Showing papers on "Transmission electron microscopy published in 1998"

Ultrafast Studies of Photoexcited Electron Dynamics in γ- and α-Fe2O3 Semiconductor Nanoparticles

Abstract: The ultrafast dynamics of photoexcited electrons in the semiconductor iron oxides, γ-Fe2O3 and α-Fe2O3, have been measured using femtosecond laser spectroscopy. Transmission electron microscopy shows the γ-Fe2O3 particles are spherical, with 1−2 nm average diameter, and the α-Fe2O3 are spindle-shaped, with average dimensions of 1 × 5 nm. Static electronic absorption measurements of the colloids suggest that they may be in the quantum confined regime. Steady-state emission measurements show that the <400 nm direct transitions are moderately emissive, but the indirect transitions in the visible do not produce measurable emission. The ultrafast transient absorption decay profiles measured for the γ-Fe2O3 and α-Fe2O3 samples synthesized in our lab as well as a commercial sample of γ-Fe2O3 are the same and are fit best with three exponentials with 0.36, 4.2, and 67 ps time constants. The decay profiles are independent of pump power, probe wavelength, and pH and were not affected by lattice doping with other me...

Sonochemical Preparation and Size-Dependent Properties of Nanostructured CoFe2O4 Particles

Abstract: Nanostructured CoFe2O4 particles were prepared by a sonochemical approach, first by preparation of the amorphous precursor powders, followed by heat treatment at relatively very low temperatures. The precursor was prepared by sonochemical decomposition of solutions of volatile organic precursors, Fe(CO)5 and Co(NO)(CO)3, in Decalin at 273 K, under an oxygen pressure of 100−150 kPa. The amorphous nature of these particles was confirmed by various techniques, such as scanning and transmission electron microscopy (SEM and TEM), electron microdiffraction, and X-ray diffractograms. Magnetic measurements, Mossbauer, and electron paramagnetic resonance (EPR) spectral studies indicated that the as-prepared amorphous particles were superparamagnetic. The Mossbauer parameters and the significantly low (45 emu/g) observed saturation of magnetization of the annealed sample, compared to that of the bulk sample (72 emu/g), reflected its nanocrystalline nature.

Octahedral boron nitride fullerenes formed by electron beam irradiation

Abstract: Here we report on the formation of fullerenes with a reduced number of layers (typically ≤3) in boron nitride (BN) which was subjected to in situ electron irradiation at 20 and 490°C in a high resolution 300 kV transmission electron microscope (HRTEM). The BN fullerenes exhibited B/N stoichiometry of ∼1 as confirmed by electron energy loss spectroscopy using a 1 nm electron probe. The fullerene HRTEM images revealed rectangle-like shapes when viewed in specific projections, unlike the quasispherical carbon fullerene morphology. The octahedral BN fullerene model [O. Stephan, Y. Bando, A. Loiseau, F. Willaime, N. Shramchenko, T. Tamiya, and T. Sato, Appl. Phys. A 67, 107 (1998)] is verified by the BN fullerene observations at different viewing angles.

Aluminum-induced crystallization of amorphous silicon on glass substrates above and below the eutectic temperature

Abstract: The achievement of high-quality continuous polycrystalline silicon (poly-Si) layers onto glass substrates by using aluminum-induced crystallization is reported. The crystallization behavior of dc sputtered amorphous silicon on glass induced by an Al interface layer has been investigated above and below the eutectic temperature of 577 °C. Secondary electron micrographs in combination with energy-dispersive x-ray microanalysis show that annealing below this temperature leads to the juxtaposed Al and Si layers exchanging places. The newly formed poly-Si layer is fully crystallized and of good crystalline quality, according to Raman spectroscopy and transmission electron microscopy investigations. At 500 °C, the time needed to crystallize a 500-nm-thick Si layer is as short as 30 min. By annealing above the eutectic temperatures, layer exchange is not as pronounced and the newly formed Al layer is found to contain a network of crystallized Si.

Phase separation in InGaN grown by metalorganic chemical vapor deposition

Abstract: We report on phase separation in thick InGaN films with up to 50% InN grown by metalorganic chemical vapor deposition from 690 to 780 °C. InGaN films with thicknesses of 0.5 μm were analyzed by θ–2θ x-ray diffraction, transmission electron microscopy (TEM), and selected area diffraction (SAD). Single phase InGaN was obtained for the as-grown films with <28% InN. However, for films with higher than 28% InN, the samples showed a spinodally decomposed microstructure as confirmed by TEM and extra spots in SAD patterns that corresponded to multiphase InGaN.

Structural properties of microcrystalline silicon in the transition from highly crystalline to amorphous growth

Abstract: The growth of microcrystalline silicon prepared by plasma-enhanced chemical vapour deposition depends on the deposition conditions and yields films with variable content of crystalline grains, amorphous network, grain boundaries and voids. The changes in the structural properties of a series of films grown under a variation of the dilution of the process gas silane in hydrogen, which induces a transition from highly crystalline to amorphous growth, were investigated. The evolution of the crystalline volume fraction was quantitatively analysed by Raman spectroscopy and X-ray diffraction. The results confirm the need for proper correction of the Raman data for optical absorption and Raman cross-section. Transmission electron microscopy was used to investigate the characteristics and the variation in the microstructure. Upon increasing the silane concentration the strong columnar growth with narrow grain boundaries degrades towards the growth of small irregularly shaped grains enclosed in an amorpho...

Control of diamond film microstructure by Ar additions to CH4/H2 microwave plasmas

Abstract: The transition from microcrystalline to nanocrystalline diamond films grown from Ar/H2/CH4 microwave plasmas has been investigated. Both the cross-section and plan-view micrographs of scanning electron microscopy reveal that the surface morphology, the grain size, and the growth mechanism of the diamond films depend strongly on the ratio of Ar to H2 in the reactant gases. Microcrystalline grain size and columnar growth have been observed from films produced from Ar/H2/CH4 microwave discharges with low concentrations of Ar in the reactant gases. By contrast, the films grown from Ar/H2/CH4 microwave plasmas with a high concentration of Ar in the reactant gases consist of phase pure nanocrystalline diamond, which has been characterized by transmission electron microscopy, selected area electron diffraction, and electron energy loss spectroscopy. X-ray diffraction and Raman spectroscopy reveal that the width of the diffraction peaks and the Raman bands of the as-grown films depends on the ratio of Ar to H2 in...

Nanocrystalline-silicon superlattice produced by controlled recrystallization

Abstract: Nanocrystalline-silicon superlattices are produced by controlled recrystallization of amorphous-Si/SiO2 multilayers. The recrystallization is performed by a two-step procedure: rapid thermal annealing at 600–1000 °C, and furnace annealing at 1050 °C. Transmission electron microscopy, Raman scattering, x-ray and electron diffraction, and photoluminescence spectroscopy show an ordered structure with Si nanocrystals confined between SiO2 layers. The size of the Si nanocrystals is limited by the thickness of the a-Si layer, the shape is nearly spherical, and the orientation is random. The luminescence from the nc-Si superlattices is demonstrated and studied.

A transmission electron microscopy study of constituent-particle-induced corrosion in 7075-T6 and 2024-T3 aluminum alloys

Abstract: To better understand particle-induced pitting corrosion in aluminum alloys, thin foil specimens of 7075-T6 and 2024-T3 aluminum alloys, with identified constituent particles, were immersed in aerated 0.5M NaCl solution and then examined by transmission electron microscopy (TEM). The results clearly showed matrix dissolution around the iron- and manganese-containing particles (such as Al23CuFe4), as well as the Al2Cu particles. While Al2CuMg particles tended to dissolve relative to the matrix, limited local dissolution of the matrix was also observed around these particles. These results are consistent with scanning electron microscopy (SEM) observations of pitting corrosion and are discussed in terms of the electrochemical characteristics of the particles and the matrix.

Preparation of nano-crystalline diamonds using pulsed laser induced reactive quenching

Abstract: Nano-crystalline diamond is prepared with a unique method in which a graphite target is etched by a high-power pulsed laser in water. Transmission electron microscopy (TEM) and high resolution electron microscopy (HREM) indicate nano-crystalline diamond is obtained, having a hexagonal lattice or cubic lattice.

Morphology and Enzymatic Degradation of Poly(l-lactic acid) Single Crystals

Abstract: Lamellar single crystals of poly(l-lactic acid) (PLLA) were grown from a 0.05% solution of p-xylene at 90 °C. The hexagonal, truncated-lozenge and lozenge-shaped single crystals with spiral growth were prepared simultaneously, and these crystals were relatively stable under conditions of electron bombardment. The hexagonal and lozenge-shaped crystals gave well-resolved electron diffractograms from which the reciprocal lattice parameters a* = 0.935 nm-1, b* = 1.626 nm-1, and γ* = 90° could be determined, and these lattice parameters corresponded with those of the α structure of PLLA fibers. Accordingly, the hexagonal crystal was considered as pseudo-hexagonal symmetry, but actually with orthorhombic packing of the PLLA chains; i.e., the ratio of the a and b cell parameters is quite equal to √3. The enzymatic degradation of PLLA single crystals with a proteinase-K from the mold Tritirachium album in 50 mM Tris−HCl buffer at pH 8.5 and 37 °C was investigated by means of transmission electron microscopy, atom...

Synthesis of nanocrystalline diamond thin films from an Ar–CH4 microwave plasma

Abstract: Nanocrystalline diamond thin films have been synthesized in an Ar–CH4 microwave discharge, without the addition of molecular hydrogen. X-ray diffraction, transmission electron microscopy, and electron energy loss spectroscopy characterizations show that the films consist of a pure crystalline diamond phase with very small grain sizes ranging from 3 to 20 nm. Atomic force microscopy analysis demonstrates that the surfaces of the nanocrystalline diamond films remain smooth independent of the film thicknesses. Furthermore, the reactant gas pressure, which strongly affects the concentration of C2 dimer in the Ar–CH4 plasma as well as the growth rate of the films, has been found to be a key parameter for the nanocrystalline diamond thin film depositions.

Structure and properties of ion-beam-modified (6H) silicon carbide

Abstract: The ion-beam-induced crystalline-to-amorphous phase transition in single crystal ( 6 H) α -SiC has been studied as a function of irradiation temperature. The evolution of the amorphous state has been followed in situ by transmission electron microscopy in specimens irradiated with 0.8 MeV Ne + , 1.0 MeV Ar + , and 1.5 MeV Xe + ions over the temperature range from 20 to 475 K. The threshold displacement dose for complete amorphization in α -SiC at 20 K is 0.30 dpa (damage energy=15 eV atom −1 ). The dose for complete amorphization increases with temperature due to simultaneous recovery processes that can be adequately modeled in terms of a single-activated process. The critical temperature, above which amorphization does not occur, increases with particle mass and saturates at about 500 K. Single crystals of α -SiC with [0001] orientation have also been irradiated at 300 K with 360 keV Ar 2+ ions at an incident angle of 25° over fluences ranging from 1 to 8 Ar 2+ ions nm −2 . The damage accumulation in these samples has been characterized ex situ by Rutherford backscattering spectrometry–channeling (RBS/C) along the [0001] direction, Raman spectroscopy, cross-sectional transmission electron microscopy (XTEM), and mechanical microprobe measurements.

Physicochemical Characterization of Silicalite-1 Nanophase Material

Abstract: A silicalite-1 nanophase material with an elementary particle size of 18−100 nm is synthesized from clear solution and isolated and purified using supercentrifugation. The nanopowder is characterized in detail using scanning electron microscopy, high-resolution transmission electron microscopy, attenuated force microscopy, 29Si magic angle spinning NMR, 13C cross polarization magic angle spinning NMR, X-ray diffraction, dinitrogen physisorption, and thermogravimetric analysis and compared with micrometer-sized silicalite-1. The nanosized and micrometer-sized materials have many common properties including the refined structure and the nature and concentrations of tetrapropylammonium species incorporated during the synthesis. Unique properties of the nanophase are a splitting of the characteristic framework vibration at 550 cm-1 into a doublet at 555 and 570 cm-1, a high concentration of defect sites, and a strain in the crystallites along the “a” crystallographic direction. The nanophase exhibits a two-st...

Gan heteroepitaxial growth on silicon nitride buffer layers formed on si (111) surfaces by plasma-assisted molecular beam epitaxy

Abstract: Wurtzite GaN films were grown on silicon nitride buffer layers formed on Si (111) substrates by radio frequency plasma-assisted molecular beam epitaxy. Reflection high energy electron diffraction, Auger electron spectroscopy, transmission electron microscopy, and photoluminescence results indicate that the single crystalline wurtzite GaN was grown on the buffer layers of amorphouslike silicon nitride formed on Si (111) substrates by taking the following relationship with the substrate: GaN [0001]//Si [111] and GaN (1120)//Si (110). Both faces of the silicon nitride buffer layer were found to be flat and sharp, the thickness of the buffer layer (1–1.5 nm) being constant across the interface. Efficient bound exciton emission was observed at 3.46 eV. The growth technique described was found to be simple but very powerful for growing high quality GaN films on Si substrates.

Growth of Quantum-Confined Indium Phosphide inside MCM-41

Abstract: An array of analytical techniques comprising powder X-ray diffraction, solid-state NMR spectroscopy, high-resolution transmission electron microscopy, nitrogen adsorption, and UV/vis diffuse reflectance spectroscopy has been applied to study the incorporation of indium phosphide semiconductor inside MCM-41 materials by metal organic chemical vapor deposition. Line broadening in the X-ray diffraction patterns suggests the existence of both large surface deposited indium phosphide particles and nanosized indium phosphide particles deposited within the pores. High-resolution transmission electron microscopy corroborates this result: surface deposits have been imaged, and analysis of electron diffraction patterns provides evidence of the existence of nanoparticles. Nitrogen adsorption provides information on pore filling. Quantum-confinement effects, brought about by the nanoparticle size regime, are evidenced by upfield shifting of the indium phosphide resonance in the 31P magic-angle-spinning NMR spectra a...

High-Pressure, High-Temperature Synthesis and Characterization of Boron Suboxide (B6O)

Abstract: Boron suboxide, nominally B6O, was synthesized by reducing B2O3 with B up to 10 GPa in a multianvil press at temperatures between 1200 and 1800 °C. The samples were characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and parallel electron energy-loss spectroscopy (PEELS). We used high-pressure techniques to synthesize boron suboxide of improved purity and crystallinity and less oxygen-deficient (i.e., closer to the nominal B6O composition) in comparison to products of room-pressure syntheses. We describe the preparation of grains ranging from 20 nm to 40 μm in diameter, as well as the first synthesis of micrometer-sized B6O icosahedral twins and euhedral “crystals”. The best materials are obtained for starting mixtures containing an excess B2O3 reacted at 1700−1800 °C between 4 and 5.5 GPa. After the products were washed in water, well-crystallized single-phase product dominated by icosahedrally twinned particles to 30 μm in diameter was easily recove...

Magnetic Field Effects on Nickel Electrodeposition

Abstract: The effects of a superimposed magnetic field B on the structure of nickel electrodeposits prepared from a quiescent Watts solution were studied by scanning electron microscope and transmission electron microscopy investigations. It was observed that B can induce a change of the surface morphology and of the preferential growth direction of the nickel grains. In the absence of organic inhibitor, it is shown that these effects result from an inhibition of nickel electrocrystallization in relation to the mass-transport enhancement of H + ions promoted by the magnetic field. The phenomenon is much more important in the presence of a strong inhibitor such as 2-butyne-1,4-diol, the activity of which is also under mass-transport limitation. The presence of the magnetically induced convective flow is directly illustrated by the perturbations of the relief of the deposit in the vicinity of attached bubbles. These results demonstrate that most structural modifications of nickel electrodeposits observed in the presence of a magnetic field are consequences of convection phenomena induced by a magnetohydrodynamic effect.

Hydrothermal–electrochemical deposition of hydroxyapatite

Abstract: Hydroxyapatite crystals were formed on a titanium electrode using the hydrothermal-electrochemical method in an autoclave with two electrodes. The electrolyte dissolving NaCl, K 2 HPO 4 , CaCl 2 . 2H 2 O, trishydroxyaminomethane, and hydrochloric acid was maintained at 80°- 200°C. After loading of a constant current at 12.5 mA/cm 2 for 1 h, the deposited amounts were measured through the weight gain of the electrode. The deposits were characterized by X-ray diffractometry, Fourier transform infrared spectroscopy, field emission-type scanning electron microscopy, field emission-type transmission electron microscopy, and energy dispersive X-ray spectroscopy. The deposited amount increased with electrolyte temperatures up to 150°C and slightly decreased above that temperature. The deposits were identified as hydroxyapatite crystal rods grown along the c axis and perpendicular to the substrate. The crystallinity of the deposited hydroxyapatite increased continuously with the electrolyte temperature and closed to stoichiometric hydroxyapatite. At 150°-160°C, the hydroxyapatite rod grew very homogeneously perpendicular to the substrate, and the edge of the needle had a flat hexagonal plane. © 1998 John Wiley & Sons, Inc. J Biomed Mater Res, 42, 387-395, 1998.

Wet and dry sizes of atmospheric aerosol particles: An AFM‐TEM Study

Abstract: We studied the hygroscopic behavior of atmospheric aerosols by using a novel approach, the combination of atomic force microscopy (AFM) with transmission electron microscopy (TEM) imaging of the same individual particles. By comparing the dimensions of hydrated and dry ammonium sulfate particles collected above the North Atlantic Ocean, we determined that particle volumes are up to four times larger under ambient conditions (as determined by AFM) than in the vacuum of a transmission electron microscope. We interpret these changes as resulting from the loss of water. Organic films on the particles may be responsible for the relatively large water uptake at low relative humidities.

Microstructure and electronic properties of the refractory semiconductor ScN grown on MgO(001) by ultra-high-vacuum reactive magnetron sputter deposition

Abstract: ScN layers, 180 nm thick, were grown on MgO(001) substrates at 750 °C by ultra-high-vacuum reactive magnetron sputter deposition in pure N2 discharges. N/Sc ratios, determined by Rutherford backscattering spectroscopy, were 0.98±0.02. X-ray diffraction θ–2θ scans and pole figures combined with plan-view and cross-sectional transmission electron microscopy showed that the films are strongly textured, both in plane and along the growth direction, and have a columnar microstructure with an average column width near the film surface of 30±5 nm. During nucleation and the early stages of film growth, the layers consist of approximately equal volume fractions of 002- and 111-oriented grains. However, preferred orientation evolves toward a purely 111 texture within ≃40 nm as the 002 grains grow out of existence in a kinetically limited competitive growth mode. 002 grains exhibit local cube-on-cube epitaxy with an orientation relationship (001)ScN∥(001)MgO and [010]ScN∥[010]MgO while 111 grains have a complex four...