DFT study of carbon monoxide adsorption on α-Al2O3(0001)

The adsorption of CO on α-Al₂O₃(0001) was studied using the DFT-GGA computational method and on α-Al₂O₃ powder experimentally by Infra red spectroscopy. The core and valence level regions of α-Al₂O₃(0001) single crystal surface were also studied experimentally. Ar ions sputtering of the surface results in a slight but reproducible decrease in the XPS O2p lines in the valence band regions due to preferential removal of surface (and near surface) O atoms. Core level XPS O1s and Al2p further confirmed oxygen depletion with an associated surface stoichiometry close to Al₂O_2.9. The adsorption energy of CO was computed and found equal to 0.52 eV for θ = 0.25, it decreased to 0.42 eV at θ = 1. The IR frequency of νCO was also computed and in all cases it was blue shifted with respect to gas phase CO. The shift, Δν, decreased with increasing coverage where it was found equal to 56 cm^? 1 for θ = 0.25 and decreased to 30 cm^? 1 for θ = 1. Structural analyses indicated that the change in the adsorption energy and the associated frequency shift is due to surface relaxation upon adsorption. Experimentally the adsorption of CO gave rise to one main IR peak at 2154 cm^? 1 at 0.3 Torr and above. Two far smaller peaks are also seen at lower pressures of 0.03–0.2 Torr at 2189 and 2178 cm^? 1. The isosteric heat of adsorption was computed for the IR band at 2154 cm^? 1 and was found equal to 0.2 eV which did not change with coverage in the investigated range up to θ = 0.6.     

Optical and physical characteristics of In-doped ZnO nanorods

In this paper the effect of indium dopants on structure, optical, electrical and mechanical properties of ZnO nanorods are studied. The average surface potentials and the surface currents of ZnO:In nanorods were 0.25–0.84 mV and 2.2–200 MΩ-cm, respectively. The turn-on threshold field for the vertical ZnO nanorods was around 2–16 V μm⁻¹. Emission current densities of 3.3–911.4 mA cm⁻² were obtained for an electrical field of 60–160 V μm⁻¹. The photoluminescence (PL) spectrum measured at 15–300 K showed that the intensity of the peak at 2.06 eV increased with decreasing temperature, while the peak at 2.06 eV further red shifted and the peak at 3.39 eV blue shifted.     

Ultraviolet spectroscopy of Pr+3 and its use in making ultraviolet filters

Sputtered deposited thin films of AlN:Pr and GaN:Pr emit in ultraviolet–visible and visible regions of the spectrum, respectively, under electron excitation in cathodoluminescence apparatus. The goal is to study the ultraviolet emission from Pr⁺³ when doped in nitride semiconductor hosts. Luminescence peaks at a wavelength of 295 nm (4.2 eV), 335 nm (3.7 eV) and 385 nm (3.24 eV) are observed as a result from ¹S₀ → ¹G₄, ¹S₀ → ¹D₂ and ¹S₀ → ¹I₆ transitions, respectively. However the ¹S₀ → ¹G₄ and ¹S₀ → ¹D₂ transitions are not observed when Pr⁺³ is doped in GaN host. The bandgap of GaN absorbs the ultraviolet radiation emitted from Pr⁺³ and hence GaN can be used as ultraviolet filter for radiation shielding and protection purposes. AlN is transparent to ultraviolet due to its wide bandgap of 6.2 eV.     

Lithium-ionic diffusion and electrical conduction in the Li7La3Ta2O13 compounds

The electrical properties and the mechanism of lithium ionic diffusion in the Li₇La₃Ta₂O₁₃ compounds were investigated. The bulk and total conductivity at 300 K of the Li₇La₃Ta₂O₁₃ compound are about 3.3 × 10^? 6 S/cm and 2.6 × 10^? 6 S/cm, respectively. The activation energy of bulk and total conductivity is in the range of 0.38–0.4 eV. A prominent internal friction peak in Li₇La₃Ta₂O₁₃ compounds was observed around 280 K at 0.5 Hz, which is actually composed of two subpeaks (P₁ peak at lower temperature and P₂ peak at higher temperature). From the shift of peak position with frequency, the activation energy of 1.0 eV and the pre-exponential factor of relaxation time in the order of 10^? 18–10^? 21 s were obtained if one assumes Debye relaxation processes. These values of relaxation parameters strongly suggest the existence of interaction between the relaxation species (here lithium ions or vacancies). Based on the coupling model, the relaxation activation energies are deduced as 0.45 eV and the pre-exponential factor of relaxation time as 10^? 15 s. Judging from these relaxation parameters and the similarity of structure between Li₇La₃Ta₂O₁₃ and Li₅La₃Ta₂O₁₂ compounds, the P₁ and P₂ peaks are suggested to be related with the lithium ionic diffusion between 48g?48g and 24d?48g.     

Energy relaxation through the π*-state in C,F and O K-shell excited CF3COCH3

Total photoabsorption spectra of CF₃COCH₃ were measured in the C, F and O K-shell regions and the peak assignments were tentatively given. The K-shell electrons of C, F and O atoms were selectively excited into the π^* orbital. The kinetic energy (KE) distribution of CF₃⁺ formed through the π^* states gave the maxima at KE = 0 and 0.43 eV. The yield of CF₃⁺ with KE = 0 eV increased from 10 to 50% by changing the excitation sites from F 1s to O 1s. This finding was reasonably understood by considering that intramolecular energy flows from the initially excited K-shell electron to vibrational modes of CF₃ group. The KE distribution of CH₃⁺ showed a mirror image of that for CF₃⁺.     

Oxygen diffusion and surface exchange studies on (La0.75Sr0.25)0.95Cr0.5Mn0.5O3−δ

Oxygen tracer diffusion coefficient (D^⁎) and surface exchange coefficient (k^⁎) have been measured for (La_0.75Sr_0.25)_0.95Cr_0.5Mn_0.5O_3−δ using isotopic exchange and depth profiling by secondary ion mass spectrometry technique as a function of temperature (700–1000 °C) in dry oxygen and in a water vapour-forming gas mixture. The typical values of D^⁎ under oxidising and reducing conditions at ∼ 1000 °C are 4 × 10^− 10 cm² s^− 1 and 3 × 10^− 8 cm² s^− 1 respectively, whereas the values of k^⁎ under oxidising and reducing conditions at ∼ 1000 °C are 5 × 10^− 8 cm s^− 1 and 4 × 10^− 8 cm s^− 1 respectively. The apparent activation energies for D^⁎ in oxidising and reducing conditions are 0.8 eV and 1.9 eV respectively.     

Characterization of luminescent chromophore obtained from silica spheres

Blue light emitting chromophores have been separated from silica spheres by soaking them into acetone for 120 days. The luminescent chromophores were not obtained from other solvents, including ether, methanol, ethanol, 2-propanol, chloroform and tetrahydrofuran. According to the Fourier transform infrared spectrum, the luminescent material is composed of C–OH, –CH₂, –CH₃, C=O, and Si–O–Si. UV–visible absorption peak of the chromophore is at 5.17 eV (240 nm). Field emission scanning electron microscope images show small cracks on the surface of aged spheres. The luminescence peak was at 2.81 eV (441 nm) for excitation energy between 3.88 and 3.35 eV and slightly shifted toward lower energy for excitation energy lower than 3.35 eV. The deconvoluted luminescent spectrum shows two emission bands at 3.08 and 2.74 eV, which are well-matched the oxygen deficient center model. Compared to the absorption peak (5.17 eV) and the emission peak (2.81 eV), large Stokes shift (2.36 eV) is observed.     

Structural and optical properties of La-doped BaSnO3 thin films grown by PLD

In this study the structural and optical properties of lanthanum-doped BaSnO₃ powder samples and thin films deposited on fused silica were investigaed using laser ablation. Under an oxygen pressure of 5×10⁻⁴ mbar, phase pure BaSnO₃ films with a lattice constant of 0.417 nm and grain size of 21 nm were prepared at 630 °C. The band gap of BaSnO3 powder sample and thin films was calculated to be 3.36 eV and 3.67 eV, respectively. There was a progressive increase in conductivity for thin films of BaSnO₃ doped with 0~7 at% of La. The highest conductivity, 9 Scm⁻¹, was obtained for 7 at% La-doped BaSnO₃. Carrier concentration, obtained from Burstein-Moss (B-M) shift, nearly matches the measured values except for 3 at% and 10 at% La-doped BaSnO₃ thin films.     

Electrical and structural investigations of perovskite structure reactively sputter deposited coatings

Sr(Zr_0.84Y_0.16)_0.91O_3 ? δ (SZY) and Ba(Zr_0.84Y_0.16)_0.96O_3 ? δ (BZY) protonic conductor coatings were co-sputter deposited from metallic targets in argon–oxygen reactive gas mixtures. The chemical and structural features were investigated by energy dispersive X-ray spectroscopy and X-ray diffraction, and their morphology was assessed by scanning electron microscopy of the surface and of brittle fracture cross sections. The electrical properties of the coating were determined by complex impedance spectroscopy as a function of temperature in air. Relationships are established between the electrical properties and the morphology of the coatings. The SZY as deposited coatings is amorphous and crystallises under the convenient perovskite structure after annealing treatment at 873 K under air. The BZY as deposited coatings is crystallised at 523 K in situ under perovskite structure and a further annealing treatment increases the grain size. Conductivities and activation energies of crystallised coatings were 3.1 · 10^? 5 S cm^? 1/2 · 10^? 5 S cm^? 1 and 0.65 eV/0.71 eV after stabilization at 773 K for strontium and barium zirconate, respectively.     

Electret properties of ethylene–propylene random co-polymer

Thermally stimulated current (TSC) spectra were examined for ethylene–propylene (EP) random co-polymer at different charging voltages V_p with positive and negative polarities. Observed TSC spectra showed two well-separated TSC bands, B_L and B_H, which respectively appeared in the temperature regions below and above 100 °C. Observed V_p dependence of B_L was quite different from that of typical polypropylene homo-polymer: As V_p increased, B_L band grew keeping its peak position same at 65 °C, and the band shape unchanged, as if the traps responsible for the B_L band are a single set of traps with the same trap depth and capture cross section. The trap depth of B_L was about 1.9 eV and 1.7 eV for positively charged EP and talc-containing EP samples, respectively. EP samples also showed unique TSC bands above 100 °C: one is a narrow TSC band peaked at 120 °C and the other is an unusual TSC band which was non-vanishing even at 165 °C just before destruction of samples by their melting. Consequently, the utmost stable charge density in EP co-polymer above 100 °C was found to be 3.5 × 10^?4 C/m² and 6.0 × 10 ^?4 C/m² for positively and negatively charged samples, respectively. These equivalent surface charge densities are much larger than those of usual polypropylene homo-polymer.     

On the origin of blue light emission from Ge-nanocrystals containing a-SiOxfilms

We have studied the photoluminescence of a-Si_xGe_yO_{1 − x − y}films with average Ge-nanocrystal sizes ranging from over 100 nm down to 2 nm. No systematic peak shift of the luminescence bands at 3.0 eV and 2.0 eV with the diameter of the nanocrystals is observed. Comparision with a simplified confinement model shows that quantum size effects cannot explain the blue luminescence. We propose the Ge²₀defect as a likely source for this band, based on considerations about the crystallization process.     

A time-resolved luminescence spectroscopy study of non-linear optical crystals K2Al2B2O7

We report the results of complex study of luminescence and dynamics of electronic excitations in K₂Al₂B₂O₇ (KABO) crystals obtained using low-temperature luminescence-optical vacuum ultraviolet spectroscopy with sub-nanosecond time resolution under selective photoexcitation with synchrotron radiation. The paper discusses the decay kinetics of photoluminescence (PL), the time-resolved PL emission spectra (1.2–6.2 eV), the time-resolved PL excitation spectra and the reflection spectra (3.7–21 eV) measured at 7 K. On the basis of the obtained results three absorption peaks at 4.7, 5.8 and 6.5 eV were detected and assigned to charge-transfer absorption from O^2? to Fe³⁺ ions; the intrinsic PL band at 3.28 eV was revealed and attributed to radiative annihilation of self-trapped excitons, the defect luminescence bands at 2.68 and 3.54 eV were separated; the strong PL band at 1.72 eV was revealed and attributed to a radiative transition in Fe³⁺ ion.     

Modifications in physical,optical and electrical properties of tin oxide by swift heavy Au8+ ion bombardment

Tin oxide thin films were deposited by a novel technique called as modified-SILAR. The preparative parameters were optimized to obtain good quality thin films. As-deposited films were annealed in O₂ atmosphere for 1 h at 500 °C. The annealed films were irradiated using Au⁸⁺ ions with energy of 100 MeV at different fluencies of 1 × 10¹¹, 1 × 10¹², 5 × 10¹² and 1 × 10¹³ ions/cm² using tandem pelletron accelerator. The irradiation-induced modifications in tin oxide thin films were studied using XRD, AFM, optical band gap, photoluminescence and I–V measurements. XRD studies showed formation of tin oxide with tetragonal structure. AFM revealed uniform deposition of the material with increase in grain size after irradiation. Decrease in band gap from 3.51 eV to 2.82 eV was seen with increases in fluency. A decrease in PL intensity, and an additional peak was observed after irradiation. I–V measurements showed a decrease in resistance with fluency.     

Electronic excitations and luminescence of SrAlF5 crystals doped with Ce3+ ions

This paper reports the results of a time-resolved photoluminescence and energy transfer processes study in Ce³⁺ doped SrAlF₅ single crystals. Several Ce³⁺ centers emitting near 4 eV due to 5d-4f transitions of Ce³⁺ ions substituting for Sr²⁺ in non-equivalent lattice sites were identified. The lifetime of these transitions is in the range of 25–35 ns under intra-center excitation in the energy region of 4–7 eV at T = 10 K. An effective energy transfer from lattice defects to dopant ions was revealed in the – 7–11 eV energy range. Both direct and indirect excitation channels are efficient at room temperature. Excitons bound to dopants are revealed at T = 10 K under excitation in the fundamental absorption region above 11 eV, as well as radiative decay of self-trapped excitons resulting in luminescence near 3 eV.     

Experimental and theoretical studies on bis(glycine) lithium nitrate (BGLiN): A physico-chemical approach

Good quality and bulk size single crystal (size: 20×13×8 mm³) of bis(glycine) lithium nitrate (BGLiN) was grown by a slow evaporation solution technique from the aqueous solutions at constant temperature i.e. 27 °C using synthesized materials. Crystal system and lattice parameters were determined by single crystals as well as powder X-ray diffraction analysis. The lattice parameters of the titled compound are a=10.0223 Å, b=5.0343 Å, c=17.0510 Å, and V=860.312 Å³ and it crystallized in an orthorhombic system with space group Pca2₁ obtained by single crystal XRD. Elemental composition was confirmed by energy dispersive X-ray spectroscopic analysis. Optical absorption spectrum was recorded and various optical parameters such as optical transmission (~60%), and optical band gap (4.998 eV) were calculated. Photoluminescence study shows that the grown crystal is free from major defects. Crystalline perfection of the grown crystal was assessed and found good. Ground state optimized geometry has been obtained by using DFT with 6-31G(d,p) basis set. HOMO and LUMO energy gap was found to be 6.01 eV and dipole moment was 1.65 D.     

Defect related luminescence in Hg0.2Cd0.8Te nano- and micro-crystals grown by the solvothermal method

The photoluminescence (PL) properties of nano- and micro-crystalline Hg_1?xCd_xTe (x≈0.8) grown by the solvothermal method have been studied over the temperature range 10–300 K. The emission spectra of the samples excited with 514.5 nm Ar⁺ laser consist of five prominent bands around 0.56, 0.60, 0.69, 0.78 and 0.92 eV. The entire PL band in this NIR region is attributed to the luminescence from defect centers. The features like temperature independent peak energy and quite sensitive PL intensity, which has a maximum around 50 K is illustrated by the configuration coordinate model. After 50 K, the luminescence shows a thermal quenching behavior that is usually exhibited by amorphous semiconductors, indicating that the defects are related to the compositional disorder.     

Dielectric,optical and structural properties of Bi4V2−xSrxO11−δ (0.05≤x≤0.20)

Composition Bi₄V_2−xSr_xO_11−δ (0.05≤x≤0.20) is synthesized by melt quench technique followed by heat treatment at 800 °C for 12 h. These compounds are characterised by X-ray diffraction, Fourier transform infrared (FTIR) spectroscopy, UV–visible spectroscopy, impedance spectroscopy and scanning electron microscopy. X-ray diffraction patterns of all the samples show γ-phase stabilization at room temperature except x=0.05 heat treated sample. The optical band gap of all the samples is observed in semiconducting range. The lowest and the highest optical band gap is 2.39 eV and 2.57 eV for x=0.10 heat treated and x=0.20 quenched samples, respectively. The highest value of dielectric constant is obtained ~10⁷ with very low dielectric loss for x=0.15 and 0.20 samples at ~350 °C and below 10 Hz. The grain size increases with dopant concentration leads to increase the dielectric constant.     

Crystal structures and band gap characters of h-BN polytypes predicted by the dispersion corrected DFT and GW method

Geometry optimizations are performed for three polytypes of h-BN using density functional theory with dispersion correction for the van der Walls interaction. Quasiparticle band structure calculations are carried out to solve the controversy on band gap type of h-BN. Band energies are corrected by GW method. The h-BN with B_k structure has an indirect band gap of 5.840 eV. Two kinds of h-BN polytypes are shown to be mechanically stable and have quasi-direct band gap type.     

Magnetism in nanocrystalline SiC films

Magnetism been studied in two series of nanocrystalline SiC films obtained by the method of direct deposition of ions with an energy of ~100 eV at temperatures 1150 °С and 1200 °С. There were separated the contributions of diamagnetism, paramagnetism and superparamagnetism+ferromagnetism. Magnetization value of the films correlates with the deposition temperature. In the films deposited at higher temperatures the value of magnetization was by 1.5 times lower. It was concluded that induced magnetism in nanocrystalline SiC films is caused by interaction of magnetic moments of neutral V_SiV_C divacancies in separate nanocrystals. The estimated concentration of neutral V_SiV_C divacancies in nanocrystalline SiC films is ~10²⁰ сm⁻³.     

Monochromatic soft X-ray-induced reactions of CCl2F2 adsorbed on Si(111)-7 × 7 near the Si(2p) edge

Continuous-time photoelectron spectroscopy (PES) and continuous-time core-level photon-stimulated desorption (PSD) spectroscopy were used to study the monochromatic soft X-ray-induced reactions of CCl₂F₂ molecules adsorbed on Si(111)-7 × 7 at 30 K (CCl₂F₂ dose = 2.0 × 10¹⁴ molecules/cm², ~ 0.75 monolayer) near the Si(2p) core level. Evolution of adsorbed CCl₂F₂ molecules was monitored by using continuous-time photoelectron spectroscopy at two photon energies of 98 and 120 eV to deduce the photolysis cross section as a function of energy. It was found that the photolysis cross sections for 98 and 120 eV photons are _~1.4 × 10^? 18 and ~ 8.0 × 10^? 18 cm², respectively. Sequential F⁺ PSD spectra obtained by using continuous-time core-level photon-stimulated desorption spectroscopy in the photon energy range of 98–110 eV show the variation of their shapes with photon exposure and depict the formation of surface SiF species. The dissociation of CCl₂F₂ molecules adsorbed on Si(111)-7 × 7, irradiated by monochromatic soft X-ray in the photon energy range of 98–110 eV, is mainly due to dissociative electron attachment and indirect dipolar dissociation induced by photoelectrons emitted from the silicon surface.