High pressure optical studies of α-ZnAl2S4:Cr

The effect of hydrostatic pressure (varying up to 91 kbar) at room temperature emission spectra and fluorescence lifetime of the Cr³⁺ R1 line (²E → ⁴A₂ transition) in tiospinel ZnAl₂S₄ was studied for the first time. The studies performed reveal the shift of the R1 line with increasing pressure. Such a red shift was accompanied by a significant change of the fluorescence lifetime. Both trends were successfully explained by a simple model, which resulted in adequate agreement between the calculated and the observed.     

Polarized spectroscopic properties of a potential self-frequency doubling crystal, Nd:BaCaBO3F

In this paper, the polarized absorption and fluorescence spectra of Nd³⁺ ion in the BaCaBO₃F non-linear optical crystal have been investigated at room temperature. The Judd-Ofelt theory, extended to anisotropic system, has been applied to calculate the phenomenological intensity parameters, spontaneous transition probabilities, branching ratios and radiative lifetimes. The maximum emission cross-sections have been calculated to be 7.22 × 10⁻²⁰ cm² and 15.21 × 10⁻²⁰ cm² at 1068 nm for π- and σ-polarization using the Füchtbauer-Ladenburg (F-L) equation. The fluorescence lifetime of ⁴F_3/2 manifold is equal to 57.7 μs, and the quantum efficiency is 26.8%, which is higher than those of most of other borates. The potential of Nd³⁺:BaCaBO₃F as a new self-frequency doubling laser crystal has also been evaluated preliminarily. Appreciable self-absorption at the second-harmonic wavelength of Nd³⁺:BaCaBO₃F crystal should be considered for further laser operations.     

Photoluminescence and radioluminescence of pure and Ce activated Na3Gd(PO4)2

The spectroscopic properties of Na₃Gd(PO₄)₂ and Na₃Gd(PO₄)₂:Ce³⁺ phosphors in the VUV-UV spectral range were investigated. Five excitation bands of Ce³⁺ ions at Gd³⁺ sites are observed at wavelengths of 205, 246, 260, 292, and 321 nm. Doublet Ce³⁺ 5d → 4f emission bands are observed at 341 and 365 nm with a decay constant τ_1/e around 26 ns. The X-ray excited luminescence of Na₃Gd_0.99Ce_0.01(PO₄)₂ at room temperature shows a photon yield of ∼17,000 photons/MeV of absorbed X-ray energy.     

Analysis of surface states and series resistance in Au/n-Si Schottky diodes with insulator layer using current-voltage and admittance-voltage characteristics

In order to good interpret the experimentally observed Au/n-Si (metal-semiconductor) Schottky diodes with thin insulator layer (18 Å) parameters such as the zero-bias barrier height (Φ_bo), ideality factor (n), series resistance (R_s) and surface states have been investigated using current-voltage (I-V), capacitance-frequency (C-f) and conductance-frequency (G-f) techniques. The forward and reverse bias I-V characteristics of Au/n-Si (MS) Schottky diode were measured at room temperature. In addition, C-f and G-f characteristics were measured in the frequency range of 1 kHz-1 MHz. The higher values of C and G at low frequencies were attributed to the insulator layer and surface states. Under intermediate forward bias, the semi-logarithmic Ln (I)-V plot shows a good linear region. From this region, the slope and the intercept of this plot on the current axis allow to determine the ideality factor (n), the zero-barrier height (Φ_bo) and the saturation current (I_S) evaluated to 2.878, 0.652 and 3.61 × 10⁻⁷ A, respectively. The diode shows non-ideal I-V behavior with ideality factor greater than unity. This behavior can be attributed to the interfacial insulator layer, the surface states, series resistance and the formation barrier inhomogeneity at metal-semiconductor interface. From the C-f and G-f characteristics, the energy distribution of surface states (N_ss) and their relaxation time (τ) have been determined in the energy range of (E_c − 0.493E_v)-(E_c − 0.610) eV taking into account the forward bias I-V data. The values of N_ss and τ change from 9.35 × 10¹³ eV⁻¹ cm⁻² to 2.73 × 10¹³ eV⁻¹ cm⁻² and 1.75 × 10⁻⁵ s to 4.50 × 10⁻⁴ s, respectively.     

Studying trivalent/bivalent metal ion doped TiO2 as p-TiO2 in bipolar heterojunction devices

Trivalent/bivalent metal ions doped TiO₂ thin films (M_xTi_1−xO₂, M = Cr³⁺, Fe³⁺, Ni²⁺, Co²⁺, Mn²⁺ and x = 0.01, 0.05, 0.1, 0.15, 0.2) were deposited on Indium–tin oxide (ITO) coated glass substrates by spin coating technique. X-ray photoelectron spectroscopy (XPS) showed Ti⁴⁺ oxidation state of the Ti2p band in the doped p-TiO₂. The homogenous M_xTi_1−xO₂ was used to support n-ZnO thin films with thickness ∼40–80 nm and vertically aligned n-ZnO nanorods (NR) with length ∼300 nm and 1.5 μm. Current (I)–voltage (V) characteristics for the Ag/n-ZnO/M_xTi_1−xO₂/ITO/glass assembly showed rectifying behavior with small turn-on voltages (V₀) < 1 V. The ideality factor (η) and the resistances in both forward and reverse bias were calculated. The temperature dependence performance of these bipolar devices was performed and variation of the parameters with temperature was studied.     

Polarized spectroscopic properties of Ho ions in NaLa(MoO4)2 crystal

A Ho³⁺-doped NaLa(MoO₄)₂ single crystal was grown by the Czochralski method. The polarized absorption spectra, polarized fluorescence spectra, and fluorescence decay curves of the crystal were measured at room temperature. The spontaneous emission probabilities, radiative lifetimes, and fluorescence branching ratios of the typical fluorescence multiplets of Ho³⁺ ions were calculated. The polarized stimulated emission and gain cross-sections of the ⁵I₇ → ⁵I₈ transition were obtained. The results show that the Ho³⁺:NaLa(MoO₄)₂ crystal is a promising gain medium for tunable and ultrashort pulse lasers operating around 2.0 μm.     

Growth and spectroscopic properties of Er-doped Li3Ba2Y3(MoO4)8 crystal

Er³⁺:Li₃Ba₂Y₃(MoO₄)₈ crystal has been grown by the top seeded solution growth method (TSSG) from a flux of Li₂MoO₄ and its morphology was analyzed. The polarized absorption spectra, fluorescence spectra and fluorescence decay curves of the crystal were measured. Based on the Judd-Ofelt (J-O) theory, spectroscopic parameters of Er³⁺:Li₃Ba₂Y₃(MoO₄)₈ crystal, including the oscillator intensity parameters Ω_t (t = 2, 4, 6), spontaneous emission probabilities, fluorescence branching ratios, and radiative lifetimes were calculated and analyzed. Stimulated emission cross-sections of the ⁴I_13/2 → ⁴I_15/2 transition were estimated by the reciprocity method (RM) and the Fuchtbauer-Ladenburg (F-L) formula. Five up-conversion fluorescence bands around 490, 530, 550, 660 and 800 nm were observed with 977 nm excitation, and the possible up-conversion mechanisms were proposed.     

Crystal growth of Nd: NaLu(WO4)2

Nd: NaLu(WO₄)₂ single crystals have been grown by the top seeded solution growth (TSSG) method. The peaks shown in the X-ray powder diffraction pattern were assigned. The crystal belongs to the space group of I4₁/a and the unit-cell parameters were calculated as a = b = 5.168 Å, c = 11.174 Å, V = 298.46 Å³. It is a tetragonal scheelite-like single crystal. The DSC (differential scanning calorimeter) curve proved that the as-grown crystal had a glass transition at the temperature range of 733.8 °C-781.4 °C which was lower than its melting point at 1149.3 °C. The absorption and fluorescence spectra were measured at room temperature.     

Novel Dy-doped Ca2Gd8(SiO4)6O2 white light phosphors for Hg-free lamps application

The luminescent properties of Ca₂Gd_8(1−x)(SiO₄)₆O₂:xDy³⁺ (1% ≤ x ≤ 5%) powder crystals with oxyapatite structure were investigated under vacuum ultraviolet excitation. In the excitation spectrum, the peaks at 166 nm and 191 nm of the vacuum ultraviolet region can be assigned to the O²⁻ → Gd³⁺, and O²⁻ → Dy³⁺ charge transfer band respectively, which is consistent with the theoretical calculated value using Jφrgensen's empirical formula. While the peaks at 183 nm and 289 nm are attributed to the f-d spin-allowed transitions and the f-d spin-forbidden transitions of Dy³⁺ in the host lattice with Dorenbos's expression. According to the emission spectra, all the samples exhibited excellent white emission under 172 nm excitation and the best calculated chromaticity coordinate was 0.335, 0.338, which indicates that the Ca₂Gd₈(SiO₄)₆O₂:Dy³⁺ phosphor could be considered as a potential candidate for Hg-free lamps application.     

Fabrication and laser behavior of the Yb:YAG ceramic microchips

Highly transparent 5 at.% Yb:YAG ceramic microchips were fabricated by vacuum sintering technology. The calculated cross section of the absorption σ_abs and emission σ_em were 6.1 × 10⁻²¹ cm² at 940 nm and 1.9 × 10⁻²⁰ cm² at 1030 nm, respectively. The fluorescence lifetime of the unannealed sample was 0.618 ms, and became 1.229 ms after annealing. Laser oscillation around 1037 nm was observed when pumped by the fiber-coupled laser with 970 nm. The threshold power was 3.0 W and the slope efficiency was about 12% when using the transmission of 2.6% output coupler.     

Structural and spectroscopic characterization of Yb doped Lu2O3 transparent ceramics

Yb³⁺ doped Lu₂O₃ transparent ceramics were fabricated by the solid-state reaction method and sintered in H₂ atmosphere. Structural and spectroscopic properties of Yb:Lu₂O₃ ceramics were studied. The Yb:Lu₂O₃ ceramic structure, and the lattice parameter are refined with the Rietveld method. Yb:Lu₂O₃ has broad absorption and emission bands with a long fluorescence lifetime (1.31 ms). The energy level diagram is calculated based on the absorption and emission spectra, Yb³⁺ in Lu₂O₃ ceramics exhibits a big splitting energy of the ²F_7/2 ground state (1023 cm⁻¹). Furthermore, the gain cross-section (σ_g) is estimated with different β values.     

Crystal structure, magnetic and thermal properties of LaFeTeO6

LaFeTeO₆ was prepared by solid state reaction of La₂O₃, Fe₂O₃ and TeO₂ in 1:1:2 molar ratios and characterized by powder X-ray diffraction, thermogravimetry and magnetometry. The detailed crystal structure analysis was carried out by Rietveld refinement. LaFeTeO₆ crystallizes in a trigonal lattice with unit cell parameters: a = 5.2049(1) Å and c = 10.3457(2) Å, V = 242.73(2) Å³. The crystal structure is built from sheets of the edge shared FeO₆ and TeO₆ octahedra stacked along the c-axis. These sheets are connected together by La³⁺ ions. Thermogravimetric analysis of the compound showed it to be thermally stable up to 1323 K and continuous loss of TeO₂ was observed above 1323 K leading to the formation of LaFeO₃. High temperature XRD studies revealed a normal expansion behavior of the compound. Temperature and field dependent magnetization of LaFeTeO₆ showed paramagnetic behavior in the temperature range of 3-300 K. The effective magnetic moment per Fe³⁺ ion (5.14 μB) indicates the high spin d⁵ state of Fe³⁺ ion.     

Low loss Sr1−xCaxLa4Ti5O17 microwave dielectric ceramics

Microwave dielectric ceramics in the Sr_1−xCa_xLa₄Ti₅O₁₇ (0 ≤ x ≤ 1) composition series were prepared through a solid state mixed oxide route. All the compositions formed single phase ceramics within the detection limit of in-house X-ray diffraction when sintered in the temperature range 1450-1580 °C. Theoretical density and molar volume decreased due to the substitution of Ca²⁺ for Sr²⁺ which was associated with a decrease in the dielectric constant (?_r) and temperature coefficient of resonant frequency (τ_f) but an increase in quality factor, Qf_o. Optimum properties were achieved for Sr_0.4Ca_0.6La₄Ti₅O₁₇ which exhibited, ?_r ∼ 53.7, Qf_o ∼ 11,532 GHz and τ_f ∼ −1.4 ppm/°C.     

Lattice thermal expansion studies of V2GeO4F2: A topaz type oxide-fluoride

A new compound V₂GeO₄F₂ was earlier found to exist in the V₂O₃-VF₃-GeO₂ system and the structure elucidation revealed it to be iso-structural to the mineral topaz. Herein, we report the lattice thermal expansion data of this compound. The lattice thermal expansion of V₂GeO₄F₂ was studied in the temperature range of 298-873 K under a flowing helium atmosphere by the high temperature XRD (HTXRD). The coefficients of axial thermal expansions of V₂GeO₄F₂ were found to be as: α_a = 3.5 × 10⁻⁶, α_b = 6.1 × 10⁻⁶ and α_c = 7.6 × 10⁻⁶ K⁻¹. The coefficient of volume (α_V) thermal expansion was 17.3 × 10⁻⁶ K⁻¹, which is relatively low compared to many analogues silicates.     

Crystal growth and spectral properties of Pr:La2(WO4)3

Pr³⁺-doped La₂(WO₄)₃ single crystal with dimensions up to Ø 20 mm × 35 mm has been grown by the Czochralski method. The structure of the Pr³⁺:La₂(WO₄)₃ crystal was determined by the X-ray powder diffraction and the Pr³⁺ concentration in this crystal was determined. The absorption and fluorescence spectra of Pr³⁺:La₂(WO₄)₃ crystal were measured at room temperature, and the fluorescence lifetime of main emission multiplets were estimated from the recorded decay curves. The spectral properties related to laser performance of the crystal were evaluated.     

Low temperature luminescence of KMgF3:Eu crystal

Paper presents luminescence spectra and time resolved spectra of KMgF₃:Eu²⁺ system obtained at different temperatures and pressures, under excitation with 325 nm. At temperatures between 200 K and 292 K the spectra consist of sharp line peaked at 27,830 cm⁻¹ related to ⁶P_7/2 → ⁸S_7/2 transition in Eu²⁺ accompanied by the phonon sideband. Under pressure the red spectral shift with the rate equal to −0.6 cm⁻¹/kbar is observed. Luminescence decay is single-exponential with the lifetime equal to 5.2 ms independent of pressure and temperature. The emission spectra obtained at temperatures lower than 125 K consist of 5 sharp lines peaked at 27,590 cm⁻¹, and 27,670 cm⁻¹, 27,722 cm⁻¹, 27,766 cm⁻¹ and 27,809 cm⁻¹, that relative intensity depends on temperature. Pressure shift of these lines was found to be equal to −0.6 cm⁻¹/kbar; the same as ⁶P_7/2 → ⁸S_7/2 transition in Eu²⁺, whereas their lifetime is shorter and is equal to 0.7 ms at 100 K. These new lines disappear at temperature greater than 200 K. We tentatively related them to the luminescence of Eu²⁺-F center (fluorine vacancy with electron) complex.     

Crystal structure and aggregation behavior in water of ionic liquid 1-hexadecyl-3-methylimidazolium Bromide

A large-transparent single crystal of 1-hexadecyl-3-methylimidazolium bromide monohydrate ([C₁₆mim]Br·H₂O) with 4-mm length was firstly obtained in the water-trichloromethane-toluene growth system (V_water:V_{trichloromethane}:V_toluene = 0.1:1:2). The crystal structure was determined by single crystal X-ray diffraction method. It crystallizes in the triclinic system, space group P-1, with unit cell parameters a = 5.4962(15) Å, b = 7.839(2) Å, c = 27.279(8) Å, α = 94.375°(4), β = 91.500°(5), γ = 101.999°(4), Z = 2, V = 1145.2(5)ų, Dc = 1.176 Mg/m³, μ = 1.804 mm^− 1, F(000) = 436, the final R1 = 0.0497, wR2 = 0.1154. The 3D supramolecular structure is constructed through the weak interactions between imidazolium cations, Br⁻ anions and lattice water molecules. The long alkyl chain to imidazolium ring and lattice water molecules play an important role in the self-assembled process. Moreover, it is proposed that [C₁₆mim]Br has the aggregation behavior in water at the higher concentration and the possible self-assembled structure is the interdigitated pattern.     

Study on ZnGa2O4:Cr a.c. powder electroluminescent device

An a.c. powder electroluminescent (EL) device using ZnGa₂O₄:Cr³⁺ phosphor was fabricated by the screen printing method. Optical and electrical properties of the device were investigated. The fabricated device shows a red emission at 695 nm driven by the a.c. voltage. The emission is attributed to the energy transfer from hot electrons to Cr³⁺ centers via self-activated Ga-O groups. Luminance (L) versus voltage (V) matches the well-known equation of L = L₀exp(− bV ^− 1 / 2) and luminance increases proportionally with frequency due to the increase of excitation probability of host lattice or Cr³⁺ centers. The diagram of the charge density (Q) versus applied voltage (V) is based on a conventional Sawyer-Tower circuit. At 280 V and 1000 Hz, the luminance and the luminous efficiency of the fabricated powder EL device are about 1.0 cd/m² and 13 lm/W, respectively. And under the high field, the device fabricated with the oxide-based phosphor of ZnGa₂O₄:Cr³⁺ shows excellent stability in comparison with the conventional sulfide powder EL device.     

Eu-Mössbauer spectroscopic and X-ray diffraction study on EuyM1−yO2−x (0 ≤ y ≤ 1.0) (M = Th, U)

¹⁵¹Eu-Mössbauer spectroscopic and powder X-ray diffraction (XRD) study has been performed for the Eu_yM_1−yO_2−x (M = Th and U) systems over the entire composition range of 0 ≤ y ≤ 1.0. The XRD results of the Eu-Th system showed that a very wide defect-fluorite (DF) type phase in which oxygen vacancies (V_O) are disordered (x = y/2) is formed for 0 ≤ y < 0.5 and that two-phase regions sandwitching a narrow C-type (C) single phase around y ≈ 0.8 appear for 0.5 < y < 0.8 (DF + C) and 0.82 < y < 1.0 (C + B-type (monoclinic) Eu₂O₃). The Mössbauer results show that the isomer shifts (ISs) of Eu³⁺ in this system smoothly increase with Eu composition, y. The decrease of average coordination number (CN) of O²⁻ around Eu³⁺ with increasing y (CN = 8 − 2y) (x = y/2) results in the decrease of the average EuO bond length, which is due to the decrease of repulsion force between O²⁻ anions. This result confirms that the IS of Eu³⁺ correlates well with the average EuO bond length in oxide systems. For the Eu-U system, the lattice parameter, a₀, of the system decreases almost linearly with y, in accordance with the calculated a₀ versus y curve for the oxygen-stoichiometric (i.e. x = 0) fluorite-type dioxide (CN = 8). The ISs of Eu³⁺ in this composition range remain almost constant around 0.5 mm/s, which is comparable to those of pyrochlore oxides (Eu₂Zr₂O₇ and Eu₂Hf₂O₇ (y = 0.5)) with O²⁻-eight-fold coordinated Eu³⁺(CN = 8).     

Synthesis and luminescent properties of novel Ba2−xEuxZr2−yHfySi3O12 phosphor

A series of Eu²⁺ activated luminescent materials according to the composition of Ba_2−xEu_xZr_2−yHf_ySi₃O₁₂ were synthesized using a high temperature solid-state reaction method starting from metal oxides and carbonates. Single phase powders were obtained using two annealing steps and boric acid as a flux. Firstly, starting materials were sintered at 1450 °C for 5 h under CO atmosphere and subsequently annealed at 1200 °C for 5 h under N₂/H₂ (95%/5%) gas flow. All samples were characterized by powder X-ray diffraction (XRD) analysis, thermal quenching (TQ), fluorescence lifetime measurements and photoluminescence (PL) techniques. Moreover, emission colour points, luminous efficacies and quantum efficiencies (QE) were calculated and discussed as a function of Eu²⁺ concentration and Zr/Hf ratio of the host lattice.