THERMOLUMINESCENCE SPECTRA AND DOSE RESPONSES OF SrSO4 PHOSPHORS DOPED WITH RARE EARTHS (Eu, Dy, Tm) AND PHOSPHORUS.

The thermoluminescence (TL) spectra and dose responses of strontium sulphate doped with rare earth ions show that the SrSO4:Eu2+ phosphor might reasonably be assumed a isoelectronic trap sample which has unique TL characteristics: there is only one obvious glow peak at 385 nm, 489 K in the TL 3D emission spectra and its TL dose response is linear-sublinear. However, there are several elementary glow peaks in the TL 3D emission spectra and their TL dose responses are linear-supralinear for SrSO4:RE3+ (RE = Dy, Tm). These TL peaks occurred from low to high temperature indicate that the traps are distributed in different energy levels. When a suitable amount of other impurities co-doped into these SrSO4:RE3+, such as phosphorus, the relative intensities of these elementary glow peaks are changed significantly, especially the TL peak ~500 K is enhanced while the peaks at lower temperature are suppressed. The enhanced peak of SrSO4:RE3+,P is attributed to the deep traps. Their dose responses remain in nonlinearity when co-doped phosphorus. The above results and the luminescence properties of other sulphate doped rare earths impurities illustrate that the TL characteristics depend on the structure of defect complexes which can be assumed the basic elements in the TL multi-stage processes.

Controlling the Electronic Structures of Perovskite Oxynitrides and their Solid Solutions for Photocatalysis.

Band-gap engineering of oxide materials is of great interest for optoelectronics, photovoltaics, and photocatalysis applications. In this study, electronic structures of perovskite oxynitrides, LaTiO2 N and SrNbO2 N, and solid solutions, (SrTiO3 )1-x (LaTiO2 N)x and (SrTiO3 )1-x (SrNbO2 N)x , are investigated using hybrid density functional calculations. Band gaps of LaTiO2 N and SrNbO2 N are much smaller than that of SrTiO3 owing to the formation of a N 2p band, which is higher in energy than the O 2p band. The valence- and conduction-band offsets of SrTiO3 /LaTiO2 N and SrTiO3 /SrNbO2 N are computed, and the adequacy for H2 evolution is analyzed by comparing the positions of the band edges with respect to the standard hydrogen electrode (SHE). The band gap of (SrTiO3 )1-x (LaTiO2 N)x and (SrTiO3 )1-x (SrNbO2 N)x solid solutions are also discussed.

Dosimetric investigations of Tb3+-doped strontium silicate phosphor.

Tb(3+)-doped SrSiO(3) phosphor synthesised by co-precipitation technique exhibits intense green emission due to cross-relaxation phenomena between Tb(3+) ions. Dosimetric properties of this phosphor have been investigated using thermoluminescence (TL) technique. A dosimetrically useful glow peak observed was at 581 K along with a linear dose response over the wide dose range (100 mGy-4 Gy). TL parameters such as trap depth (E), frequency factor (s) and the order of kinetics (b) are determined by different methods such as Chen's peak shape, initial rise, isothermal decay and variable heating rate methods. Results of these methods are compared and reported in this study.

Propagation characteristics of a focused laser beam in a strontium barium niobate photorefractive crystal under reverse external electric field.

The propagation characteristics of a focused laser beam in a SBN:75 photorefractive crystal strongly depend on the signal-to-background intensity ratio (R=Is/Ib) under reverse external electric field. In the range 20>R>0.05, the laser beam shows enhanced self-defocusing behavior with increasing external electric field, while it shows self-focusing in the range 0.03>R>0.01. Spatial solitons are observed under a suitable reverse external electric field for R=0.025. A theoretical model is proposed to explain the experimental observations, which suggest a new type of soliton formation due to "enhancement" not "screening" of the external electrical field.

The structure and activation of substrate water molecules in Sr(2+)-substituted photosystem II.

The mechanism of solar water oxidation by photosystem II (PSII) is of fundamental interest and it is the object of extensive studies both in the past and present. The solar water oxidation reaction of PSII occurs in the oxygen-evolving complex (OEC). The OEC consists of a tetranuclear manganese calcium-oxo (Mn4Ca-oxo) cluster that is surrounded by amino acid residues and inorganic cofactors. The role of the Ca(2+) ion in the water oxidation reaction is one of the most interesting questions that is yet to be answered. In this study, we probe the structural and functional differences induced by metal ion substitution in the Mn4Ca-oxo cluster by substituting the Ca(2+) ion in the OEC by a Sr(2+) ion. We apply two-dimensional (2D) hyperfine sublevel correlation (HYSCORE) spectroscopy to detect weak magnetic interactions between the paramagnetic Mn4Sr-oxo cluster and the surrounding protons in the S2 state of the OEC of Sr(2+)-substituted PSII. We identify three groups of protons that are magnetically interacting with the Mn4Sr-oxo cluster. Using the recently reported 1.9 Å resolution X-ray structure of the OEC in the S1 state [Umena et al.] and the high-resolution 2D HYSCORE spectroscopy studies of the S2 state of the OEC of Ca(2+)-containing PSII [Milikisiyants et al., Energy Environ. Sci., 2012, 5, 7747], we discuss the assignments of the three groups of protons that are magnetically coupled to the Mn4Sr-oxo cluster. Since hyperfine interactions are highly sensitive to small perturbations in the electronic and geometric structure of paramagnetic centers, a comparison of the 2D HYSCORE spectra of Sr(2+)-substituted and Ca(2+)-containing PSII allows us to draw important conclusions with respect to the structure of the substrate water molecules in the OEC and the role of the Ca(2+) ion in the water oxidation reaction. In addition, for the first time, we determine the experimental value of the spin projection factor for the Mn(III) ion of the Mn4Ca-oxo cluster as ρ1 = ±1.7 from the assignment of the hyperfine interaction of the paramagnetic cluster with the protons of the D1-His332 residue of PSII.

Hierarchical assembly of TiO2-SrTiO3 heterostructures on conductive SnO2 backbone nanobelts for enhanced photoelectrochemical and photocatalytic performance.

Heterostructures can play a role in enhanced photoinduced electrochemical and catalytic reactions due to the advantageous combination of two compounds. Herein, we demonstrate the fabrication of Sb:SnO2@TiO2-SrTiO3 3D heterostructures via a simple hydrothermal method using a conductive Sb:SnO2@TiO2 nanobelt electrode as a template. XRD, FESEM, and TEM analyses confirm that a well-dispersed and crystalized SrTiO3 layer is formed on the surface of TiO2 nanorods. The photoelectrochemical (PEC) performance of the heterostructure is optimized by controlling the reaction time. Details about the effect of the hydrothermal reaction time on the PEC performance are discussed. The optimized Sb:SnO2@TiO2-SrTiO3 heterostructure exhibited a higher onset potential and a saturated photocurrent in comparison to the Sb:SnO2@TiO2 nanostructure. The result is attributed to a Fermi level shift and a blocking layer effect caused by the SrTiO3. Furthermore, the photocatalytic degradation of methylene blue was significantly enhanced on the optimized Sb:SnO2@TiO2-SrTiO3. This work demonstrates that a synergetic effect between three-dimensional nanoarchitecturing and a heterojunction structure is responsible for enhanced PEC as well as improved photocatalytic performance levels, both of which can be extended to other metal-oxide and/or ternary compounds.

Direct approach for flexoelectricity from first-principles calculations: cases for SrTiO3 and BaTiO3.

Understanding the nature of flexoelectricity, which is the linear response of electric polarization to a strain gradient, has recently become crucial for nanostructured dielectrics and ferroelectrics because of their complicated strain distribution. This paper presents a direct and full approach at the atomic level to predict flexoelectricity for dielectrics based on first-principles calculations. The flexoelectric coefficients of BaTiO3 and SrTiO3 are directly calculated as the representatives of ferroelectric and paraelectric materials, respectively. For SrTiO3, the flexoelectric coefficients predicted from our approach are in good agreement with the experimental measurements. For BaTiO3, our predictions have a large discrepancy from the experimental measurements. In a practical situation, defect and surface effects are inevitable, and have a significant influence on the flexoelectricity. Direct methods have the advantage of including the extrinsic contributions from surface and defect effects.

Polarization dependence of the self-organized microgratings induced in SrTiO3 crystal by a single femtosecond laser beam.

In this paper, self-organized microgratings are fabricated in SrTiO(3) crystal just by scanning the focus of a tightly-focused linearly-polarized femtosecond laser beam to form a single line. The polarization direction of the laser beam is rotated by a λ/2 waveplate to check the effect of the polarization azimuth on the micrograting morphology. Fourier analyzing of the microscopic images of the microgratings indicates that the polarization plane azimuth of the laser beam does have influence on the microgratings in the aspects of groove orientation and groove spacing. A possible mechanism of polarization dependence is also proposed.

Unusual ultraviolet photoconductivity in single crystalline SrTiO3.

In this paper, ultraviolet photoconductivity of single crystalline SrTiO(3) is studied. It is found that the photoconductivity of SrTiO(3) arises from intrinsic band-to-band excitation, and photoconductivity increases monotonously with decreasing temperature and undergoes six or seven orders of magnitude enhancement from room temperature to 10 K. In particular, at and below 105 K, the structural transition temperature, SrTiO(3) is highly ultraviolet-sensitive, and the photoconductivity increases drastically so that a TCR (temperature coefficient of resistance) ∼30% is observed. Moreover, it is the first time that an anomaly is observed in photoconductivity at around 65 K and 37 K respectively. These results will shed light on our understanding of the band structure of tetragonal-phase SrTiO(3), and the physics of SrTiO(3) at lower temperatures.

Nanoscale strontium titanate photocatalysts for overall water splitting.

SrTiO(3) (STO) is a large band gap (3.2 eV) semiconductor that catalyzes the overall water splitting reaction under UV light irradiation in the presence of a NiO cocatalyst. As we show here, the reactivity persists in nanoscale particles of the material, although the process is less effective at the nanoscale. To reach these conclusions, Bulk STO, 30 ± 5 nm STO, and 6.5 ± 1 nm STO were synthesized by three different methods, their crystal structures verified with XRD and their morphology observed with HRTEM before and after NiO deposition. In connection with NiO, all samples split water into stoichiometric mixtures of H(2) and O(2), but the activity is decreasing from 28 µmol H(2) g(-1) h(-1) (bulk STO), to 19.4 µmol H(2) g(-1) h(-1) (30 nm STO), and 3.0 µmol H(2) g(-1) h(-1) (6.5 nm STO). The reasons for this decrease are an increase of the water oxidation overpotential for the smaller particles and reduced light absorption due to a quantum size effect. Overall, these findings establish the first nanoscale titanate photocatalyst for overall water splitting.

Ultraviolet photodetector with high internal gain enhanced by TiO2/SrTiO3 heterojunction.

In this letter, TiO2 nanocrystalline film was prepared on SrTiO3 (001) substrate to form an n-n heterojunction active layer. Interdigitated Au electrodes were deposited on the top of TiO2 film to fabricate modified HMSM (heterojunction metal-semiconductor-metal) ultraviolet photodetector. At 10 V bias, the dark current of the detector was only 0.2 nA and the responsivity was 46.1 A/W at 260 nm. The rise and fall times of the device were 3.5 ms and 1.4 s, respectively. The TiO2/SrTiO3 heterojunction contributed a lot to the high responsivity and reduced the fall time, which improved the device performance effectively. These results demonstrate the excellent application of TiO2/SrTiO3 heterojunction in fabricating high performance UV photodetectors.

Terahertz dielectric response of ferroelectric Ba(x)Sr(1-x)TiO3 thin films.

Terahertz time-domain spectroscopy has been used to investigate the dielectric and optical properties of ferroelectric Ba(x)Sr(1-x)TiO(3) thin films for nominal x-values of 0.4, 0.6, and 0.8 in the frequency range of 0.3 to 2.5 THz. The ferroelectric thin films were deposited at approximately 700 nm thickness on [001] MgO substrate by pulsed laser deposition. The measured complex dielectric and optical constants were compared with the Cole-Cole relaxation model. The results show that the Cole-Cole relaxation model fits well with the data throughout the frequency range and the dielectric relaxation behavior of ferroelectric Ba(x)Sr(1-x)TiO(3) thin films varies with the films compositions. Among the compositions of Ba(x)Sr(1-x)TiO(3) films with different Ba/Sr ratios, Ba(0.6)Sr(0.4)TiO(3) has the highest dielectric constants and the shortest dielectric relaxation time.

Combinatorial search for iron/titanium-based ternary oxides with a visible-light response.

A combinatorial approach has been carried out to systematically investigate visible-light responsiveness of Fe-Ti-M (M: various metal elements) oxides for photoelectrochemical water splitting. Among the 25 elements tested, strontium was the most effective. A ternary metal oxide with the composition Fe(86.1)Ti(9.6)Sr(4.3)O(x) has been identified as a new lead structure for a visible-light responsive, n-type semiconductor. We have conducted various kinds of characterization of the Fe-Ti-Sr oxide semiconductor and discussed the reason why Sr in the Fe-Ti oxide gave the highest photocurrent.

Syntheses and characterization of Sr(OH)2 and SrCO3 nanostructures by ultrasonic method.

The Sr(OH)(2) and SrCO(3) nanostructures were synthesized by reaction of strontium(II) acetate and sodium hydroxide or tetramethylammonium hydroxide (TMAH) via ultrasonic method. Reaction conditions, such as the concentration of the Sr(2+) ion, aging time, power of the ultrasonic device and alkali salts show important roles in the size, morphology and growth process of the final products. The pure crystalline SrCO(3) were obtained by heating of product at 400 degrees C. The Sr(OH)(2) and SrCO(3) nanostructures were characterized by scanning electron microscopy (SEM), X-ray powder diffraction (XRD), thermal gravimetric (TG), differential thermal analyses (DTA) and the infrared spectroscopy (IR).

(86)Y production via (86)Sr(p,n) for PET imaging at a cyclotron.

Excitation functions of (86)Y production via (86)Sr(p,xn), (86)Sr(d,xn), (85)Rb(alpha,xn), (85)Rb((3)He,xn), and (nat)Zr(d,alphaxn) reactions were studied by means of ALICE-ASH code and the results were compared with ALICE-91 code and experimental data. The greatest nuclear reaction of cyclotron (86)Y production was found out as (86)Sr(p,n)(86)Y process. (86)Y production yield was calculated too. A SrCO(3) thick film was deposited on a copper substrate by sedimentation method. The deposited (nat)SrCO(3) was irradiated with 15MeV proton at 30microA current beam. The separation of Y from Cu and Sr was carried out by means of dual ion exchange chromatography.

Targetry of SrCO(3) on a copper substrate by sedimentation method for the cyclotron production no-carrier-added (86)Y.

Strontium carbonate deposition on copper substrate was carried out by the sedimentation method in order to produce yttrium-86. Natural strontium carbonate thick layer was prepared with 480 mg SrCO(3), 220 mg ethyl cellulose, and 7.5 mL acetone. This optimum condition is a result of several repeated experiments with different amount of ethyl cellulose and acetone. Target quality control was done by SEM photomicrograph and thermal shock test. The deposited target was irradiated at 30 microA current and 15 MeV proton beam for 12 min.

Dielectric properties of Ba0.6Sr0.4TiO3-Sr(Ga0.5Ta0.5)O3 solid solutions.

Ba(0.6)Sr(0.4)TiO(3)-Sr(Ga(0.5)Ta(0.5))O(3) solid solutions are prepared by a solid-state reaction method, and their dielectric and tunable characteristics are investigated. The solid solutions with cubic perovskite structures are obtained for compositions of 10-50 mol% Sr(Ga(0.5)Ta(0.5))O(3). It is observed that the addition of Sr(Ga(0.5)Ta(0.5))O(3) into Ba(0.6)Sr(0.4)TiO(3) causes a shift in the phase transition peak to a lower temperature. Ba(0.6)Sr(0.4)TiO(3)-Sr(Ga(0.5)Ta(0.5))O(3) solid solutions exhibit depressed and broadened phase transition peaks, resulting in decreased dielectric constants and dielectric losses at room temperature. With the increase of Sr(Ga(0.5)Ta(0.5))O(3) content, the dielectric constant, loss tangent, and tunability are decreased. 0.9Ba(0.6)Sr(0.4)TiO(3)-0.1Sr(Ga(0.5)Ta(0.5))O(3) has a dielectric constant epsilon = 534 and a tunability of 16% at 100 kHz under 2.63 kV/mm. The dielectric characteristics of Ba(0.6)Sr(0.4)TiO(3)-Sr(Ga(0.5)Ta(0.5))O(3) ceramics at microwave frequencies are also evaluated.

Refractive index mismatch and monomer reactivity influence composite curing depth.

Limited cure depth is a drawback of light-activated composites. We hypothesize that curing light transmission and cure depth are influenced by monomer reactivity and filler/resin refractive index mismatch. Light transmission throughout cure was recorded for composites based on strontium (refractive index 1.51) or barium (refractive index 1.53) glass fillers. Fillers were mixed (70 wt%) with 4 bisphenol-A diglycidyl-ether-dimethacrylate (bis-GMA):triethylene glycol dimethacrylate (TEGDMA) formulations with refractive indices ranging from 1.4703 to 1.5370. Following polymerization, cure depth and pre- and post-cure translucency parameters were determined. Transmission changes and cure depths related to monomer reactivity and filler/resin refractive index mismatch with significant interaction. Composites became more opaque or translucent on curing. Optimizing filler/resin refractive index mismatch provides increased curing depth and assists shade-matching.

Creation of multiple nanodots by single ions.

In the search to develop tools that are able to modify surfaces on the nanometre scale, the use of heavy ions with energies of several tens of MeV is becoming more attractive. Low-energy ions are mostly stopped by nuclei, which causes the energy to be dissipated over a large volume. In the high-energy regime, however, the ions are stopped by electronic excitations, and the extremely local (approximately 10 nm3) nature of the energy deposition leads to the creation of nanosized 'hillocks' or nanodots under normal incidence. Usually, each nanodot results from the impact of a single ion, and the dots are randomly distributed. Here we demonstrate that multiple, equally spaced dots, each separated by a few tens of nanometres, can be created if a single high-energy xenon ion strikes the surface at a grazing angle. By varying this angle, the number of dots, as well as their spacing, can be controlled.

Evolution of nanodomains in the uniaxial relaxor Sr0.61Ba0.39Nb2O6:Ce.

The evolution of the nanodomain pattern of the uniaxial relaxor ferroelectric strontium barium niobate doped with cerium was studied by piezoresponse force microscopy (PFM). The fractal-like nanodomains observed at room temperature decay on heating. At temperatures up to about 15 K above the Curie temperature, Tc = 320 K, areas of correlated polarization are still visible. On cooling from the paraelectric state to below Tc, a slow isothermal growth of nanodomain was found. The mean domain size increases according to a logarithmic law as predicted for the three-dimensional random field Ising model.