Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session N18: Focus Session: Carbon Nanotubes: Transport I |
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Sponsoring Units: DMP Chair: Jun Kono, Rice University Room: Baltimore Convention Center 315 |
Wednesday, March 15, 2006 8:00AM - 8:36AM |
N18.00001: Quantum Interference in Multiwall Carbon Nanotubes Invited Speaker: Recent low temperature conductance measurements on multiwall carbon nanotubes in perpendicular and parallel magnetic field are reported. An efficient gating technique allows for a considerable tuning of the nanotube doping level. This enables us to study extensively the signature of nanotube bandstructure in electron quantum interference effects like weak localization, universal conductance fluctuations and the Aharonov-Bohm effect. We show that the weak localization is strongly suppressed at peaks at certain gate voltages which can be linked with the bottoms of one-dimensional electronic subbands. This assignment allows a detailed comparison of theoretical calculations with the experimental data. In agreement with the theory, we find clear indications for a pronounced energy dependence of the elastic mean free with a strong enhancement close to the charge neutrality point. In large parallel magnetic field, we observe a superposition of h/2e-periodic Altshuler-Aronov-Spivak oscillations and an additional h/e-periodic contribution. The latter contribution shows a diamond-like pattern in the B/V$_{gate}$-plane, which reflects the magnetic field dependence of the density of states of the outermost shell of the nanotube. [Preview Abstract] |
Wednesday, March 15, 2006 8:36AM - 8:48AM |
N18.00002: Three-dimensional images of contact geometry between carbon nanotubes and metal contacts using electron tomography Judy Cha, Matthew Weyland, James Sethna, David Muller A significant barrier to the widespread application of carbon nanotube transistors is the variability in contact resistance between metallic leads and nanotubes. Varying by orders of magnitude, the contact resistance has recently been reported to depend on the size of the nanotube. To understand why, we study the three-dimensional contact geometry between metal contacts and nanotubes using electron tomography. Spatially resolved core-level electron energy-loss spectroscopy reveals a change in the local electronic structure of the nanotube in contact with gold-palladium We report successful three-dimensional reconstructions of the metal-nanotube interface for gold, gold-palladium and titanium contacts that explain the change in the electronic structure of the nanotube. [Preview Abstract] |
Wednesday, March 15, 2006 8:48AM - 9:00AM |
N18.00003: Electrically Tunable Magnetic Properties of Defective Metallic Carbon Nanotubes Young-Woo Son, Marvin L. Cohen, Steven G. Louie We present a first-principles study of the magnetic properties of metallic carbon nanotubes with various defects under a homogeneous transverse electric field. Single carbon adatoms, hydrogen passivated single carbon adatoms, and the various vacancies in (10,10) nanotubes are shown to play the role of magnetic impurities. The relative energy levels of quazi-localized states of such magnetic impurities with respect to the Fermi level are changeable with the application of a transverse electric field so that the corresponding magnetic ground states are shown to be tunable. Our results suggest that a pure organic nanomagnet could be realizable and their magnetic properties are controllable by electric fields. [Preview Abstract] |
Wednesday, March 15, 2006 9:00AM - 9:12AM |
N18.00004: Band structure modulation in carbon nanotube-metal junction Vadim Puller, Slava V. Rotkin In transport experiments carbon nanotubes (NTs) are embedded in metallic contacts. Distortion of NT shape within the NT-metal junction region may result in the change of NT electronic structure: the deformation can open a band gap in otherwise metallic nanotube, and reduce or completely close the gap in a semiconducting one. Local band structure modulation has non-negligible effect on the electron transmission through NT-metal interface, e.g. the Shottky barrier transparency is changing exponentially with the gap width. To analyze the deformation-induced change of the NT band structure in the contact region, we develop a comprehensive theory of the inhomogeneous deformations in NT, which combines the tight-binding approach with continuous description of the deformation field, and is applicable for NT of any chirality. We calculate the band gap variation in the contact region, and discuss the effect of the deformation on the NT-metal contact resistance. [Preview Abstract] |
Wednesday, March 15, 2006 9:12AM - 9:24AM |
N18.00005: Ab initio Calculation of Contact Effects on Transport Properties of Carbon Nanotubes Connected to Metallic Electrodes Nobuhiko Kobayashi, Taisuke Ozaki, Kenji Hirose Recently, a number of studies have been performed to make carbon nanotube devices. One of the important issues in the developments of the carbon nanotube devices is the control of contact effects of the electrodes. To detect electric signals through nanotubes, electrodes must be connected to the nanotubes. Contact with the electrodes sensitively influences the transport properties. Therefore, it is important to discuss the transport properties on the basis of the detailed electronic state calculation that includes the effect of contact with the electrodes. We have developed a first-principles method of analyzing quantum transport in nanometer-scale systems between electrodes. The electronic states are calculated using a numerical atomic orbital basis set in the framework of the density functional theory, and the conductance is calculated using the Green's function method. We apply the method to calculating transmission spectra of carbon nanotubes connected to metallic electrodes, and discuss the contact effect of the electrodes on the transport properties in the finite size of metallic and semiconducting nanotubes. [Preview Abstract] |
Wednesday, March 15, 2006 9:24AM - 9:36AM |
N18.00006: Electronic Structure of Metal-covered Semiconducting Carbon Nanotubes Wenguang Zhu, Efthimios Kaxiras Carbon nanotube field-effect transistor (CNFET) are regarded as potential building blocks for future nanoelectronics. The interaction between a carbon nanotube and metal contacts and the resulting electronic structure effects are crucial for device properties. In this talk, we present recent results on the properties of semiconducting single wall carbon nanotubes in contact with Pd, in a fully covered geometry that resembles experimental setups. We use first-principles calculations to determine the electronic structure, charge transfer effects, electrostatic potential and Fermi level alignment at the interfaces between the metal contact and various semiconducting single-wall carbon nanotubes. [Preview Abstract] |
Wednesday, March 15, 2006 9:36AM - 9:48AM |
N18.00007: Atomic-Scale Theory and Modeling of Electronic and Molecular Transport through Carbon Nanotubes Yongqiang Xue In this talk, we investigate the application of carbon nanotubes as novel transport channels for electrons and molecules using atomistic simulation. (1) Electronic transport: In this talk, we present a Green's function based self-consistent tight-binding study of electron transport through SWNT junction devices, which takes fully into account the 3D atomistic nature of the electronic processes. We discuss insights obtained from such atomistic study on the contact/diameter dependence of junction conductance and self-consistent study of current transport through metal-SWNT-metal junctions. (2) Molecular transport: Carbon nanotube could also be used to build assemblies for controlled transport of biomolecules for nanofluidic devices. Water confinement inside such nanoscale cylindrical core plays a significant role in determining the insertion and flow of biomolecules through the nanotube channel, which can be strongly affected by surface functionalization. Molecular dynamics simulations have been carried out to study the structure and thermodynamics of water in carbon nanotube and its effect on the spontaneous insertion of DNA molecules inside the nanotube channel. The simulations can provide valuable insights into the transport of molecules through nanoscale pore or channel structures. [Preview Abstract] |
Wednesday, March 15, 2006 9:48AM - 10:00AM |
N18.00008: Signatures of Chemical Defects in Carbon Nanotube Electronic Devices Brett R. Goldsmith, A. Kane, Philip G. Collins The study of chemical defects in carbon nanotubes has important implications for their operation as electronic devices, and many synthesis and fabrication techniques for such devices result in non-zero defect densities. Scanned probe microscopy techniques are particularly useful for identifying these sites and then examining their electronic properties. We have examined a number of electronic devices in which single defects play important roles in determining the two- and three-terminal device behavior. Using conducting-tip atomic force microscopy to measure local electronic properties, we distinguish one type of defect from another and correlate this with the device characteristics. Furthermore, we can chemically modify and reinvestigate the same defect site. Ultimately, the goal is to use the device characteristics as a kind of signature to reliably infer the presence of particular chemical defects. This work is partly supported by NSF grant DMR-0239842. [Preview Abstract] |
Wednesday, March 15, 2006 10:00AM - 10:12AM |
N18.00009: Gated Spin Transport through an Individual Single Wall Carbon Nanotube Bhaskar Nagabhirava, Tanesh Bansal, Gamini Sumanasekera, Lei Liu, Bruce Alphenaar We report on the fabrication and characterization of ferromagnetically contacted ``short channel'' SWNT devices that show clear hysteretic switching in the magnetoresistance, and provide strong evidence for SWNT spin transport. The main difference between our work and previous studies is that we have greatly reduced the transport length separating the ferromagnetic contacts to distances on the order of 10 nm. Preliminary measurements demonstrate this reduction to be extremely beneficial. We have observed clear hysteretic switching in the magnetoresistance in 75{\%} of our devices, and are able to modify the magnetoresistance between +15{\%} and -10{\%} as a function of gate voltage. The gate mediated change in magnitude \textit{and sign} of the magnetoresistance switching allows us to discount other non-spin related sources for the observed signal and provides the basis for the first SWNT spin transistor. We note that the short channel contacting scheme is generally applicable to non-ferromagnetic contacts as well, and provides a straightforward technique for fabricating SWNT quantum dot devices. [Preview Abstract] |
Wednesday, March 15, 2006 10:12AM - 10:24AM |
N18.00010: Spin transport studies in mesoscopic graphite Barbaros Oezyilmaz, Philip Kim We present experimental studies on spin transport in mesoscopic graphite. Two dimensional graphite sheets have been fabricated by means of micromechanical exfoliation. Spin injection has been achieved by employing ferromagnetic Co electrodes. We use the shape anisotropy of the electrodes to uniquely define the magnetic state of the device. Typical two terminal resistances are in the order of 1 k$\Omega$. We will discuss the switching behavior of the device magnetoresistance as a function of temperature, the gate bias voltage and of the source drain bias. [Preview Abstract] |
Wednesday, March 15, 2006 10:24AM - 10:36AM |
N18.00011: Ballistic conductance in narrow graphene strips D. Areshkin, J.W. Mintmire, C.T. White With structures making them suitable for in-plane device processing, high aspect ratio graphene strips with widths down to tens of nanometers or smaller could ultimately provide important components in carbon-based quantum electronics. However, in comparison to corresponding single-wall carbon nanotubes, such strips will likely have a higher degree of imperfection due to variations in their widths and interactions with the substrate which will degrade their conductance. Also, unlike nanotubes, they can exhibit highly localized edge states which are degenerate with their more extended states at or near the Fermi level. On the other hand, their more extended states near the Fermi level have properties similar to those exhibited by related states in nanotubes, which should suppress the effects of back scattering both due to short and long-range disorder. Stimulated by these observations and recent experiments on graphene sheets, simulations were performed to assess the effects of various types of disorder on the conductance of narrow graphene strips. The results indicate that these strips can exhibit ballistic conductance over large distances in the presence of reasonable disorder making them excellent synthetic targets for carbon-based device applications. [Preview Abstract] |
Wednesday, March 15, 2006 10:36AM - 10:48AM |
N18.00012: Probing Biological Processes on Supported Lipid Bilayers with Single-Walled Carbon Nanotube Field-Effect Transistors Xinjian Zhou, Jose Manuel Moran-Mirabal, Harold Craighead , Paul McEuen We have formed supported lipid bilayers (SLBs) by small unilamellar vesicle fusion on substrates containing single-walled carbon nanotube field-effect transistors (SWNT-FETs). We are able to detect the self-assembly of SLBs electrically with SWNT-FETs since their threshold voltages are shifted by this event. The SLB fully covers the NT surface and lipid molecules can diffuse freely in the bilayer surface across the NT. To study the interactions of important biological entities with receptors imbedded within the membrane, we have also integrated a membrane protein, GT1b ganglioside, in the bilayer. While bare gangliosides can diffuse freely across the NT, interestingly the NT acts as a diffusion barrier for the gangliosides when they are bound with tetanus toxin. This experiment opens the possibility of using SWNT-FETs as biosensors for label-free detection. [Preview Abstract] |
Wednesday, March 15, 2006 10:48AM - 11:00AM |
N18.00013: First Principles Properties of Polymeric Photovoltaic Materials T. Jayasekera, J. W. Mintmire Recent reports suggest that the acceptor-donor junction for bulk heterojunction photovoltaic devices can be achieved using single wall carbon nanotubes (SWNT) and polymers such as poly-3-octothiophenes (P3OT). Optical excitation is believed to occur in the organic polymer which acts as a good hole conductor, with electron transfer to the SWNT which acts as a good electron conductor. An appropriate theoretical understanding of the photovoltaic effect requires knowledge of the electronic states near the Fermi level in these materials. We calculate the electronic structure of infinitely long quasi one-dimensional nanostructures such as carbon nanotubes or electroactive chain polymers, such as polythiophenes using a first principles, all electron, self consistent local density functional (LDF) approach. We present and compare electronic structure calculations for SWNTs and poly-3-alkyl-thiophenes. Further we discuss the variation of effective mass of charge carriers in polymers and SWNTs in the vicinity of Fermi level. This work was supported by the US Office of Naval Research and the DoD HPCMO CHSSI program through the Naval Research Laboratory. [Preview Abstract] |
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