Selected Publications

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By R. Vanfleet (et al.)
Abstract: The magnetic properties and microstructure of p-type Si (100) implanted with 1.0 x 10(15) cm(-2) of Cr ions at 200 keV have been investigated by a superconducting quantum interference device (SQUID) magnetometer, scanning electron microscope (SEM) and transmission electron microscopy (TEM). The magnetic hysteresis loops and saturation magnetization of 0.67-0.75 emu/g in a wide temperature range are observed in the as-implanted sample. Annealing of the as-implanted sample modifies the microstructure and therefore weakens the magnetic exchange interaction. TEM observations show that the as-implanted silicon layer is amorphous. After annealing at temperature >= 800 degrees C, the SEM showed that the implanted profile layer became blurred and narrow, the ferromagnetism was weakened, which should have resulted from the re-crystallization of the implanted amorphous layer. These results were further compared with magnetic hysteresis observed in Mn-implanted silicon. (C) 2008 Elsevier Ltd. All rights reserved.
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By R. Vanfleet (et al.)
Abstract: To better understand the mechanism of the reported “quasi‐ferromagnetism” observed in Si ions self‐implanted or irradiated silicon, we carry out high resolution transmission electron microscopy (HRTEM), magnetization measurements using superconducting quantum interference device (SQUID) magnetometer, and ferromagnetic resonance (FMR) measurements of the magnetic interaction of the defect‐associated sites in silicon damaged by silicon self‐implantation or energetic particle beams. The SQUID measurements showed that the silicon self‐implanted sample has paramagnetic ordering. FMR measurements indicated the He++ irradiated sample has a ferromagnetic interaction and yields a Lande g‐factor of 2.35.
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By R. R. Vanfleet (et al.)
Abstract: A scanning transmission electron microscope (STEM) study of silicon-germanium alloying using annular dark field (ADF) or Z-contrast imaging and electron energy loss spectroscopy (EELS) is presented. Results and techniques are discussed. Growth of 11 equivalent monolayers of germanium on silicon at 650 degrees C results in dome-shaped islands or quantum dots that contain up to similar to 40% silicon. The interface between the as-grown island and substrate shows a highly disordered or amorphous zone similar to 1.5-nm wide directly under the island. Annealing for 60 min at 650 degrees C gives larger pyramidal islands with diffuse crystalline interfaces and an equilibrium distribution of up to similar to 70% silicon in the islands.
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Abstract: A modified back-etch method is described that has been successfully used to prepare samples of thin films and nanoparticles on Si wafer substrates for examination by high-resolution transmission electron microscopy ( HRTEM). This process includes ultrasonic cutting, abrasive pre-thinning and a two-stage etching procedure. Unlike previous reports of back-etching methods, tetramethyl ammonium hydroxide, which has a very high-etching selectivity of Si to SiO2, is used for the final etching to allow removal of the Si without degradation of the SiO2 membrane. An innovative wrapping method is also described. This novel approach reduces the preparation time for HRTEM samples to < 1 h per sample for groups of 10 or more samples. As an example, the preparation of FePt nanoparticle samples for HRTEM imaging is described.
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By R. R. Vanfleet (et al.)
Abstract: 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 (< 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. (C) 2006 American Institute of Physics.
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By Richard R. Vanfleet and Daniel Richardson (et al.)
Abstract: The long-range order parameter of [001]-oriented FePt L1(0) nanoparticles has been determined by measurement of convergent-beam electron diffraction (CBED) intensities of single particles using scanning transmission electron microscopy (STEM) and comparison of the measured intensities to simulations of diffracted-beam intensities. The nanoparticles were fabricated by co-sputtering high-purity Fe and Pt targets onto SiO2-Si substrates and annealing the deposited material.