Carbon Nanotube Devices: Growth, Imaging, and Electronic Properties

This dissertation focuses on growth, fabrication, and electronic characterization of carbon nanotube (CNT) devices. A technique for imaging CNTs on insulating substrates with the scanning electron microscope (SEM) will be described. This technique relies on differential charging of the CNT relative to the surrounding insulator. In addition, it is not only quicker than using scanning probe microscopy (SPM), but is also useful for identifying conducting pathways within an assortment of CNTs and metallic contacts. CNT field effect transitors (FETs) fabricated on strontium titanate gate dielectric show transconductances normalized by channel width of 8900 S/m, greatly exceeding that in Si FETs. Intriguingly, the transconductance cannot be explained within the conventional FET or Schottky-barrier models. To explain this, it is proposed that there is Schottky-barrier lowering due to high electric fields at the nanotube/contact interface. Exploring novel CNT-FET lithography, I demonstrate focused electron beam induced deposition (FEBID) of pure gold for CNT device electrodes. In examination of the CNT/electrode interface, equivalence between FEBID leads and leads deposited using conventional electron beam lithography is found with the majority device resistance in the CNT. Lastly, CNTs are suspended across wide trenches (>100 microns). These trenches are formed without lithography or etching and have metallic leads on either side of the trench for electrical transport measurements. Using a mechanical probe as a mobile gate, electrical transport can be performed on these suspended CNT devices, which show minimal hysteresis consistent with the absence of charge trapping

Optical Dark-Field and Electron Energy Loss Imaging and Spectroscopy of Symmetry-Forbidden Modes in Loaded Nanogap Antennas

We have produced large numbers of hybrid metal–semiconductor nanogap antennas using a scalable electrochemical approach and systematically characterized the spectral and spatial character of their plasmonic modes with optical dark-field scattering, electron energy loss spectroscopy with principal component analysis, and full wave simulations. The coordination of these techniques reveal that these nanostructures support degenerate transverse modes which split due to substrate interactions, a longitudinal mode which scales with antenna length, and a symmetry-forbidden <i>gap-localized transverse</i> mode. This gap-localized transverse mode arises from mode splitting of transverse resonances supported on both antenna arms and is confined to the gap load enabling (i) delivery of substantial energy to the gap material and (ii) the possibility of tuning the antenna resonance <i>via</i> active modulation of the gap material’s optical properties. The resonant position of this symmetry-forbidden mode is sensitive to gap size, dielectric strength of the gap material, and is highly suppressed in air-gapped structures which may explain its absence from the literature to date. Understanding the complex modal structure supported on hybrid nanosystems is necessary to enable the multifunctional components many seek

CO \u3c inf\u3e 2 electroreduction to hydrocarbons on carbon-supported Cu nanoparticles

© 2014 American Chemical Society. Activities of Cu nanoparticles supported on carbon black (VC), single-wall carbon nanotubes (SWNTs), and Ketjen Black (KB) toward CO2 electroreduction to hydrocarbons (CH4, C2H2, C2H4, and C2H6) are evaluated using a sealed rotating disk electrode (RDE) setup coupled to a gas chromatograph (GC). Thin films of supported Cu catalysts are deposited on RDE tips following a procedure well-established in the fuel cell community. Lead (Pb) underpotential deposition (UPD) is used to determine the electrochemical surface area (ECSA) of thin films of 40 wt % Cu/VC, 20 wt % Cu/SWNT, 50 wt % Cu/KB, and commercial 20 wt % Cu/VC catalysts on glassy carbon electrodes. Faradaic efficiencies of four carbon-supported Cu catalysts toward CO2 electroreduction to hydrocarbons are compared to that of electrodeposited smooth Cu films. For all the catalysts studied, the only hydrocarbons detected by GC are CH4 and C2H4. The Cu nanoparticles are found to be more active toward C2H4 generation versus electrodeposited smooth copper films. For the supported Cu nanocatalysts, the ratio of C2H4/CH4 Faradaic efficiencies is believed to be a function of particle size, as higher ratios are observed for smaller Cu nanoparticles. This is likely due to an increase in the fraction of under-coordinated sites, such as corners, edges, and defects, as the nanoparticles become smaller

Ultraviolet and Visible Photochemistry of Methanol at 3D Mesoporous Networks: TiO₂ and Au–TiO₂

Comparison of methanol photochemistry at three-dimensionally (3D) networked aerogels of TiO₂ or Au–TiO₂ reveals that incorporated Au nanoparticles strongly sensitize the oxide nanoarchitecture to visible light. Methanol dissociatively adsorbs at the surfaces of TiO₂ and Au–TiO₂ aerogels under dark, high-vacuum conditions. Upon irradiation of either ultraporous material with broadband UV light under anaerobic conditions, adsorbed methoxy groups act as hole-traps and extend conduction-band and shallow-trapped electron lifetimes. A higher excited-state electron density arises for UV-irradiated TiO₂ aerogel relative to commercial nanoparticulate TiO₂, indicating that 3D networked TiO₂ more efficiently separates electron–hole pairs. Upon excitation with narrow-band visible light centered at 550 nm, long-lived excited-state electrons are evident on CH₃OH-exposed Au–TiO₂ aerogelsbut not on identically dosed TiO₂ aerogelsverifying that incorporated Au nanoparticles sensitize the networked oxide to visible light. Under aerobic conditions (20 Torr O₂) and broadband UV illumination, surface-sited formates accumulate as adsorbed methoxy groups oxidize, at similar rates, on Au–TiO₂ and TiO₂ aerogels. Moving to excitation wavelengths longer than ∼400 nm (i.e., the low-energy range of UV light) dramatically decreases methoxy photoconversion for methanol-saturated TiO₂ aerogel, while Au–TiO₂ aerogel remains highly active for methanol photooxidation. The wavelength dependence of formate production on Au–TiO₂ tracks the absorbance spectrum for this material, which peaks at λ = 550 nm due to resonance with the surface plasmon in the Au particles. The photooxidation rate for Au–TiO₂ aerogel at 550 nm is comparable to that for TiO₂ aerogel under broadband UV illumination, indicating efficient energy transfer from Au to TiO₂ in the 3D mesoporous nanoarchitecture