Time resolved ultraviolet photoluminescence of mesoporous silica

We studied the optical properties of sol-gel synthesized porous silica excited by synchrotron radiation in the 4-10 eV range. The spectral and temporal characteristics of the ultraviolet photoluminescence at about 3.7 eV are reported. The UV emission results from the contribution of two different centers: the first one centered at 3.7 eV with a decay time of 2.0 ns and the second one peaked at 3.9 eV with a decay time of 20 ns. We propose to assign the observed luminescence to different interacting surface silanols.     

Hydrogen-Related Paramagnetic Centers in Ge-Doped Sol-Gel Silica Induced by γ-Ray Irradiation

We have studied the generation mechanisms of H(II) paramagnetic centers in Ge-doped silica by investigating up to 10⁴ mol ppm sol-gel Ge-doped silica materials. We have considered materials with the same concentrations of Ge but that are produced by two different densification routes that give rise to different concentrations of Ge-related oxygen deficient centers (GeODC(II)). These centers are characterized by an optical absorption band at ∼5.2 eV (B_2β band) and two related emissions at ∼3.2 eV and ∼4.3 eV. The GeODC(II) content was estimated by absorption and emission measurements. The H(II) centers were induced by room temperature γ-ray irradiation and their concentration was determined by electron paramagnetic resonance measurements. The comparison between the two kinds of materials, obtained by different preparations, shows that the GeODC(II) concentration increases with the Ge content and is enhanced by vacuum densification. The comparison of irradiated samples shows that beyond the already known process of conversion of preexisting GeODC(II) into H(II) centers, another generation process of H(II) centers is effective that involves irradiation induced GeODC(II).     

Preparation and optical absorption of electrodeposited or sputtered,dense or porous nanocrystalline CuInS2 thin films

Copper indium disulphide thin films were obtained by one-step deposition with two different techniques. Films are synthesised by electrodeposition using a single electrolytic bath and by r.f. sputtering using a single target. Deposition rates were about 75 nm/min and 2.5–6.5 nm/min, respectively. Electrodeposited films have rough and porous surfaces, with no preferential orientation. Smooth or particle-covered surfaces were observed for sputtered films with a highly (112)-oriented chalcopyrite structure. Absorption coefficients calculated from transmittance spectra have high values in visible range. Electrodeposited samples present higher absorption coefficients on a larger wavelength range. A relationship between morphology and optical properties was found; absorption coefficients increase with porosity and roughness of the films. Band gap values of about 1.3 eV for electrodeposited and 1.5 eV for sputtered thin films were calculated.     

Synthesis and properties of a new polydiacetylene: Poly[1,6-di(N-carbazolyl)-2,4-hexadiyne]

The solid-state synthesis and properties are reported for a new polydiacetylene: poly[1,6-di(N-carbazolyl)-2,4-hexadiyne]. The monomer crystals polymerize quantitatively with γ irradiation or thermal annealing. An Autocatalytic effect is observed in both γ-ray polymerization and thermal polymerization and is attributed to an increase in chain propagation length at about 5% conversion. The activation energy for thermal polymerization is about 25 kcal/mole, independent of the degree of conversion to polymer. The exceptional thermal stability of the polymer crystals allowed a thermomechanical analysis over a large temperature range, ?50 to 300°C. With increasing temperature, the polymer contracts in the chain direction linearly with temperature over the entire range, yielding a thermal expansion coefficient of (?2.32 ± 0.02) × 10^?5°C^?1. Photoconductivity action spectra are reported for the polymer crystals. The energies for the photoconductivity onset (ca. 2.3 eV) and for the lowest energy optical transition (1.89 eV) are the lowest reported for the polydiacetylenes. The photoconduction onset is blue-shifted with respect to optical absorption—a result which is consistent with the excitonic assignment for the lowest energy optical transition in the polydiacetylenes.     

Effect of the annealing temperature on crystalline phase of copper oxide nanoparticle by copper acetate precursor and sol–gel method

A sol–gel route to synthesize copper oxide nanoparticles with an average size of ca. 63 nm from copper acetate precursor and monoethanolamine as the capping agent is reported. Structural characterization showed the formation of a cubic phase for CuO. The effect of annealing temperature on formation of crystalline phases was investigated. Characterization of the products was performed using thermo-gravimetric analysis, X-ray diffraction, field emission scanning electron microscopy, and diffuse reflectance. The results showed that there are significant differences in the morphological, crystallographic, structural, and optical properties of the nanostructures prepared at different annealing temperatures. The optical properties and band gap of CuO nanoparticles were studied by UV–Vis spectroscopy. According to the results of the optical measurements, the band gap is estimated to be 1.41 eV. These results showed that the band gap energy changed with increase of annealing temperature, which can be attributed to the change in grain size of the samples.     

Optical properties of Se2- and Se2 color centers in the red selenium ultramarine with the sodalite structure

The unique optical properties of Se2- radicals located in the cages of the sodalite structure are reported. By means of luminescence, photoluminescence excitation, and absorption spectroscopy, three different centers are identified. Two of them are Se2- anions in sites with presumably a tetrahedral Na4(4+) coordination and a Na3(3+) environment with cation deficiency, respectively, giving rise to a red luminescence band with two different progressions. The third center is the intermediate Se2 molecule, created photochemically by UV laser excitation. It induces an additional blue luminescence. The electronic properties of the Se2- centers, particularly in the excited states, are significantly influenced by steric constraints imposed by the limited space in the sodalite host polyhedra. Thus, the sodalite structure can be viewed as a model system for studying effects of this kind on chromophores imbedded in the cages of the zeolite-type lattice.     

High-pressure dissociation of crystalline para-diiodobenzene: optical experiments and Car-Parrinello calculations

We have investigated the high-pressure properties of the molecular crystal para-diiodobenzene, by combining optical absorption, reflectance, and Raman experiments with Car-Parrinello simulations. The optical absorption edge exhibits a large red shift from 4 eV at ambient conditions to about 2 eV near 30 GPa. Reflectance measurements up to 80 GPa indicate a redistribution of oscillator strength toward the near-infrared. The calculations, which describe correctly the two known molecular crystal phases at ambient pressure, predict a nonmolecular metallic phase, stable at high pressure. This high-density phase is characterized by an extended three-dimensional network, in which chemically bound iodine atoms form layers connected by hydrocarbon bridges. Experimentally, Raman spectra of samples recovered after compression show vibrational modes of elemental solid iodine. This result points to a pressure-induced molecular dissociation process which leads to the formation of domains of iodine and disordered carbon.     

Use of cathode lens in scanning electron microscope for low voltage applications

At a landing energy of 10 eV it is possible to achieve spatial resolution of the same order as at the nominal energy, which is usually 15 keV in the classical scanning electron microscope, by taking advantages of the optical properties of the cathode lens. Two different types of the detection system were designed and tested to learn as much about the optical properties of this system as possible and to start to understand the contrast mechanisms at very low energies. Great changes in the contrast take place when the landing energy is changed from 10 eV to an energy of about 2 keV.     

Enhancement on the structural,functional, optical and morphological properties of temperature treated silver sulfide nanostructures

The Silver Sulfide (Ag₂S) nanostructures were synthesized via the facile co-precipitation method. Thorough study and analysis were carried out to reveal and compare the structural, optical, functional, and morphological characteristics of as-synthesized samples annealed at various temperatures. The XRD analysis characterized the structural properties of Ag₂S nanoparticles, which unveiled the excellent crystallinity and monoclinic structure. The as-synthesized samples show an average crystallite size of 52 nm–41.7 nm. The modes of vibration and peak position of metal sulfides in Ag₂S nanoparticles were investigated through the FTIR technique. The optical attributes of prepared samples were scrutinized using UV–Vis analysis, which portrays the cut-off wavelength in the range of 1192–1223 nm for non-annealed and annealed Ag₂S nanoparticles, alongside the optical band gap is about 0.86 eV–0.96 eV. This work elucidates a novel approach to synthesis and scrutinises the characteristics of Ag₂S nanoparticles by subjecting them to distinct annealing temperatures precisely, as-prepared, 200 °C and 400 °C.     

Identification of defect sites on MgO(100) thin films by decoration with Pd atoms and studying CO adsorption properties.

CO adsorption on Pd atoms deposited on MgO(100) thin films has been studied by means of thermal desorption (TDS) and Fourier transform infrared (FTIR) spectroscopies. CO desorbs from the adsorbed Pd atoms at a temperature of about 250 K, which corresponds to a binding energy, E(b), of about 0.7 +/- 0.1 eV. FTIR spectra suggest that at saturation two different sites for CO adsorption exist on a single Pd atom. The vibrational frequency of the most stable, singly adsorbed CO molecule is 2055 cm(-)(1). Density functional cluster model calculations have been used to model possible defect sites at the MgO surface where the Pd atoms are likely to be adsorbed. CO/Pd complexes located at regular or low-coordinated O anions of the surface exhibit considerably stronger binding energies, E(b) = 2-2.5 eV, and larger vibrational shifts than were observed in the experiment. CO/Pd complexes located at oxygen vacancies (F or F(+) centers) are characterized by much smaller binding energies, E(b) = 0.5 +/- 0.2 or 0.7 +/- 0.2 eV, which are in agreement with the experimental value. CO/Pd complexes located at the paramagnetic F(+) centers show vibrational frequencies in closest agreement with the experimental data. These comparisons therefore suggest that the Pd atoms are mainly adsorbed at oxygen vacancies.     

Electrical and optical characterization of PMMA doped with Y0.0025Si0.025Ba0.9725 (Ti(0.9)Sn0.1)O3 ceramic

<正>Poly(methylmetacrylate)(PMMA)/Y_(0.0025)Si_(0.025)Ba_(0.9725)(Ti_((0.9))Sn_(0.1))O_3(YBTS) composites were prepared at different weight ratios of YBTS(0 wt%,5 wt%,10 wt%and 20 wt%YBTS) in order to investigate effect of YBTS additions on the electrical and optical properties of PMMA host.The electrical properties(impedance,dielectric constant dielectric loss and AC conductivity) were studied in the frequency range 10 kHz-1 MHz and in temperature range 20-90℃.Upon increasing the contents of YBTS ceramic,we observed a reduction in the impedance and an increase in the dielectric constant,dielectric loss and AC conductivity of PMMA host.We also found that the relaxation process at high YBTS contents was due to relaxation in the ionic conductivity.The absorption coefficient(α) has been determined in the wavelength range 230-800 nm at room temperature for all YBTS-PMMA composites.Moreover,the addition of YBTS ceramic highly enhances the UV absorption of PMMA host especially below 300 nm.Addition of 20 wt%YBTS ceramic to PMMA host decreases the optical energy gap from 5 eV to 3.41 eV.Correlations between electrical,optical and SEM results are reported.     

Tuning Optical Properties of Si Quantum Dots by π‐Conjugated Capping Molecules

The absorption and photoluminescence (PL) properties of silicon quantum dots (QDs) are greatly influenced by their size and surface chemistry. Herein, we examined the optical properties of three Si QDs with increasing σ–π conjugation length: octyl‐, (trimethylsilyl)vinyl‐, and 2‐phenylvinyl‐capped Si QDs. The PL photon energy obtained from as‐prepared samples decreased by 0.1–0.3 eV, while the PL excitation (PLE) extended from 360 nm (octyl‐capped Si QDs) to 400 nm (2‐phenylvinyl‐capped Si QDs). A vibrational PL feature was observed in all samples with an energy separation of about 0.192±0.013 eV, which was explained based on electron–phonon coupling. After soft oxidization through drying, all samples showed blue PL with maxima at approximately 410 nm. A similar high‐energy peak was observed with the bare Si QD sample. The changes in the optical properties of Si QDs were mainly explained by the formation of additional states arising from the strong σ–π conjugation and QD oxidation.     

Band gap engineering of bulk ZrO2 by Ti doping

It has been experimentally observed that Ti doping of bulk ZrO(2) induces a large red-shift of the optical absorption edge of the material from 5.3 to 4.0 eV [Livraghi et al., J. Phys. Chem. C, 2010, 114, 18553-18558]. In this work, density functional calculations based on the hybrid functional B3LYP show that Ti dopants in the substitutional position to Zr in the tetragonal lattice cause the formation of an empty Ti 3d band about 0.5 eV below the bottom of the conduction band. The optical transition level ε(opt)(0/-1) from the topmost valence state to the lowest empty Ti impurity state is found at 4.9 eV in a direct band gap of 5.7 eV. The calculated shift is consistent with the experimental observation. The presence of Ti(3+) species in Ti-doped ZrO(2), probed by means of electron paramagnetic resonance (EPR), is rationalized as the result of electron transfers from intrinsic defect states, such as oxygen vacancies, to substitutional Ti(4+) centers.     

Investigation of non-diamond carbon phases and optical centers in thermochemically polished polycrystalline CVD diamond films

Polycrystalline chemical vapor deposition (CVD) diamonds films grown on silicon substrates using the microwave-enhanced CVD technique were polished using the thermochemical polishing method. The surface morphology of the samples was determined by optical and scanning electron microscopes before and after polishing. The average surface roughness of the as-grown films determined by the stylus profilometer yielded 25 μm on the growth side and about 7 μm on the substrate side. These figures were almost uniform for all the samples investigated. Atom force microscopic measurements performed on the surface to determine the average surface roughness showed that thermochemical polishing at temperatures between 700 °C and 900 °C reduced the roughness to about 2.2 nm on both the substrate and growth sides of the films. Measurements done at intermittent stages of polishing using confocal micro-Raman spectroscopy showed that thermochemical polishing is accompanied by the establishment of non-diamond carbon phases at 1353 cm⁻¹ and 1453 cm⁻¹ at the initial stage of polishing and 1580 cm⁻¹ at the intermediate stage of polishing. The non-diamond phases vanish after final fine polishing at moderate temperatures and pressures. Photoluminescence of defect centers determined by an Ar⁺ laser (λl_exct= 514.532 nm) showed that nitrogen-related centers with two zero-phonon lines at 2.156 eV and 1.945 eV and a silicon-related center with a zero-phonon line at 1.681 eV are the only detectable defects in the samples. Received: 26 July 1999 / Accepted: 15 November 1999     

ZnO and MgO nanoparticles: Synthesis and comparative study on their properties

We report the optical and structural properties of ZnO and MgO nanoparticles. The samples are obtained by a simple method using a new template of hexamethylene tetramine. The optical properties of the samples are studied by UV-visible spectroscopy. Their crystal structure and morphology are studied by XRD and scanning electron microscopy. The absorption spectra of MgO and ZnO show that the optical band gaps are 4.27 eV and 3.02 eV, respectively. In this investigation the photocatalytic degradation of indigo carmine (IC) in water is studied. The effects of some parameters such as pH, amount of catalyst, initial concentration of dye, are examined.     

Electrical and optical properties of inorganic complexes C36H76N2O9ClNa and C14H12N2O4TeBr2

The electrical properties of some inorganic samples were studied as functions of temperature and the results were analyzed. These samples activation energies were calculated from the electrical conductivity measurements. Also, a detailed study of optical absorption is presented. The optical absorption spectra are measured in the UV+invisible wavelength range 200–1100 nm. The absorption coefficient and value optical energy gaps were determined from the absorption spectra. The complex C₃₆H₇₆N₂O₉ClNa is characterized by direct optical absorption with an optical edge at 4.49 eV. However, the complex C₁₄H₁₂N₂O₄TeBr₂ is characterized by indirect optical absorption with an optical edge at 1.45 eV, and a nearby direct one at hν=1.80 eV.     

基于吡啶甲酮衍生物的蓝色磷光主体材料的合成及性质

以4-苯甲酰吡啶、吩噁嗪和9,9-二甲基吖啶为原料,合成了两种蓝色磷光主体材料:[(2-溴-5-吩噁嗪-10-基)苯基](吡啶-4-基)甲酮(BPPPM),[2-溴-5-(9,9-二甲基-9H-吖啶-10-基)苯基](吡啶-4-基)甲酮(BDPPM)。并用~1H NMR、~(13)C NMR和元素分析对其结构进行了表征,研究了它们的光电性质和热性质。结果表明,BPPPM和BDPPM的光学带隙(Eg)分别为3.31和2.64 e V;它们的发射峰分别位于405和435 nm,发深蓝和蓝色荧光;它们均具有较高的三线态能级(2.70,2.65e V),可与蓝色磷光客体材料FIrpic(2.65e V)的三线态能级相匹配;它们均具有匹配的HOMO(-5.45、-5.35e V)与LUMO能级(-2.14、-2.82e V),且其HOMO、LUMO轨道的电子云分离突出,具有良好的双极性质;此外它们均具有较好的热稳定性和成膜性。     

聚对苯二甲酸乙二酯光学特性的椭圆偏振光谱研究

概要介绍椭圆偏振光谱 (SE)的原理与特点 ,并用椭偏光谱测定了一组结晶度不同的无取向PET薄膜的光学常数谱 ,研究半结晶性高聚物PET不同结晶形态对其光学性能的影响 ,发现随结晶度的增加 ,其光学常数显著增大 ,并趋于晶态的光学常数 .光学性质的改变可能与微晶的尺寸有关     

Luminescence and color of radiation-modified polytetrafluoroethylene in blocks

Spectral-luminescent analysis has shown that γ-irradiation of bulky PTFE samples (blocks) in the range 330–340°C initiates generation of the same optical centers as in thin films. The irradiated samples become fluorescent and acquire a color ranging from light yellow to yellow. The spectral composition of fluorescence and the color of the samples depend on the absorbed dose and the irradiation temperature. The spectrum of fluorescence excited at a wavelength of 313 nm lies within 350–600 nm, and it is determined by various optical emitting centers. The fluorescence and color centers are distributed over the bulk of the samples in different ways. The concentration of optical centers in the near-surface layers of the samples (to a depth of 0.3–1.0 mm, depending on the sample) is higher than that in layers remote from the surface. In the latter layers, the spectral composition of fluorescence is primarily determined by shorter wavelength fluorescence centers. Almost no fluorescence is emitted from the central zone of the sample, and this zone is colorless. It is inferred that the nonuniform distribution of optical centers is related to the concentration gradient of molecular oxygen in the bulk of the samples appearing in the course of their radiation treatment (the inner layers are depleted of oxygen, while the surface layers are enriched in oxygen). New evidence is derived that polyene structures responsible for the fluorescent behavior and color of the modified PTFE are formed in main chains. The structure of these moieties is described by the structural formula ...CF₂-(CF=CF)n-CF₂, where ...n = 4–7.     

Hybrid organic-inorganic conductor with a magnetic chain anion: kappa-BETS2[Fe(III)(C2O4)Cl2] [BETS = bis(ethylenedithio)tetraselenafulvalene

The synthesis, crystal structure, and electrical, optical, and magnetic properties of kappa-BETS2[Fe(III)(C2O4)Cl2], where BETS is bis(ethylenedithio)tetraselenafulvalene, are reported. The black plate crystals consist of parallel donor layers, two per unit cell, displaying a kappa-type packing of BETS(0.5+) within the bc plane and anionic magnetic chains, [Fe(C2O4)Cl2-]n, running along the c axis. It displays metallic behavior down to 4.2 K, and analysis of the optical reflectivity data gives unscreened plasma energies of 0.69 eV (E parallel c) and 0.40 eV (E perpendicular c). The optical anisotropy is larger than that seen for other kappa phases and is described well by transfer integrals obtained from extended Hückel calculations. However, the transfer integrals need to be scaled down uniformly by a factor of 1.21 to reproduce the absolute experimental plasma frequencies. The band structure consists of a one-dimensional (1D) band and a hole pocket, characteristics of kappa phases. The magnetic properties were modeled by the sum of a 1D antiferromagnetic chain contribution from the d spins of Fe3+, a temperature-independent paramagnetic contribution, and a Curie impurity term. At 4.5 K, there is a signature of long-range magnetic ordering to a canted-antiferromagnetic state in the zero-field-cooled-field-cooled magnetizations, and at 2 K, a small hysteresis loop is observed.