Johnson, A T

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Now showing 1 - 10 of 19
  • Publication
    Probing Spin-charge Relation by Magnetoconductance in One-dimensional Polymer Nanofibers
    (2012-10-12) Choi, A.; Kim, K. H; Hong, S. J; Goh, M.; Akagi, K.; Kaner, R. B; Johnson, A.T. Charlie; Kirova, N. N; Bonnell, Dawn A; Brazovskii, S. A; Mele, Eugene J.; Park, Y. W
    Polymer nanofibers are one-dimensional organic hydrocarbon systems containing conducting polymers where the nonlinear local excitations such as solitons, polarons, and bipolarons formed by the electron-phonon interaction were predicted. Magnetoconductance (MC) can simultaneously probe both the spin and charge of these mobile species and identify the effects of electron-electron interactions on these nonlinear excitations. Here, we report our observations of a qualitatively differentMC in polyacetylene (PA) and in polyaniline (PANI) and polythiophene (PT) nanofibers. In PA, the MC is essentially zero, but it is present in PANI and PT. The universal scaling behavior and the zero (finite)MC in PA (PANI and PT) nanofibers provide evidence of Coulomb interactions between spinless charged solitons (interacting polarons which carry both spin and charge).
  • Publication
    Nanoparticle Shape Selection by Repulsive Interactions: Metal Islands on Few-Layer Graphene
    (2010-01-01) Somers, Luke A; Johnson, Charlie; Mele, Eugene J; Zimbovskaya, Natalya A
    Metal atoms adsorbed on few-layer graphenes condense to form nanometer-size droplets whose growth is size limited by a competition between the surface tension and repulsive electrostatic interactions from charge transfer between the metal droplet and the graphene. For situations where the work-function mismatch is large and the droplet surface tension is small, a growing droplet can be unstable to a family of shape instabilities. We observe this phenomenon for Yb deposited and annealed on few-layer graphenes and develop a theoretical model to describe it by studying the renormalization of the line tension of a two-dimensional droplet by repulsive interparticle interactions. Our model describes the onset of shape instabilities for nanoparticles where the growth is size limited by a generic repulsive potential and provides a good account of the experimentally observed structures for Yb on graphene.
  • Publication
    DNA-Decorated Carbon Nanotubes as Sensitive Layer for AlN Contour-Mode Resonant-MEMS Gravimetric Sensor
    (2009-01-25) Zuniga, Chiara; Rinaldi, Matteo; Khamis, Samuel M; Jones, Timothy S; Johnson, A T; Piazza, Gianluca
    In this work a nano-enabled gravimetric chemical sensor prototype based on single-stranded DNA (ss-DNA) decorated single-walled carbon nanotubes (SWNT) as nano-functionalization layer for Aluminun Nitride (AIN) contour-mode resonant-MEMS gravimetric sensors has been demonstrated. Two resonators fabricated on the same silicon chip and operating at different resonance frequencies, 287 and 450 MHz, were functionalized with this novel bio-coating layer to experimentally prove the capability of two distinct single strands of DNA bound to SWNT to enhance differently the adsorption of volatile organic compounds such as dinitroluene (DNT, simulant for explosive vapor) and dymethyl-methylphosphonate (DMMP, a simulant for nerve agent sarin). The introduction of this bio-coating layer addresses the major drawbacks of recovery time (50% recovery in less than 29 seconds has been achieved) and lack of selectivity associated with gas sensor based on polymers and pristine carbon nanotube functionalization layers.
  • Publication
    Gas sensing properties of single conducting polymer nanowires and the effect of temperature
    (2009-10-02) Dan, Yaping; Cao, Yanyan; Mallouk, Tom E; Johnson, A.T. Charlie; Evoy, Stephane
    We measured the electronic properties and gas sensing responses of template-grown poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS)-based nanowires. The nanowires had a 'striped' structure (gold–PEDOT/PSS–gold), and were typically 8 µm long (1 µm–6 µm–1 µm for the sections, respectively) and 220 nm in diameter. Single-nanowire devices were contacted with pre-fabricated gold electrodes using dielectrophoretic assembly. A polymer conductivity of 11.5 ± 0.7 S cm−1 and a contact resistance of 27.6 ± 4 kΩ were inferred from measurements on nanowires of varying length and diameter. The nanowire sensors detected a variety of odors, with rapid response and recovery (seconds). The response (ΔR/R) varied as a power law with analyte concentration. The power law exponent was found to increase with the molecular weight of the analyte and as a function of temperature. The detection limits are set by noise intrinsic to the device and are at the ppm level even for very volatile analytes.
  • Publication
    Fabrication of Photonic Crystals with high refractive index
    (2007-03-09) Xu, Yongan; Yang, Shu; Moon, Jun Hyuk; Johnson, Alan T; Dan, Yaping; Perry, Joseph W; Adibi, Ali; Hotchkiss, Peter; Marder, Seth
    • Complete photonic bandgap • High contrast of refractive index (RI) • Polymer material with a low RI • Inorganic material with a higher RI, such as silicon, titania. • Fabrication of diamond-like PCs by MBIL, • Fabrication of high RI inorganic PCs via double templating, • Core-shell morphology of replica • Pinch-off problem • Development of combined level-surface to address pinch-off problem • Electrodeposition of titania 3D structure • Electrophoretic deposition of surface charged nanoparticles
  • Publication
    Effect of Thermal Treatments on the Transduction Behaviors of Conductometric Hydrogen Gas Sensors Integrated with HCl-Doped Polyaniline Nanofibers
    (2008-01-01) Dan, Yaping; Somers, Luke A; Wang, Pen-Cheng; Johnson, A.T. Charlie; MacDiarmid, Alan G
    We present the effect of thermal treatments on the transduction behaviors of HCl-doped polyaniline (PANI) nanofibers integrated in conductometric devices upon exposure to 1% H2 (carried by N2). After drying in N2 at 25ºC for 12 hours, devices showed a ~10% decrease in electrical resistance upon exposure to 1% H2. However, devices subject to 12-hour drying in N2 at 25ºC followed by further thermal treatments in N2 at 100ºC, 164ºC or 200ºC for 30 minutes showed different transduction behaviors. Specifically, devices subject to thermal treatments at 100ºC and 164ºC showed a decrease in electrical resistance by ~7% and <0.5%, respectively. More interestingly, the device subject to thermal treatment at 200ºC showed a transduction behavior with opposite polarity, i.e. a ~5% increase in electrical resistance upon exposure to 1% H2. SEM, FTIR and TGA were employed to investigate the effect of thermal treatments on the morphology and chemical characteristics of HCl-doped polyaniline nanofibers. The results indicated that the change in the devices? interesting transduction behaviors might be related to the thermal treatment effects on the HCl-doped PANI nanofibers in (i) removal of adsorbed water, and (ii) crosslinking and/or degradation of polymer backbones.
  • Publication
    Optimized Photolithographic Fabrication Process for Carbon Nanotube Devices
    (2011-04-14) Khamis, Samuel M.; Johnson, A. T. Charlie; Jones, R. A.
    We have developed a photolithographic process for the fabrication of large arrays of single walled carbon nanotube transistors with high quality electronic properties that rival those of transistors fabricated by electron beam lithography.Abuffer layer is used to prevent direct contact between the nanotube and the novolac-based photoresist, and a cleaning bake at 300C effectively removes residues that bind to the nanotube sidewall during processing. In situ electrical measurement of a nanotube transistor during a temperature ramp reveals sharp decreases in the ON-state resistance that we associate with the vaporization of components of the photoresist. Data from nearly 2000 measured nanotube transistors show an average ON-state resistance of 250 ± 100 kΩ. This new process represents significant progress towards the goal of highyield production of large arrays of nanotube transistors for applications including chemical sensors and transducers, as well as integrated circuit components.
  • 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
    Gate Coupling to Nanoscale Electronics
    (2009-05-07) Datta, Sujit S.; Strachan, Douglas R.; Johnson, A.T. Charlie
    The realization of single-molecule electronic devices, in which a nanometer-scale molecule is connected to macroscopic leads, requires the reproducible production of highly ordered nanoscale gaps in which a molecule of interest is electrostatically coupled to nearby gate electrodes. Understanding how the molecule-gate coupling depends on key parameters is crucial for the development of high-performance devices. Here we directly address this, presenting two- and three-dimensional finite-element electrostatic simulations of the electrode geometries formed using emerging fabrication techniques. We quantify the gate coupling intrinsic to these devices, exploring the roles of parameters believed to be relevant to such devices. These include the thickness and nature of the dielectric used, and the gate screening due to different device geometries. On the singlemolecule ( ~ 1 nm) scale, we find that device geometry plays a greater role in the gate coupling than the dielectric constant or the thickness of the insulator. Compared to the typical uniform nanogap electrode geometry envisioned, we find that nonuniform tapered electrodes yield a significant 3 orders of magnitude improvement in gate coupling. We also find that in the tapered geometry the polarizability of a molecular channel works to enhance the gate coupling.
  • Publication
    Controlled Doping of Graphene Using Ultraviolet Irradiation
    (2012-06-20) Luo, Zhentang; Pinto, Nicholas J.; Davila, Yarely; Johnson, A.T. Charlie
    The electronic properties of graphene are tunable via doping, making it attractive in low dimensional organic electronics. Common methods of doping graphene, however, adversely affect charge mobility and degrade device performance. We demonstrate a facile shadow mask technique of defining electrodes on graphene grown by chemical vapor deposition (CVD) thereby eliminating the use of detrimental chemicals needed in the corresponding lithographic process. Further, we report on the controlled, effective, and reversible doping of graphene via ultraviolet (UV) irradiation with minimal impact on charge mobility. The change in charge concentration saturates at ~2 x 1012cm-2 and the quantum yield is 10-5 e/photon upon initial UV exposure. This simple and controlled strategy opens the possibility of doping wafer-size CVD graphene for diverse applications.