Compositional stability of FePt nanoparticles on SiO2/Si during annealing

The loss of Fe due to oxidation or diffusion into the substrate can prevent the successful preparation of well-ordered, stoichiometric, FePt nanoparticles. In this work we report the composition changes during annealing observed for small ( \u3c 10 nm) FePt nanoparticles on thermally grown SiO2 layers on Si wafer substrates. Additionally, we describe the use of a controlled reducing gas mixture, Ar+H-2+H2O, to reduce the loss of Fe

Diffusion profiles of high dosage Cr and V ions implanted into silicon

The depth profiles of high dosage Cr-52(+) and V-51(+) ions implanted in (100) crystalline silicon after thermal anneal at temperatures between 300 degreesC and 1000 degreesC are studied by secondary ion mass spectrometry and cross-sectional transmission electron microscopy. At dosages of 1x10(15) ions/cm(2) and above, the surface layer of silicon substrate is amorphorized. During the subsequent thermal annealing, the depth profiles of the implanted ions are strongly coupled with the solid phase epitaxial growth of amorphous silicon. Silicide precipitate formation is important to understand the differences between Cr and V diffusion. After anneal of the 1x10(15) ions/cm(2) implanted samples at 900 degreesC and 1000 degreesC, most of the Cr has left the silicon, but only 10% of the V has escaped. The 1x10(14) ions/cm(2) Cr-implanted sample shows Cr ions exist only near the surface after 1000 degreesC anneal. The V-implanted sample, on the other hand, only shows a narrowing of the V profile after 1000 degreesC anneal

Antiphase ordering and surface phases in lithium aluminate

Antiphase domains are seen in single crystal gamma lithium aluminate (gamma-LiAlO(2)) with 16.7 nm periodicity in the \u3c 110 \u3e direction. Alternate domains have a 1/2 [001] shift. Beta phase lithium aluminate (beta-LiAlO(2)) is seen to form on the surface of the as-received wafers with an epitaxial strain limited relationship with the bulk gamma phase. The orthorhombic beta phase aligns with the a and b axes (0.528 and 0.630 nm) matching with the tetragonal gamma phase\u27s a and c axes (0.5168 and 0.6268 nm). The gamma and beta phases are seen to have different etch rates. The beta phase converts back to the gamma phase above 450 degrees C

Defects in m-face GaN films grown by halide vapor phase epitaxy on LiAlO2

Free-standing wafers (50 mm diameter) of GaN were grown by halide vapor phase epitaxy on lattice-matched gamma-LiAlO2. We report a transmission electron microscopy study of defects and defect densities in these wafers. The growth direction is [10 (1) over bar0]. Stacking faults in the basal plane are seen when viewing the specimen in the [1 (2) over bar 10] direction with an average spacing of less than 100 nm. Convergent beam electron diffraction measurements show no switch in the polarity and thus the faults are proposed to be ABABACAC changes in the stacking. Threading dislocations are found to have a correlated arrangement with a density of 3x10(8) cm(-2) when viewing the [1 (2) over bar 10] direction and widely varying (depending upon location) when viewing in the [0001] direction. These dislocations act as seeds for postgrowth surface features that directly exhibit the correlated nature of these threading dislocations

Porous Silica Nanotube Thin Films as Thermally Insulating Barrier Coatings

The fabrication and examination of a porous silica thin film, potentially for use as an insulating thin film, were investigated. A vertically aligned carbon nanotube (CNT) forest, created by chemical vapor deposition (CVD), was used as scaffolding to construct the porous film. Silicon was deposited on the CNT forest using low-pressure CVD (LPCVD) and then oxidized to remove the CNTs and convert the silicon to silica for electrical or thermal passivation (e.g., thermal barrier). Thermal conductivity was determined using a 1D heat-transfer analysis that equated radiative heat loss in a vacuum with conduction through the substrate and thin film stack. A comparison of the surface temperature differences between a sample film and a reference of comparable thermal resistance enabled determination of the increase in the thermal resistance and of the thermal conductivity of the films. For film thicknesses of approximately 55 μm, the cross-plane thermal conductivity was found to be 0.054−0.071 W m−1 K−1 over 378−422 K. This thermal conductivity value is in the range of other silica aerogels and consistent with the low gravimetric density of 0.15 g cm−3 for the samples. The film is also relatively smooth and flat, with an average arithmetic mean roughness of 1.04 μm

High Aspect Ratio, Carbon Nanotube Membranes Decorated with Pt Nanoparticle Urchins for Micro Underwater Vehicle Propulsion \u3cem\u3evia\u3c/em\u3e H\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e2\u3c/sub\u3e Decomposition

The utility of unmanned Micro Underwater Vehicles (MUVs) is paramount for exploring confined spaces, but their spatial agility is often impaired when maneuvers require burst-propulsion. herein we develop high-aspect ratio (150:1), multi-walled carbon nanotube microarray membranes (CNT-MMs) for propulsive, MUV thrust generation by the decomposition of hydrogen peroxide (H2O2). The CNT-MMs are grown via chemical vapor deposition with diamond shaped pores (nominal diagonal dimensions of 4.5 × 9.0 [µm]) and subsequently decorated with urchin-like, platinum (Pt) nanoparticles via a facile, electroless, chemical deposition process. The Pt-CNT-MMs display robust, high catalytic ability with an effective activation energy of 26.96 [kJ mol-1] capable of producing oa thrust of 0.209 ± 0.049 [N] from 50 % [w/w] H2O2 decomposition within a compact reaction chamber of eight Pt-CNT-MMs in series

Disordering of small metal particles in a scanning transmission electron microscope

Small metal particles in the range of a few nanometers in diameter are seen to progressively disorder when the 100 keV electron beam of a Scanning Transmission Electron Microscope (STEM) is held stationary on the particle. The diffraction pattern of the individual particle is seen to progress from an initial array of indexable diffraction spots to a mixture of diffraction spots and amorphous-like rings and finally to rings with no persistent diffraction spots. Only particles below a critical size are seen to fully disorder. We have observed this disordering in Platinum, Palladium, Rhodium, and Iridium and have developed a model for the disordering process. In this model, electrons scattering from surface atoms transfer enough energy to break the surface atoms from their binding site. A competing process of disordered atoms rebinding to crystalline sites is also included. Because small particles have large fractions of their atoms on the surface, the beam driven disorder, under certain conditions, is able to propagate into the core of the particle. For Platinum, surface disordering requires energy transfers from the electrons to the Platinum atoms of0.54 eV.U of I Onlydissertation/thesi

Electron Beam Driven Disordering in Small Particles

AbstractSmall metal particles in the range of a few nanometers in diameter are seen to progressively disorder when the 100 keV electron beam of a Scanning Transmission Electron Microscope (STEM) is held stationary on the particle. The diffraction pattern of the individual particle is seen to progress from an initial array of indexable diffraction spots to a mixture of diffraction spots and amorphous-like rings and finally to rings with no persistent diffraction spots. After the electron beam is removed, the particles will recrystallize after minutes or hours. Only particles below a critical size are seen to fully disorder. We have observed this in Platinum, Palladium, Rhodium, and Iridium and based on our model of disordering process believe it is a universal effect. It has also been observed with a Platinum Ruthenium alloy. We discuss the mechanism of this disordering and the structure of the resulting disordering particle for the case of Platinum clusters.</jats:p

Prospects For Imaging of Single Dopant Atoms in Silicon by ADF Stem

The demands of the National Technology Roadmap for Semiconductors will necessitate measurement of dopant concentrations with greater spatial resolution than now possible. Current experimental and simulation experience indicate that Annular Dark Field (ADF) imaging in a Scanning Transmission Electron Microscope (STEM) should be able to determine dopant distributions with near atomic resolution. The ADF signal is derived from electrons diffusely scattered to high angles, resulting in contrast due to atomic number (Z-contrast) and defects in the crystal lattice. Thus, heavy atoms can be imaged by their Z-contrast and small atoms by the misfit strain induced in the silicon lattice. Atomic number scattering is proportional to Zn where n is between 1.5 and 1.9 depending upon the inner detector angle of the ADF detector.</jats:p