Showing papers on "Transmission electron microscopy published in 1995"

Energy-filtering transmission electron microscopy

Abstract: Publisher Summary In energy-filtering transmission electron microscopy (EFTEM), the zero-loss electrons or electrons passing an energy-loss window of the electron energy-loss spectroscopy (EELS) are used for image formation. This can be achieved by using the scanning mode in a dedicated scanning transmission electron microscope (STEM) or in a TEM with a spectrometer behind the camera chamber or by using an imaging filter lens in the column of a TEM. The conventional TEM and STEM modes can be combined in this way with the mode of electron spectroscopic imaging (ESI) and electron spectroscopic diffraction (ESD), and different modes can be used to record an EELS spectrum. An EFTEM can therefore make full use of elastic and inelastic electron-specimen interactions. This chapter provides an overview of the physical background and the possibilities of EFTEM. The relevant physics of elastic and inelastic scattering is also discussed followed by the instrumentation of EFTEM. The theoretical approaches for understanding the contrast and examples of application are presented for ESI and for ESD.

Structure and Morphology of Poly(tetramethylene succinate) Crystals

Abstract: Crystals of an aliphatic polyester, poly(tetramethylene succinate) (PBS) are investigated using transmission electron microscopy. Single crystals grown from a 0.01 wt % dichlorobenzene solution show a terrace-like morphology above 65 °C and a leaflike one at lower temperatures. The molecules are packed perpendicular to the basal plane of the single crystals, and twin crystals with a (110) twin plane are frequently observed. The thickness of the single crystal lamellae increases smoothly with increasing crystallization temperature. Lattice parameters of the PBS crystal in the monoclinic unit cell are determined from the electron diffraction patterns of the single crystals and stretched films as a = 0.523 nm, b = 0.908 nm, c = 1.079 nm, and β = 123.87°. The dimension of the c-axis is shorter than the value calculated from a fully extended chain conformation, as has already been found for other aliphatic polyesters. Two types of negative spherulites are observed according to growth temperature.

Cryo-TEM and SANS Microstructural Study of Pluronic Polymer Solutions

Abstract: Cryogenic temperature transmission electron microscopy (cryo-TEM) and small-angle neutron scattering (SANS) were combined to give complete microstructural characterization of aqueous solutions of Pluronic F127, an (EO) 99 (PO) 65 (EO) 99 triblock copolymer. The images of vitrified specimens observed by cryo-TEM provided direct information about the building blocks of these systems, i.e., spheroidal micelles. This confirmed the structural model obtained from analysis of SANS data, which, in addition, provide quantitative information about the system. Occasionally spheroidal micelles were pressed closer to each other during specimen preparation, thus giving rise locally to higher concentration. The cubic phase that was thus formed was directly imaged. The existence of this lyotropic liquid crystalline phase was confirmed by SANS of an originally higher concentration solution. When exposed to shear, the polycrystalline phase transforms into a monodomain crystal with cubic symmetry. The system was also used to demonstrate the potential of selective electron beam radiolysis to enhance contrast in radiation-sensitive, inherently low-contrast systems.

Open‐core screw dislocations in GaN epilayers observed by scanning force microscopy and high‐resolution transmission electron microscopy

Abstract: Structural investigations of organometallic vapor phase epitaxy grown α‐GaN films using high‐resolution transmission electron microscopy and scanning force microscopy have revealed the presence of tunnel‐like defects with 35–500 A radii that are aligned along the growth direction of the crystal and penetrate the entire epilayer. These defects, which are termed ‘‘nanopipes,’’ terminate on the free surface of the film at the centers of hexagonal growth hillocks and form craters with 600–1000 A radii. Either one or two pairs of monolayer‐height spiral steps were observed to emerge from the surface craters which allowed us to conclude that nanopipes are the open cores of screw dislocations. The measured dimensions of the defects are compared to Frank’s theory for the open‐core dislocation.

Morphology and Structure of Oxide Films Formed on Magnesium by Exposure to Air and Water

Abstract: Oxide films formed naturally on pure Mg are investigated by the use of ultramicrotomed cross sections and transmission electron microscopy. The film formed in air immediately after scratching the metal surface is initially thin, dense, amorphous, and relatively dehydrated. Continuing exposure to humid air or exposure to water leads to the formation of a thicker hydrated film adjacent to the metal. The film formed in water contains an additional top layer characterized by a plateletlike morphology. The structure of these layers and their significance in corrosion protection are discussed. The changes occurring in these structures as a result of exposure to the electron beam are reported.

Morphology, structure, and growth of nanoparticles produced in a carbon arc.

Abstract: The morphology and crystalline microstructure of carbon-encapsulated nanoparticles produced in a Huffman-Kr\"atschmer fullerene reactor are studied systematically as a function of location within the reactor. X-ray powder diffraction and high-resolution transmission electron microscopy are used to characterize powder harvested from the reactor walls and the inner and outer cores of the cathode deposit. We observe increased graphitization and crystallinity, more faceting, and more gaps between the nanoparticle and the encapsulating carbon cages in the cathode deposit when compared to the wall powder. We propose a growth model based on gas-phase nucleation to explain these observations, linking carbon arc nanoparticle synthesis to existing work on gas aggregation cluster sources.

Initial bone matrix formation at the hydroxyapatite interface in vivo

Abstract: Dense, sintered, slip-cast hydroxyapatite rods were implanted transfemorally in young adult rats. The femora were excised after 2 and 4 weeks and, following fixation, either embedded in methyl methacrylate for light microscopy, decalcified and prepared for transmission electron microscopy, or freeze fractured in liquid nitrogen for scanning electron microscopic analysis. The latter was performed on the two tissue fragments that remained after freeze fracturing, from which the first contained the implants and the second comprised tissue that had been immediately adjacent to the hydroxyapatite rods. Undecalcified light microscopic sections revealed extensive bone tissue formation around and in contact with the hydroxyapatite rods. The initial bone matrix apposed to the implant surface, as demonstrated with scanning electron microscopy, was either composed of globular deposits or an organized network of collagen fibers. The deposits, which ranged in size from 0.1–1.1 μm, fused to form a cement-like matrix to which collagen fibers were attached. Degradation of the hydroxyapatite surface resulted in the presence of unidirectionally aligned crystallites, with which the newly formed bone matrix was closely associated. Ultrastructural analysis of the bone-hydroxyapatite interface with transmission electron microscopy revealed a 50–600-nm-wide collagen-free granular zone, comprising one or more 40–100-nm-thick electron-dense layer(s). These structural arrangements most probably partially represent the globular deposits and proteinaceous material adsorbed onto and partially in the degrading hydroxyapatite surface. Although the latter change in surface topography may have enhanced bonding of the cement-like matrix to the hydroxyapatite, the cause for this change in topography and the type of bond formed are, at present, unknown. © 1995 John Wiley & Sons, Inc.

Influence of misfit dislocations on the surface morphology of Si1−xGex films

Abstract: The influence of misfit dislocations on the surface morphology of partially strain relaxed Si1−xGex films is studied by atomic force microscopy and transmission electron microscopy. Surface steps arising from the formation of single and multiple 60° dislocations are identified. The role of such steps in the development of a cross‐hatch pattern in surface morphology is discussed.

Epitaxial growth of PbTiO3 thin films on (001) SrTiO3 from solution precursors

Abstract: A mixed alkoxide liquid precursor was used to form epitaxial PbTiO3 thin films by spin-coating on cubic (001) SrTiO3 substrates. The films were heat-treated at temperatures between 380 °C/1 h and 800 °C/1 h. X-ray diffraction, atomic force microscopy, scanning and transmission electron microscopy were used to characterize the microstructure of the films and to evaluate the epitaxial phenomena. At ∼400 °C/1 h, a polycrystalline, metastable Pb-Ti fluorite crystallizes from the pyrolyzed amorphous precursor. At slightly higher temperatures (∼420 °C/1 h), the thermodynamically stable phase with the perovskite structure epitaxially nucleates at the film/substrate interface. A small number of epitaxial grains grow through the film toward the surface and consume the nanocrystalline fluorite grains. Coarsening of the perovskite grains leads to a reduction in mosaic spread during further heating. Pores, which concurrently coarsen with grain growth, produce a pitted surface as they disappear from within the film. At 800 °C/1 ha dense epitaxial PbTiO3 film with a smooth surface is observed. Parameters governing the formation of a- and c-domains are discussed as well as the small tilts of the domain axes away from the substrate normal.

Growth and characterization of carbon nitride thin films prepared by arc‐plasma jet chemical vapor deposition

Abstract: Carbon nitride thin films have been successfully grown on nickel substrates by a novel arc‐plasma jet chemical vapor deposition. These films were characterized by Auger electron spectroscopy, transmission electron microscopy, and Raman spectroscopy. Small grains (∼0.1 μm) as well as nanocrystallites found in the films were identified to be β‐C3N4. Raman spectroscopy also confirmed the existence of β‐C3N4 phase in the films through five pronounced Raman bands as expected from the Hooke’s law approximation based on the vibrational frequencies obtained in analogous compound, β‐Si3N4.

Characterization of Zeolite L Nanoclusters

Abstract: Zeolite L (structure code LTL) crystals are synthesized starting from homogeneous potassium aluminosilicate solutions. Particle size and shape of the formed zeolite L are examined by high-resolution transmission electron microscopy, field emission-scanning electron microscopy, X-ray diffraction, and dynamic light scattering. The synthesis conditions reported lead to nanoclusters of aligned zeolite crystalline domains of dimensions {approximately}40 nm in the channel direction and {approximately}15 nm in the direction perpendicular to the zeolite channels. The nanoclusters have sizes of {approximately}60 nm. Electron microscopy of the nanoclusters reveals the presence of inhomogeneities (intercrystalline porosity) on length scales ranging from 2 to 60 nm. The N{sub 2} and cyclohexane adsorption capacities and the thermal stability of the crystals synthesized are correlated with their microstructure. The preparation of colloidal suspensions of the zeolite L nanoclusters in water is described along with their use in seeded growth and film formation. 16 refs., 12 figs.

Step bunching in chemical vapor deposition of 6H– and 4H–SiC on vicinal SiC(0001) faces

Abstract: Step bunching and terrace widening on 6H– and 4H–SiC epilayers on vicinal {0001} faces have been studied with atomic force microscopy (AFM) and transmission electron microscopy (TEM). Macroscopically, hill‐and‐valley structures are observed on off‐oriented (0001)Si faces, whereas surfaces are rather flat on off‐oriented (0001)C faces. High‐resolution TEM analysis revealed that 3 bilayer‐height steps are dominant on 6H–SiC and 4 bilayer‐height steps on 4H–SiC. The distribution of step height and terrace width are also investigated.

The effect of trace additions of sn on precipitation in Al-Cu alloys: An atom probe field ion microscopy study

Abstract: We have studied the early stages of θ′ precipitation in an Al-Cu-Sn alloy by atom probe field ion microscopy (APFIM) and transmission electron microscopy (TEM). Clusters of Sn atoms were observed in as-quenched samples, and their formation clearly precedes the formation of the θ′ phase. Transmission electron microscopy also revealed that elevated temperature aging leads to the formation of spherical particles. Atom probe analysis and microbeam electron diffraction suggested that these particles were β-Sn (I41/amd, α = 0.583 nm, and c = 0.318 nm). The TEM observations showed that θ′ nucleated heterogeneously on these particles. We have also examined the θ′/α interface following further precipitate growth to check for possible Sn segregation. Atom probe analysis of both the broad face and the narrow rim of the platelike θ′ precipitates has shown no evidence of Sn segregation at θ′/α interfaces. It was also found that cold work prior to aging inhibits the formation of Sn particles resulting in a lower number of these types of nucleation sites for the θ′ phase. Atom probe analysis has also revealed solute depletion at grain boundaries during the early stages of aging in the Al-Cu-Sn alloy, and this, in conjunction with vacancy depletion, explains the formation of narrow precipitate-free zones observed following further aging.

Immobilization of small proteins in carbon nanotubes: high-resolution transmission electron microscopy study and catalytic activity

Abstract: The morphologies of the small proteins Zn2Cd5-metallothionein, cytochrome c3 and β-lactamase I immobilized inside carbon nanotubes have been studied by high-resolution transmission electron microscopy (HRTEM); single protein molecules and their associated forms were clearly observed inside the central cavity and a significant amount remained catalytically active indicating that no drastic conformational change had taken place.

Nanoapatite and organoapatite implants in bone: Histology and ultrastructure of the interface

Abstract: This article reports on the reaction of bone to a new family of nanocrystalline hydroxyapatite biomaterials with crystal sizes similar to those of human bone. Pure nanoapatite cylinders and organoapatite cylinders containing a synthetic nanopeptide were analyzed 28 days after implantation into the spongy bone of Chinchilla rabbits. The experimental techniques used for analysis were light microscopy, scanning electron microscopy, and transmission electron microscopy. Both implant types were well incorporated, and interface events were found to be similar to those observed on human bone surfaces with regard to resorption by osteoclast-like cells and bone formation by osteoblasts. Different types of giant cells were observed resorbing the outermost surfaces of implants. There seemed to be both dissolution of the implant and particulate biodegradation leading to less dense implant regions near the interface, whereas the bulk of the implants remained denser. Transmission electron micrographs revealed that bone bonding occurred with and without an afibrillar intervening layer. Given the biologic reaction observed, these implant materials should be suitable for bone replacement and the organoapatite form could be useful for additional functions such as the release of drugs and optimized release of antibiotics, growth factors, or other substances. The organic component can also be used to control physical properties in a bony implantation bed. © 1995 John Wiley & Sons, Inc.

Growth and structure of copper thin films deposited on (0001) sapphire by molecular beam epitaxy

Abstract: The structure of copper films grown on (0001) sapphire by molecular beam epitaxy at 200°C and 600°C was studied by in situ reflection high-energy electron diffraction (RHEED) and transmission electron microscopy (TEM). Both RHEED and TEM confirm that growth is epitaxial at 200°C, but {111}Cu textured at 600°C. However, in both instances growth occurs with the copper (111) plane parallel to the interface. In the case of the 200°C films the close-packed directions of copper lie parallel to the close-packed directions of sapphire in the interfacial plane; these films contain low angle grain boundaries with the interface normal to the axis of rotation. High resolution and atomic resolution electron microscopy of the interface viewed in cross-section suggest that the atomic structure of the copper/sapphire interface is incoherent. The textured growth at 600°C and near single crystal growth at 200°C are explained in terms of entropy contributions to the free energy of the Cu/Al2O3 interface.

InAs/GaAs quantum dots radiative recombination from zero‐dimensional states

Abstract: Nanometer-scale quantum dots are fabricated using the Stranski-Krastanov growth mode of InAs on GaAs(001) For an average coverage of four monolayers, 10 11 cm -2 pyramidal-shaped dots (12±1 nm base along , 5±1 nm high) are formed as observed in plane-view and cross-section transmission electron microscopy The quantum dots exhibit short-range order, aligning along rows in directions The three-dimensional confinement of the wave function results in ultrasharp luminescence lines (full width at half maximum <015 meV) from individual dots as revealed with highly spatially resolved cathodoluminescence Even at elevated temperatures extremely sharp lines are observed, proving the δ-like zero-dimensional electronic density of states

Electron Microscopy Investigation of the Microstructure of Nafion Films

Abstract: Transmission electron microscopy (TEM) and transmission electron diffraction were used to study the microstructure of recast Nafion films. Zero-loss bright-field (BF) images were obtained, as well as Dage SIT low-light images (minimum specimen damage) and specific sulfur imaging. The results show a nonrandom distribution of the -SOsgroups in the polymer film, primarily as -5 nm clusters. Electron diffraction of deposited Nafion films shows the existence of single crystals, randomly distributed in the film, with an average distance of several microns. The diffraction pattern indicates an orthorhombic crystal structure which is similar to that of polyethylene (PE). The calculated d-spacings of the corresponding lattice planes for the two polymers are also quite close, suggesting similarity in their crystal structures. This result indicates that the fluorocarbon backbone of Nafion is in the form of a linear zigzag chain as in PE and not a twisted chain as in polytetrafluoroethylene (PTFE), despite the