Showing papers on "Transmission electron microscopy published in 1999"

Silver-based crystalline nanoparticles, microbially fabricated

Abstract: One mechanism of silver resistance in microorganisms is accumulation of the metal ions in the cell. Here, we report on the phenomenon of biosynthesis of silver-based single crystals with well-defined compositions and shapes, such as equilateral triangles and hexagons, in Pseudomonas stutzeri AG259. The crystals were up to 200 nm in size and were often located at the cell poles. Transmission electron microscopy, quantitative energy-dispersive x-ray analysis, and electron diffraction established that the crystals comprise at least three different types, found both in whole cells and thin sections. These Ag-containing crystals are embedded in the organic matrix of the bacteria. Their possible potential as organic-metal composites in thin film and surface coating technology is discussed.

Mechanism of Zeolite A Nanocrystal Growth from Colloids at Room Temperature

Abstract: The formation and growth of crystal nuclei of zeolite A from clear solutions at room temperature were studied with low-dose, high-resolution transmission electron microscopy in field emission mode and with in situ dynamic light scattering Single zeolite A crystals nucleated in amorphous gel particles of 40 to 80 nanometers within 3 days at room temperature The resulting nanoscale single crystals (10 to 30 nanometers) were embedded in the amorphous gel particles The gel particles were consumed during further crystal growth at room temperature, forming a colloidal suspension of zeolite A nanocrystals of 40 to 80 nanometers On heating this suspension at 80°C, solution-mediated transport resulted in additional substantial crystal growth

Cobalt nanosized particles organized in a 2d superlattice : synthesis, characterization, and magnetic properties

Abstract: Colloidal assemblies are used to synthesize FCC cobalt nanoparticles. The particles are coated, extracted from micelles, and characterized by transmission electron microscopy, small angle X-ray scattering, and electron and X-ray diffraction spectroscopy. These cobalt metal particles are stable in air, have a narrow size distribution, and on deposition on a graphite support, spontaneously form a 2D hexagonal network. The magnetic properties are compared when they are dispersed in a solvent and organized in 2D superlattices. Changes in the hysteresis loop and in the blocking temperature are observed and attributed to collective flip of the magnetization of adjacent particles.

Solder reaction-assisted crystallization of electroless Ni-P under bump metallization in low cost flip chip technology

Abstract: Solder reaction-assisted crystallization of electroless Ni–P under bump metallization in the Si/SiO2/Al/Ni–P/63Sn–37Pb multilayer structure was analyzed using transmission electron microscopy, scanning electron microscopy, energy dispersive x-ray, and electron probe microanalyzer. The electroless Ni–P had an amorphous structure and a composition of Ni85P15 in the as-plated condition. Upon reflow, the electroless Ni–P transformed to Ni3Sn4 and Ni3P. The crystallization of electroless Ni–P to Ni3P was induced by the depletion of Ni from electroless Ni–P to form Ni3Sn4. The interface between electroless Ni–P and Ni3P layer was planar. From the Ni3P thickness-time relationship, the kinetics of crystallization was found to be diffusion controlled. Conservation of P occurs between electroless Ni–P and Ni3P, meaning that little or no P diffuses into the molten solder. Combining the growth rates of Ni3Sn4 and Ni3P, the consumption rate of electroless Ni–P was determined. Based upon microstructural and diffusion r...

A study of low temperature crystallization of amorphous thin film indium–tin–oxide

Abstract: Deposition of tin-doped–indium-oxide (ITO) on unheated substrates via low energy processes such as electron-beam deposition can result in the formation of amorphous films The amorphous-to-crystalline transformation was studied in this system using in situ resistivity, time resolved reflectivity, glancing incidence angle x-ray diffraction, and transmission electron microscopy The resistivity of 180 nm thick In2O3(99 wt %SnO2) was monitored during isothermal anneals at 125, 135, 145, and 165 °C The dependence of the resistance on the volume fraction of crystalline phase was established using glancing incidence angle x-ray diffraction and a general two phase resistivity model for this system was developed These studies show that, upon annealing, as-deposited amorphous ITO undergoes both a structural relaxation and crystallization Structural relaxation of the amorphous material includes local ordering that increases the ionized vacancy concentration which, in turn, increases the carrier density in the

Large-scale synthesis of single crystalline gallium nitride nanowires

Abstract: Large-scale synthesis of single crystalline GaN nanowires in anodic alumina membrane was achieved through a gas reaction of Ga2O vapor with a constant flowing ammonia atmosphere at 1273 K X-ray diffraction, Raman backscattering spectroscopy, scanning electron microscopy, and transmission electron microscopy indicated that those GaN nanowires with hexagonal wurtzite structure were about 14 nm in diameter and up to several hundreds of micrometers in length The growth mechanism of the single crystalline GaN nanowires is discussed

Polybenzimidazole nanofiber produced by electrospinning

Abstract: Nanofibers of aromatic heterocyclic PBI (poly(2,2′(m-phenylene)-5,5′ bibenzimidazole)) polymer were made by an electrospinning process. The diameter of the fibers was around 300 nanometers. A liquid jet of a polymer solution, formed when electrical force overcame surface tension, stretched and dried as the solvent evaporated. The resulting electrically charged nanofibers were attracted to and collected on a rotating cylinder covered with aluminum foil that was electrically grounded. The fibers were collected in non-woven sheets about 20 centimeters wide and 100 centimeters long, with a compressed thickness of a few tens of microns. The mass per unit area of the nonwoven sheets was 15 to 50 g/m2. The nonwoven fabric of PBI nanofibers was treated with sulfuric acid, and then washed, dried and heated to improve its strength. Optical microscopy showed that electrospun PBI nanofibers fibers were birefringent. Other morphological features of these fibers were observed with scanning electron microscopy, transmission electron microscopy and atomic force microscopy.

Photoinduced energy and electron transfer reactions in lamellar polyanion/polycation thin films: Toward an inorganic 'leaf'

Abstract: Sequential adsorption of polyanions and polycations was used to make a five-component energy/electron-transfer cascade, which mimics some of the functions of natural photosynthetic assemblies. The photon antenna part of the system consists of coumarin- and fluorescein-derivatized poly(allylamine hydrochloride) (Coum-PAH and Fl-PAH), palladium(II)tetrakis(4-N,N,N-trimethylanilinium) porphyrin (PdTAPP4+) or palladium(II)tetrakis(4-sulfonatophenyl) porphyrin (PdTSPP4-) layers, interleaved with anionic Zr(HPO4)2·H2O (α-ZrP) sheets. α-ZrP or HTiNbO5 sheets separate the porphyrin electron donor from a polyviologen electron acceptor layer. Layer-by-layer growth of these thin film assemblies was characterized by atomic force microscopy (AFM) and ellipsometry on planar supports, and by elemental analysis, surface area measurements, and transmission electron microscopy high on surface area silica supports. UV−vis absorption and steady-state emission spectroscopies showed that the overall energy/electron-transfer re...

Electron microscopy and catalytic study of silver catalysts : structure sensitivity of the hydrogenation of crotonaldehyde

Abstract: Conventional (CTEM) and high-resolution transmission electron microscopy (HRTEM) of Ag/SiO2 and Ag/TiO2 catalysts were combined with reaction studies of the hydrogenation of crotonaldehyde to examine the influence of the silver particle size on activity and selectivity toward the unsaturated alcohol. Nanostructural features of the catalysts, as their silver particle size distribution, mean particle size, and dispersion were markedly dependent on the preparation method. For silica-supported Ag catalysts the selectivity to the unsaturated alcohol was found to be (59 ± 3)%, independent of the particle size in the range of 3.7 nm ≤ dAg ≤ 6.3 nm. Moreover, the specific activities were similar in magnitude and exhibit no clear trend with particle size. Consequently, the hydrogenation of crotonaldehyde over these Ag/SiO2 catalysts appears to be structure-insensitive. Titania-supported silver catalysts reduced in hydrogen at low temperature (473 K, LTR) or high temperature (773 K, HTR), however, showed a quite d...

Synthesis of Nanoscale Platinum Colloids by Microwave Dielectric Heating

Abstract: Polymer-stabilized platinum colloids with nearly uniform spherical shape were prepared by microwave dielectric heating. The average diameters of the as-prepared platinum colloids were 2−4 nm with a...

Formation and morphology of calcium sulfate nanoparticles and nanowires in water-in-oil microemulsions

Abstract: Nanosized insoluble metal sulfate aggregates, predominantly CaSO4, have been prepared in a variety of water-in-oil (w/o) microemulsions stabilized by either nonionic or ionic surfactants. Particles were visualized by transmission electron microscopy (TEM) and the identities of the aggregates confirmed by energy-dispersive X-ray analysis (EDXA). BaSO4 prepared in n-heptane microemulsions stabilized by the sodium salt of Aerosol-OT (AOT) appeared as slightly irregular aggregates, 8−50 nm in diameter. In contrast, BaSO4 in n-heptane microemulsions stabilized by ammonium diethylhexyl phosphate (NH4DEHP) existed in the form of submicron-sized “flocs” comprising nanospheres 5−7 nm in diameter. BaSO4 synthesis in cyclohexane microemulsions stabilized by tetraethylene glycol monododecyl ether (C12E4), produced discrete essentially monodisperse nanospheres 8−10 nm in diameter. A wider variety of morphologies were encountered in the synthesis of CaSO4 which produced nanospheres, ellipsoids, rods, nanohairs, nanowir...

Chemical vapor synthesis and luminescence properties of nanocrystalline cubic y2o3:eu

Abstract: Nanocrystalline europium doped yttria was synthesized using a chemical vapor technique. The powder was characterized by x-ray diffraction, transmission electron microscopy, and ultraviolet spectroscopy. For the first time it was possible to obtain single phase Y2O3:Eu nanoparticles crystallized in the cubic structure with an average particle size of only 10 nm. The reflection, excitation, and emission spectra were studied. The nanoparticles show blue shifted absorption bands with respect to coarse grained material.

Origin of Solid‐State Activated Sintering in Bi2O3‐Doped ZnO

Abstract: Activated sintering in Bi2O3-doped ZnO has been studied with emphasis on the mechanistic role of intergranular amorphous films. The atomic-level microstructures and bismuth solute distributions in doped powders have been investigated using high-resolution electron microscopy and scanning transmission electron microscopy. Densification is observed to be significant below the bulk eutectic temperature in the presence of Bi2O3 concentrations as low as 0.58 mol%. Transmission electron microscopy of as-calcined and sintered powders shows that significant neck growth and particle coarsening occur in the solid state. Intergranular amorphous films of ∼1 nm thickness, terminating in wetting menisci at sinter-necks, are observed to form concurrently with the onset of activated sintering. In a few instances, amorphous films are also observed at surfaces of the ZnO particles. These films appear to be the free-surface counterpart to equilibrium-thickness intergranular films. Activated sintering in this binary system is attributed to rapid mass transport through subeutectic, equilibrium-thickness intergranular films, with the amorphous phase also providing capillary pressure.

Ultrastructural investigation of pellicle morphogenesis at two different intraoral sites during a 24-h period

Abstract: The purpose of the present in vivo study was to examine salivary pellicle formation on enamel surfaces at two different intraoral sites for periods of 1 min up to 24 h by means of transmission electron microscopy. Bovine enamel specimens were attached to the buccal and lingual surfaces of the upper first molars in three subjects using removable intraoral splints. Specimens were carried over periods of 1, 10, 30 and 60 min, 2, 6 and 24 h and were processed for transmission electron microscopy. After 1 min, an electron dense pellicle layer, 10–20 nm thick, was observed on the enamel surfaces. The subsequent adsorption of salivary biopolymers was governed by local influences of the oral cavity. Specimens located on the lingual aspect were covered within 2 h by a 20- to 80-nm-thick, homogeneous, predominantly granular-structured pellicle. The thickness of the surface coatings that were adsorbed on lingually carried specimens increased to 100–200 nm after 24 h. In contrast, on the buccally mounted specimen surfaces, a variably structured pellicle with granular and globular components could be detected after intraoral exposure for 2 h. The thickness of the 2-h buccal pellicles ranged between 200 and 700 nm. After 24 h, the buccally positioned specimens were covered by a dense globular pellicle layer varying in thickness from 1000 to 1300 nm. It is suggested that in vivo pellicle formation is initiated by adsorption of an electron-dense layer of salivary proteins. Further adsorption of salivary biopolymers leads to the formation of an outer loosely arranged pellicle layer. Under oral conditions, the locally available salivary biopolymers and the influence of locally effective shearing forces are of significance for the ultrastructural pattern and extent of pellicle formation.

Molecular beam epitaxy growth of ferromagnetic single crystal (001) Ni2MnGa on (001) GaAs

Abstract: The ferromagnetic shape memory alloy Ni2MnGa has been grown on GaAs by molecular beam epitaxy. In situ reflection high energy electron diffraction, ex situ x-ray diffraction, and transmission electron microscopy selective area electron diffraction indicate the single crystal growth of a pseudomorphic tetragonal phase of Ni2MnGa on (001) GaAs. Both vibrating sample magnetometry and superconducting quantum interference device magnetometry measurements show that the Ni2MnGa film is ferromagnetic with in-plane magnetization and has a Curie temperature of ∼320 K.

Surfactant-assisted synthesis of highly dispersed molybdenum sulfide

Abstract: A simple preparation method of high surface area MoS2 using reactions in aqueous solution in the presence of an organic surfactant has been reported. Molybdenum sulfide was obtained from the reactions of aqueous (NH4)2MoS4 with N2H4 or NH2OH·H2SO4, followed by thermal treatment under nitrogen flow. The solids were characterized by X-ray diffraction, chemical analysis, surface and porosity measurements, photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HREM). Addition of the cetyltrimetylammonium chloride surfactant to the reaction mixtures led to the increase of specific surface area up to 210 m2/g and disappearance of MoS2 layers stacking. Single layer, short fringes of MoS2 were observed by HREM. The solids contained 4−8 wt % of carbon impurity, which probably acts as a textural stabilizer.

Effects of Al content on hardness, lattice parameter and microstructure of Ti1-xAlxN films

Abstract: Ti1−xAlxN films were synthesized by the arc ion plating method using Ti1−xAlx alloy targets with differing Al contents. X-ray diffraction patterns from films indicated that the NaCl structure for x≤0.6 changed into wurtzite structure for x≥0.7. For films with x≤0.6, the lattice parameter decreased in proportion to the x value, and correspondingly, the hardness gradually increased from ∼2000 HV for x=0 up to 3200 HV for x=0.6. On the other hand, the hardness of films with x≥0.7 abruptly deceased from ∼3000 HV for x=0.7 to 1400 HV for x=1. Further, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations showed that the films with x=0.6 had a typical columnar structure with grain sizes of 100–200 nm. While for films with x≥0.7, a columnar structure disappeared and excess Al atoms did not segregate at the grain boundaries as AlN, but Ti and Al were uniformly dissolved and distributed as nitride grains with wurtzite structure.

Characterisation of ZnO-based varistors prepared from nanometre Precursor powders

Abstract: Nanometre precursor powders of ZnO and various additives are synthesised by a new technology, namely chemical coprecipitation and plasma pyrolysis Transmission electron microscopy (TEM) and the Brunauer-Emmett-Teller (BET) method are employed to illustrate the precursor powders' properties, The ZnO and additive composite powders are composed of spherical particles whose size is about 10-50 nm, Varistors are prepared by sintering the precursor powders at 1050 degrees C, Scanning electron microscopy (SEM) results show that the average grain size of the varistors is about 10 mu m The prepared ZnO-based varistors have excellent electronic properties Analytical results reveal that the breakdown voltage is 5000 V mm(-1) and the non-linear coefficient is about 540 Copyright (C) 2000 John Wiley a Sons, Ltd

Abrupt PbTiO3/SrTiO3 superlattices grown by reactive molecular beam epitaxy

Abstract: PbTiO3/SrTiO3 superlattices were grown on (001) SrTiO3 substrates by reactive molecular beam epitaxy (MBE). Sharp superlattice reflections were observed by x-ray diffraction. High-resolution transmission electron microscopy of a [(PbTiO3)10/(SrTiO3)10]15 superlattice revealed that the PbTiO3/SrTiO3 interface structure is atomically sharp. The superlattice interfaces are fully coherent; no misfit dislocations or other crystal defects were observed in the superlattice by transmission electron microscopy. Selected area electron diffraction patterns indicated that the PbTiO3 layers are oriented with the c axis parallel to the growth direction. The dimensional control and interface abruptness achieved in this model system indicate that MBE is a viable method for constructing oxide multilayers on a scale where enhanced dielectric effects are expected.

Fabrication of CdS Nanoparticle Chains along DNA Double Strands

Abstract: Positively charged CdS nanoparticles having diameter of 3.0 ± 0.2 nm were prepared by the chemical modification of their surfaces with thiocholine. Chains of size-quantized CdS nanoparticles were prepared by using the electrostatic interaction between positively charged nanoparticle surfaces and the phosphate groups of DNA molecules. The observation by transmission electron microscopy revealed that the CdS nanoparticles were arranged in a quasi one dimension with dense packing. The line width of a nanoparticle array was equal to the diameter of CdS nanoparticles that was ca. 3.0 nm. The average distance between the centers of the adjacent nanoparticles was estimated to be 3.5 nm, which was almost equal to the length of 10 base pairs in DNA double strands.