ZnO/SiO2的激光诱导发光光谱研究

用激光诱导发光光谱技术对一系列不同ZnO担载量的ZnO/SiO2及在不同焙烧气氛下制备的ZnO/SiO2材料进行了深入的研究,发现在空气气氛中焙烧的ZnO/SiO2主要出现ZnO的橙色发光带(中心位于670nm),但在氩气气氛中焙烧制得的样品主要呈现ZnO的绿色发光带(中心位于540nm),且明显观察到橙色发光带和绿色发光带之间的转化.ZnO的可见光区发光带可能是由ZnO中的氧缺陷引起的,高浓度的氧缺陷引发绿色发光带,而低浓度的氧缺陷引发橙色发光带.结果表明,激光诱导发光光谱是一种表征缺陷的有力工具,常规表征技术很难对材料的缺陷状态进行表征.XRD衍射图谱表明,样品中ZnO是以纤锌矿结构存在的,紫外-可见漫反射光谱和拉曼光谱显示,ZnO主要以大颗粒状态覆盖在SiO2表面上.     

Collisional deactivation mechanism of luminescence in hydrogen-loaded Ge-doped fibers

We report experimental results on the temperature and Ge-content dependencies of the decay times of the 3.1 eV emission of twofold-coordinated germaniums, excited at 3.51 eV, in both virgin and H2-loaded germanosilicate optical fibers. For T<110 K, the lifetimes are temperature invariant in the two kinds of fibers; at higher temperatures, they are progressively shortened in the H2-loaded fibers whereas they remain unaltered in the virgin fibers. On increasing the Ge content in the fiber core the lifetime decreases in the whole temperature range. We also found a direct correlation in H2-loaded samples between the dependencies on temperature of the emission lifetime and the diffusion lengths of H2 molecules in silica glass determined from earlier work. These experimental features are explained by assuming an elastic-collisional deactivation mechanism due to interaction with diffusing H2 molecules, described in terms of an Arrhenius law with activation energy approximately 64 meV that operates in competition with the normal radiative process.     

Physicochemical and Redox Characteristics of Fe Ion‐doped CeO2 Nanoparticles

We report the structural, thermal, optical, and redox properties of Fe‐doped cerium oxide (CeO₂) nanoparticles, obtained using the polyol‐co‐precipitation process. X‐ray diffraction data reveal the formation of single‐phase structurally isomorphous CeO₂. The presence of Fe³⁺ may act as electron acceptor and/or hole donor, facilitating longer lived charge carrier separation in Fe‐doped CeO₂ nanoparticles as confirmed by optical band gap energy. The increased content of localized defect states in the ceria gap and corresponding shift of the optical absorption edge towards visible range in Fe‐doped samples can significantly improve the optical activity of nanocrystalline ceria. The better‐quality redox performances of the Fe‐doped CeO₂ nanoparticles, compared with undoped CeO₂ nanoparticles, were ascribed mainly to a decrease in band gap energy and an increase in specific surface area of the material. As observed from TPR studies all Fe ‐doped CeO₂ nanoparticles, particularly the 10 mol % Fe doped CeO₂ nanoproduct, exhibit excellent reduction performance.     

Optical reorientation in cholesteric nematic mixtures

Abstract

The optical Fréedericksz transition for linearly polarized light at normal incidence is studied in mixtures of nematic E7 and cholesteric C15 in cells coated for homeotropic alignment. The reorientation process is found to be dramatically different from the case of pure nematic samples showing the phenomenon of optical phase locking and large hysteresis. These effects are ascribed to the occurrence of self-induced stimulated light scattering, which does not occur in pure nematics.     

Photosensitive polyimides for applications in electrical and optical interconnection technology

The application of photosensitive polyimides in electrical and optical interconnection technology is discussed. Critical properties of two photosensitive polyimide formulations with significantly different structural chemistries have been compared. Polyamic ester based formulations such as Selectilux® HTR3 show significant distortions in the photopatterned features upon high-temperature baking, which can be ascribed to anisotropic shrinkage. A BTDA-alkylated diamine based preimidized formulation (Probimide®), on the other hand, shows a more uniform shrinkage of the photopatterned features. HTR3 films interacts strongly with metals such as copper, which adversely affect the photopatternability of these materials. Planarization behavior in multiple layer structures has also been investigated. We explored the viability of these materials for applications in optical interconnection. Probimide® materials exhibit low scattering losses and appear to be promising candidates for the development of a MCM-compatible optical interconnection technology. We also describe here a novel technique for optical recording of refractive index patterns in Probimide® films.     

Synthesis of InP nanotubes

Indium phosphide (InP) nanotubes have been synthesized via the vapor-liquid-solid (VLS) growth mechanism. The nanotubes are crystalline and have the (bulk) zinc blende structure and therefore represent a new class of tube materials. The tubes show photoluminescence, which is considerably blue-shifted with respect to bulk emission, indicating that the optical properties are not dominated by defect states. They are formed at higher temperatures than those at which nanowires are fabricated. A simple model for the formation of the nanotubes is presented. The wall thickness can be controlled by the synthesis temperature and is in the range of 2-20 nm.     

Stress-optical coefficients of swollen polymer networks

Stress-optical coefficients have been determined for crosslinked samples of polyethylen (PE) and polystyrene (PS) at high temperatures, i.e., in the rubberlike state, and when swollen in a variety of liquids. For PE, swelling liquids with long straight molecules gave large values of optical anisotropy whereas liquids with more symmetrical molecules gave minimum values, as found previously for cis- polyisoprene and trans-polyisoprene. This solvent effect is attributed to short-range orientational order in molecularly asymmetric media. Sizes of the equivalent random link for unperturbed molecules of these three polymers were deduced from the minimum values of optical anisotropy. Measures of shape asymmetry were also obtained by matching the optical anisotropy of samples when unswollen with that observed when swollen with a liquid of known molecular asymmetry. Reasonable agreement was found to hold between the two methods. In contrast, the optical anisotropy of swollen PS was found to be substantially independent of the swelling liquid. The apparent absence of a molecular ordering effect in this case is attributed to the bulky nature of the PS molecule. A marked reduction in optical anisotropy on swelling is ascribed to increased phenyl group motion.     

Cu2+–Mn2+-Co-doped CdO nanocrystallites: comprehensive research on phase,morphology and optoelectronic properties

In this study, firstly cadmium hydroxide nanopowder was evolved by cost-effective wet chemical co-precipitation method. The transformation of nanocrystalline Cu²⁺–Mn²⁺-co-doped CdO occurred via thermal decomposition of the obtained hydroxide at 750 °C. The structural, optical and electrical behavior of nanocrystallites was analyzed by different complementary measuring tools. DTA of the as-prepared sample exhibited an endothermic peak at 240 °C attributed to crystallization. XRD analysis depicted a multiphase structure in the as-prepared sample, and pure rocksalt structure was obtained after annealing. Cu²⁺–Mn²⁺-co-doped cubic CdO has been achieved first time which was further confirmed by FTIR with various stretching and bending vibrations of Cd–O at 720, 625 and 460 cm^?1. SEM–TEM images demonstrated the brain-like morphology of different hexagonal and spherical nanocrystallites with an average size of ~?35 nm. In addition, optical band gap energy was found in the range 2.14–2.44 eV by Tauc’s plot. In photoluminescence results, emission spectra have many bands at 420, 480, 550 nm originated from excitonic transition, structural defects and oxygen vacancies, while intense peak at 450, 520 nm may be ascribed to Cu²⁺ and Mn²⁺ dopants, respectively. Hall measurements demonstrated that the Cu²⁺–Mn²⁺-co-doped CdO with a pure cubic phase has superior semiconducting behavior. The homogeneous codoping of Cu²⁺–Mn²⁺ leads to efficient modification in structural, optical and electrical parameters of CdO which would make such materials attractive for semiconductor and photovoltaic industry, etc.

     

Studies on the defect structure for Cu2+ in CdSe nanocrystals

The defect structure for Cu²⁺ in CdSe nanocrystals is theoretically studied by analyzing the spin Hamiltonian parameters of this impurity center. This center is ascribed to Cu²⁺ occupying the octahedral interstitial site, rather than the tetrahedral substitutional Cd²⁺ site proposed by previous work. The Cu²⁺ center exhibits slight tetragonal elongation distortion (characterized by the elongation parameter ρ ≈ 0.03) due to the Jahn–Teller effect. The theoretical spin Hamiltonian parameters and optical transition show good agreement with the experimental data. The above unusual defect structure (occupation and symmetry) for Cu²⁺ in CdSe nanocrystals is discussed, as compared with the conventional trigonally distorted tetrahedral Cu²⁺ centers in bulk II–VI semiconductors.     

Probing defect-originated properties,actinide-lanthanide doping,and gamma irradiation effect in Lu2Hf2O7 pyrochlore nanocrystals for phosphor and nuclear applications

Multifunctional materials are in high demand these days by virtue of their efficacy to perform more than one role and are expected to ease the high global materials crunch. This work is directed towards probing defect emission in undoped and doping of uranium (U), plutonium (Pu), curium (Cm), cerium (Ce), and samarium (Sm) Lu₂Hf₂O₇ (LuHO) pyrochlore nanocrystals (NCs) for photoluminescence and radioactive waste immobilization. Density of state calculations shows abundant defects in the undoped LuHO NCs. Multiple visible luminescence spanning violet to red in the undoped LuHO pyrochlore NCs upon ultraviolet irradiation was ascribed to the presence of neutral and ionized oxygen vacancies. The doped LuHO NCs show typical blue (Ce³⁺), orange-red (Sm³⁺), and green (U⁶⁺) emissions endowed by 4f→5d, 4f→4f, and charge transfer transitions, respectively. Moreover, uranium is stabilized as UO₆^6? ion (octahedral uranate) in the LuHO pyrochlore lattice. Luminescence lifetime spectroscopy and density functional theory (DFT) calculated defect formation energies suggest that Ce, Sm, Cm, and Pu ared stabilized at Lu³⁺ site whereas U is stabilized at Hf⁴⁺ site in the LuHO NCs. The LuHO pyrochlore NCs also demonstrate high radiation stability on gamma exposure retaining the crystallinity and undergoing structural phase transition from defect fluorite to ideal pyrochlore structure. Considering the significant health hazard posed by radioactive elements present in the spent fuel, these LuHO NCs possess high potential to immobilize U, Pu, Cm, Ce, and Sm. This work poses high significance for immobilizing radioactive nuclear waste and designing tunable luminescent solid-state NCs.     

Synthesis and characterization of Co3O4 nanoparticles by a simple method

Using solid complex molecular precursor [bis(salicylaldehyde)ethylenediiminecobalt(II)], [Co(salen)], a simple and surfactant-free method to synthesize Co₃O₄ nanoparticles was proposed. Cubic-phase Co₃O₄ nanoparticles of size 30–50-nm could be produced by thermal treatment of the Co(salen) in the air at 500 °C for 5 h. The as-prepared samples were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The optical absorption spectrum indicates that the direct band gaps of Co₃O₄ nanoparticles are 1.53 and 2.02 eV. The optical property test indicates that the absorption peak of the nanoparticles shifts towards short wavelengths, and the blue shift phenomenon might be ascribed to the quantum effect. The hysteresis loops of the obtained samples reveal their ferromagnetic behavior, an enhanced coercivity (H_c) and a decreased saturation magnetization (M_s) as compared to their respective bulk materials.     

Promoting electrochemical conversion of CO2 to formate with rich oxygen vacancies in nanoporous tin oxides

Defect engineering, especially oxygen vacancies (O-vacancies) introduction into metal oxide materials has been proved to be an effective strategy to manipulate their surface electron exchange processes. However, quantitative investigation of O-vacancies on CO₂ electroreduction still remains rather ambiguous. Herein, a series of nanoporous tin oxide (SnO_x) materials have been prepared by thermal treatment at various temperatures and reaction conditions. The annealing temperature dependent O-vacancies property of the SnOx was revealed and attributed to the balance tunning of the desorption of oxygen species and the continous oxidation of SnO_x. The as-prepared nanoporous SnO_x with 300 °C treatment was found to be highest O-vacant material and showed an impressive CO₂RR activity and selectivity towards the conversion of CO₂ into formic acid (up to 88.6%), and superior HCOOH incomplete current density to other samples. The ideal performance of the O-vacancies rich SnO_x-300 material can be ascribed to the high delocalized electron density inducing much enhanced adsorption of CO₂ with O binding and benefiting the subsequent reduction with high selectively forming of formic acid.     

Luminescence and photocatalytic activity of ZnO nanocrystals: correlation between structure and property

Low-dimensional ZnO nanocrystals with controlled size, aspect ratio, and oxygen defects (e.g., type and concentration) are successfully prepared through simple solvothermal and thermal treatment methods. The structure of the as-synthesized samples is characterized by XRD, N2 physical adsorption, TEM, and IR and XPS spectra. The results show that the aspect ratio and size of the as-synthesized ZnO nanocrystals increase with increasing [OH-]/[Zn2+]; the morphology evolves from nanorod to nanoparticle with an increase in the annealing temperature; the BET surface areas of the corresponding samples decrease during these processes, respectively; and different oxygen defects, which are likely to be oxygen vacancy (Vo**) and interstitial oxygen (Oi'), are formed in our experiments accordingly. With evolution of the structure, IR absorption bands and visible photoluminescence emission peaks of the synthesized ZnO nanocrystals shift and split, which is ascribed to the change of oxygen defects. In addition, it is found that the photocatalytic activity of the synthesized ZnO nanocrystals is mainly dependent on the type and concentration of oxygen defects. The relationship of structure-property and the possible photocatalytic mechanism are discussed in detail.     

Practical applications of neutron activation and radioisotope techniques in optical waveguide research and development

Gamma-ray spectrometric survey analyses were used to screen raw materials available during the infancy of the optical waveguide research program. Examinations of γ-ray spectra and semiquantitative survey analyses showed most samples to be insufficiently pure for waveguide applications. Highly pure samples have been prepared successfully under carefully controlled conditions when the purification process is adequately monitored by analyses. Radioisotope techniques and neutron activation analyses were vital in the development of successful procedures for purifying various reagents from which glass could be subsequently fabricated by melting techniques. Contamination sources during fiber production have also been detected by neutron activation analyses.     

Characterization of paramagnetic hole-centers in Al2O3 and Al2O3:Fe by optical and EPR absorption

The electron paramagnetic resonance (EPR) and optical absorption spectra of nominally pure and Fe-doped single crystals of Al₂O₃ have been examined before and after gamma-ray irradiation at 77°K. The EPR spectra for doped and irradiated crystals were found to be a function of the iron concentration. Analysis of the optical and EPR spectra observed on the same Fe-doped crystals which had been subjected to gamma-ray irradiations suggests that the center which accounts most satisfactorily for the details of these spectra is: a single trapped-hole localized on an anion which is adjacent to a substitutional divalent iron impurity atom. The optical absorption band ascribed to this center occurs at 3.08 eV.     

Influence of Size and Shape on the Photocatalytic Properties of SnO2 Nanocrystals

Tuning the functional properties of nanocrystals is an important issue in nanoscience. Here, we are able to tune the photocatalytic properties of SnO₂ nanocrystals by controlling their size and shape. A structural analysis was carried out by using X‐ray diffraction (XRD)/Rietveld and transmission electron microscopy (TEM). The results reveal that the number of oxygen‐related defects varies upon changing the size and shape of the nanocrystals, which eventually influences their photocatalytic properties. Time‐resolved spectroscopic studies of the carrier relaxation dynamics of the SnO₂ nanocrystals further confirm that the electron–hole recombination process is controlled by oxygen/defect states, which can be tuned by changing the shape and size of the materials. The degradation of dyes (90 %) in the presence of SnO₂ nanoparticles under UV light is comparable to that (88 %) in the presence of standard TiO₂ Degussa P‐25 (P25) powders. The photocatalytic activity of the nanoparticles is significantly higher than those of nanorods and nanospheres because the effective charge separation in the SnO₂ nanoparticles is controlled by defect states leading to enhanced photocatalytic properties. The size‐ and shape‐dependent photocatalytic properties of SnO₂ nanocrystals make these materials interesting candidates for photocatalytic applications.     

Defect intergrowths in barium polytitanates: 1. Ba2Ti9O20

Using high-resolution transmission electron microscopy, we have investigated the mechanisms of defect formation in samples of the microwave dielectric material, Ba₂Ti₉O₂₀. We found that materials prepared by a variety of different techniques all show considerable structural disorder. The most prevalent intergrowth involved formation of a new triclinic polytype with an ionic arrangement closely related to that in the accepted structure. Defects also resulted from considerable microtwinning and were observed mainly in the samples prepared from a vanadate flux. The degree of nonstoichiometric defect formation was small in comparison to the stoichiometric intergrowths. In this case defects appeared to result from the incorporation of excess vacancies into the close-packed layers of the structure. Barium-deficient surface phases were also formed via a similar mechanism.     

1,3-二苯基-2-吡唑啉纳米带的制备及其光波导性质

以1,3-二苯基-2-吡唑啉(DP)为目标化合物, 利用再沉淀方法, 以混合溶剂作为不良溶剂, 制备了形貌均一、宽度约2 μm、厚度200 nm和长度数十微米的一维纳米带状结构. 选区电子衍射结果证实, DP分子在纳米带中因为强的分子间π-π相互作用而沿着晶体[100]方向优势生长. 稳态光谱结果表明, DP纳米带具有不同于分子和体相材料的介观特性. 由于J-聚体在DP纳米带中优势形成, 其480 nm的发射峰与分子和体相材料相比分别红移了30和20 nm. 利用扫描近场光学显微镜进一步发现, 一维DP纳米带类似于天然的亚波长尺度谐振腔, 紫外激发DP分子发射的荧光被限域在DP纳米带中, 沿一维方向传导并在两端耦合输出.     

Layered oxysulfides Sr2MnO2Cu2m-0.5Sm+1 (m = 1, 2, and 3) as insertion hosts for Li ion batteries

The layered oxysulfides Sr2MnO2Cu2m-0.5Sm+1 (m = 1-3) consist of alternating perovskite-type Sr2MnO2 layers and copper sulfide layers. The copper ions can be replaced electrochemically and reversibly by Li. The lithiated materials were studied by Li MAS NMR, and Li resonances were observed with shifts that could be rationalized based on the number of sulfide layers. The materials were cycled versus Li and showed enhanced capacity retention in comparison to pure Cu2S; the good electrochemical performance was ascribed to the presence of the layered framework structure and rapid Li+ and Cu+ conductivity in the sulfide layers.     

Steady-state photoinduced absorption of CdSe/CdS octapod shaped nanocrystals

Colloidal branched nanocrystals have been attracting increasing attention due to evidence of an interesting relationship between their complex shape and charge carrier dynamics. Herein, continuous wave photoinduced absorption (CW PIA) measurements of CdSe/CdS octapod-shaped nanocrystals are reported. CW PIA spectra show strong bleaching due to the one-dimensional (1D) CdS pod states (480 nm) and the zero-dimensional (0D) CdSe core states (690 nm). The agreement with previously reported ultrafast pump-probe experiments indicates that this strong bleaching signal may be assigned to state filling. Additional bleaching features at 520 and 560 nm are characterized by a longer lifetime and are thus ascribed to defect states, localized at the pod-core interface of the octapod, showing that some of the initially photogenerated carriers get quickly trapped into these long-lived defect states. However, we remark that a relevant part of electrons remain untrapped: this opens up the opportunity to exploit octapod shaped nanocrystals in photovoltaics applications, as electron acceptor materials, considering that several efficient hole extracting materials are already available for the realization of a composite bulk heterojunction.