Small Area Electron Diffraction Research Articles

Synthesis and Characterization of Gallium Oxide Nanostructures via a Soft-Chemistry Route

Nano- to microsized gallium oxide was prepared with and without surfactant via a hydrothermal route at low temperature through different synthesis procedures. Rodlike GaOOH crystals with average length of ∼2.5 μm and width of 1.5 μm were prepared when the initial molar ratio of Ga to OH was 1:3. β-Ga2O3 materials were derived from GaOOH by calcination at 900 °C, and the initial morphology was retained. γ-Ga2O3 nanotubes up to 65 nm in length, with internal and external diameters of around 0.8 and 3 nm, were achieved directly in solution with and without surfactant under hydrothermal treatment condition at 100 °C when the initial molar ratio of Ga to OH was 1:5. The combination of X-ray diffraction (XRD), transmission electron microscopy (TEM), N2 adsorption, small area electron diffraction (SAED), and energy dispersive X-ray analysis (EDX) were employed to characterize the resulting nano- to microsized structures. Cationic and nonionic surfactants were used in this study. Detailed results are presented.

Synthesis and Characterization of TiO2 Nanocrystals Prepared from n-Octadecylamine−Titanyl Oxalate Langmuir−Blodgett Films

TiO2 nanosized clusters have been prepared by the Langmuir−Blodgett technique starting from the n-octadecylamine−titanyl oxalate complex. The floating film morphology has also been investigated by Brewster angle microscopy (BAM). The presence of TiO2 in the subphase promotes the formation of aggregates of n-octadecylamine on the subphase surface. LB films initially contain TiO2 nanosized clusters embedded in the organic amorphous matrix; their structural evolution induced by thermal annealing has been investigated. UV−vis spectroscopy, high-resolution transmission electron microscopy (HRTEM), and small area electron diffraction (SAD) characterization methods are used as synergic techniques giving complementary information regarding both structural features and photophysical properties.

Structural and electrical properties of In2O3/SeO2 thin filmsfor gas-sensing applications

In this work, we report about the analysis of structural, physical,and gas-sensing properties of a new sensing material made by mixingIn2O3 and SeO2. The In2O3/SeO2 thin films have been obtainedby thermal evaporation of In:Se in a 2:1 ratio and subsequent thermalannealing in an oxygen flow. In order to investigate the structure and themorphology of the films obtained, high-resolution transmission electronmicroscopy, small-area electron diffraction, and digital image processing wereperformed on thermally annealed films. The Hall effect and resistivitymeasurements of such films have been carefully analysed. In addition,In2O3/SeO2 mixed oxide films were tested in a controlled atmosphereand the electrical conductance changes were measured as a function of temperature.The sensing behaviour of such films has been analysed according to anadsorption kinetic model, which related the conductivity variation to gasconcentrations, time exposure, and working temperature.

Protected magnetic particles: silica on yttrium iron garnet

The formation of silica on core yttrium iron garnet presents a variety of different applications as corrosion resistance and stabilization of magnetic properties. Well-defined magnetic particles were prepared by heterocoagulating silica on yttrium iron garnet to protect the core. Yttrium iron garnet was obtained using a homogeneous nucleation process by controlling the chemical routes from cation hydrolysis in acid medium. The heterocoagulation was induced by tetraethyl orthosilicate hydrolysis in appropriate yttrium iron garnet dispersion medium. The presence of silica on yttrium iron garnet was characterized by vibrating sample magnetometry, X-ray photoemission spectroscopy, transmission electron microscopy, small area electron diffraction and differential thermal analysis.

Structural and electrical properties of In2O3–SeO2 mixed oxide thin films for gas sensing applications

In 2 O 3 – SeO 2 mixed oxide thin films have been obtained by thermal evaporation of a stoichiometric In–Se starting charge and subsequent thermal annealing in an oxygen flow. High-resolution transmission electron microscopy, small area electron diffraction, and digital image processing have been employed in order to investigate the structure and the morphology of the films obtained. Hall effect and resistivity measurements in a vacuum and in a controlled atmosphere have been performed to obtain information about physical properties of these films. The experimental data show evidence that the electrical conductivity of these films undergoes a remarkable variation due to exposure to small concentrations of NO in dry synthetic air or argon. To interpret the behavior of such films, an adsorption kinetic model has been developed and the conductivity variation as a function of gas concentrations, time, and temperature has been derived. The fitting of the theoretical and experimental behavior allows us to determine the adsorption kinetic parameters.

General Principle of Ordered Apatitic Crystal Formation in Enamel and Collagen Rich Hard Tissues

The biomineralization processes in different hard tissues like enamel, circumpulpal dentine, epiphyseal growth plates were analyzed morphologically and ultrastructurally by an energy filtering transmission electron microscope. In the primary stage of crystal formation Ca-and phosphate-ions accumulate at charged sites, “active sites”, along the fiber matrix-molecules of the extracellular matrix. After exceeding the critical radius for nucleation, crystal nuclei appear that develop to “chains” of stable nanometer-sized paracrystalline particles. In the latest studies of small area electron diffraction it was found that in the earliest stage of crystal formation these mineral chains show a parallel orientation in the direction of the c-axis of apatite. This was supported by a texture of the 002 reflection in the corresponding diffraction patterns. Since apatite is bipolar in this direction crystal growth would be in like manner in both directions. Thus the center-to-center distances between nucleating sites along the matrix macromolecules show with the chains of nanometer islands the same process of biomineralization in the different mineralizing hard tissue systems. This way of crystal formation might be a general principle of apatitic biomineralization.

Analysis of a general principle of crystal nucleation, formation in the different hard tissues.

We have found, at high EM magnification, on ultrathin sections of shock-frozen, freeze-dried, embedded pieces of the developing hard tissues, that the primary crystallites consist of strands composed of nanometer-sized apatitic islands, which rapidly coalesce to needles and afterward to platelets. By small-area electron diffraction, with energy-filtered electrons, it was clarified that these strands are already crystallographically oriented along the bipolar c-axis so that the center-to-center distances between the islands would reflect the distances between crystal-nucleating sites along the matrix. The EM analysis of the cross-cut stained unmineralized and of the unstained mineralized collagen fibers of turkey tibia tendon shows that the staining "nuclei" and the early crystallites, appearing as dark dots, surround "light" round structures, which we interpret as the collagen microfibrils, surrounded by the apatitic crystallites.

Titanium oxide thin films for NH3 monitoring: Structural and physical characterizations

Titanium oxide thin films have been deposited by thermal evaporation in vacuum and then have been analyzed before and after a suitable thermal annealing in order to test their application in NH3 gas-sensing technology. In particular, spectrophotometric and conductivity measurements have been performed in order to determine the optical and electrical properties of titanium oxide thin films. The structure and the morphology of such material have been investigated by high resolution electron microscopy and small area electron diffraction.

The Evolutionary Process during Pyrolytic Transformation of Poly(N‐methylsilazane) from a Preceramic Polymer into an Amorphous Silicon Nitride/Carbon Composite

The pyrolytic evolution of poly(N‐methylsilazane), –[H2SiN‐Me]x–, from preceramic polymer to ceramic product is followed by heating samples of the partially cross‐linked polymer, in 200°C increments, from ambient temperature to 1400°C. The intermediate products are characterized by chemical analysis, diffuse reflectance Fourier transform IR spectroscopy (DRIFTS), Raman spectroscopy, and 29Si and 13C magic‐angle spinning (MAS) solid‐state NMR. Spectro‐scopic characterization indicates that the 1400°C pyrolysis products are amorphous silicon nitride mixed with amorphous and graphitic carbon (as determined by Raman spectroscopy), rather than silicon carbide nitride, as expected based on the presence of up to 20 mol% retained carbon. Efforts to crystallize the silicon nitride through heat treatments up to 1400°C do not lead to any crystalline phases, as established by transmission electron microscopy (TEM) and small‐area electron diffraction (SAD). It appears that the presence of free carbon, along with the absence of oxygen, strongly inhibits crystallization of amorphous silicon nitride. These results contrast with the isostructural poly‐(Si‐methylsilazane), –[MeHSiNH]x–, which is reported to form silicon carbide nitride on pyrolysis.

Waveguiding epitaxial LiNbO3 layers deposited by radio frequency sputtering

Thin films of LiNbO3 were rf sputter deposited onto a number of different substrates and were investigated with a variety of characterization methods. The morphology of the films was investigated by the scanning electron microscope and the crystal structure was determined by x-ray diffraction and small area electron diffraction. The composition was obtained from electron spectroscopy for chemical analysis and was confirmed by Rutherford backscattering. Some optical properties of the film were also investigated, for example, the refractive index which was determined from the coupling angles or the losses of the guided modes measured by scanning a fiber probe along the mode. These analyses showed that best layers can be obtained when depositing LiNbO3 layers with a thickness of approximately 200 nm on (001)-sapphire at a temperature of 500 °C and a deposition rate of 0.2–0.5 nm/min. Using these parameters, single crystalline epitaxial films can be deposited having stoichiometric composition and minimal surface roughness. It was also possible to excite waveguide modes in these films with losses less than 5 dB/cm.