Fischer, John E
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Publication EWGP--a work in progress....(2007-03-09) Fischer, John EThe need for energy affects almost every aspect of modern society. Indeed, the advent of coal as a widely-used energy source is sometimes viewed as the spark that started the Industrial Revolution. However, we are about to enter a new era due to increased competition from developing nations for the world's dwindling energy supplies and to the growing recognition that our current energy usage is unsustainable and is affecting the world's climate. It is apparent that research on energy-related issues will become increasingly important in the coming years.Publication Imaging, Structural and Chemical Analysis of Silicon Nanowires(2002-12-02) Fischer, John E; Barsotti, R. J.; Mahmood, J.; Lee, C. H.; Adu, Kofi W.; Eklund, Peter CLaser ablation has been used to grow silicon nanowires with an average diameter of 6.7 nm ± 2.7 nm surrounded by an amorphous SiOx sheath of 1-2 nm. This paper reports the imaging, chemical and structural analysis of these wires. Due to the growth temperature and the presence of calcium impurities and trace oxygen, two distinct types of wires are found. They appear to grow by two different processes. One requires a metal catalyst, the other is catalyzed by oxygen.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 CElectrochemical 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 EFibers 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 EWe 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.Publication Synthesis and Post-growth Doping of Silicon Nanowires(2005-11-01) Byon, Kumhyo; Tham, Douglas; Fischer, John E; Johnson, Alan THigh quality silicon nanowires (SiNWs) were synthesized via a thermal evaporation method without the use of catalysts. Scanning electron microscopy and transmission electron microscopy showed that SiNWs were long and straight crystalline silicon with an oxide sheath. Field effect transistors (FETs) were fabricated to investigate the electrical transport properties. Devices on as-grown material were p-channel with channel mobilities 1 - 10 cm2 V-1 s-1. Post-growth vapor doping with bismuth converted these to n-channel behavior.Publication Carbide-Derived Carbons: Effect of Pore Size on Hydrogen Uptake and Heat of Adsorption(2006-10-27) Yushin, Gleb; Dash, Ranjan; Jagiello, Jacek; Fischer, John E; Gogotsi, YuryCryoadsorption is a promising method of enhancing gravimetric and volumetric onboard H2 storage capacity for future transportation needs. Inexpensive carbide-derived carbons (CDCs), produced by chlorination of metal carbides, have up to 80 % open-pore volume with tunable pore size and specific surface area (SSA). Tuning the carbon structure and pore size with high sensitivity by using different starting carbides and chlorination temperatures allows rational design of carbon materials with enhanced C-H2 interaction and thus increased H2 storage capacity. A systematic experimental investigation of a large number of CDCs with controlled pore size distributions and SSAs shows how smaller pores increase both the heat of adsorption and the total volume of adsorbed H2. It has been demonstrated that increasing the average heat of H2 adsorption above 6.6 kJ mol-1 substantially enhances H2 uptake at 1 atm (1 atm = 101 325 Pa) and -196 °C. The heats of adsorption up to 11 kJ mol-1 exceed values reported for metal-organic framework compounds and carbon nanotubes.Publication Single-walled carbon nanotubes in superacid: X-ray and calorimetric evidence for partly ordered H2SO4(2005-07-01) Zhou, Wei; Fischer, John E; Heiney, P. A; Fan, H.; Davis, Virginia A; Pasquali, M.; Smalley, Richard ELiquid anhydrous sulfuric acid forms a partly ordered structure in the presence of single-walled carbon nanotubes (SWNTs). X-ray scattering from aligned fibers immersed in acid shows the formation of molecular shells wrapped around SWNTs. Differential scanning calorimetry of SWNT-acid suspensions exhibits concentration-dependent supercooling/melting behavior, confirming that the partly ordered molecules are a new phase. We propose that charge transfer between nanotube π electrons and highly oxidizing superacid is responsible for the unique partly ordered structure.Publication Charge transfer and Fermi level shift in p-doped single-walled carbon nanotubes(2005-06-22) Zhou, Wei; Vavro, Juraj; Nemes, Norbert M.; Fischer, John E; Borondics, F.; Kamarás, K.; Tanner, D. B.The electronic properties of p-doped single-walled carbon nanotube (SWNT) bulk samples were studied by temperature-dependent resistivity and thermopower, optical reflectivity and Raman spectroscopy. These all give consistent results for the Fermi level downshift (δ EF) induced by doping. We find δ EF ≈ 0.35 eV and 0.50 eV for concentrated nitric and sulfuric acid doping respectively. With these values, the evolution of Raman spectra can be explained by variations in the resonance condition as EF moves down into the valence band. Furthermore, we find no evidence for diameter-selective doping, nor any distinction between doping responses of metallic and semiconducting tubes.Publication Applications of electron microscopy to the characterization of semiconductor nanowires(2006-11-01) Tham, Douglas; Nam, Chang-Yong; Byon, Kumhyo; Kim, Jinyong; Fischer, John EWe review our current progress on semiconductor nanowires of β-Ga2O3, Si and GaN. These nanowires were grown using both vapor–solid (VS) and vapor–liquid–solid (VLS) mechanisms. Using transmission electron microscopy (TEM) we studied their morphological, compositional and structural characteristics. Here we survey the general morphologies, growth directions and a variety of defect structures found in our samples. We also outline a method to determine the nanowire growth direction using TEM, and present an overview of device fabrication and assembly methods developed using these nanowires.