Showing papers on "Transmission electron microscopy published in 2000"

Laser-Assisted Catalytic Growth of Single Crystal GaN Nanowires

Abstract: Xiangfeng Duan and Charles M. Lieber*Department of Chemistry and Chemical BiologyHarVard UniVersity, Cambridge, Massachusetts 02138ReceiVed October 18, 1999Herein we report the bulk synthesis of single crystalline GaNnanowires. Laser ablation of a composite target of GaN and acatalytic metal generates liquid nanoclusters that serve as reactivesites confining and directing the growth of crystalline nanowires.Field emission scanning electron microscopy (FE-SEM) showsthat the product primarily consists of wire-like structures. PowderX-ray diffraction (PXRD) analyses of a bulk nanowire samplecan be indexed to the GaN wurtzite structure, and indicate >95%phase purity. Transmission electron microscopy (TEM), conver-gent beam electron diffraction (CBED), and energy-dispersiveX-ray fluorescence (EDX) analyses of individual nanowires showthat they are GaN single crystals with a [100] growth direction.Nanostructured GaN materials have attracted extensive interestover the past decade due to their significant potential foroptoelectronics.

Structure and stability of ultrathin zirconium oxide layers on Si(001)

Abstract: We have examined the structure of ultrathin ZrO2 layers on Si(001) using medium energy ion scattering and cross-sectional transmission electron microscopy. Films can be deposited on SiO2 layers with highly abrupt interfaces by atomic layer deposition. On HF stripped Si(001), nucleation was inhibited, resulting in poorer film morphology. ZrO2 showed remarkable stability against silicate formation, with no intermixing even after high temperature oxidation. The oxide is vulnerable to high temperature vacuum annealing, with silicidation occurring at temperatures above 900 °C.

Identification of Electron Donor States in N-doped Carbon Nanotubes

Abstract: Nitrogen doped carbon nanotubes have been synthesized using pyrolysis and characterized by Scanning Tunneling Spectroscopy and transmission electron microscopy. The doped nanotubes are all metallic and exhibit strong electron donor states near the Fermi level. Using tight-binding and ab initio calculations, we observe that pyridine-like N structures are responsible for the metallic behavior and the prominent features near the Fermi level. These electron rich structures are the first example of n-type nanotubes, which could pave the way to real molecular hetero-junction devices.

Germanium Nanowire Growth via Simple Vapor Transport

Abstract: Germanium nanowires are synthesized in bulk quantities and high purity using a simple vapor transport process. X-ray diffraction, scanning electron microscopy and transmission electron microscopy studies show that these wires are single crystalline with [111] growth direction; they have diameters ranging from 5 to 300 nm, and lengths up to hundreds micrometer. With brief vacuum thermal treatment, the average diameters of the wires can be further reduced to 16 nm.

Formation Process of CdS Nanorods via Solvothermal Route

Abstract: The formation process of CdS nanorods prepared by the reaction of thiourea and cadmium nitrate in ethylenediamine was investigated in detail by X-ray powder diffraction, transmission electron microscopy, selected-area electron diffraction, and IR techniques. An accordion-like folding process was proposed to account for the formation of CdS nanorods. Further studies indicate that the dissociation of ethylenediamine molecule adsorbed on the surface of CdS results in the formation of CdS nanorods. The obtained CdS single-crystal nanorods showed an abnormal electron diffraction that was explained by the double diffraction of the incident electron inside the sample. The result from electron diffraction confirmed that CdS nanorods grew along c axis.

Sonochemical synthesis and characterization of pure nanometer-sized Fe3O4 particles

Abstract: Sonochemical synthesis of pure nanometer-size Fe3O4 powder with particle size of ca 10 nm is reported in this article. Fe3O4 can be simply synthesized by sonication of iron(II)acetate in water under an argon atmosphere. The properties of pure nanometer-size Fe3O4 particles were characterized by X-ray diffraction, Mossbauer spectroscopy, transmission electron microscopy (TEM), thermogravimetric analysis (TGA) with an external magnetic field, and quantum design SQUID magnetization measurements. The prepared Fe3O4 nanoparticles are superparamagnetic and its magnetization at room temperature is very low (

Thermodynamic stability of amorphous oxide films on metals: Application to aluminum oxide films on aluminum substrates

Abstract: It has been shown on a thermodynamic basis that an amorphous structure for an oxide film on its metal substrate can be more stable than the crystalline structure. The thermodynamic stability of a thin amorphous metal-oxide film on top of its single-crystal metal substrate has been modeled as a function of growth temperature, oxide-film thickness, and crystallographic orientation of the metal substrate. To this end, expressions have been derived for the estimation of the energies of the metal-substrate amorphous-oxide film interface and the metal-substrate crystalline-oxide film interface as a function of growth temperature, and crystallographic orientation of the substrate (including the effect of strain due to the lattice mismatch). It follows that, up to a certain critical thickness of the amorphous oxide film, the higher bulk Gibbs free energy of the amorphous oxide film, as compared to the corresponding crystalline oxide film, can be compensated for by the lower sum of the surface and interfacial energies. The predicted occurrence of an amorphous aluminum-oxide film on various crystallographic faces of aluminum agrees well with previous transmission electron microscopy observations.

Carbon nanotube atomic force microscopy tips: Direct growth by chemical vapor deposition and application to high-resolution imaging

Abstract: Carbon nanotubes are potentially ideal atomic force microscopy probes because they can have diameters as small as one nanometer, have robust mechanical properties, and can be specifically functionalized with chemical and biological probes at the tip ends. This communication describes methods for the direct growth of carbon nanotube tips by chemical vapor deposition (CVD) using ethylene and iron catalysts deposited on commercial silicon-cantilever-tip assemblies. Scanning electron microscopy and transmission electron microscopy measurements demonstrate that multiwalled nanotube and single-walled nanotube tips can be grown by predictable variations in the CVD growth conditions. Force-displacement measurements made on the tips show that they buckle elastically and have very small (≤ 100 pN) nonspecific adhesion on mica surfaces in air. Analysis of images recorded on gold nanoparticle standards shows that these multi- and single-walled carbon nanotube tips have radii of curvature of 3–6 and 2–4 nm, respectively. Moreover, the nanotube tip radii determined from the nanoparticle images are consistent with those determined directly by transmission electron microscopy imaging of the nanotube ends. These molecular-scale CVD nanotube probes have been used to image isolated IgG and GroES proteins at high-resolution.

Core−Shell (Ag)Au Bimetallic Nanoparticles: Analysis of Transmission Electron Microscopy Images

Abstract: Layered core−shell bimetallic silver−gold colloids in the size range of 10−16 nm have been prepared by the seed-growth method. Silver nuclei were covered by gold shells of various thicknesses without any stabilization agent. Interfacial (Ag)Au colloid−2,2‘-bipyridine films were prepared from these bimetallic colloids and used for the purpose of analysis of transmission electron microscopy (TEM) images and electron diffraction. Both observed and calculated TEM images were used to characterize the prepared nanoparticles. On the basis of the analysis of TEM images, the calculated TEM image contrast, and results obtained by electron diffraction, energy-dispersive X-ray analysis, and other experiments, the core−shell structure of the prepared (Ag)Au nanoparticles was revealed. Particles were found to consist of a silver core and a gold shell enriched with silver.

Residual stress, microstructure, and structure of tungsten thin films deposited by magnetron sputtering

Abstract: The residual stress and structural properties of tungsten thin films prepared by magnetron sputtering as a function of sputtering-gas pressure are reported. The films were analyzed in situ by a cantilever beam technique, and ex situ by x-ray diffraction, cross-sectional transmission electron microscopy (TEM), x-ray photoelectron spectroscopy, electron energy-loss spectrometry, and energy-filtered electron diffraction. It is found that the residual stress, microstructure, and surface morphology are clearly correlated. The film stresses, determined in real time during the film formation, depend strongly on the argon pressure and change from highly compressive to highly tensile in a relatively narrow pressure range of 12–26 mTorr. For pressures exceeding ∼60 mTorr, the stress in the film is nearly zero. It is also found that the nonequilibrium A15 W structure is responsible for the observed tensile stress, whereas the stable bcc W or a mixture of bcc W and A15 W are in compression. Cross-sectional TEM eviden...

Growth and characterization of hypothetical zinc-blende ZnO films on GaAs(001) substrates with ZnS buffer layers

Abstract: A stable wurtzite phase of ZnO is commonly observed. In this letter, we report the growth and characterization of zinc-blende ZnO on GaAs(001) substrates. The ZnO films grown on GaAs(001) substrates using microwave-plasma-assisted metalorganic molecular-beam epitaxy were characterized by reflection high-energy electron diffraction, x-ray diffraction, transmission electron microscope, and atomic force microscope measurements. The use of a ZnS buffer layer was found to lead to the growth of the zinc-blende ZnO films. Although the zinc-blende ZnO films were polycrystalline with columnar structures, they showed bright band-edge luminescence at room temperature.

Growth of large-scale GaN nanowires and tubes by direct reaction of Ga with NH3

Abstract: Large-scale wurtzite GaN nanowires and nanotubes were grown by direct reaction of metal gallium vapor with flowing ammonia in an 850–900 °C horizontal oven. The cylindrical structures were as long as 500 μm with diameters between 26 and ∼100 nm. Transmission electron microscopy, scanning electron microscopy, and x-ray diffraction were used to measure the size and structures of the samples. Preliminary results show that the size of the nanowires depends on the temperature and the NH3 flow rate. The growth mechanism is discussed briefly. The simple method presented here demonstrates that GaN nanowires can be grown without the use of a template or catalyst, as reported elsewhere.

The true structure of hexagonal mesophase-templated silica films as revealed by X-ray scattering : Effects of thermal treatments and of nanoparticle seeding

Abstract: This work describes the detailed structural investigation of mesophase-templated mesoporous silica films by 1D and 2D X-ray scattering techniques and transmission electron microscopy. The films are prepared by sol−gel dip coating with 2D hexagonal templating mesophases, yielding 2D mesoporous structures consisting of cylindrical pores whose axes are aligned parallel to the surface. It is shown that drying and thermal treatments induce an unidirectional shrinkage of the layers in the direction of the normal of the film. The true rectangular symmetry is only evidenced by 2D X-ray scattering in two different scattering geometries. 1D diffraction gives only an apparent hexagonal symmetry. It is furthermore shown that although the cylinder axes are randomly orientated within the plane parallel to the surface, there are large domains with well aligned 2D planar unit cells perpendicular to the surface. It is demonstrated that this preferential ordering is destroyed by nanoparticle seeding with amorphous silica o...

Carbon with an onion-like structure obtained by chlorinating titanium carbide

Abstract: Nanoporous carbon prepared by chlorination of TiC powder has been characterized by high-resolution transmission electron microscopy (HRTEM) and showed a disordered amorphous structure of graphite planes with areas of an onion-like structure, where the graphite planes are ordered with concentric spherical graphitic shells (15–35 nm) interpenetrating and overlapping each other.

Deposition of aligned bamboo-like carbon nanotubes via microwave plasma enhanced chemical vapor deposition

Abstract: Aligned multiwall carbon nanotubes have been grown on silicon substrates by microwave plasma enhanced chemical vapor deposition using methane/ammonia mixtures Scanning electron microscopy shows that the nanotubes are well aligned with high aspect ratio and growth direction normal to the substrate Transmission electron microscopy reveals that the majority phase has a bamboo-like structure Data are also presented showing process variable effects on the size and microstructure of the aligned nanotubes, giving insight into possible nucleation and growth mechanisms for the process

Characterization of copper layers produced by cold gas-dynamic spraying

Abstract: The cold gas-dynamic spray method produces coatings or deposits by introducing solid feedstock particles into a supersonic gas stream developed through the use of a converging-diverging (de Laval) nozzle. The particles thus accelerated impact on a substrate surface and develop into a dense deposit through a process believed to be similar to cold compaction. The work reported here explores the internal nature and physical characteristics of copper deposits produced by the cold gas-dynamic spray method using two vastly different starting powders: in one case, a “spongy” copper obtained by a direct-reduction process, and in the second, a denser, more spheroidal particulate produced by gas atomization. Optical and electron microscopies (scanning electron microscopy [SEM] and transmission electron microscopy [TEM]) were used to observe details of microstructure in the feedstock particles and deposits. Young’s modulus and residual stress measurements for the deposits were obtained through mechanical means, and measurements of hardness and electrical conductivity are reported. The internal structure of the cold-spray deposit was influenced by the surface purity of the feedstock material.

Magnetic properties and magnetization reversal of α-Fe nanowires deposited in alumina film

Abstract: Uniform arrays of Fe nanowires were prepared by electrochemical deposition of iron into nanoporous anodic aluminum oxide films. The microstructure and crystal structures of the nanowires were studied by transmission electron microscopy and electron diffraction. It was found that each nanowire looked like a chain of dots and each dot in the chain was supposed to be a single crystal of α-Fe. Each dot was shown to be a single magnetic domain. The magnetic properties of a uniform array of Fe nanowires and the magnetization reversal in a Fe nanowire were investigated by Mossbauer spectroscopy and vibrating sample magnetometry, which demonstrated that the film of Fe nanowires in alumina had superior perpendicular magnetic characteristics. The magnetic studies also revealed that the moments of each single domain dot were oriented along the chain. Experimental results could be interpreted by the reversal model of “chains of spheres” with the symmetric fanning mechanism.

Nanocrystalline Tungsten Oxide Thin Film: Preparation, Microstructure, and Photochromic Behavior

Abstract: A nanocrystalline tungsten oxide photochromic thin film was prepared by colloid chemistry method. The microstructure, phase transition involved in the solution process, photochromic behavior, and mechanism of the film were investigated by means of transmission electron microscope, x-ray diffraction, ultraviolet-visible absorption spectra, and x-ray photoelectron spectra. It was found that the particle size and crystallinity of the thin film could be easily controlled by adjusting the concentration of oxalic acid in the colloid solution of tungsten oxide hydrate. With the increase of the oxalic acid concentration, the size of nanoparticles in the film decreased sharply, and meanwhile, a blue shift of the absorption peaks caused by the quantum size effect was observed accordingly. With the increase of the pH in the solution, tungsten oxide hydrate was gradually transformed into an oxided 12-tungstate with Keggin structure, which led to the change of photochromic property of the films. The photochromism of the film is believed to be due to the electron transfer between the different valence states of tungsten ions located in adjacent sites.

Microstructural, electrical, and optical properties of SnO2 nanocrystalline thin films grown on InP (100) substrates for applications as gas sensor devices

Abstract: SnO2 thin films were grown on p-InP (100) substrates by using radio-frequency magnetron sputtering at low temperature. Transmission electron microscopy (TEM) and electron diffraction pattern measurements showed that these SnO2 thin films were nanocrystalline. The capacitance–voltage measurements at room temperature showed that the type and the carrier concentration of the nominally undoped SnO2 film were n type and approximately 1.62×1016 cm−3, respectively. Raman scattering measurements showed that the grain sizes of the nanocrystalline films were below 10 nm, which was in reasonable agreement with the result obtained from the high-resolution TEM measurements. Photoluminescence measurements showed a broad peak below the band-to-band emission. These results can help improve the understanding of SnO2 nanocrystalline films grown on p-InP (100) substrates for applications in high-sensitivity gas sensors.

Synthesis, Isolation, and Chemical Reactivity Studies of Nanocrystalline Zinc Oxide

Abstract: Nanocrystals of ZnO have been produced by an alkoxide based synthesis involving diethylzinc, tert-butyl alcohol, ethanol, and water. The resulting ZnO is in the form of a powder made up of zincite crystallites in the size range of 3−5 nm. These crystallites aggregate together to form larger spherical particles. These spherical particles have been studied by transmission electron microscopy (TEM) and Brunauer−Emmet−Teller (BET) methods and were found to contain many pores and tunnels. It is because of this that an uncharacteristically high surface area is found, averaging about 120 m2/g. As seen with other metal oxides, once they are made as nanoparticles, their reactivity is greatly enhanced. This is thought to be due to morphological differences, whereas larger crystallites have only a small percentage of reactive sites on the surface, smaller crystallites will possess much higher surface concentration of such sites. Elemental analysis, X-ray diffraction, and infrared spectroscopy have been used to chara...

Lorentz microscopy of circular ferromagnetic permalloy nanodisks

Abstract: Circular permalloy elements were fabricated by a combination of electron beam lithography, thermal evaporation and liftoff technique on electron transparent membrane substrates. The magnetic properties have been studied by Lorentz transmission electron microscopy. In situ magnetizing experiments have been carried out to obtain information about the nucleation and propagation of magnetic domains within the permalloy nanodisks and to determine the nucleation and saturation fields. The diameter of the patterned elements has been varied between 180 and 950 nm, the height was 15 nm. The experiments showed that the vortex configuration is the most favorable state in zero field conditions of all investigated permalloy nanodisks.

Evolution of the microstructure in microcrystalline silicon prepared by very high frequency glow-discharge using hydrogen dilution

Abstract: A series of samples was deposited by very high frequency glow discharge in a plasma of silane diluted in hydrogen in concentrations SiH4/(SiH4+H2) varying from 100% to 1.25%. For silane concentrations below 8.4%, a phase transition between amorphous and microcrystalline silicon occurs. Microcrystalline silicon has been characterized by transmission electron microscopy (TEM) and x-ray diffraction. The medium-resolution TEM observations show that below the transition, the microstructure of microcrystalline silicon varies in a complex way, showing a large variety of different growth structures. For the sample close to the phase transition, one observes elongated nanocrystals of silicon embedded in an amorphous matrix followed at intermediate dilution by dendritic growth, and, finally, at very high dilution level, one observes columnar growth. X-ray diffraction data evidence a (220) crystallographic texture; the comparison of the grain sizes as evaluated from TEM observations and those determined using Scherr...

Focused electron beam induced deposition of gold

Abstract: Codeposition of hydrocarbons is a severe problem during focused electron beam writing of pure metal nanostructures. When using organometallic precursors, a low metal content carbonaceous matrix embedding and separating numerous nanosized metal clusters is formed. In this work, we present a new and easy approach to obtain high purity gold lines: the use of inorganic PF3AuCl as a precursor. Electrical resistivities as low as 22 μΩ cm at 295 K (ten times the bulk Au value) were obtained. This is to our knowledge the best value for focused electron beam deposition obtained from the vapor phase so far. No special care was taken to prevent hydrocarbon contamination. The deposited nanostructure consists of gold grains varying in size and percolation with beam parameters.

Structure of synthetic 2-line ferrihydrite by electron nanodiffraction

Abstract: Comparison of experimental single-crystal electron diffraction patterns of synthetic two-line ferrihydrite (2LFh) with simulated single-crystal electron-diffraction patterns indicates that a synthetic 2LFh sample contains highly disordered material and nanocrystals with structures based on hexagonal (ABAB) and cubic (ABC) stacking of close-packed layers of O2− and OH− ions. An apparently continuous variation in ordering exists between the highly disordered material and each of the crystalline structures, suggesting that both nanocrystalline structures represent local extremes of three-dimensional ordering. Experimental diffraction patterns were obtained using electron nanodiffraction, a technique in which the finely focused beam from a field-emission gun in an electron microscope can be used to produce diffraction patterns from areas <1 nm across. Nanodiffraction patterns from the highly disordered material have diffuse streaks rather than distinct reflections, and are consistent with a two-dimensional structure that consists of close-packed anionic layers with essentially complete stacking disorder and nearly random distribution of Fe atoms. The structure with cubic stacking is similar to maghemite and has ~25% of the Fe in tetrahedral sites. The structure with hexagonal stacking consists of double chains of face-sharing Fe octahedra; each octahedron shares one face, two edges, and three corners with adjacent octahedra. Previous results from transmission electron microscopy, powder X-ray and electron diffraction, and synchrotron-based techniques reflect the overall high degree of structural disorder rather than the characteristics of the maghemite-like and double-chain structures.

Structure and properties of hard and superhard Zr–Cu–N nanocomposite coatings

Abstract: Zr‐Cu‐N nanocomposite films represent a new material of the type-nanocrystalline transition metal nitride (nc-MeN):metal. In the present work, films were deposited onto steel substrates using unbalanced dc reactive magnetron sputtering of a Zr‐Cu (62:38 at.%) target. Film structure, chemical composition, mechanical and optical properties were investigated by means of X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, wavelength dispersive electron probe microanalysis, depth-sensing microindentation and spectroscopic ellipsometry. It was found that (i) there is a strong correlation between the film structure, Cu content and film properties and (ii) either hard or superhard Zr‐Cu‐N films can be formed. The superhard coatings with hardness H\ 40 GPa are characterized by a columnar structure, a strong 111 XRD peak from ZrN grains and no diffraction peaks from Cu. These films exhibit a high elastic recovery of about 80% and contain a very low amount of Cu, approximately 1‐2 at.%. In contrast, the hard (B 40 GPa) Zr‐Cu‐N films are characterized by many diffraction peaks from polyoriented ZrN and Cu grains, a more random microstructure and a Cu content higher than 2 at.%. The optical properties of nanocomposite Zr‐Cu‐N films depend on the stoichiometry of the hard ZrNx compound and the content of Cu in the film. © 2000 Elsevier Science S.A. All rights reserved.

Nanocrystalline Ni–Cu alloy plating by pulse electrolysis

Abstract: Pulse electrolysis and the effect of pulse parameters on the composition of Ni–Cu alloys deposited from a citrate bath has been studied. Coherent, smooth and bright coating is obtained by precise control of the pulse time, relaxation time and peak current density. Stirring, high pH and high temperature is shown to increase the copper content of the deposit. X-Ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies revealed that the deposited Ni–Cu alloy is nanocrystalline in nature (crystallite size ∼2.5–28.5 nm) and it exists in a single FCC-phase. The strain and mean crystallite size has also been estimated from X-ray diffraction line broadening analysis. Knoop microhardness for pulse current (PC) plated samples is higher than the direct current (DC) plated sample and the internal stress is lower for the PC sample. The corrosion resistance of the pulse-plated Ni–35.8 wt.% Cu alloy, as evaluated by potentiodynamic polarisation studies in deaerated 3 wt.% NaCl solution at 50°C, is better than that of the DC-plated alloy and the commercial Monel-400.