Synthesis of nanocrystalline ytterbium-doped yttria by citrate-gel combustion method and fabrication of ceramic materials

Nanosized ytterbium doped yttria powders were prepared by citrate-gel combustion techniques. As-synthesized precursor and calcined powders were characterized for their crystalline structure, particle size and morphologies. Nanocrystalline Yb³⁺:Y₂O₃ powders with pure cubic yttria crystal structure were obtained by calcination of as-prepared precursors at 1100 °C for 3 h. Powders obtained were well dispersed with an average particle size of 60 nm. By using the obtained powders, nearly full dense Yb³⁺:Y₂O₃ ceramics were produced by vacuum sintering at 1800 °C for 12 h. The emission spectrum of the sintered ceramics under the excitation wavelength of 905 nm illustrates that there are three fluorescence peaks locating at 976 nm, 1030 nm and 1075 nm respectively, all corresponding to the ²F_5/2 → ²F_7/2 transitions of ytterbium ion.     

Deep red upconversion photoluminescence in Er3+-doped Yb3Ga5O12 nanocrystalline garnet

The nanocrystalline single-phase Er³⁺-doped Yb₃Ga₅O₁₂ garnets have been prepared by the sol-gel combustion technique with a crystallite size of ≈30 nm. The presence of Yb³⁺ in garnet hosts allows their efficient excitation at the ≈977 nm wavelength. The Er³⁺ doping of Yb₃Ga₅O₁₂ garnet host results in deep red Er³⁺: ⁴F_9/2 → ⁴I_15/2 upconversion photoluminescence (UCPL) emission. The dominance of the red UCPL emission over the green Er³⁺: ⁴F_7/2/²H_11/2/⁴S_3/2 → ⁴I_15/2 component was investigated using the measurement of the steady-state and time-dependent Er³⁺ and Yb³⁺ emission spectra in combination with the power-dependent UCPL emission intensity. The proposed upconversion mechanism is discussed in terms of the Er³⁺ → Yb³⁺ energy back transfer process as well as Yb³⁺(Er³⁺) → Er³⁺ energy transfer and Er³⁺ ↔ Er³⁺ cross-relaxation processes. The studied Er³⁺-doped Yb₃Ga₅O₁₂ garnet may be utilized as a red upconversion emitting phosphor.     

Tb3+/Yb3+ co-doped Y2O3 upconversion transparent ceramics: Fabrication and characterization for IR excited green emission

Tb³⁺/Yb³⁺ co-doped Y₂O₃ transparent ceramics were fabricated by vacuum sintering of the pellets (prepared from nanopowders by uniaxial pressing) at 1750 °C for 5 h. Zr⁴⁺ and La³⁺ ions were incorporated in Tb³⁺/Yb³⁺ co-doped Y₂O₃ nanoparticle to reduce the formation of pores which limits the transparency of ceramic. An optical transmittance of ∼80% was achieved in ∼450 to 2000 nm range for 1 mm thick pellet which is very close to the theoretical value by taking account of Fresnel’s correction. High intensity luminescence peak at 543 nm (green) was observed in these transparent ceramics under 976 and 929 nm excitations due to Yb–Tb energy transfer upconversion.     

Carbonate-precipitation synthesis of Yb:Y2O3 nanopowders and its characteristics

Ytterbium-doped yttria (Yb³⁺:Y₂O₃) nanopowders for transparent ceramics were synthesized by using a carbonate-precipitation method. The characteristics of precursor and powders calcined at different temperatures were investigated. The pure yttria phase can form through calcining at 700 °C. The Yb³⁺:Y₂O₃ nanopowders calcined at 1100 °C were well dispersed with a spherical morphology, and had a narrow particle size distribution with a mean particle size of about 70 nm. By using 1100 °C-calcined powders, nearly full dense Yb³⁺:Y₂O₃ ceramics were fabricated at 1750 °C for 8 h without any additives under vacuum conditions. The fluorescence spectrum of the sintered ceramics illustrates that there are two emission peaks locating at 1028 and 1071 nm respectively, all corresponding to the ²F_5/2 → ²F_7/2 transitions of Yb³⁺ ion. Homogeneous Yb³⁺:Y₂O₃ nanopowders synthesized by carbonate-precipitation method are suitable for the fabrication of IR-transparent ceramics.     

Spectroscopic properties of Yb2+ in aluminosilicate glass

In contrast to Yb³⁺ which is a well-investigated optically active ion, the spectroscopic properties of its divalent counterpart Yb²⁺ in glasses are hardly investigated, although Yb²⁺ might have a notable influence on the luminescence properties of Yb³⁺-doped glasses even at low Yb²⁺ concentrations because of its strong f-d transitions. In this paper, we report on the preparation and spectroscopic properties of Yb²⁺-doped aluminosilicate glasses that were produced using the normal melt-quench technique. The glass composition is 20CaO·20Al₂O₃·60SiO₂ (mol%). To achieve a sufficient amount of Yb²⁺ in the samples, different methods were applied to influence the red-ox equilibrium during glass melting: firstly, under argon atmosphere and additional argon bubbling, and secondly by the addition of metallic aluminum powder to the batch. The strongly reduced samples show a greenish to brownish yellow coloring which could be attributed to the strong absorption of Yb²⁺ ions in the UV to blue spectral range. The absolute Yb²⁺ concentration in the glass and the molar extinction coefficient of Yb²⁺ was obtained by spectroscopic measurements. If irradiated with UV light, the Yb²⁺-doped samples show a broad fluorescence emission in the wavelength range from 450 to 700 nm with a peak at around 515 nm.     

Blue Upconversion Emission in Germanate Glass Co-Doped with Yb3+/Tm3+ Ions

In the paper, the upconversion luminescence of 70GeO₂–30[Ga₂O₃–BaO–Na₂O] glass system co-doped with Yb³⁺/Tm³⁺ ions was investigated. Strong blue emission at 478 nm corresponding to the transition ¹G₄ → ³H₆ in thulium ions was measured under the excitation of 976-nm diode laser. The dependence of the upconversion emission upon the thulium ion concentration was studied to determine the optimal conditions of energy transfer between energy levels of active dopants. The most effective energy transfer Yb³⁺ → Tm³⁺ was obtained in glass co-doped with molar ratio of dopant 0.7 Yb₂O₃/0.07 Tm₂O₃. The increase in thulium concentration more than 0.07 mol% results in the reverse energy transfer from Tm³⁺ → Yb³⁺, which leads to rapid quenching of the luminescence line at the wavelength 478 nm. In germanate glass co-doped with 0.7Yb₂O₃/0.07Tm₂O₃, the longest lifetime of ¹G₄ level equal 278 μs was achieved. The presented results indicate that elaborated germanate glass co-doped with Yb³⁺/Tm³⁺ ions is a promising material that can be used to produce fiber lasers and superluminescent fiber sources generating radiation in the visible spectrum.     

Effect of Ce dopant on upconversion and temperature sensing performances in homogeneous ultrasmall Y2O3:Yb3+/Ho3+ nanoparticles through flame aerosol synthesis

Flame aerosol synthesis (FAS) is an excellent strategy for continuous, fast, and mass production of small-size upconversion nanoparticles (UCNPs), which have high potential applications in fields like biological imaging, colour display and optical temperature sensing. However, flame-made UCNPs have received less attention, and relevant studies are limited. Herein, for the first time, we successfully fabricated cerium (Ce)-doped homogeneous ultrasmall Y₂O₃:Yb³⁺/Ho³⁺ UCNPs using a liquid-fed FAS method. Ce was doped to improve the upconversion luminescence (UCL) of the Y₂O₃:Yb³⁺/Ho³⁺ UCNPs. The overall UCL intensity was enhanced ～77.9-fold for an optimal concentration of 20 mol% Ce-doped UCNPs, compared with the UCNPs without Ce doping with a relatively homogeneous ultrasmall size of 8–10 nm. Further studies confirmed that both trivalent (Ce³⁺) and tetravalent (Ce⁴⁺) simultaneously exist in the Y₂O₃ hosts and are critical in enhancing the UCL properties. In addition, the fluorescence intensity ratio (FIR) method was used to evaluate the thermal properties of the fabricated UCNPs. Ce doping significantly improved the thermal sensitivity of Y₂O₃:Yb³⁺/Ho³⁺ UCNPs. An excellent relative sensitivity (S_R) of 0.622% K^?1 at 598 K was obtained for flame-made UCNPs doped with 20 mol% Ce.     

Structural-phase state and lasing of 5–15 at% Yb3+:Y3Al5O12 optical ceramics

Yttrium aluminum garnet (Yb³⁺:Y₃Al₅O₁₂) laser ceramics doped by 5, 10 and 15 at% of ytterbium ions were obtained by reactive sintering. Optimal sintering temperature range for the formation of highly-dense transparent Yb³⁺:Y₃Al₅O₁₂ ceramics under normal recrystallization conditions was found to be T = 1750–1800 °C. The influence of Yb³⁺ ions on structural-phase state, phase composition, microstructure, optical and luminescent properties of sintered samples was experimentally investigated. It was shown that lattice parameter a of Yb³⁺:Y₃Al₅O₁₂ ceramics decreases linearly with increasing of Yb³⁺ concentration in a good agreement with L. Vegard’s rule, that indicates to the formation of (Y_1−xYb_x)₃Al₅O₁₂ (х = 0.05–0.15) substitutional solid solutions. No concentration quenching of Yb³⁺ luminescence was observed in Yb³⁺:Y₃Al₅O₁₂ within the 5–15 at% doping range. Quasi-CW lasing of Yb³⁺:Y₃Al₅O₁₂ ceramics was studied under diode-pumping at 970 nm. A highest slope efficiency of about 50% was obtained for 15 at%-doped Yb³⁺:Y₃Al₅O₁₂ ceramics sintered at T = 1800 °C for 10 h.     

Near-IR Emission and upconversion luminescent properties of Tb3+/Yb3+ Co-doped HfO2/SiO2 nanoparticles

The luminescent characteristics of spherical hafnia/silica (HfO₂/SiO₂) nanoparticles (NP?s) co-doped with Tb³⁺/Yb³⁺ were analysed. These NP?s were synthesized using the spray pyrolysis technique. The addition of SiO₂ and Tb³⁺/Yb³⁺ was found to induce a cubic phase in HfO₂. The luminescent spectra presented the characteristic emission peaks for inter-electronic energy levels transitions of the Tb³⁺ and Yb³⁺ ions, with an excitation band centred at 270 nm. Under solid-state laser excitation at 980 nm an upconversion emission related to the Tb³⁺ ion was observed. The maximum emission peak in the visible region was at 543 nm, associated with ⁵D₄ → ⁷F₅ transitions of the Tb³⁺ ions and an IR emission peak at 970 nm (²F_5/2 → ²F_7/2) pertaining to Yb³⁺, with irradiation at 270 nm (UV). The energy transfer mechanism from Tb³⁺→Yb³⁺ (excitation at 270 nm), is discussed based on the time decay of the luminescence intensity analysis and the energy transfer efficiency (η_ET) and was determined to be in the range of 29.2% to40.8%.     

Sintering of Yb3+:Y2O3 transparent ceramics in hydrogen atmosphere

Highly transparent Yb³⁺:Y₂O₃ ceramics with doping concentration up to 40.0 at.% had been fabricated successfully via hydrogen atmosphere sintering, where the raw powders were synthesized by co-precipitation method. The sintering temperature is about 600 °C lower than its melting temperature. SEM investigation revealed the average grain size of Yb³⁺:Y₂O₃ ceramics sintered at 1850 °C for 9 h was about 7 μm. The highest transmittance of as-prepared 1 mm thickness samples around wavelength of 1050 nm reached 80%, which is close to the theoretical value of Y₂O₃. The optical spectroscopic properties of Yb³⁺:Y₂O₃ transparent ceramics have also been investigated, which shows that it is a very good laser material for diode laser pumping and short pulse mode-locked laser.     

Phase formation and lattice distortion of Lu2O3-doped ZrO2 in comparison with Y2O3-doped ZrO2

Lu₂O₃ and Y₂O₃ doping of 8, 11, and 18 mol% in ZrO₂ were prepared by solid solution reaction, aiming to study the phase stabilization of Lu₂O₃-doped ZrO₂ and Y₂O₃-doped ZrO₂ in terms of phase formation and lattice distortion. The Rietveld refinement results indicated that Lu₂O₃-doped ZrO₂ and Y₂O₃-doped ZrO₂ followed the same trend in terms of cubic phase fraction, increasing from 25%–30% (8 mol%) to 95%–100% (11 and 18 mol%). This phase formation was confirmed by observing the same diffraction ring pattern observed for the Lu₂O₃-doped ZrO₂ and Y₂O₃-doped ZrO₂. The Vickers hardness of the Lu₂O₃-doped ZrO₂ was 4.3% higher than that of Y₂O₃-doped ZrO₂ at 8 mol%, but 9.7% and 14.8% lower at 11 and 18 mol%, respectively. This was likely caused by the lattice distortion effect of Y₂O₃ doping overpowering the field strength difference between Lu³⁺ and Y³⁺.     

Enhancement of up-conversion emission and emerging cool white light emission in co-doped Yb3+/ Er3+: Li2O-LiF-B2O3-ZnO glasses for photonic applications

A series of co-doped (Yb³⁺/Er³⁺): Li₂O-LiF-B₂O₃-ZnO glasses were prepared by standard melt quenching technique. Structural and morphological studies were carried out by XRD and FESEM. Phonon energy dynamics have been clearly elucidated by Laser Raman analysis. The pertinent absorption bands were observed in optical absorption spectra of singly doped and co-doped Yb³⁺/Er³⁺: LBZ glasses. We have been observed a strong up-conversion red emission pertaining to Er³⁺ ions at 1.0 mol% under the excitation of 980 nm. However, the up-conversion and down conversion (1.53 µm) emission intensities were remarkably enhanced with the addition of Yb³⁺ ions to Er³⁺: LBZ glasses due to energy transfer from Yb³⁺ to Er³⁺. Up-conversion emission spectra of co-doped (Yb³⁺/Er³⁺): LBZ glasses exhibits three strong emissions at 480 nm, 541 nm and 610 nm which are assigned with corresponding electronic transitions of ²H_9/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 respectively. Consequently, the green to red ratio values (G/R) also supports the strong up-conversion emission. The Commission International de E′clairage coordinates and correlated color temperatures (CCT) were calculated from their up-conversion emission spectra of co-doped (Yb³⁺/Er³⁺): LBZ glasses. The obtained chromaticity coordinates for optimized glass (0.332, 0.337) with CCT value at 5520 K are very close to the standard white colorimetric point in cool white region. These results could be suggested that the obtained co-doped (Yb³⁺/Er³⁺): LBZ glasses are promising candidates for w-LEDs applications.     

Yb3+ concentration on emission color,thermal sensing and optical heater behavior of Er3+ doped Y6O5F8 phosphor

Up-conversion phosphor is a potential candidate as non-contact temperature sensor because of its unjammable and unique detection abilities. In this work, we investigate the influence of Yb³⁺ concentration on the emission color, thermal sensing and optical heater behavior of Er³⁺ doped Y₆O₅F₈ phosphor. Our results show that the emission color of Er³⁺ and Yb³⁺ co-doped Y₆O₅F₈ powder changes from green to yellow with the Yb³⁺ concentration increasing. Importantly, the temperature sensing sensitivities of Er³⁺ and Yb³⁺ co-doped Y₆O₅F₈ powder reach 0.008, 0.009, 0.010 and 0.011 K⁻¹ as the sample doped with 2%, 5%, 8% and 11% Yb³⁺ at 476 K, respectively. Moreover, the temperature of high Yb³⁺ concentration sample shows preferable optical heating behavior, whose temperature is ascended by a large value of 94 K when the excitation pump power density changes from 1.0 to 13.1 W cm⁻². These results suggest Er³⁺ and Yb³⁺ co-doped Y₆O₅F₈ powder has great potential in colorful display, temperature sensing and optical heating.     

Preparation and photoluminescence properties of Eu3+-doped ZrO2 nanotube arrays

Zr–Eu alloy containing 3 at% Eu was prepared by a powder metallurgical method and Eu³⁺-doped ZrO₂ nanotube arrays were prepared by anodising the Zr–Eu alloy. The properties of Eu³⁺-doped ZrO₂ nanotube arrays were studied in contrast to undoped ZrO₂ nanotube arrays under different annealing temperatures. Results showed that the Eu³⁺ ions could not only stabilise the tetragonal phase of zirconium oxide, but also make the crystallite sizes smaller. Annealing temperature exerted a significant influence on the absorbance value, as well as the intensity and position of the photoluminescence peaks. When the excitation wavelength was either 248 nm or 270 nm, the sample annealed at 600 °C displayed the strongest emission peak; while under excitation at 232 nm, the sample annealed at 400 °C exhibited the strongest emission peak.     

Intense upconversion luminescence and energy‐transfer mechanism of Ho3+/Yb3+ co‐doped SrLu2O4 phosphor

A series of novel SrLu₂O₄: x Ho³⁺, y Yb³⁺ phosphors (x=0.005‐0.05, y=0.1‐0.6) were synthesized by a simple solid‐state reaction method. The phase purity, morphology, and upconversion luminescence were measured by X‐ray diffraction (XRD), scanning electron microscopy (SEM), and photoluminescence (PL) spectroscopy. The doping concentrations and sintering temperature were optimized to be x=0.01, y=0.5 and T=1400°C to obtain the strongest emission intensity. Under 980 nm laser diode excitation, the SrLu₂O₄:Ho³⁺, Yb³⁺ phosphors exhibit intense green upconversion (UC) emission band centered at 541 nm (⁵F₄,⁵S₂→⁵I₈) and weak red emission peaked at 673 nm (⁵F₅→⁵I₈). Under different pump‐power excitation, the UC luminescence can be finely tuned from yellow‐green to green light region to some extent. Based on energy level diagram, the energy‐transfer mechanisms are investigated in detail according to the analysis of pump‐power dependence and luminescence decay curves. The energy‐transfer mechanisms for green and red UC emissions can be determined to be two‐photon absorption processes. Compared with commercial NaYF₄:Er³⁺, Yb³⁺ and common Y₂O₃:Ho³⁺, Yb³⁺ phosphors, the SrLu_1.49Ho_0.01Yb_0.5O₄ sample shows good color monochromaticity and relatively high UC luminescence intensity. The results imply that SrLu₂O₄:Ho³⁺, Yb³⁺ can be a good candidate for green UC material in display fields.     

Role of Li+ ion in improved crystallization and the luminescence enhancement of up and down conversion process in Er3+/Yb3+ doped in Y4O(OH)9NO3 and Y2O3 nanoparticles

Here, the up-and down-conversion emissions enhancement in Er³⁺/Yb³⁺/Li⁺ co-doped Y₄O(OH)₉NO₃ and Y₂O₃ nanoparticles was studied in function of the calcination temperature. Tri-doped Y₄O(OH)₉NO₃ nanophosphors were synthesized by the hydrothermal method and then calcined to yield Y₂O₃ nanocrystalline structures. The phase evolution from Y₄O(OH)₉NO₃ to Y₂O₃ was investigated by XRD analysis, and the quantification of Li ⁺ species in the nanoparticles lattice was done by FTIR. Both techniques allow explaining the photoluminescence enhancement in terms of the calcination temperature required to achieve optimum lithium incorporation. SEM analysis was used to monitor changes in the morphology with the thermal treatment temperature. The resulting photoluminescence emission spectra of the Er³⁺/Yb³⁺ doped Y₄O(OH)₉NO₃ and Y₂O₃ nanocrystals were composed of emission bands associated with the Er^{3+ 2}H_11/2, ⁴S_3/2 → ⁴I_15/2 (green), ⁴F_9/2 → ⁴I_15/2 (red), and ⁴I_13/2 → ⁴I_15/2 (infrared) intraelectronic transitions, and its relative contribution was found to be sensitive to the calcination temperature. Thus, tunable color emission from yellowish-orange to yellow-green light with a color temperature of 2581–4433 K upon 980 nm excitation, respectively, was observed.     

White luminescence of bismuth and samarium codoped Y2O3 phosphors

In this work Sm³⁺ (0–2.0 at%) and Bi³⁺ (0–2.0 at%) doped Y₂O₃ luminescent powders were prepared by a sol–gel method from yttrium acetylacetonate, samarium and bismuth nitrates as metal sources. The as prepared powders (chemical composition is close to stoichiometric Y₂O₃) present the cubic structure from 700 °C, and at 900 °C are characterized by the presence of rounded particles with heterogeneous size of 42.9 nm. Luminescent effect of ions of Sm³⁺ and Bi³⁺ into Y₂O₃ host as was studied on heat treated powders from 800 to 1100 °C. The combination of the red luminescence from the Sm³⁺ ions and the bluish from Bi³⁺, makes the synthesized phosphors candidates to be used in fabrication of phosphor-converted light-emitting diodes (LEDs).     

Effect of Li+ Ions Doping on Microstructure and Upconversion Emission of Y2Ti2O7:Er3+/Yb3+ Nanophosphors Synthesized Via a Sol–Gel Method

Er³⁺/Yb³⁺/Li⁺‐tridoped Y₂Ti₂O₇ nanophosphors were synthesized via a facile sol–gel process. The samples were characterized by the inductively coupled plasma atomic emission spectrometer (ICP‐AES), X‐ray diffraction (XRD), transmission electron microscopy (TEM), and infrared‐to‐visible upconversion (UC) luminescence spectra. XRD analysis showed that the crystallization temperature of pyrochore‐type Y₂Ti₂O₇ was reduced due to the flux effect of Li⁺ ions, whereas TEM measurements confirmed that the particles size of (Y_0.815Er_0.01Yb_0.075Li_0.10)₂Ti₂O₇ was about 30–40 nm when calcining at 800°C for 1.0 h. The calcining temperature and Li⁺ ion concentration dependence on UC luminescence spectra were investigated. It was found that, when incorporating 10.0 mol% Li⁺ ion, the UC red and green emission intensity was drastically increased by a factor of 18.6 and 8.3, respectively. The enhancement of UC emission may be mainly attributed to the modification of local symmetry around Er³⁺ ions by tridoping Li⁺ ions. And also, the pump power dependence of the emission intensity was investigated to understand the fundamental UC mechanism.     

Fabrication and optical properties of Y2O3-based ceramics with broad emission bandwidth

In this article, we report on the fabrication and optical properties of highly transparent yttria ceramics for lasers active media with broadband gain profile. Laser synthesis method was used to produce Y₂O₃-based nanopowders doped with 1 mol.% Nd³⁺ or Yb³⁺ for these transparent ceramics. The additives of sesquioxides Lu₂O₃ and Sc₂O₃ were used along with ZrO₂ to disorder the crystalline structure. The porosity and average grain size decrease with these additives and the emission bandwidths of Nd³⁺ (⁴F_3/2 → ⁴I_11/2) and Yb³⁺ (²F_5/2 → ²F_7/2) transitions widen to 40 and 60 nm, respectively. Laser operation with the slope efficiency of 29% was obtained in [(Yb_0.01Lu_0.24Y_0.75)₂O₃]_0.88(ZrO₂)_0.12 ceramic sample.     

Molten salt synthesis and optical properties of Eu3+, Dy3+or Nd3+ doped NiNb2O6 columbite-type phosphors

Pure, Eu³⁺, Dy³⁺ or Nd³⁺-doped NiNb₂O₆ powders have been prepared by a molten salt synthesis method by using Li₂SO₄–Na₂SO₄ salt mixture as a flux at relatively low temperatures as compared to the solid state reaction method. X-ray diffraction patterns of pure NiNb₂O₆ samples indicated an orthorhombic single phase. For Eu³⁺-doped NiNb₂O₆ samples, the luminescence of Eu³⁺ was observed at 615 nm as red emission while Dy³⁺-doped NiNb₂O₆ showed yellow emission at 577 nm and Nd³⁺ doped sample exhibited a typical emission at 1064 nm varying with the Eu³⁺ or Nd³⁺ doping concentrations. These luminescence characteristics of the doped samples may be attributed to the energy transfer between rare earth ions and NiO₆ octahedral groups in the columbite structure.