Structural and optical properties of Bi<Subscript>3.25</Subscript>La<Subscript>0.75</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> ferroelectric thin films prepared by chemical solution methods

Ferroelectric Bi_3.25La_0.75Ti₃O₁₂ (BLT) thin films have been grown on Pt/Ti/SiO₂/Si substrates by chemical solution methods. X-ray diffraction analysis shows that BLT thin films are polycrystalline with (171)-preferential orientation. Atomic force microscopy investigation shows that they have large grains about 120 nm in size. A Pt/BLT/Pt capacitor has been fabricated and showed excellent ferroelectricity, with a remnant polarization and coercive field of 24 μC/cm² and 116 kV/cm, respectively. The capacitor shows no polarization fatigue up to 10⁹ switching cycles. The optical constants (n,k) of the BLT thin films in the wavelength range 0.35–1.7 μm were obtained by spectroscopic ellipsometry measurements, and the band-gap energy was found to be about 3.25 eV. Received: 16 October 2001 / Accepted: 6 January 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +86-21/65830-734, E-mail: gswang@mail.sitp.ac.cn     

Luminescence of CaGa<Subscript>2</Subscript>Se<Subscript>4</Subscript>:Eu crystals

We have studied photoluminescence and thermoluminescence (PL and TL) in CaGa₂Se₄:Eu crystals in the temperature range 77–400 K. We have established that broadband photoluminescence with maximum at 571 nm is due to intracenter transitions 4f⁶ 5d–4f⁷ (⁸S_7/2) of the Eu²⁺ ions. From the temperature dependence of the intensity (log I–10³/T), we determined the activation energy (E _a = 0.04 eV) for thermal quenching of photoluminescence. From the thermoluminescence spectra, we determined the trap depths: 0.31, 0.44, 0.53, 0.59 eV. The lifetime of the excited state 4f6 5d of the Eu²⁺ ions in the CaGa₂Se₄ crystal found from the luminescence decay kinetics is 3.8 μsec. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 1, pp. 112–116, January–February, 2009.     

Magnetic ground state and the spin-state transitions in YBaCo<Subscript>2</Subscript>O<Subscript>5.5</Subscript>

The crystal and magnetic structure of the perovskite-like, oxygen deficient cobalt oxide YBaCo₂O_5.5 has been studied by means of neutron and X-ray diffraction in the 10–300 K temperature range. The magnetic ground state is characterized by a coexistence of two distinct antiferromagnetic phases. In the first one, the ionic moments of high-spin Co³⁺ ions in the pyramidal sites are ordered in a spiral arrangement, while octahedral sites are non-magnetic due to presence of low-spin Co³⁺ ions. The arrangement in the second phase is collinear of the G-type, with non-zero moments both in pyramidal (high-spin Co³⁺ ions) and octahedral sites (presumably a mixture of the low- and high-spin states). With increasing temperature, at 260–300 K, the system develops a gradual structural transformation, which is associated with appearance of spontaneous magnetic moment. This process is related to a thermally induced reversion of low- and high-spin states at the octahedral sites to the intermediate-spin Co³⁺ states, resulting in an insulator-metal transition at T_C ≈ T_IM ≈ 295 K.     

Application of MEH-PPV/SnO<Subscript>2</Subscript> bilayer as hybrid solar cell

The effects of oxygen content in the sputtering gas on the crystallographic and optoelectronic properties of 210 nm-thick Zr–doped In₂O₃ (Zr–In₂O₃) films by rf magnetron sputtering were initially studied. The results of X-ray diffraction show that the Zr–In₂O₃ films grown on glass substrates exhibit mixed crystallographic orientations. Moreover, the Zr–In₂O₃ film grown in an Ar atmosphere promotes the appearance of crystallographic orientation of (222). The surface of the Zr–In₂O₃ film becomes rougher as the oxygen content in the sputtering gas decreases; the current images obtained by conductive atomic force microscopy reveal that the surfaces of the Zr–In₂O₃ films exhibit a distribution of coexisting conducting and nonconducting regions, and that the area of the nonconducting surface increases with the oxygen content in the sputtering gas. The resistivity is minimized to 3.51×10⁻⁴ Ω cm when the Zr–In₂O₃ film is grown in an Ar atmosphere and the average transmittance in the visible light region is ∼85%. The optical band gap decreases as the oxygen content in the sputtering gas increases.     

Spectroscopic properties of Nd<Superscript>3+</Superscript>:CaNb<Subscript>2</Subscript>O<Subscript>6</Subscript> crystal

The absorption spectra, fluorescence spectrum and fluorescence decay curve of Nd³⁺ ions in CaNb₂O₆ crystal were measured at room temperature. The peak absorption cross section was calculated to be 6.202×10⁻²⁰ cm² with a broad FWHM of 7 nm at 808 nm for E//a light polarization. The spectroscopic parameters of Nd³⁺ ions in CaNb₂O₆ crystal have been investigated based on Judd-Ofelt theory. The parameters of the line strengths Ω _t are Ω ₂=5.321×10⁻²⁰ cm²,Ω ₄=1.734×10⁻²⁰ cm²,Ω ₆=2.889×10⁻²⁰ cm². The radiative lifetime, the fluorescence lifetime and the quantum efficiency are 167 μs, 152 μs and 91%, respectively. The fluorescence branch ratios are calculated to be β ₁=36.03%,β ₂=52.29%,β ₃=11.15%,β ₄=0.533%. The emission cross section at 1062 nm is 9.87×10⁻²⁰ cm².     

Spectroscopic Properties and Energy Transfer Analysis of Tm<Superscript>3+</Superscript>-Doped BaF<Subscript>2</Subscript>-Ga<Subscript>2</Subscript>O<Subscript>3</Subscript>-GeO<Subscript>2</Subscript>-La<Subscript>2</Subscript>O<Subscript>3</Subscript> Glass

This paper reports on the spectroscopic properties and energy transfer analysis of Tm³⁺-doped BaF₂-Ga₂O₃-GeO₂-La₂O₃ glasses with different Tm₂O₃ doping concentrations (0.2, 0.5, 2.0, 2.5, 3.0, 3.5, 3.5, 4.0 wt%). Mid-IR fluorescence intensities in the range of 1,300 nm−2,200 nm have been measured when excited under an 808 nm LD for all the samples with the same pump power. Energy level structure and Judd-Ofelt parameters have been calculated based on the absorption spectra of Tm³⁺, cross-relaxation rates and multi-phonon relaxation rates have been estimated with different Tm₂O₃ doping concentrations. The maximum fluorescence intensity at around 1.8 μm has been obtained in Tm₂O₃-3 wt% sample and the maximum value of calculated stimulated emission cross-section of Tm³⁺ in this sample is about 0.48 × 10⁻²⁰ cm² at 1,793 nm, and there is not any crystallization peak in the DSC curve of this sample, which indicate the potential utility of Tm³⁺-doped BaF₂-Ga₂O₃-GeO₂- La₂O₃ glass for 2.0-μm optical fiber laser.     

Large optical nonlinearity in CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> thin films

CaCu₃Ti₄O₁₂ (CCTO) thin films were successfully prepared on LaAlO₃ substrates by pulsed laser deposition technique. We measured the nonlinear optical susceptibility of the thin films using Z-scan method at a wavelength of 532 nm with pulse durations of 25 ps and 7 ns. The large values of the third-order nonlinear optical susceptibility, χ ⁽³⁾, of the CCTO film were obtained to be 2.79×10⁻⁸ esu and 3.30×10⁻⁶ esu in picosecond and nanosecond time regimes, respectively, which are among the best results of some representative nonlinear optical materials. The origin of optical nonlinearity of CCTO films was discussed. The results indicate that the CCTO films on LaAlO₃ substrates are promising candidate materials for applications in nonlinear optical devices.     

DFT study of BaTiO<Subscript>3</Subscript> (001) surface with O and O<Subscript>2</Subscript> adsorption

Progress of scanning tunneling microscopy (STM) allowed to handle various molecules adsorbed on a given surface. New concepts emerged with molecules on surfaces considered as nano machines by themselves. In this context, a thorough knowledge of surfaces and adsorbed molecules at an atomic scale is thus particularly invaluable. In this work, within the framework of density functional theory (DFT), we present an electronic and structural ab initio study of a BaTiO₃ (001) surface (perovskite structure) in its paraelectric phase. As far as we know the atomic and molecular adsorption of oxygen at surface is then analyzed for the first time in the literature. Relaxation is taken into account for several layers. Its analysis for a depth of at least four layers enables us to conclude that a reasonable approximation for a BaTiO₃ (001) surface is provided with a slab made up of nine plans. The relative stability of two possible terminations is considered. By using a kinetic energy cut off of 400 eV, we found that a surface with BaO termination is more stable than with TiO₂ termination. Consequently, a surface with BaO termination was chosen to adsorb either O atom or O₂ molecule and the corresponding calculations were performed with a coverage 1 on a (1×1) cell. A series of cases with O₂ molecule adsorbed in various geometrical configurations are also analyzed. For O₂, the most favorable adsorption is obtained when the molecule is placed horizontally, with its axis, directed along the Ba-Ba axis and with its centre of gravity located above a Ba atom. The corresponding value of the adsorption energy is -9.70 eV per molecule (-4.85 eV per O atom). The molecule is then rather extended since the O–O distance measures 1.829 ?. By comparison, the adsorption energy of an O atom directly located above a Ba atom is only -3.50 eV. Therefore we are allowed to conclude that the O–O interaction stabilizes atomic adsorption. Also the local densities of states (LDOS) corresponding to various situations are discussed in the present paper. Up to now, we are not aware of experimental data to be compared to our calculated results.     

O<Subscript>2</Subscript> photodesorption from a Ag<Subscript>8</Subscript>O<Stack><Subscript>2</Subscript><Superscript>-</Superscript></Stack> cluster

The decay path of an Ag₈(O₂)^- cluster photoexcited by a 3.1 eV photon is elucidated using time-resolved photoelectron spectroscopy. Photoabsorption results in the formation of an excited state giving rise to a peak in the photoelectron spectra with well-resolved vibrational finestructure. With a lifetime of about 100 fs this bound state decays into an anti-bonding state which dissociates into O₂ and Ag₈^- on a timescale of 10 ps. In the photoelectron spectra, this corresponds to a broad maximum shifting gradually towards higher binding energy while the O₂ and Ag₈^- separate. Finally, the spectrum of bare Ag₈^- appears. This process is unique to small clusters, because on metal surfaces excited state lifetimes are too short to allow for direct dissociation.     

Impurity band in SnBi<Subscript>4</Subscript>Se<Subscript>7</Subscript>: thermoelectric power and electrical resistivity measurements

Thermoelectric power and electrical resistivity measurements on polycrystalline samples of Bi₂Se₃ and stoichiometric ternary compound in the quasi-binary system SnSe–Bi₂Se₃ in the temperature range of 90–420 K are presented and explained assuming the existence of an impurity band. The variation of the electron concentration with temperature above 300 K is explained in terms of the thermal activation of a shallow donor, by using a single conduction band model. The density of states effective mass m ^*=0.15m ₀ of the electrons, the activation energy of the donors, their concentration, and the compensation ratio are estimated. The temperature dependence of the electron mobility in conduction band is analyzed by taking into account the scattering of the charge carriers by acoustic phonon, optical phonon, and polar optical phonon as well as by alloy and ionized impurity modes. On the other hand, by considering the two-band model with electrons in both the conduction and impurity bands, the change in the electrical resistivity with temperature between 420 and 90 K is explained.     

Photoluminescence of nanoparticles of (Ga<Subscript>2</Subscript>S<Subscript>3</Subscript>)<Subscript>0.95</Subscript>(Eu<Subscript>2</Subscript>O<Subscript>3</Subscript>)<Subscript>0.05</Subscript> solid solution

We have ground bulk samples to obtain nanoparticles of (Ga₂S₃)_1–x (Eu₂O₃) _x solid solutions, the sizes of which were determined using an atomic force microscope. The photoluminescence spectra of the nanoparticles were studied in the temperature interval 77–300 K. We have established the mechanisms for emission and transfer of energy from the matrix to the rare-earth ion, and we determined the Stokes shift (ΔS = 0.7 eV), the Huang–Rhys parameter (S = 16), and the optical phonon energy (ħ−ω = 23 meV).     

Infrared absorption spectra of pure and doped YAl<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> single crystals

Several weak absorption bands have been observed in the optical absorption spectra of pure and rare-earth-doped YAl₃(BO₃)₄ single crystals in the 3350– 3650 cm⁻¹ wave number region. Two of them, peaking at about 3377 cm⁻¹ and 3580 cm⁻¹ in the 8 K spectra, appear in most of the samples. They are tentatively attributed to the stretching mode of OH⁻ ions incorporated in the crystal during the growth. An additional absorption band at about 5250 cm⁻¹ at 8 K has also been detected in almost all samples. The temperature and polarization dependences of these bands, and their possible origin, are discussed.     

Application of R-matrix method to electron-H<Subscript>2</Subscript>S collisions in the low energy range

Electron-H₂S collision process is studied using the R-matrix method. Nine low-lying states of H₂S molecule are considered in the R-matrix formalism to obtain elastic integral, differential, momentum transfer and excitation cross sections for this scattering system. We have represented our target states using configuration interaction (CI) wavefunctions. We obtained adequate representation of vertical spectrum of the target states included in the scattering calculations. The cross sections are compared with the experiment and other theoretical results. We have obtained good agreement for elastic and momentum transfer cross sections with experiment for entire energy range considered. The differential cross sections are in excellent agreement with experiment in the range 3–15 eV. A prominent feature of this calculation is the detection of a shape resonance in ²B₂ symmetry which decays via dissociative electron attachment (DEA). Born correction is applied for the elastic and dipole allowed transition to account for higher partial waves excluded in the R-matrix calculation. The electron energy range is 0.025–15 eV.     

Impedance spectroscopy study of Pb<Subscript>2</Subscript>P<Subscript>2</Subscript>O<Subscript>7</Subscript> compound

The lead pyrophosphate, Pb₂P₂O₇, compound was prepared by conventional solid-state reaction and identified by X-ray powder diffractometer. Pb₂P₂O₇ has a triclinic structure whose electrical properties were studied using impedance spectroscopy technique. Both impedance and modulus analysis exhibit the grain and grain boundary contribution to the electrical response of the sample. The temperature dependence of the bulk and grain boundary conductivity were found to obey the Arrhenius law with activation energies E _g = 0.66 eV and E _gb = 0.67 eV, respectively. The scaling behavior of the imaginary part of the complex impedance suggests that the relaxation describes the same mechanism at various temperatures.     

Growth of AgInSe<Subscript>2</Subscript> on Si(100) substrate by thermal evaporation technique

AgInSe₂ films were prepared by a thermal evaporation technique onto Si(100) substrates at a pressure of 10⁻⁵ mbar. Structural and optical properties of films deposited at 300 and 473 K have been investigated. The film composition was studied by energy dispersive analysis through X-rays. X-ray diffraction patterns indicate that AgInSe₂ films have chalcopyrite structure with strong preferred orientation in the (112) direction. Average vertical crystallite size of 25 nm was observed. The optical energy gaps of 1.20 and 1.90 eV were obtained due to the fundamental absorption edge and a transition originating from crystal field splitting, respectively. Field emission scanning electron microscopy shows loosely packed grains of spherical symmetry with some facets.     

Influence of carbon additive on the properties of spherical Li<Subscript>4</Subscript>Ti<Subscript>5</Subscript>O<Subscript>12</Subscript> and LiFePO<Subscript>4</Subscript> materials for lithium-ion batteries

Preparing spherical particles with carbon additive is considered as one effective way to improve both high rate performance and tap density of Li₄Ti₅O₁₂ and LiFePO₄ materials. Spherical Li₄Ti₅O₁₂/C and LiFePO₄/C composites are prepared by spray-drying–solid-state reaction method and controlled crystallization–carbothermal reduction method, respectively. The X-ray diffraction characterization, scanning electron microscope, Brunauer–Emmett–Teller, alternating current impedance analyzing, tap density testing, and electrochemical property measurements are investigated. After hybridizing carbon with a proper quantity, the crystal grain size of active materials is remarkably decreased and the electrochemical properties are obviously improved. The Li₄Ti₅O₁₂/C and LiFePO₄/C composites prepared in this work are spherical. The tap density and the specific surface area are as high as 1.71 g cm⁻³ and 8.26 m² g⁻¹ for spherical Li₄Ti₅O₁₂/C, which are 1.35 g cm⁻³ and 18.86 m² g⁻¹ for spherical LiFePO₄/C powders. Between 1.0 and 3.0 V versus Li, the reversible specific capacity of the Li₄Ti₅O₁₂/C is more than 150 mAh g⁻¹ at 1.0-C rate. Between 2.5 and 4.2 V versus Li, the reversible capacity of the LiFePO₄/C is close to 140 mAh g⁻¹ at 1.0-C rate.     

Fast ionic conduction in cubic hafnium garnet Li<Subscript>7</Subscript>La<Subscript>3</Subscript>Hf<Subscript>2</Subscript>O<Subscript>12</Subscript>

A new member of the family of garnets with fast lithium ion conduction has been found with the composition Li₇La₃Hf₂O₁₂. The anion arrangement corresponds to the oxygen framework in garnets, e.g., in Ca₃Fe₂Si₃O₁₂. Hafnium is coordinated octahedrally while the lanthanum environment can be described as a distorted cube. Lithium occupies a large number of positions with tetrahedral, trigonal planar, and metaprismatic coordination. Li₇La₃Hf₂O₁₂ shows a lithium bulk ion conductivity of 2.4 × 10⁻⁴ Ω⁻¹ cm⁻¹ at room temperature with an activation energy of 0.29 eV.     

Tunable optical parametric oscillator based on a KTP crystal,pumped by a pulsed Ti<Superscript>3+</Superscript>:Al<Subscript>2</Subscript>O<Subscript>3</Subscript> laser

We present the characteristics of an optical parametric oscillator based on a KTP crystal, pumped with noncritical phase matching by a pulsed Ti³⁺:Al₂O₃ laser, tunable in the range 677–970 nm. Tunable generation of signal and idler waves is obtained in the ranges 1030–1390 nm and 2690–3050 nm respectively. The efficiency of conversion of the pump to the signal wave is ≈23%, which for pulses of duration ≈8 nsec ensures an energy in the range 1.0–11.5 mJ. The width of the emission spectrum for the signal wave is within the range 0.8–1.8 nm and is predominantly determined by the linewidth of the Ti³⁺:Al₂O₃ pump laser. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 3, pp. 351–356, May–June, 2007.     

Experimental evidence for molecular ultrafast dissociation in O<Subscript>2</Subscript> clusters

Resonant Auger spectra of O₂ clusters excited at the O1s edge are reported. After excitation to the repulsive 1s^-13σ^* state, the resulting resonant Auger spectrum displays features that remain constant in kinetic energy as the photon energy is detuned. The shift between known atomic fragment features and these features is consistent with that observed for atoms and clusters in singly charged states in direct photoemission. These findings are strong evidence for the existence of molecular ultrafast dissociation processes within the clusters or on their surface.     

Synthesis and luminescent properties of Eu<Superscript>3+</Superscript>- and Eu<Superscript>2+</Superscript>-activated sodium pyrophosphate Na<Subscript>4</Subscript>P<Subscript>2</Subscript>O<Subscript>7</Subscript>

The luminescent properties of Eu³⁺ and Eu²⁺ ions in sodium pyrophosphate, Na₄P₂O₇, have been studied. The excitation spectrum of the Eu³⁺ emission in Na₄P₂O₇ consists of several sets of bands in the range 280–535 nm due to 4f–4f transitions of Eu³⁺ ions and a broad band with a maximum at about 240 nm interpreted to be due to a charge transfer (CT) transition from oxygen 2p states to empty states of the Eu³⁺ 4f⁶-configuration. Although the CT band energy is large enough, the quantum efficiency (η) of the Eu³⁺ emission in Na₄P₂O₇ under CT excitation was estimated to be very low (η ≤ 0.01). In terms of a configurational coordinate model, this fact is interpreted as a result of the high efficiency of a radiationless relaxation from the CT state to the ⁷F₀ ground state of Eu³⁺ ions occupying sodium sites in Na₄P₂O₇. A strong reducing agent is required in order to stabilize Eu²⁺ ions in Na₄P₂O₇ during the synthesis. Several nonequivalent Eu²⁺ luminescence centers in Na₄P₂O₇ were found.