Fischer, John E

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Now showing 1 - 10 of 37
  • Publication
    Unusually low thermal conductivity of gallium nitride nanowires
    (2008-03-28) Guthy, Csaba; Nam, Chang-Yong; Fischer, John E
    We report measurements of thermal conductivity κ on individual gallium nitride nanowires (GaN NWs) with diameters ranging from 97 to 181 nm grown by thermal chemical vapor deposition. We observed unexpectedly small kappa values, in the range of 13–19 W/m K at 300 K, with very weak diameter dependence. We also observe unusual power law κ~Tn behavior with n=1.8 at low temperature. Electron-energy-loss-spectroscopy measurements indicate Si and O concentrations in the ranges of 0.1–1 and 0.01–0.1 at. %, respectively. Based on extensive numerical calculations, we conclude that both the unexpectedly low κ and the T1.8 dependence are caused by unusually large mass-difference scattering, primarily from Si impurities. Our analysis also suggests that mass-difference scattering rates are significantly enhanced by the reduced phonon group velocity in nanoscale systems. Planar defects running the length of the NW, previously characterized in detail, may also play a role in limiting the phonon mean free path.
  • Publication
    Small angle neutron scattering from single-wall carbon nanotube suspensions: evidence for isolated rigid rods and rod networks
    (2004-01-19) Zhou, Wei; Islam, M. F; Wang, H.; Ho, D. L; Winey, Karen I; Yodh, A. G; Fischer, John E
    We report small angle neutron scattering (SANS) from dilute suspensions of purified individual single wall carbon nanotubes (SWNTs) in D2O with added sodium dodecylbenzene sulfonate (NaDDBS) ionic surfactant. The scattered intensity scales as Q-1 for scattered wave vector, Q, in the range 0.005 < Q < 0.02 Å-1. The Q-1 behavior is characteristic of isolated rigid rods. A crossover of the scattered intensity power law dependence from Q-1 to Q-2 is observed at ~0.004 Å-1, suggesting the SWNTs form a loose network at 0.1 wt% with a mesh size of ~160 nm. SANS profiles from several other dispersions of SWNTs do not exhibit isolated rigid rod behavior; evidently the SWNTs in these systems are not isolated and form aggregates.
  • Publication
    Carbide-Derived Carbons with Tunable Porosity Optimized for Hydrogen Storage
    (2007-05-16) Fischer, John E; Gogotsi, Yury; Yildirim, Taner
    Relevance: Improvements in gravimetric and volumetric capacity were realized by processes which increase pore volume, heat of adsorption and powder density. Volumetric capacity was more than doubled by rolling peels with PTFE binder and pellet pressing. Even larger gains may be achieved with bulk precursors. Approach: A suite of post-processing strategies were developed and optimized for specific precursors. Technical Accomplishments and Progress: Excess H2 adsorption over 4.3 wt.% and 0.034 kg/L was demonstrated in as-produced CDC having a moderate SSA and pore volume @ (77K, 55 atm). Max heat of H2 adsorption up to 11 kJ/mol (with average values ~ 8 kJ/mol) demonstrated. Proposed Future Research: Further science-based modification of CDC porosity, microstructure and chemistry for improved H2uptake.
  • Publication
    Orientational Phase Transition in NaxC60 (1 < x < 3)
    (1993-08-30) Yildirim, Taner; Fischer, John E; Harris, A. Brooks; Stephens, Peter W; Smith, Amos B; Liu, Dan; Brard, Laurent; Strongin, Robert M
    X-ray diffraction and calorimetry data on cubic NaxC60(1
  • Publication
    Systematic study of contact annealing: Ambipolar silicon nanowire transistor with improved performance
    (2007-04-05) Byon, Kumhyo; Fischer, John E; Tham, Douglas; Johnson, Alan T
    High performance ambipolar silicon nanowire (SiNW) transistors were fabricated. SiNWs with uniform oxide sheath thicknesses of 6–7 nm were synthesized via a gas-flow-controlled thermal evaporation method. Field effect transistors (FETs) were fabricated using as-grown SiNWs. A two step annealing process was used to control contacts between SiNW and metal source and drain in order to enhance device performance. Initially ρ-channel devices exhibited ambipolar behavior after contact annealing at 400 ºC. Significant increases in on/off ratio and channel mobility were also achieved by annealing.
  • Publication
    Effect of the polar surface on GaN nanostructure growth and morphology
    (2004-12-06) Nam, Chang-Yong; Tham, Douglas; Fischer, John E
    Wurtzite gallium nitride nanostructures were grown by thermal reaction of gallium oxide and ammonia. The resulting morphology varied depending on ammonia flow rate. At 75 sccm only nanowires were obtained, while polyhedral crystals and nanobelts were observed at 175 sccm. Scanning electron microscopy and transmission electron microscopy revealed both thin smooth and thick corrugated nanowires. The growth axes of most of the smooth ones, as well as the nanobelts, were perpendicular to the c-axis (<0001>), while the corrugated nanowires and the large polyhedra grew parallel to <0001>. We propose a model to explain these morphology variations in terms of the Ga/N ratio and the different characteristic lengths of {0001} polar surfaces in the different nanostructures.
  • Publication
    Titanium Carbide Derived Nanoporous Carbon for Energy-Related Applications
    (2006-10-01) Dash, Ranjan; Chmiola, John; Yushin, Gleb; Gogotsi, Yury; Laudisio, Giovanna; Singer, Jonathan; Fischer, John E; Kucheyev, Sergei
    High surface area nanoporous carbon has been prepared by thermo-chemical etching of titanium carbide TiC in chlorine in the temperature range 200–1200 °C. Structural analysis showed that this carbide-derived carbon (CDC) was highly disordered at all synthesis temperatures. Higher temperature resulted in increasing ordering and formation of bent graphene sheets or thin graphitic ribbons. Soft X-ray absorption near-edge structure spectroscopy demonstrated that CDC consisted mostly of sp2 bonded carbon. Small-angle X-ray scattering and argon sorption measurements showed that the uniform carbon-carbon distance in cubic TiC resulted in the formation of small pores with a narrow size distribution at low synthesis temperatures; synthesis temperatures above 800 °C resulted in larger pores. CDC produced at 600–800 °C show great potential for energy-related applications. Hydrogen sorption experiments at −195.8 °C and atmospheric pressure showed a maximum gravimetric capacity of ∼ 330 cm3/g (3.0 wt.%). Methane sorption at 25 °C demonstrated a maximum capacity above 46 cm3/g (45 vol/vol or 3.1 wt.%) at atmospheric pressure. When tested as electrodes for supercapacitors with an organic electrolyte, the hydrogen-treated CDC showed specific capacitance up to 130 F/g with no degradation after 10 000 cycles.
  • Publication
    In-Situ Raman Scattering Studies of Alkali-Doped Single Wall Carbon Nanotubes
    (2001-01-05) Claye, Agnes; Rahman, S.; Fischer, John E; Sirenko, A.; Sumanasekera, G. U; Eklund, Peter C
    Electrochemical doping and in-situ Raman scattering were used to study charge transfer in K- and Li-doped single wall carbon nanotubes (SWNT) as a function of alkali concentration. An 8 cm-1 downshift was observed for the tangential phonon mode of SWNT doped to stoichiometries of KC24 and Li1.25C6. The shift in both systems is reversible upon de-doping despite an irreversible loss of crystallinity. These results indicate that the tangential mode shifts result from electron transfer from alkali dopants to the SWNT, and that these modes are only weakly affected by long-range order within the ropes.
  • Publication
    Single Wall Carbon Nanotube Fibers Extruded from Super-Acid Suspensions: Preferred Orientation, Electrical and Thermal Transport
    (2004-01-15) Zhou, Wei; Vavro, Juraj; Winey, Karen I; Guthy, K.; Fischer, John E; Ericson, Lars M; Ramesh, Sivarajan; Saini, Rajesh K; Davis, Virginia A; Kittrell, Carter; Pasquali, M.; Hauge, Robert H; Smalley, Richard E
    Fibers of single wall carbon nanotubes extruded from super-acid suspensions exhibit preferred orientation along their axes. We characterize the alignment by x-ray fiber diagrams and polarized Raman scattering, using a model which allows for a completely unaligned fraction. This fraction ranges from 0.17 to 0.05±0.02 for three fibers extruded under different conditions, with corresponding Gaussian full widths at half-maximum (FWHM) from 64o to 44o±2o. FWHM, aligned fraction, electrical and thermal transport all improve with decreasing extrusion orifice diameter. Resistivity, thermoelectric power and resonant-enhanced Raman scattering indicate that the neat fibers are strongly p-doped; the lowest observed ρ is 0.25mΩcm at 300 K. High temperature annealing increases ρ by more than 1 order of magnitude and restores the Raman resonance associated with low-energy van Hove transitions, without affecting the nanotube alignment.
  • Publication
    Single Wall Carbon Nanotubes Filled with Metallocenes: a First Example of Non-Fullerene Peapods
    (2001-11-26) Stercel, Ferenc; Nemes, Norbert M; Fischer, John E; Luzzi, David E
    We report the synthesis and analysis of metallocenes (ferrocene, chromocene, ruthenocene, vanadocene, tungstenocene-dihydride) encapsulated in single wall carbon nanotubes (SWNTs). In the case of ferrocene, efficient filling of the SWNTs was accomplished from both the liquid and the vapor phase. The other two metallocenes were filled from the vapor phase. High resolution transmission electron microscopy reveals single molecular chains of metallocenes inside SWNTs. Molecules move under the electron beam in the SWNTs indicating the absence of strong chemical bonds between each other and the SWNT wall. Their movement freezes after short illumination as a result of irradiation damage. Energy dispersive X-ray spectrometry confirms the presence of iron, chromium, ruthenium, vanadium and tungsten.