrated that color-tuned luminescence can be obtained by changing doping concentration of Er³⁺ ions, which could be applied in the fields of optical telecommunication and optoelectronic devices. The UC luminescent mechanism of LaOCl:Er³⁺ nanostructures were also proposed.     

Color tunability of upconversion emission in YBO3: Yb, Er inverse opal

Upconversion (C) light-emitting photonic band gap materials (YBO₃: Yb, Er) with inverse opal structure were prepared by a self-assembly technique in combination with a sol-gel method. The effect of the photonic stop-band on the upconversion luminescence of Er³⁺ ions has been investigated in the YBO₃: Yb, Er inverse opals. Significant suppression of the green or red UC emission was detected if the photonic band-gap overlaps with the Er³⁺ ions emission band. We successfully achieved the color tuning of the UC optical properties of the inverse opal by controlling the structure of the photonic crystal.     

High angle annular dark field-scanning transmission electron microscopy and high-resolution transmission electron microscopy studies in the Er2O3-ZrO2 system

ZrO₂:Er³⁺ nanocrystals synthesized by the sol-gel method using a non-ionic (PLURONIC P-127) surfactant in order to produce single nanocrystals were characterized. High angle annular dark field-scanning transmission electron microscopy (HAADF-STEM), energy dispersive X-ray spectroscopy and high resolution transmission electron microscopy were used to characterize the ZrO₂:Er³⁺ nanoparticles annealed at 1000 °C for 5 h. Single nanocrystals of monoclinic zirconia phase well-dispersed with a faceted morphology were obtained after the calcination. Er atoms were randomly integrated in the zirconia lattice as a subtitutional solid solution. Segregation of Er₂O₃ phase at the annealing temperature was not observed. Substitution of some Zr by Er atoms in the monoclinic zirconia lattice was directly observed by HAADF-STEM analysis.     

Unusual broadening of the NIR luminescence of Er-doped Nb2O5 nanocrystals embedded in silica host: Preparation and their structural and spectroscopic study for photonics applications

This paper reports on the preparation of novel sol-gel erbium-doped SiO₂-based nanocomposites embedded with Nb₂O₅ nanocrystals fabricated using a bottom-up method and describes their structural, morphological, and luminescence characterization. To prepare the glass ceramics, we synthesized xerogels containing Si/Nb molar ratios of 90:10 up to 50:50 at room temperature, followed by annealing at 900, 1000, or 1100 °C for 10 h. We identified crystallization accompanying host densification in all the nanocomposites with orthorhombic (T-phase) or monoclinic (M-phase) Nb₂O₅ nanocrystals dispersed in the amorphous SiO₂ phase, depending on the niobium content and annealing temperature. A high-intensity broadband emission in the near-infrared region assigned to the ⁴I_13/2 → ⁴I_15/2 transition of the Er³⁺ ions was registered for all the nanocomposites. The shape and the bandwidth changed with the Nb₂O₅ crystalline phase, with values achieving up to 81 nm. Er³⁺ ions were located mainly in Nb₂O₅-rich regions, and the complex structure of the different Nb₂O₅ polymorphs accounted for the broadening in the emission spectra. The materials containing the T-phase, displayed higher luminescence intensity, longer ⁴I_13/2 lifetime and broader bandwidth. In conclusion, these nanostructured materials are potential candidates for photonic applications like optical amplifiers and WDM devices operating in the S, C, and L telecommunication bands.     

Spectroscopic properties of Pr3+ and Er3+ ions in lead-free borate glasses modified by BaF2

Lead-free oxyfluoride borate glasses singly doped with Pr³⁺ and Er³⁺ were prepared and next investigated using absorption and luminescence spectroscopy. In the studied glass system, barium oxide was substituted by BaF₂. Two luminescence bands of Pr³⁺ located at visible spectral region are observed, which correspond to ³P₀–³H₄ (blue) and ¹D₂–³H₄ (reddish orange) transitions, respectively. The luminescence bands due to ¹D₂–³H₄ transition of Pr³⁺ are shifted to shorter wavelengths, when BaO was substituted by BaF₂. Near-infrared luminescence spectra of Er³⁺ ions in lead-free borate glasses modified by BaF₂ correspond to ⁴I_13/2–⁴I_15/2 transition. Their spectral linewidths increase with increasing BaF₂ concentration. The changes in measured lifetimes of rare earth ions are well correlated with the bonding parameters calculated from the optical absorption spectra.     

Effects of aging time on phase,morphology, and luminescence by two-photon processes of BiPO4:Er3+, Yb3+ in the solvothermal synthesis

BiPO₄:Er³⁺, Yb³⁺ phosphors were synthesized by the solvothermal process. The phase transformation, morphology, and UC luminescent property were characterized by different analytical techniques. The aging time has obvious influence on the phase, morphology, and luminescence of the samples. With the extension of aging time, the phase of BiPO₄:Er³⁺, Yb³⁺ phosphors changes from hexagon to monocline. The morphology changes from nanorods through nanorugbies to microoctahedra. Under the excitation at 980 nm, BiPO₄:Er³⁺, Yb³⁺ phosphors show green and red UC emissions, which originate from the (²H_11/2, ⁴S_3/2) → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺ ions. The green and red UC emission intensities increase gradually with the increase of pumping power. On the basis of the luminescent properties, one can conclude that the two-photon process is involved in green and red UC emissions.     

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.      

Relationship between the local structure and spontaneous emission probability of Er3+ in silicate,borate, and phosphate glasses

Extended X-ray absorption fine structure (EXAFS) measurements have been performed on Er³¹ in silicate, borate, and phosphate glasses in order to investigate the local structure surrounding the Er³¹.Er³¹ ions coordinate to non-bridging oxygen ion sites, where alkali or alkaline earth ions terminate the network structure of silicate glasses. In borate glasses, the local structure surrounding Er³¹ ions is altered by the structural change ofthe borate anion. Er³¹ ions coordinate to non-bridging oxygen ion sites and BO₄ structural units in the cases with and without the formation of non-bridging oxygen, respectively. The former is similar to the case in silicate glasses. Er³¹ ions selectively coordinate to the PyO site regardless of the glass composition variation. A correlation was observed between the spontaneous emission probability for ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ and the average Er–O distance calculated by EXAFS analysis. It shows the maximum value near 2.32 Å, and we conclude that the overlapping radial integral of the 4f and 5d orbitals of Er³⁺ would be the largest at the optimum Er³¹– O²² distance 2.3 Å.     

Luminescence properties of Yb:Er:KY3F10 nanophosphor and thermal treatment effects

In this work, we present the spectroscopic properties of KY₃F₁₀ nanocrystals activated with erbium and codoped with ytterbium ions. The most important processes that lead to the erbium upconversion of green and red emissions of Er³⁺ were identified. A time-resolved luminescence spectroscopy technique was employed to measure the luminescence decays of ⁴S_3/2 and ⁴F_9/2 excited levels of Er³⁺ and to determine the upconversion processes and the luminescence efficiencies of erbium in the visible. Analysis of the luminescence kinetics in Yb:Er:KY₃F₁₀ shows a rapid upconversion (Up₁) for the green emission with a time constant of 0.31 μs after pulsed laser excitation at 972 nm for as synthesized nanocrystals, which is faster than the time constant measured for the bulk crystal (23 μs). In addition, it is observed a second upconversion process (non-resonant) (Up₂) responsible for the red emission (Er³⁺), which competes with Up₁ process. However, the luminescence efficiency of the green emission (⁴S_3/2) is observed to be very low (1.6%) for the as synthesized nanocrystal (25 °C). Nevertheless, it increases with the nanopowder heat treatment reaching an efficiency of 99% (T = 550 °C) relative to the bulk crystal. Similar luminescence behavior was observed for the ⁴F_9/2 level (Er³⁺) that emits red emission. X-ray diffraction analysis of nanopowder by Rietveld method reveled that the mean crystallite size remains unchanged (8.3–12.3 nm) after thermal treatments with T ∼ 400 °C, while the ⁴S_3/2 luminescence efficiency strongly increases to 20%. The luminescence dynamics indicates that Er³⁺ ions distribution plays a determinant role in the luminescence efficiency of green and red emissions of Er³⁺ besides also the strong influence on the upconversions processes. The observed luminescence effect is caused by the non-uniform Er³⁺ (and Yb³⁺) ions distribution due to the nanocrystal grown, which introduces a concentration gradient that increases towards the nanoparticle surface. This concentration effect produces strong (Er × Er) cross-relaxations depleting the excited states populations of ⁴S_3/2 and ⁴F_9/2 levels and their luminescence efficiencies in KY₃F₁₀ nanocrystals. The concentration gradient is very accentuated in the as synthesized nanocrystal and gradually decreases with the thermal treatments where the dopant ions can migrate through the lattice towards the nanocrystal’s interior to get a more uniform and random distribution, which is reached after heat treatment to T = 550 °C.     

Yb3+ concentration dependence of upconversion luminescence in Y2Sn2O7:Yb3+/Er3+ nanophosphors

Pyrochlore Y₂Sn₂O₇ nanophosphors codoped with Er³⁺ (fixed 2 at.%) and Yb³⁺ ions (2–16 at.%) were synthesized via hydrothermal process followed by heat treatment. We investigate the infrared-to-visible upconversion (UC) luminescence properties of Er–Yb codoped Y₂Sn₂O₇. Upon 980 nm excitation at room temperature, green (at ~522 and 544 nm) and red (at ~661 nm) UC emissions were observed, which are ascribed to the (²H_11/2, ⁴S_3/2) → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions, respectively. It has been found that the Yb³⁺-doping concentrations have greatly influenced on the UC luminescence intensity and the emission ratio of the red and green in Y₂Sn₂O₇:Yb³⁺/Er³⁺ nanophosphors. The tunable emission is due to the energy back transfer from Er³⁺ to Yb³⁺ and the cross relaxation between the two neighboring Er³⁺ ions. It is expected that the achieved single and intense red emission band may have potential application for in vivo bioimaging.     

Fluoride and tellurite glasses for thin-film IR viewers

We have studied optical characteristics of Er³⁺-, Ho³⁺-, and Yb³⁺-doped ZBLAN and TWL glasses. Their luminescence was excited at wavelengths of 975, 378, and 449 nm. The 975-nm radiation excited the Yb³⁺ (² F _7/2 → ² F _5/2), and the excitation energy was then transferred to the Er³⁺ and Ho³⁺. The short-wavelength excitation led to cross-relaxation processes: (⁴ F _7/2, ² F _7/2) → (⁴ I _11/2, ² F _5/2) for the Er³⁺-Yb³⁺ pair (378 nm) and (⁵ F ₃, ² F _7/2) → (⁵ I ₅, ² F _5/2) for the Ho³⁺-Yb³⁺ (449 nm). At the three excitation wavelengths, we observed green luminescence in the range 525–550 nm. Using the glasses studied here, we prepared thin colorless lacquer films potentially attractive for hidden information recording and hidden labeling of various objects and materials.     

NIR luminescent Er/Yb co-doped SiO2–ZrO2 nanostructured planar and channel waveguides: Optical and structural properties

Optical and structural properties of planar and channel waveguides based on sol–gel Er³⁺ and Yb³⁺ co-doped SiO₂–ZrO₂ are reported. Microstructured channels with high homogeneous surface profile were written onto the surface of multilayered densified films deposited on SiO₂/Si substrates by a femtosecond laser etching technique. The densification of the planar waveguides was evaluated from changes in the refractive index and thickness, with full densification being achieved at 900 °C after annealing from 23 up to 500 min, depending on the ZrO₂ content. Crystal nucleation and growth took place together with densification, thereby producing transparent glass ceramic planar waveguides containing rare earth-doped ZrO₂ nanocrystals dispersed in a silica-based glassy host. Low roughness and crack-free surface as well as high confinement coefficient were achieved for all the compositions. Enhanced NIR luminescence of the Er³⁺ ions was observed for the Yb³⁺-codoped planar waveguides, denoting an efficient energy transfer from the Yb³⁺ to the Er³⁺ ion.     

Preparation and photoluminescence properties of fluorosilicate glass ceramics containing CeOF: Dy nanocrystals

The Ce³⁺ and Dy³⁺ co-doped oxyfluoride glasses and glass ceramics containing CeOF or CeF₃ nanocrystals have been prepared in the reducing atmosphere. The crystallinity increased significantly with the Ce³⁺ concentration, while the crystal size of nanocrystals is mainly influenced by the annealing temperatures. The glasses and glass ceramics emitted white light, deriving from a combination of the Ce³⁺ blue and the Dy³⁺ yellow light. The emission intensity and CIE chromaticity coordinates of the Ce³⁺ and Dy³⁺ co-doped glasses can be tuned by adjusting the ratio of Ce³⁺/Dy³⁺ concentration or the annealing temperature.     

Infrared-to-visible conversion luminescence of Er3+ ions in lead borate transparent glass-ceramics

Transparent glass-ceramics were successfully prepared during controlled heat treatment of lead borate glasses. The PbF₂ particles were dispersed into a borate glass matrix which was evidenced by X-ray diffraction analysis. The phase identification revealed that crystalline peaks can be related to the orthorhombic PbF₂ phase. Green up-conversion luminescence due to the ⁴S_3/2–⁴I_15/2 transition of Er³⁺ ions was registered. In comparison to the precursor glass the luminescence intensity was considerably higher, whereas the luminescence linewidth slightly decreased in the studied oxyfluoride transparent glass-ceramics. It indicated that a part of the trivalent erbium was incorporated into the PbF₂ crystalline phase.     

Synthesis and Upconversion Emission of β-Ca2SiO4: (Er3+, Yb3+)

The β-Ca₂SiO₄: (Er³⁺, Yb³⁺) powders were synthesized by the simple solid-state process. The obtained samples were given characterizations of X-ray diffraction, Fourier-transform infrared, transmission electron microscopy, and luminescence. The samples have monoclinic parawollastonite phase and irregular morphology. Under the excitation at 980 nm, the obtained β-Ca₂SiO₄: (Er³⁺, Yb³⁺) samples show the intense upconversion (UC) emission. The dosage of Yb³⁺ has obvious influence on the emission intensities of β-Ca₂SiO₄: (Er³⁺, Yb³⁺) samples. Also, the emission intensity increases gradually with the increasing pump power from 350 to 600 mW. On the basis of luminescent properties of samples, we can conclude that the UC emission originates from the biphotonic process.