Fischer, John E
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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 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 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 Out-of-plane mosaic of single-wall carbon nanotube films(2004-03-22) Winey, Karen I; Zhou, Wei; Fischer, John E; Sreekumar, T. V.; Kumar, S.; Kataura, H.For single-wall carbon nanotube (SWNT) films deposited from suspension onto filter membranes, or by drop casting or spin coating onto flat substrates, the tube axes lie preferentially in the film plane. Using x-ray scattering and a two-dimensional detector, we show that this out-of-plane mosaic spread can be easily and accurately quantified. It varies significantly with deposition conditions, and the aligning effects of deposition and external force in the film plane (e.g., magnetic field) are additive. Films from well-dispersed tubes show better alignment than from poor dispersions. The finite out-of-plane mosaic in C60@SWNT films enables quantitative separation of one-dimensional diffraction (chains of C60 peas) from the 2D rope lattice diffraction.Publication Electrical and thermal properties of C60-filled single-wall carbon nanotubes(2002-02-25) Vavro, Juraj; Llaguno, Mark C; Luzzi, David E; Satishkumar, B. C; Fischer, John EWe report measurements of electrical resistivity, thermopower, and thermal conductivity of highly C60-filled single-wall carbon nanotubes and unfilled controls, from 1.5 to 300 K. The data suggest that the C60 chains provide additional conductive paths for charge carriers, increase the rate of phonon scattering, and block interior sites from sorbing other gas molecules.Publication Silicon Nanowires: Doping Dependent N- And P- Channel FET Behavior(2004-11-29) Byon, Kumhyo; Fischer, John E; Adu, Kofi W; Eklund, Peter C.The electrical transport properties of field effect transistor (FET) devices made of silicon nanowires (SiNWs) synthesized by pulsed laser vaporization (PLV) were studied. From as-grown PLV-SiNW FET, we found p-channel FET behavior with low conductance. To improve conductance, spin on glass (SOG) and vapor doping were used to dope phosphorus and indium into SiNW, respectively. From doping after synthesis, we could successfully make both n- and p-channel FET devices.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.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 Carbon nanotube composites for thermal management(2002-04-15) Biercuk, M. J; Llaguno, Mark C; Radosavljevic, M.; Johnson, Alan T; Hyun, J. K; Fischer, John ESingle-wall carbon nanotubes (SWNTs) were used to augment the thermal transport properties of industrial epoxy. Samples loaded with 1 wt% unpurified SWNT material show a 70% increase in thermal conductivity at 40K, rising to 125% at room temperature; the enhancement due to 1 wt% loading of vapor grown carbon fibers is three times smaller. Electrical conductivity data show a percolation threshold between 0.1 and 0.2 wt% SWNT loading. The Vickers hardness rises monotonically with SWNT loading up to a factor of 3.5 at 2 wt%. These results suggest that the thermal and mechanical properties of SWNT-epoxy composites are improved, without the need to chemically functionalize the nanotubes.