Amorphous to crystalline transition of selenium thin films of different thicknesses

Thin films of amorphous selenium obtained by vacuum evaporation display an increase of “optical gap” E_g^opt with an increase of thickness of the film. From the observed dependence of E_g^opt on the thickness of the film, the influence of the thickness on the gap states is interpreted in terms of the density of states model proposed by Mott and Davis. The amorphous to crystalline transition obtained by heat treatment of the specimen is also investigated. The minimum temperature for an appreciable change in crystallisation determined by the transmission of light through selenium films is also a function of the thickness and binding energy of the films. The crystalline structures resulting from heat treatment at different temperatures have been identified by scanning electron microscopy. The generation of different crystalline structures is reported in terms of the thickness and preparation conditions of the amorphous films.     

Preparation and growth mechanism of clustered one-dimensional SiOx amorphous nanowires by catalytic pyrolysis of a polymer precursor

Large-scale synthesis of clustered one-dimensional amorphous silica nanowires was achieved by simple thermal pyrolysis of an amorphous preceramic powder from perhydropolysilazane on alumina wafers coated with catalyst FeCl₂. Scanning electron microscopy and transmission electron microscopy observations showed that the silica nanowires had smooth surface, and lengths of hundreds of micrometers and diameters in the range of 30–40 nm. Energy dispersive X-ray spectroscopy revealed that these nanowires consisted of Si and O elements in an atomic ratio of approximately 1:2, consistent with the stoichiometric formula SiO₂. The two amorphous bulges in Raman spectrum at the centers of around 260 cm^?1 and 800 cm^?1 were identified to be those of amorphous silica. The growth mechanism of the as-produced silica nanowires could be attributed to vapor–liquid–solid mechanism. These results provide an alternative and simple preparation procedure for nanostructures with controlled morphology, and it will be helpful to understand the growth mechanism of one-dimensional SiO₂ nanostructures.     

Preparation of amorphous carbon nanotubes using attapulgite as template and furfuryl alcohol as carbon source

Amorphous carbon nanotubes were synthesized by vapor deposition polymerization (VDP) method using attapulgite as template and furfuryl alcohol as carbon source. The morphology and microstructure analysis of the as-prepared samples showed that highly pure amorphous carbon nanotubes were obtained, and determined by scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SEAD) and energy dispersive X-ray (EDX) spectrum. The Brunauer–Emmett–Teller (BET) surface area analysis indicated that the specific surface area of the as-prepared amorphous carbon nanotubes reaches up to 503.1 m²/g. A hypothesis about the formation mechanism of the amorphous carbon nanotubes was also proposed accordingly.     

Complementary study of the internal porous silicon layers formed under high-dose implantation of helium ions

The surface layers of Si(001) substrates subjected to plasma-immersion implantation of helium ions with an energy of 2–5 keV and a dose of 5 × 10¹⁷ cm^–2 have been investigated using high-resolution X-ray reflectivity, Rutherford backscattering, and transmission electron microscopy. The electron density depth profile in the surface layer formed by helium ions is obtained, and its elemental and phase compositions are determined. This layer is found to have a complex structure and consist of an upper amorphous sublayer and a layer with a porosity of 30–35% beneath. It is shown that the porous layer has the sharpest boundaries at a lower energy of implantable ions.     

Influence of temperature on the microcrystalline structure of thermally evaporated Sb2S3 thin films

Thin films of antimony trisulfide (Sb₂S₃) were prepared by thermal evaporation under vacuum (p=5×10^–5 torr) on glass substrates maintained at various temperatures between 293 K and 523 K. Their microstructural properties have obtained by transmission electron microscopy (TEM). The electron diffraction analysis showed the occurrence of amorphous to polycrystalline transition in the films deposited at higher temperature of substrates (523 K). The polycrystalline thin films were found to have an orthorhombic structure. The interplanar distances and unit‐cell parameters were determined by high‐resolution transmission electron microscopy (HRTEM) and compared with the standard values for Sb₂S₃. The surface morphology of Sb₂S₃ thin films was investigated by scanning electron microscopy (SEM). The optical transmission spectra at normal incidence of Sb₂S₃ thin films have been measured in the spectral range of 400–1400 nm. The analysis of the absorption spectra revealed indirect energy gaps, characterizing of amorphous films, while the polycrystalline films exhibited direct energy gap. From the photon energy dependence of absorption coefficient, the optical band gap energy, E_g, were calculated for each thin films. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Threefold coordinated model structure of amorphous GeS,GeSe and GeTe

The black P structure is presented as a model for the structures of amorphous GeS, GeSe and GeTe. It is shown that the short interatomic distances, low near neighbor coordinations and high covalencies of the amorphous materials, relative to the crystalline, can be rationalized with the model. When scaled to the near neighbor interatomic distances in the amorphous materials, the model yields satisfactory agreement with the observed position and area of the second neighbor X-ray radial distribution function peaks. The model predicts: (a) A first neighbor peak area for GeS which is significantly different from that predicted by the random covalent model and (b) phase separation in certain composition regions which, for the Ge-S system, should be observable by means of transmission electron microscopy. An explanation of why phase separation is not likely to be observable through transmission electron microscopy studies of amorphous GeTe and GeSe is also presented.     

Amorphous and quasicrystalline Ti–Ni–Zr powders synthesized by mechanical alloying

The formation of amorphous and quasicrystalline phases in the Ti₄₅Zr₃₈Ni₁₇ system both directly by mechanical alloying and after subsequent annealing was studied. The presence of amorphous, icosahedral quasicrystalline and the Ti₂Ni-type with a fcc structure phases together with the initial metallic components was found in as-milled samples by X-ray diffraction. An increase of the milling time results in an increase of the amorphous phase content. Icosahedral quasicrystalline phases of Ti–Ni–Zr system were produced by mechanical alloying and subsequent annealing. Differential scanning calorimetry studies up to 520 °C showed an extended exothermal effect starting from 300 °C, which corresponds to the crystallization of the as-milled samples. The shape and size of the particles of the alloys were investigated by scanning electron microscopy and argon adsorption. The Specific area surface of the as-milled sample was rather small, in agreement with scanning electron microscopy data. The kinetics of the hydrogenation of the amorphous alloy Ti₄₅Zr₃₈Ni₁₇ at different temperatures was studied.     

CdS纳米晶的制备及其荧光研究

以醋酸镉、L-半胱氨酸为主要原料,采用水热法制备了尺寸小于10nm、具有强光致荧光的纤锌矿结构CdS半导体纳米晶.水热法可以将晶核形成与晶体生长阶段较好地分开,加之提供的高温熟化条件,可以得到粒度小而均匀、结构良好的纳米晶.用高分辨透射电镜(HRTEM)、XRD对产品的晶体大小、结构进行了详细地表征,分析了影响纳米晶尺寸的因素,用相关性较好的荧光激发与发射光谱研究了硫化镉纳米晶的光致荧光性能.制备的硫化镉(CdS)纳米晶结构良好、粒度均匀、荧光激发专一,最大激发波长在338nm,其发射荧光的波长位于419nm,发射强度大.     

Characterization of mixed phase silicon by Raman spectroscopy

We have examined the common methods for determination of the crystallinity of mixed phase silicon thin films from the TO–LO phonon band in Raman spectra. Spectra are decomposed into contributions of amorphous and crystalline phase and empirical formulas are used to obtain crystallinity either from the integral intensities (peak areas) or from magnitudes (peak maxima). Crystallinity values obtained from Raman spectra excited by Ar⁺ laser green line (514.5 nm) for a special sample with a profile of structure from amorphous to fully microcrystalline were compared with surface crystallinity obtained independently from atomic force microscopy (AFM). Analysis of the Raman collection depth in material composed of grains with absorption depth 1000 nm in an amorphous matrix (absorption depth 100 nm), was used to explain reasons for systematic difference between surface and Raman crystallinities. Recommendations are given for obtaining consistent results.     

Effect of melt treatment on the microstructure and magnetic properties of Nd2Fe14B/α-Fe nanocomposites

Melt-spun Nd_9.5Fe₈₁Zr₃B_6.5 ribbons were prepared under different quenching temperature. The effect of melt treatment on the microstructure and magnetic properties of Nd₂Fe₁₄B/α-Fe nanocomposites was studied by X-ray diffraction, scanning electron microscopy (SEM), differential scanning calorimeter, transmission electron microscopy observations, and magnetization measurements. It was found that melt spinning at different quenching temperature caused the as-quenched ribbons to have distinctive structure. Depending on the quenching temperature, nanocrystalline structure, partially amorphous structure containing nanophases or entirely amorphous structure could be obtained. Moreover, with increasing initial quenching temperature, the microstructure of optimally heat treated ribbons becomes coarser and more irregular, and the magnetic properties of them deteriorated. It is believed that the alteration of melt characteristics which are highly sensitive to the melt temperature may be the cause for the change of glass forming ability, the microstructure and magnetic properties of the ribbons.     

Influence of high temperature deformation on the fracture behavior of a bulk Zr-based metallic glass

Nanocrystalline/amorphous matrix composites obtained by isothermal compression at high temperatures and low strain rates were characterized using transmission electron microscopy. To study the influence of high temperature deformation on the fracture behavior and room temperature plasticity, compression tests with a constant strain rate of 1 × 10⁻⁴ s⁻¹ were applied to the deformed samples. Fracture features of as-cast alloy and deformed samples were analyzed using scanning electron microscopy. Compared with the as-cast alloy, the room temperature plasticity of deformed sample is not destroyed both in the range of 370-395 °C at 1 × 10⁻³ s⁻¹ and at 395 °C in 1 × 10⁻² to 1 × 10⁻³ s⁻¹, and deteriorated at higher temperatures and lower strain rates. Corresponding to the TEM images, the homogenously dispersed nanocrystals with small size contribute to the compressive plasticity, and the aggregated large nanoparticles destroy the plasticity of the sample after high temperature deformation.     

Comparative Analysis of Different Methods of Scanning Electron Microscopy and Test Preparation in Biological Tissue Studies

Crystallography Reports - Different methods of scanning electron microscopy in combination with different sample preparation techniques have been compared by an example of decellularized rat...     

Peculiarities of the microstructure of a nanoscale modification of η-TiO2

Samples with nanoscale η-TiO₂ phase have been obtained by sulfate and modified sulfate methods and are characterized by a complex of techniques: X-ray diffraction, electron diffraction, small-angle X-ray scattering (SAXS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy, and X-ray energy-dispersive analysis. The effect of sample formation conditions on the size and shape of crystallites, content of the amorphous component in the samples, and their elemental composition has been established. A significant change (depending on the synthesis conditions) in the parameters of the diffraction reflection with d ～ 17–21 Å (intensity and interplanar spacing d, Å), pronounced for η-TiO₂, is revealed. This change is most likely related to the variation in the content of water molecules in the interlayer space of η-TiO₂ structure and/or the change in crystallite shape.     

Study of the structure of alkali–silica reaction gel by high-resolution NMR spectroscopy

The alkali–silica reaction is a deleterious chemical process that can occur in concrete. The product of the reaction is an amorphous silicate material with gel characteristics, whose high expansion properties may cause cracking in the matrix and in the discrete aggregate of particles, leading to severe deterioration of the concrete structure. Structural information of this gel at the atomic scale can provide critical information on how to develop appropriate repair of the affected structure. Samples of this gel, produced under in-service conditions in a large concrete dam, were studied by ²⁹Si and ²³Na high-resolution nuclear magnetic resonance spectroscopy, triple quantum magic angle spinning ²³Na nuclear magnetic resonance, scanning electron microscopy, Fourier transform infrared spectroscopy and X-ray diffraction. The short-range atomic structure of the compound was determined as an amorphous potassium–hydroxide–silicate glass, with a Q³-like dominant silicate connectivity, having a silicate speciation highly disproportioned when compared with potassium–silicate glasses with the same K₂O content. Sodium ions are mostly segregated from the bulk amorphous silicate network, forming crystal domains attributed to the trona compound (sodium sesquicarbonate, Na₂CO₃ · NaHCO₃ · 2H₂O). The structural picture at atomic scale obtained in this study gives support for double-layer models of the expansive properties of the alkali–silica reaction gel.     

Crystallization of silicon thin films by current‐induced joule heating using a tungsten overcoat

A modified crystallization process using current‐induced joule heating under vacuum is presented. A thin layer of high temperature resistant tungsten was sputtered on the amorphous silicon as the conducting and annealing medium. The thin film thickness was measured by α‐stepper. The high current density provided effective means in crystallizing the amorphous silicon layer. The crystalline morphology was studied by scanning electron microscopy (SEM) after Secco‐etch, transmission electron microscopy (TEM), and x‐ray diffraction (XRD), under different annealing conditions. The grain size was controlled in the range of 0.1‐0.5 μm and could be increased with annealing time. No tungsten silicide was found. Some defects were formed due to electron‐migration effect near the electrodes. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nucleation and grain growth in as deposited and ion implanted GeTe thin films

The crystallization dynamic of amorphous GeTe 50 nm thick films deposited on a SiO₂/Si substrate by RF magnetron sputtering, either ion implanted by Ge⁺ ions or not, has been analyzed in situ by optical microscopy during annealing in the 143-155 °C temperature range. Raman spectroscopy has been also performed in as deposited, ion implanted (i.i.) and melt quenched (m.q.) amorphous samples to compare the local order among the different amorphous structure. Nucleation and growth rates, for i.i. and as deposited samples, have been observed and directly compared by optical microscopy in a region of about 5 × 10⁴ μm². From these data, the activation energy and pre-exponential terms of each process have been calculated. The nucleation rate and growth velocity of the i.i. films increased by a factor thirteen and a factor three with respect to the as deposited samples. This evidence, in agreement with Raman spectroscopy data, suggests that implantation, providing kinetic energy by collision cascade, induces a local atomic rearrangement towards more relaxed amorphous states. As a result the crystallization kinetic is enhanced by the reduction of wrong bonds formed during sputter deposition, a process which occurs far from equilibrium conditions.     

Raman study of gallium arsenide thin films

The influence of deposition parameters on the structure of the gallium arsenide thin films was investigated by Raman scattering. The study was based on the analysis of the first-order Raman spectra which allows for a differentiation between the amorphous component and crystallites of various sizes. The amorphous and crystalline volume fractions were calculated from the integrated intensities of the deconvoluted peaks. It was demonstrated that a transition occurs from μ-GaAs to a-GaAs for particular plasma conditions and substrate temperature. As a function of the deposition parameters the entire range from mostly microcrystalline to completely amorphous films can be obtained. These properties were consistent with the results obtained on the same samples by transmission high-energy electron diffraction and conventional transmission electron microscopy.     

Synthesis and characterization of phosphate glass microspheres for radiotherapy applications

Glass microspheres containing the radioisotope ³²P, a β^? particle emitter, and half-life of 14.3 days, can be easily introduced in specific human organs such as liver, pancreas, and uterus to kill cancer cells. In the present work phosphate glass microspheres were produced with different compositions and particle size distribution in the range of 20– 30 μm. Two different thermal processes were used to spherodize glass particles originally with irregular shapes. Samples were characterized by X-rays diffraction to check the amorphous structure, energy dispersive X-rays fluorescence spectroscopy to determine the final glass composition, and Fourier transformed infrared spectroscopy to determine the structural groups in the glass structure. The dissolution rate of glass samples in water was determined at 90 °C, and in simulated body fluid (SBF) at 37 °C. Glasses with dissolution rates close to 10^?5 g/(cm² day) were obtained, which make them suitable for the present application. Scanning electron microscopy was used to evaluate the shape of the microspheres before and after the dissolution tests. The cytotoxicity tests showed that these microspheres can be used for biological applications.     

Characterization of the protocrystalline silicon multilayer

The protocrystalline silicon (pc-Si:H) multilayer solar cell is very promising owing to its fast stabilization with low degradation against light irradiation. However, the pc-Si:H multilayers have not extensively been investigated in detail on its material characteristics yet. We present the material characteristics of pc-Si:H multilayers using a transmission electron microscopy (TEM), Fourier transform infra (FTIR) spectroscopy, Raman spectroscopy, constant photocurrent method (CPM) and quantum efficiency (QE) measurement. A TEM micrograph shows that a pc-Si:H multilayer has a repeatedly layered structure consisting of low hydrogen-diluted and highly hydrogen-diluted sublayers. FTIR spectra depict the strong vibration mode at 2090 cm⁻¹ which is attributed to hydrogen-rich amorphous silicon (a-Si:H) regions of highly hydrogen-diluted sublayers. Based on these results, excellent light-soaking behavior of the pc-Si:H multilayers is primarily due to the repeatedly layered structure that improves a structural order in the material.