Dielectric parameters in Se70Te30 and Se70Te28Zn2 chalcogenide glasses

Temperature and frequency dependence of dielectric constant (ε′) and dielectric loss (ε″) are studied in glassy Se₇₀Te₃₀ and Se₇₀Te₂₈Zn₂. The measurements have been made in the frequency range (8-500 kHz) and in the temperature range 300 to 350 K. An analysis of the dielectric loss data shows that the Guintini's theory of dielectric dispersion based on two-electron hopping over a potential barrier is applicable in the present case.No dielectric loss peak is observed in glassy Se₇₀Te₃₀. However, such loss peaks exist in the glassy Se₇₀Te₂₈Zn₂ in the above frequency and temperature range. The Cole-Cole diagrams have been used to determine some parameters such as the distribution parameter (α), the macroscopic relaxation time (τ₀), the molecular relaxation time (τ) and the Gibb's free energy for relaxation (ΔF).     

Spectral analysis of Pr-, Sm- and Dy-doped transparent GeO2-BaO-TiO2 glass ceramics

In this paper, we present the photoluminescence properties of Pr³⁺-, Sm³⁺- and Dy³⁺-doped germanate glasses and glass ceramics. From the X-ray diffraction measurement, the host glass structure was determined. These glasses have shown strong absorption bands in the near-infrared (NIR) region. Compared to Pr³⁺-, Sm³⁺- and Dy³⁺-doped glasses, their respective glass ceramics have shown stronger emissions due to the Ba₂TiGe₂O₈ crystalline phase. For Pr³⁺-doped glass and glass ceramic, emission bands centered at 530 nm (³P₀→³H₅), 614 nm (³P₀→³H₆), 647 nm (³P₀→³F₂) and 686 nm (³P₀→³F₃) have been observed with 485 nm (³H₄→³P₀) excitation wavelength. Of them, 647 nm (³P₀→³F₂) has shown bright red emission. Emission bands of ⁴G_5/2→⁶H_5/2 (565 nm), ⁴G_5/2→⁶H_7/2 (602 nm) and ⁴G_5/2→⁶H_9/2 (648 nm) for the Sm³⁺:glass and glass ceramic, with excitation at ⁶H_5/2→⁴F_7/2 (405 nm) have been recorded. Of them, ⁴G_5/2→⁶H_7/2 (602 nm) has shown a bright orange emission. With regard to the Dy³⁺:glass and glass ceramic, a bright fluorescent yellow emission at 577 nm (⁴F_9/2→⁶H_13/2) has been observed, apart from ⁴F_9/2→⁶H_11/2 (667 nm) emission transition with an excitation at 454 nm (⁶H_15/2→⁴I_15/2) wavelength. The stimulated emission cross-sections of all the emission bands of Pr³⁺, Sm³⁺ and Dy³⁺:glasses and glass ceramics have been computed based on their measured full-width at half-maxima (FWHM, Δλ) and lifetimes (τ_m).     

Spectroscopic investigations of Nd-, Er-, Er/Yb-, and Tm-ions doped SiO2-Al2O3-CaF2-GdF3 glasses

This paper reports on the absorption, visible and near-infrared luminescence properties of Nd³⁺, Er³⁺, Er³⁺/2Yb³⁺, and Tm³⁺ doped oxyfluoride aluminosilicate glasses. From the measured absorption spectra, Judd-Ofelt (J-O) intensity parameters (Ω₂, Ω₄ and Ω₆) have been calculated for all the studied ions. Decay lifetime curves were measured for the visible emissions of Er³⁺ (558 nm, green), and Tm³⁺ (650 and 795 nm), respectively. The near infrared emission spectrum of Nd³⁺ doped glass has shown full width at half maximum (FWHM) around 45 nm (for the ⁴F_3/2→⁴I_9/2 transition), 45 nm (for the ⁴F_3/2→⁴I_11/2 transition), and 60 nm (for the ⁴F_3/2→⁴I_13/2 transition), respectively, with 800 nm laser diode (LD) excitation. For Er³⁺, and Er³⁺/2Yb³⁺ co-doped glasses, the characteristic near infrared emission bands were spectrally centered at 1532 and 1544 nm, respectively, with 980 nm laser diode excitation, exhibiting full width at half maximum around 50 and 90 nm for the erbium ⁴I_13/2→⁴I_15/2 transition. The measured maximum decay times of ⁴I_13/2→⁴I_15/2 transition (at wavelength 1532 and 1544 nm) are about 5.280 and 5.719 ms for 1Er³⁺ and 1Er³⁺/2Yb³⁺ (mol%) co-doped glasses, respectively. The maximum stimulated emission cross sections for ⁴I_13/2→⁴I_15/2 transition of Er³⁺ and Er³⁺/Yb³⁺ are 10.81×10⁻²¹ and 5.723×10^-21 cm². These glasses with better thermal stability, bright visible emissions and broad near-infrared emissions should have potential applications in broadly tunable laser sources, interesting optical luminescent materials and broadband optical amplification at low-loss telecommunication windows.     

Cooperative emission study in ytterbium-doped Y2SiO5

Ytterbium ions infrared and visible cooperative luminescences, resulting from YAG laser and selective site excitations, in (6%) Yb-doped Y₂SiO₅ thin film are analyzed. Magnetically coupled Yb-Yb ion pairs seem to play a major role in energy transfer and cooperative emission, confirming the prevalence of superexchange mechanisms.     

Ferromagnetism at 300 K in spin-coated films of Co doped anatase and rutile-TiO2

Thin films of Ti_1−xCo_xO₂ (x=0 and 0.03) have been prepared on sapphire substrates by spin-on technique starting from metalorganic precursors. When heat treated in air at 550 and 700 °C, respectively, these films present pure anatase and rutile structures as shown both by X-ray diffraction and Raman spectroscopy. Optical absorption indicate a high degree of transparency in the visible region. Such films show a very small magnetic moment at 300 K. However, when the anatase and the rutile films are annealed in a vacuum of 1×10⁻⁵ Torr at 500 and 600 °C, respectively, the magnetic moment, at 300 K, is strongly enhanced reaching 0.36μ_B/Co for the anatase sample and 0.68μ_B/Co for the rutile one. The ferromagnetic Curie temperature of these samples is above 350 K.     

Photoluminescence at room temperature in polycrystalline BaxSr1−xTiO3 ceramics obtained by solid-state reaction method

Stoichiometric Ba_xSr_1−xTiO₃ ceramics (x=0.7, 0.8, and 0.9) have been prepared by solid-state reaction method. It has been observed in our experiments that the photoluminescence (PL) spectra in such samples at room temperature were centered around 800 nm; the results have been found to differ from observations in current literature. An explanation of this phenomenon was given, and the origin of such photoluminescence is ascribed to certain defects existing in these compounds, such as oxygen vacancies.     

Room-temperature electrosynthesized ZnO thin film with strong (0 0 2) orientation and its optical properties

ZnO thin film with strong orientation (0 0 2) and smooth surface morphology was electrosynthesized on ITO-coated glass substrate at room temperature under pulsed voltage. Photoluminescence (PL) shows two obvious peaks: violet band and strong green band. The former is due to the free-excitonic transition and the latter is believed to arise from the single ionized oxygen vacancy (V_O⁺). Raman scattering reveals that the 580 cm⁻¹ mode and the shoulder peak mode at 550 cm⁻¹ originate from the N-related local vibration mode (LVM) and E₁ (LO) mode, respectively.     

Formation of nano-hillocks by impact of swift heavy ions on thin films of TiO2

Amorphous thin films of TiO₂ are irradiated by swift heavy ion (SHI) beam. Surface topography is studied by atomic force microscopy (AFM). Formation of nanosized oblate hillocks on the surface of irradiated films is investigated by AFM studies. After irradiation, amorphous to crystalline phase transition is observed in glancing angle X-ray diffraction (GAXRD) and Raman spectroscopy studies. Photoluminescence (PL)-spectroscopy is carried out for optical characterization. Threshold value necessary for emergence of hillocks is estimated.     

Temperature-dependent photoluminescence spectra of Er-Tm-codoped Al2O3 thin film

Er-Tm-codoped Al₂O₃ thin films with different Tm to Er concentration ratios were synthesized by cosputtering from separated Er, Tm, Si, and Al₂O₃ targets. The temperature dependence of photoluminescence (PL) spectra was studied. A flat and broad emission band was achieved in the 1.4-1.7 μm and the observed 1470, 1533 and 1800 nm emission bands were attributed to the transitions of Tm³⁺: ³H₄ → ³F₄, Er³⁺: ⁴I_13/2 → ⁴I_15/2 and Tm³⁺: ³F₄ → ³H₆, respectively. The temperature dependence is rather complicated. With increasing measuring temperature, the peak intensity related to Er³⁺ ions increases by a factor of five, while the Tm³⁺ PL intensity at 1800 nm decreases by one order of magnitude. This phenomenon is attributed to a complicated energy transfer (ET) processes involving both Er³⁺ and Tm³⁺ and increase of phonon-assisted ET rate with temperature as well. It should be helpful to fully understand ET processes between Er and Tm and achieve flat and broad emission band at different operating temperatures.     

Domain structures in tetragonal Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals studied by piezoresponse force microscopy

The domain structures in (001) surface of Pb(Mg_1/3Nb_2/3)O₃-40% PbTiO₃ single crystals were investigated by piezoresponse force microscopy. Both micron-sized fingerprint 180° and parallel 90° domains were observed in the sample. Different sets of favourable {110} oriented domain patterns were found to meet, intersect or grow through each other. In addition, the piezoelectricity decreases sharply at the domain walls in 180° structures, but does not in the 90° domain structures.     

A combinatorial approach to the discovery and optimization of YCa4O(BO3)3-based luminescent materials

Thin films of YCa₄O(BO₃)₃ (YCOB)-based new luminescent materials were explored by the combinatorial pulsed laser deposition (PLD) method which enabled us to fabricate continuous composition spread film libraries. Strong red and green luminescence were found in the Y_1−xEu_xCOB (0 ≤ x ≤ 1), (YEuCOB) and Y_1−yTb_yCOB (0 ≤ y ≤ 1) (YTbCOB) films, respectively. The film libraries were characterized by photoluminescence (PL), PL decay, an electron-probe microanalyzer and an electron diffraction analysis. The luminescent intensities in the amorphous film libraries strongly depended on the chemical composition of each rare-earth (RE) ion. The optimum concentration of rare-earth ions in YEuCOB and YTbCOB were experimentally determined to be Eu = 7.5% and Tb = 20-30%, respectively.     

Micro-Raman spectroscopy of disordered and ordered sulfur phases on a passivated GaAs surface

The depletion layer width and band bending of passivated n-type Sn doped GaAs(1 0 0) between subsequent steps of chemical treatment as well as after a single run treatment were investigated by micro-Raman light scattering by longitudinal optical phonons and coupled phonon-plasmon modes. Experiments were carried out ex situ at room temperature. We conclude that all observed lineshape changes are due to band bending and to an amorphous surface phase represented by a broad spectral component. We applied two passivation methods. One was based on (NH₄)₂S_x solution and lasted 30 min. The second was based on the S₂Cl₂ solution and lasted 10 s. These enabled identification of surface regions of different amorphousness and for faster passivation places of enlarged and completely reduced band bending.     

A comparative study of microstructure of RuO2 nanorods via Raman scattering and field emission scanning electron microscopy

Raman scattering (RS) and field emission scanning electron microscopy (FESEM) have been used to extract microstructural information of RuO₂ nanorods (NRs) and a two-phase system comprising NRs embedded in polycrystalline matrix deposited on different substrates by the metal-organic chemical vapor deposition method. The red shifts and asymmetric broadening of the Raman line shape for the NRs are analyzed by the spatial correlation model. The deduced spatial correlation length l is found to be much smaller than that of the average size L₀ estimated from the FESEM images. The Raman features for the two-phase system can be resolved into two parts: a Lorentzian line shape feature corresponding to the polycrystallite at higher frequency side and an asymmetrically broadened NRs' signature located at lower frequency end. The volume fraction of NRs in the two-phase system can be determined from the analysis. These results demonstrate the significance of RS as a structural characterization method when used in conjunction with FESEM.     

Synthesis and luminescence properties of Tb:NaGd(WO4)2 novel green phosphors

Tb³⁺:NaGd(WO₄)₂ (Tb:NGW) phosphors with different Tb³⁺ concentrations have been synthesized by a mild hydrothermal process directly without further sintering treatment. X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence excitation and emission spectra and decay curve were used to characterize the Tb:NGW phosphors. XRD analysis confirmed the formation of NGW with scheelite structure. SEM study showed that the obtained Tb:NGW phosphors appeared to be nearly spherical and their sizes ranged from 1 to 1.5 μm. The excitation spectra of these systems showed an intense broad band with maximum at 270 nm related to the O→W ligand-to-metal charge-transfer state. Photoluminescence spectra indicated the phosphors emitted strong green light centered at 545 nm under UV light excitation. Analysis of the photoluminescence spectra with different Tb³⁺ concentrations revealed that the optimum dopant concentration for Tb³⁺ is about 15 at% of Tb³⁺ ions in Tb:NGW phosphors.     

The influence of H2/(H2 + Ar) ratio on microstructure and optoelectronic properties of microcrystalline silicon films deposited by plasma-enhanced CVD

Hydrogenated microcrystalline silicon films were deposited by glow discharge decomposition of SiH₄ diluted in mixed gas of Ar and H₂. By investigating the dependence of the film crystallinity on the flow rates of Ar and H₂, we showed that the addition of Ar in diluted gas markedly improves the crystallinity due to an enhanced dissociation of SiH₄. The infrared-absorption spectrum reveals that the fraction of SiH bonding increases with increasing the rate ratio of H₂/(H₂ + Ar). The surface roughness of the films increases with increasing the flow rate ratio of H₂/(H₂ + Ar), which is attributed to the decrease of massive bombardment of Ar ions in the plasma. Refractive index and absorption coefficient of the films were obtained by simulating the optical transmission spectra using a modified envelope method. Electrical measurements of the films show that the dark conductivity increases and the activation energy decreases with the ratio of H₂/(H₂ + Ar). A reasonable explanation is presented for the dependence of the microstructure and optoelectronic properties on the flow rate ratio of H₂/(H₂ + Ar).     

Effects of substrates on the structure and dielectric properties of Ba(Sn0.15Ti0.85)O3 thin films

Ba(Sn_0.15Ti_0.85)O₃ (BTS) thin films were grown on Pt(1 1 1)/Ti/SiO₂/Si and LaNiO₃(LNO)/Pt(1 1 1)/Ti/SiO₂/Si substrates by a sol-gel processing technique, respectively. The BTS thin films deposited on annealed Pt(1 1 1)/Ti/SiO₂/Si and annealed LNO/Pt(1 1 1)/Ti/SiO₂/Si substrates exhibited strong (1 1 1) and perfect (1 0 0) orientations, respectively. The BTS thin films grown on un-annealed Pt(1 1 1)/Ti/SiO₂/Si substrates showed random orientation with intense (1 1 0) peak, while the films deposited on un-annealed LNO/Pt(1 1 1)/Ti/SiO₂/Si substrate exhibited random orientation with intense (1 0 0) peak, respectively. The dielectric constant of the BTS films deposited on annealed Pt(1 1 1)/Ti/SiO₂/Si, annealed LNO/Pt(1 1 1)/Ti/SiO₂/Si, un-annealed Pt(1 1 1)/Ti/SiO₂/Si and un-annealed LNO/Pt(1 1 1)/Ti/SiO₂/Si substrates was 512, 565, 386 and 437, respectively, measured at a frequency of 100 kHz. A high tunability of 49.7% was obtained for the films deposited on annealed LNO/Pt(1 1 1)/Ti/SiO₂/Si substrate, measured at the frequency of 100 kHz with an applied electric field of 200 kV/cm. The high tunability has been attributed to the (1 0 0) texture of the films and larger grain sizes.     

Epitaxial growth and structural characterization of Pb(Fe1/2Nb1/2)O3 thin films

We have grown lead iron niobate thin films with composition Pb(Fe_1/2Nb_1/2)O₃ (PFN) on (0 0 1) SrTiO₃ substrates by pulsed laser deposition. The influence of the deposition conditions on the phase purity was studied. Due to similar thermodynamic stability spaces, a pyrochlore phase often coexists with the PFN perovskite phase. By optimizing the kinetic parameters, we succeeded in identifying a deposition window which resulted in epitaxial perovskite-phase PFN thin films with no identifiable trace of impurity phases appearing in the X-ray diffractograms. PFN films having thicknesses between 20 and 200 nm were smooth and epitaxially oriented with the substrate and as demonstrated by RHEED streaks which were aligned with the substrate axes. X-ray diffraction showed that the films were completely c-axis oriented and of excellent crystalline quality with low mosaicity (X-ray rocking curve FWHM?0.09°). The surface roughness of thin films was also investigated by atomic force microscopy. The root-mean-square roughness varies between 0.9 nm for 50-nm-thick films to 16 nm for 100-nm-thick films. We also observe a correlation between grain size, surface roughness and film thickness.     

Spectroscopic investigation of Tm:TeO2-WO3 glass

We studied the spectroscopic characteristics of telluride glass with the host composition (0.85)TeO₂-(0.15)WO₃, containing 0.25 and 1.0 mol% thulium oxide (Tm₂O₃). By analyzing the absorption spectra with the Judd-Ofelt theory, the average radiative lifetimes of 305±7.5 μs and 1.95±0.02 ms were determined for the ³F₄ and ³H₄ levels, respectively. Measured fluorescence lifetime of the ³F₄ level decreased from 218 to 51 μs for the 0.25 and 1.0 mol% Tm₂O₃ doped samples, respectively, indicating the effect of boosted non-radiative decay at higher doping concentrations. A similar trend was observed for the ³H₄ level, where the fluorescence lifetime decreased from 1.86 ms to 350 μs at these concentrations. The quenching of the 1460 nm (³F₄→³H₄) emission in favor of the 1800 nm (³H₄→³H₆) emission due to cross relaxation was further evident in the fluorescence spectra of the samples. The calculated stimulated emission cross sections (3.73±0.1×10⁻²¹ cm² at 1460 nm and 6.57±0.07×10⁻²¹ cm² at 1808 nm) reveal the potential importance of the Tm³⁺:(0.85)TeO₂-(0.15)WO₃ glass for applications in fiber-optic amplifiers and fiber lasers.     

First-principles study of structural, optical and elastic properties of cubic HfO2

The structural, optical and elastic properties of cubic HfO₂ were studied using the plane-wave ultrasoft pseudopotential technique based on the first-principles density-functional theory (DFT). The ground-state properties such as lattice parameter and bulk modulus were calculated and these results are in favorable agreement with the previous work. The complex dielectric function, refractive index, extinction coefficient, complex conductivity function, energy-loss spectrum, absorption coefficient and optical reflectivity are calculated and the peak position distributions of imaginary parts of the complex dielectric function have been explained. The calculated elastic properties are consistent with other calculated results.