Extremely large binding energy of biexcitons in an organic-inorganic quantum-well material (C4H9NH3)2PbBr4

We have confirmed biexciton formation in an organic-inorganic hybrid quantum-well material (C₄H₉NH₃)₂PbBr₄ by photoluminescence and two-photon absorption measurements. The biexciton has extremely large binding energy, 60 meV, which to our knowledge is the largest value ever reported for a semiconductor. By analyzing the spectrum of biexciton luminescence, the biexciton gas temperature was found to be much higher than the bath temperature due to a higher local temperature arising from the large biexciton binding energy.     

Synthesis and luminescence properties of lead-halide based organic–inorganic layered perovskite compounds (CnH2n+1NH3)2PbI4 (n=4, 5, 7, 8 and 9)

This paper describes the synthesis and characterization of organic–inorganic layered perovskite compounds, (C_nH_2n+1NH₃)₂PbI₄ (n=4, 5, 7, 8 and 9). The effect of the number of carbon atoms on luminescence properties has been examined. Thin films of microcrystalline (C_nH_2n+1NH₃)₂PbI₄ fabricated by spin-coating are highly oriented, with the c-axis perpendicular to the substrate surface. Temperature-dependent optical absorption spectra reveal that (C_nH_2n+1NH₃)₂PbI₄ films (n=4, 7, 8 and 9) show the structural phase transitions. The excitonic structures of (C_nH_2n+1NH₃)₂PbI₄ vary with the number of carbon atoms of the alkyl chain length. At low temperatures below 100 K, the lowest-energy free-exciton band of (C_nH_2n+1NH₃)₂PbI₄ (n=7, 8 and 9) split into three fine-structure levels. In contrast to (C_nH_2n+1NH₃)₂PbBr₄ films, (C_nH_2n+1NH₃)₂PbI₄ (n=7, 8 and 9) shows no triplet exciton emission, but it shows the Stokes-shifted emission from bound excitons.     

Optical spectroscopy of Yb/Er codoped NaY(WO4)2 crystal

Erbium and ytterbium codoped double tungstates NaY(WO₄)₂ crystals were prepared by using Czochralski (CZ) pulling method. The absorption spectra in the region 290-2000 nm have been recorded at room temperature. The Judd-Ofelt theory was applied to the measured values of absorption line strengths to evaluate the spontaneous emission probabilities and stimulated emission cross sections of Er³⁺ ions in NaY(WO₄)₂ crystals. Intensive green and red lights were measured when the sample were pumped by a 974 nm laser diode (LD), especially, the intensities of green upconversion luminescence are very strong. The mechanism of energy transfer from Yb³⁺ to Er³⁺ ions was analyzed. Energy transfer and nonradiative relaxation played an important role in the upconversion process. Photoexcited luminescence experiments are also fulfilled to help analyzing the transit processes of the energy levels.     

Sr2CeO4:Eu and Sr2CeO4:5 mol% Eu, 3 mol% Dy microphosphors: Wet chemistry synthesis from hybrid precursor and photoluminescence properties

In this article, Sr₂CeO₄:x mol% Eu³⁺ and Sr₂CeO₄:5 mol%Eu³⁺, 3 mol% Dy³⁺ phosphors were synthesized from assembling hybrid precursors by wet chemical method. As-prepared samples present uniform grain-like morphology and the particle size is about 0.2 μm. The luminescence spectra of Sr₂CeO₄:x mol% Eu³⁺ have been measured to examine the influence of the intensity of red emission lines for Eu³⁺ on the concentration of Eu³⁺, showing that the intensity of the red emission increases with an increase of the concentration from 1 to 5 mol%. Additionally, from the emission spectra of Sr₂CeO₄:5 mol%Eu³⁺, 3 mol% Dy³⁺ phosphors, the characteristic lines of Dy³⁺ have also been observed. This result indicates that there also exists an energy transfer process between Sr₂CeO₄ and Dy³⁺.     

Synthesis process and the luminescence properties of rare earth doped NaLa(WO4)2 nanoparticles

Rare earth doped NaLa(WO₄)₂ nanoparticles have been prepared by a simply hydrothermal synthesis procedure. The X-ray diffraction (XRD) pattern shows that the Eu³⁺-doped NaLa(WO₄)₂ nanoparticles with an average size of 10-30 nm can be obtained via hydrothermal treatment for different time at 180 °C. The luminescence intensity of Eu³⁺-doped NaLa(WO₄)₂ nanoparticles depended on the size of the nanoparticles. The bright upconversion luminescence of the 2 mol% Er³⁺ and 20 mol% Yb³⁺ codoped NaLa(WO₄)₂ nanoparticles under 980 nm excitation could also be observed. The Yb³⁺-Er³⁺ codoped NaLa(WO₄)₂ nanoparticles prepared by the hydrothermal treatment at 180 °C and then heated at 600 °C shows a 20 times stronger upconversion luminescence than those prepared by hydrothermal treatment at 180 °C or by hydrothermal treatment at 180 °C and then heated at 400 °C.     

Characterization of new-layered Cr(III)-doped chlorocadmiumphosphate, Cd(HPO4)Cl·[H3N(CH2)6NH3]0.5 crystal by EPR and optical studies

Cr(III)-doped Cd(HPO₄)Cl·[H₃N(CH₂)₆NH₃]_0.5, a new-layered cadmium phosphate, is synthesized in acidic condition at room temperature. EPR and optical studies are carried out at room temperature. Polycrystalline EPR spectrum reveals the presence of two sites of Cr(III) ions in this layered phosphate lattice with zero-field splitting values of 24.24 and 7.65 mT, indicating that Cr(III) ions are in distorted octahedral sites. The optical absorption spectrum of the sample indicates near octahedral symmetry for the dopant ions. Crystal field, inter-electronic and bonding parameters are evaluated by collaborating EPR and optical data. The evaluated parameters suggest the mode of entry of Cr(III) ion into the layered phosphate as interstitial site, and bonding between the metal and ligand is partially covalent.     

EPR and optical investigation of VO(II) in Zn(C3H3O4)2(H2O)2 single crystals: An interstitial site

EPR spectroscopic investigations on single crystals of diaquabis[malonato(1-)-κ²O,O′] zinc(II) doped with VO(II) ion have been carried out at X-band frequencies and at 300 K. The single crystal, rotated along the three mutually orthogonally axes, has yielded spin-Hamiltonian parameters g and A as: g_xx=1.980, g_yy=1.972, g_zz=1.937 and A_xx=8.4, A_yy=6.1, A_zz=18.1 mT, respectively. These spin-Hamiltonian parameters reflect a slight deviation from axial symmetry to rhombic, which is elucidated by the interstitial occupation of vanadyl ions. The isofrequency plots and powder EPR spectrum have been simulated. The percentage of metal-oxygen bond has been estimated. The optical absorption spectrum exhibits four bands at 257, 592, 720 and 764 nm suggesting a C_4v symmetry. The admixture coefficients and bonding parameters have also been calculated by collaborating EPR data with optical data.     

Improved dielectric properties of (Pb,Ca)TiO3 thin films prepared by metal-organic decomposition method

(Pb,Ca)TiO₃ (PCT) thin films have been deposited on Pt/Ti/SiO₂/Si substrate by metal-organic decomposition (MOD) technique. The film processing parameters such as drying and annealing temperatures have been optimized to obtain good-quality PCT films. Compositional analysis of the film has been studied by X-ray photoelectron spectroscopy (XPS). The effect of the annealing temperature on the crystalline structure, microstructure and electrical properties have been investigated by X-ray diffraction, atomic force microscopy (AFM) and impedance analyzer, respectively. Amorphous PCT films form at 350 °C and crystallize in the perovskite phase following the isothermal annealing at ?650 °C for 3 h in oxygen ambient. Typical tetragonal structure of the PCT film is evidenced from X-ray diffraction pattern. The grain size in the PCT films increases with an increase in annealing temperature. Significant improvement in the dielectric constant value is observed as compared to other reported work on PCT films. The observed dielectric constant and dissipation factor at 100 kHz for 650 °C annealed PCT films are 308 and 0.015, respectively. The correlation of the film microstructural features and electrical behaviors is described.     

Influence of lithium phosphate concentration and temperature on the electrical conduction of (76V2O5-24P2O5)1−X (Li3PO4)X glasses

Characterization of the (76V₂O₅-24P₂O₅)_1−X (Li₃PO₅)_X, where X=0.0,0.01,0.02,0.10 and 0.15, glass has been done using X-ray diffraction and differential thermal analysis (DTA). The dc conductivity of the glass samples was studied over a temperature range from 300 to 593 K. The temperature dependence of dc conductivity shows two regions. One at relatively high temperature range, above θ_D/2, and the other at relatively low temperature range, below θ_D/2. The I-V characteristics of the glasses have been studied as a function of both temperature and Li₃PO₄ content. The I-V characteristics exhibits threshold switching with differential negative resistance. It's found that both the threshold voltage (V_th) and threshold current (I_th) are dependent on the temperature and lithium phosphate concentration.     

Synthesis and VUV spectral properties of nanoscaled Zn2SiO4:Mn green phosphor

Nanoscaled Zn₂SiO₄:Mn²⁺ green phosphor with regular and uniform morphology was synthesized by hydrothermal method at a low temperature of 140 °C. The structure and morphology of the phosphor was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The effects of the hydrothermal temperature and the time on the crystallite structure and the vacuum ultraviolet (VUV) photoluminescence (PL) properties were evaluated. The as-synthesized nanoscaled Zn₂SiO₄:Mn²⁺ phosphor exhibited intensive broad emission around 523 nm, which was attributed to the ⁴T₁→⁶A₁ transition of Mn²⁺. The PL intensity increased along with the increasing hydrothermal temperature and time. The heat-treated phosphors exhibited higher PL intensity than the corresponding samples prepared using the conventional solid-state reaction.     

Structural and optical properties of chemically synthesized monodispersed CdCr2S4 films

Alkaline chemical synthesis of amorphous CdCr₂S₄ (CCS) thin films of different thicknesses using cadmium chloride, chromic acid, disodium salt of ethylenediaminetetra acetic acid and thiourea precursors is reported, and the structural and surface morphological properties of CCS using X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM) techniques are discussed. Films of aggregated grains with some void spaces are obtained. Change in band gap energy and electrical resistivity of CCS films are discussed as a function of film thickness. n-type conductivity is confirmed from the sign of thermally generated voltage across the cold and hot junctions.     

Preparation and characterization of the layered hybrids (CnH2n+1NH3)2FeCl4

The series of the hybrids (C_nH_2n+1NH₃)₂FeCl₄, where n=2, 4, 6, 8, 10 and 12, were synthesized in HCl solutions. The optimum conditions of synthesis were investigated including the sequence of adding reactants, the stoichiometric ratio of the reactants, the reaction time and the drying procedures. The results demonstrate that the order of adding reactants, the drying temperature and drying time have little effect on the structures of the products. But for the hybrids of n?10, longer reaction time is needed to form the layered structures. The X-ray diffraction (XRD) patterns and the images of scanning electron microscopy of the products demonstrate that the hybrids crystallize well with typical layered structures. Chemical analysis of C, H and N confirms that the hybrids consist of the elements in the ratio of the molecular formulas. The d values, the interlayer distance between the two adjacent inorganic sheets of each hybrid, are obtained from the XRD patterns and they suggest that the organic chains interdigitate between the inorganic sheets. The longer the organic chains are, the more interdigitations there are. This can be explained by the different interaction strengths between the organic species when the chain lengths change. These different interactions can also explain the phenomena observed in the synthesis and the decomposing temperatures of the hybrids.     

Electrical conduction and optical properties of amorphous (Sb2Se3)100−xSnx thin films

The variation of DC electrical conductivity and the optical properties of thermally evaporated a- (Sb₂Se₃)_100−xSn_x thin films with temperature have been studied. It is found that the thermal activation energy decreases, while the optical gap first increases (up to x=1) and then decreases, with the increase in Sn content. These results have been explained by taking into consideration the structural modifications induced by the incorporation of Sn into the parent alloy. The variation in the conductivity prefactor (σ_o) with Sn addition indicates a change in the dominant conduction transport mechanism from extended states to localized states. An experimental correlation between the activation energy and the pre-exponential factor has been observed, indicating the validity of Meyer–Neldel rule in the studied samples.     

Microstructural and optical properties of strain compensated InxGa1−xAs/InyAl1−yAs multiple quantum wells

Transmission electron microscopy (TEM) and photocurrent (PC) measurements were carried out to investigate the microstructural and excitonic transitions in In_0.52Ga_0.48As/In_0.55Al_0.45As multiple quantum wells (MQWs). TEM images showed that high-quality 11-period strain-compensated In_0.52Ga_0.48As/In_0.55Al_0.45As MQWs had high-quality heterointerfaces. Based on the TEM results, a possible crystal structure for the In_0.52Ga_0.48As/In_0.55Al_0.45As MQWs is presented, and their strains are compensated. The results for the PC data at 300 K for several applied electric fields showed that several excitonic transitions shifted to longer wavelengths as the applied electric field increased. These results indicate that the strain-compensated In_0.52Ga_0.48As/In_0.55Al_0.45As MQWs hold promise for electroabsorption modulator devices.     

EPR investigations of oxidation effects in (V1−xMox)2−δO3

Electron paramagnetic resonance (EPR) investigations has been carried out on the new family of molybdenum doped vanadium sesquioxides (V_1−xMo_x)_2−δO₃. The oxidation effects were monitored from the rate of paramagnetic V⁴⁺ created when the sample is exposed to the air. The effects of the oxidation time, sample temperature, and annealing at 1000 °C under a diluted hydrogen atmosphere on the EPR signal features are analyzed. The V⁴⁺ concentration in the oxidized samples is determined and the relaxation effects driven by the conduction electrons are pointed out from the thermal behaviour of the EPR line features. EPR spectra of all the oxidized samples also reveal a small ferromagnetic contribution strongly correlated with the V⁴⁺ content.     

Photoluminescence of Y2O3:Eu thin film phosphors by sol-gel deposition and rapid thermal annealing

Y₂O₃:Eu³⁺ phosphor films have been developed by using the sol-gel process. Comprehensive characterization methods such as Photoluminescent (PL) spectroscopy, X-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) spectroscopy were used to characterize the Y₂O₃:Eu³⁺ phosphor films. In this experiment, the XRD profiles show that the Y₂O₃:Eu³⁺ phosphor films crystallization temperature and optimum annealing temperature occur at about 650 and 750 °C, respectively. The optimum dopant concentration is 12 mol% Eu³⁺ and the critical transfer distance (R_c) among Eu³⁺ ions is calculated to be about 0.84 nm. Vacuum environment is more efficient than oxygen and nitrogen to eliminate the OH content and hence yields higher luminescent phosphor films. The PL emission intensity of Y₂O₃:Eu³⁺ phosphor films is also dependent on the annealing time. It was found that the H₂O impurities were effectively eliminated after annealing time of 25 s at 750 °C in vacuum environment. From the experiment results, the schematic energy band diagram of Y₂O₃:Eu³⁺ phosphor films is constructed.     

Studies on the influence of lithium incorporation in the photoluminescence of Y2O3:Eu thin films

Nanostructured europium-doped yttrium oxide thin films with lithium as a co-dopant were prepared using pulsed laser ablation technique. X-ray diffraction studies of the films indicated amorphous nature of the as deposited films and a transformation to crystalline phase with increase of annealing temperature. In this transformation, lithium co-doped films showed early crystallization. Lithium substitution resulted not only in enhancement of photoluminescence at 612 nm, resulting from ⁵D₀-⁷F₂ transition within europium, but also found to reduce the required processing temperature for intense photoemission. The deviation observed in the value of lattice constant of films annealed at different temperatures is found to be sensitive to annealing temperature. In the light of this, the dependence of photoluminescence intensity on the magnitude of lattice imperfection is also discussed. The morphology and transmittance of the films are also found to be sensitive to annealing process and lithium doping.     

Electrical and electrochemical properties of molten salt-synthesized Li4Ti5−xSnxO12 (x=0.0, 0.05 and 0.1) as anodes for Li-ion batteries

Submicron-sized polyhedral Li₄Ti_5−xSn_xO₁₂ (x=0.0, 0.05, and 0.1) materials were successfully prepared by a single-step molten salt method. The structural, morphological, transport and electrochemical properties of the Li₄Ti_5−xSn_xO₁₂ were studied. X-ray diffraction patterns showed the formation of a cubic structure with a lattice constant of 8.31 Å, and the addition of dopants follows Vegard's law. Furthermore, FT-IR spectra revealed symmetric stretching vibrations of octahedral groups of MO₆ lattice in Li₄Ti₅O₁₂. The formation of polyhedral submicron Li₄Ti_5−xSn_xO₁₂ particles was inferred from FE-SEM images, and a particle size reduction was observed for Sn-doped Li₄Ti₅O₁₂. The chemical composition of Ti, O and Sn was verified by EDAX. The DC electrical conductivity was found to increase with increasing temperature, and a maximum conductivity of 8.96×10⁻⁶ S cm⁻¹ was observed at 200 °C for Li₄Ti₅O₁₂. The galvanostatic charge–discharge behavior indicates that the Sn-doped Li₄Ti₅O₁₂ could be used as an anode for Li-ion batteries due to its enhanced electrochemical properties.     

Electrical and optical properties of GexFexSe100−2x chalcogenide thin films

Optical absorption at room temperature and electrical conductivity at temperatures between 283 and 333 K of vacuum evaporated Ge_xFe_xSe_100−2x (0≤x≤15) amorphous thin films have been studied as a function of composition and film thickness. It was found that the optical absorption is due to indirect transition and the energy gap increases with increasing both Ge and Fe content; on the other hand, the width of the band tail exhibits the opposite behavior. The optical band gap E_opt was found to be almost thickness independent. The electrical conductivity show two types of conduction, at higher temperature the conduction is due to extended states, while the conduction at low temperature is due to variable range hopping in the localized states near Fermi level. Increasing Ge and Fe contents were found to decrease the localized state density N(E_F), electrical conductivity and increase the activation energy for conduction, which is nearly thickness independent. Variation of the atomic densities ρ, molar volume V, glass transition temperature T_g cohesive energy C.E and number of constraints N_Co with average coordination number Z was investigated. The relationship between the optical gap and chemical composition is discussed in terms of the cohesive energy C.E, average heat of atomization and coordination numbers.     

Synthesis,photophysical properties,and photocatalytic applications of Bi doped NaTaO3 and Bi doped Na2Ta2O6 nanoparticles

Phase formation and photophysical properties of bismuth doped sodium tantalum oxide (perovskite, defect pyrochlore) nanoparticles prepared by a hydrothermal method were studied in detail. It was revealed that the synthesis conditions like NaOH concentration and bismuth precursor (NaBiO₃·2H₂O) markedly affect the crystal structure of sodium tantalum oxide. At low NaOH concentration and high bismuth precursor (NaBiO₃·2H₂O) content, Bi doped Na₂Ta₂O₆ (defect pyrochlore) phase was predominantly formed, while at higher NaOH concentration, Bi doped NaTaO₃ (perovskite) phase was formed. It was observed that the defect pyrochlore (Bi doped Na₂Ta₂O₆) phase was formed and stabilized by the presence of dopant precursor (NaBiO₃·2H₂O). The chemical analysis of the samples confirmed the doping of Bi³⁺ cations in both phases. Doping of bismuth enabled visible light absorption up to 500 nm in perovskite and defect pyrochlore type sodium tantalum oxide. Bi doped NaTaO₃ samples showed better performance for the photocatalytic degradation of rhodamine B than that of Bi doped Na₂Ta₂O₆, under visible light irritation (λ>420 nm). The present results shed light on phase formation of sodium tantalate and these results are useful in understanding properties of NaTaO₃ based compounds, synthesized by the hydrothermal method.