Conrad R. Stoldt

A low-temperature CVD process for silicon carbide MEMS

A low-temperature, single precursor CVD process for the realization of SiC-based MEMS and SiC-coated MEMS is described using 1,3-disilabutane. With this deposition method, the fabrication of an all-SiC cantilever beam array is demonstrated using standard microfabrication processes. Also, SiC coating of released Si micromechanical structures is realized using this process. The SiC-coated microstructures are shown to have superior chemical stability when compared to their Si analogs, as well as exhibit highly favorable mechanical properties.

Iron Pyrite Nanocubes: Size and Shape Considerations for Photovoltaic Application

Multiple lines of recent research indicate that iron pyrite (FeS(2)) requires a {100}-terminated crystal morphology in order to maintain semiconducting properties. Additionally, the large absorption coefficient of pyrite allows for the near complete absorption of above band gap radiation in <50 nm layers. However, to our knowledge <50 nm pyrite nanocubes have yet to be isolated. Herein, we demonstrate the synthesis of ~37 nm phase pure pyrite nanocubes by manipulating the sulfur chemical potential and ligand environment of the system. Ultraviolet-visible (UV-vis) absorption spectroscopy gives a signal of resonant light scattering indicating strong electronic coupling between nanocubes, which may allow for nanocube films with superior electron mobility. The absorption spectroscopies of cubic and irregular nanocrystals are contrasted and compared with recent theoretical work in order to investigate the effect of shape on electronic properties. Specifically, nanocubes have been found to have absorption characteristics closer to theory as compared to irregular nanocrystals, especially for UV radiation: 250-350 nm. Pyrite nanocubes display an indirect band gap at ~1.1 eV in addition to two direct transitions at ~1.9 and ~3.0 eV, correlating well to theoretical values.

Galvanic porous silicon composites for high-velocity nanoenergetics.

Porous silicon (PS) films ∼65-95 μm thick composed of pores with diameters less than 3 nm were fabricated using a galvanic etching approach that does not require an external power supply. A highly reactive, nanoenergetic composite was then created by impregnating the nanoscale pores with the strong oxidizer, sodium perchlorate (NaClO(4)). The combustion propagation velocity of the energetic composite was measured using microfabricated diagnostic devices in conjunction with high-speed optical imaging up to 930000 frames per second. Combustion velocities averaging 3050 m/s were observed for PS films with specific surface areas of ∼840 m(2)/g and porosities of 65-67%.

Renal Inflammation: Targeted Iron Oxide Nanoparticles for Molecular MR Imaging in Mice

PURPOSE To determine the feasibility of T2-weighted magnetic resonance (MR) imaging in the noninvasive quantification of renal inflammation by using superparamagnetic iron oxide (SPIO) nanoparticles targeted to tissue-bound C3 activation fragments in a mouse model of lupus nephritis. MATERIALS AND METHODS All animal procedures were approved by the University of Colorado-Denver animal care and use committee. SPIO nanoparticles were encapsulated by using amine-functionalized phospholipids. A recombinant protein containing the C3d-binding region of complement receptor type 2 (CR2) was then conjugated to the surface of the SPIO nanoparticle. Five MRL/lpr mice (a model of lupus nephritis) and six C57BL/6 wild-type mice were assessed with T2-weighted MR imaging at baseline and after SPIO injection. The same five MRL/lpr mice and three C57BL/6 mice also underwent MR imaging after injection of CR2-targeted SPIO. A series of T2-weighted pulses with 16 echo times was used to enable precise T2 mapping and calculation of T2 relaxation times in the cortex and outer and inner medulla of the kidneys, as well as in the spleen, muscle, and fat. The effects of treatment and animal genotype on T2 relaxation times were analyzed with repeated-measures analysis of variance. RESULTS At baseline, the T2-weighted signal intensity in the kidneys of MRL/lpr mice was higher than that in the kidneys of wild-type mice. Injection of untargeted SPIO did not alter the T2-weighted signal in the kidneys in either strain of mice. Injection of CR2-targeted SPIO in MRL/lpr mice, however, caused a significant accumulation of targeted iron oxide with a subsequent decrease in T2 relaxation times in the cortex and outer and inner medulla of the kidneys. No changes in T2 relaxation time were observed in the wild-type mice after injection of targeted SPIO. CONCLUSION Injection of CR2-conjugated SPIO caused a significant reduction in T2-weighted MR imaging signal and T2 relaxation time in nephritic kidneys.

Micromechanical properties of silicon-carbide thin films deposited using single-source chemical-vapor deposition

1,3-Disilabutane is used as a single-source precursor to deposit conformal silicon-carbide films on silicon atomic-force-microscopy cantilevers. By measuring the resonance frequency of the cantilever as a function of silicon-carbide film thickness and developing an appropriate model, the value of the film’s elastic modulus is determined. This value is in good agreement with those reported for silicon-carbide films deposited using conventional dual-source chemical-vapor deposition. Additionally, we comment on the feasibility of integrating this process into the fabrication technology for microelectromechanical systems.

Ripening during magnetite nanoparticle synthesis: Resulting interfacial defects and magnetic properties

The structure and magnetic properties of magnetite (Fe3O4) nanoparticles synthesized by a solvothermal processing route are investigated. The nanoparticles are grown from the single organometallic precursor Fe(III) acetylacetonate in trioctylamine (TOA) solvent at 260°C, with and without the addition of heptanoic acid (HA) as a stabilizing agent. From the temporal particle size distributions, x-ray-diffraction patterns, high-resolution transmission electron microscope tilt series experiments, and superconducting quantum interference device magnetometry, we demonstrate that HA, a strong Lewis acid stabilizing agent, slows growth processes during ripening thus reducing the formation of interfacial defects, which we observe in the TOA-only synthesis. Nanoparticles grown with HA remain single crystalline for long growth times (up to 24h), show a focused particle size distribution for intermediate growth times (3h), and possess a higher magnetic anisotropy (15.8×104J∕m3) than particles grown without the additi...

Thermal analysis of the exothermic reaction between galvanic porous silicon and sodium perchlorate.

Porous silicon (PS) films up to ∼150 μm thick with specific surface area similar to 700 m(2)/g and pore diameters similar to 3 nm are fabricated using a galvanic corrosion etching mechanism that does not require a power supply. After fabrication, the pores are impregnated with the strong oxidizer sodium perchlorate (NaClO(4)) to create a composite that constitutes a highly energetic system capable of explosion. Using bomb calorimetry, the heat of reaction is determined to be 9.9 ± 1.8 and 27.3 ± 3.2 kJ/g of PS when ignited under N(2) and O(2), respectively. Differential scanning calorimetry (DSC) reveals that the energy output is dependent on the hydrogen termination of the PS.

Nitrogen doping of polycrystalline 3C–SiC films grown by single-source chemical vapor deposition

Abstract The chemical, structural, and electrical properties of n-doped cubic silicon carbide films grown from the precursors 1,3-disilabutane and NH3 are investigated. Controlled nitrogen doping of 3C–SiC is demonstrated at 850 °C by the addition of NH3 to the reactor feed gas. This is shown to modify the growth rate, the chemical nature, and the crystalline quality of the deposited films. However, we show that an optimal conductivity is achieved without significant changes to the growth rate and crystalline quality of the 3C–SiC thin films.

Vibrational Spectra of Hydrogenated Buckminsterfullerene: A Candidate for the Unidentified Infrared Emission

A mixture of hydrides of buckminsterfullerene made by the interaction of atomic hydrogen with C60 is investigated using high-resolution electron energy loss spectroscopy (HREELS) in ultrahigh vacuum. The energy-loss spectra of partially hydrogenated C60 multilayers reveal vibrational features previously observed in infrared emission from interstellar and circumstellar dust clouds, including a broad loss envelope between 1150 and 1310 cm 21 , followed by a band at 1620 cm 21 in remarkable agreement with the canonical interstellar spacing of 300 cm 21 . Additionally, a major Ci H stretching band near 2900 cm 21 is observed and compared with the Galactic center absorption spectrum. Subject headings: dust, extinction — infrared: ISM: lines and bands — methods: laboratory

Smoluchowski ripening of Ag islands on Ag(100)

Using scanning tunneling microscopy, we study the post-deposition coarsening of distributions of large, two-dimensional Ag islands on a perfect Ag(100) surface at 295 K. The coarsening process is dominated by diffusion, and subsequent collision and coalescence of these islands. To obtain a comprehensive characterization of the coarsening kinetics, we perform tailored families of experiments, systematically varying the initial value of the average island size by adjusting the amount of Ag deposited (up to 0.25 ML). Results unambiguously indicate a strong decrease in island diffusivity with increasing island size. An estimate of the size scaling exponent follows from a mean-field Smoluchowski rate equation analysis of experimental data. These rate equations also predict a rapid depletion in the initial population of smaller islands. This leads to narrowing of the size distribution scaling function from its initial form, which is determined by the process of island nucleation and growth during deposition. Ho...