Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session P24: Focus Session: Electron Transport in Nanotubes |
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Sponsoring Units: DMP Chair: Marco Buongiorno Nardelli, North Carolina State University Room: 326 |
Wednesday, March 18, 2009 8:00AM - 8:36AM |
P24.00001: Coupling of spin and orbital motion of electrons in carbon nanotubes Invited Speaker: Electrons in atoms possess both spin and orbital degrees of freedom. In non-relativistic quantum mechanics, these are independent, resulting in large degeneracies in atomic spectra. However, relativistic effects couple the spin and orbital motion, leading to the well-known fine structure in atomic spectra. The electronic states in defect-free carbon nanotubes are widely believed to be four-fold degenerate, owing to independent spin and orbital symmetries, and also to possess electron--hole symmetry. Here we report measurements demonstrating that the spin and orbital motion of electrons are coupled, thereby breaking all of these symmetries. This spin--orbit coupling is directly observed as a splitting of the four-fold degeneracy of a single electron in ultra-clean quantum dots. The coupling favors parallel alignment of the orbital and spin magnetic moments for electrons and antiparallel alignment for holes. Our measurements are consistent with recent theories that predict the existence of spin--orbit coupling in curved graphene and describe it as a spin-dependent topological phase in nanotubes. Work done in collaboration with F. Kuemmeth, S. Ilani, and D. C. Ralph [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P24.00002: Sensitivity of carbon nanotube and graphene transistors to local ionic structure Iddo Heller, Sohail Chatoor, Jaan Mannik, Marcel A. G. Zevenbergen, Cees Dekker, Serge G. Lemay Transistors based on single-walled carbon nanotubes (SWNTs) and graphene can be operated in aqueous solution where the electrolyte acts as a highly efficient gate. We show that the composition and spatial distribution of ions in the electrolyte intricately affect the conductance of SWNT and graphene transistors. Changes in the ionic strength, pH, and surprisingly, the type of ions affect the electronic transport through the electrostatic gating effect. In addition, changing pH leads to Schottky-barrier modification, while changing ionic strength affects the gate capacitance. Interestingly, the observed electrostatic gating effect for graphene is larger than for SWNTs. Most of our data is explained by a model that considers ionizable groups on both the underlying substrate and on the carbon surfaces. Our findings have significant implications for optimizing sensing experiments with nanocarbon transistors. [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P24.00003: Boosting electronic transport in carbon nanotubes by isotopic disorder Michele Lazzeri, Niels Vandecasteele, Francesco Mauri The current/voltage curve of metallic carbon nanotubes (CNTs) displays at high bias a sudden increase of the resistivity due to the scattering of electrons with phonons [1] having an anomalously-high population (hot phonons) [2,3]. Indeed, the rate at which hot-phonons are excited by the electrons is faster than the rate at which they are deexcited. Here, we show that it is possible to improve the electrical performances of metallic CNTs by 13C isotope enrichment. In fact, isotopic disorder creates additional channels for the hot-phonon deexcitation, reduces their population and, thus, the nanotube high-bias differential-resistance. This is an extraordinary case where disorder improves the electronic transport, with important technological consequences in view of the use of metallic CNTs as interconnects in future electronic devices. [1] Z. Yao et al., Phys. Rev. Lett. 84, 2941 (2000). [2] M. Lazzeri et al., Phys. Rev. Lett. 95, 236802 (2005); M. Lazzeri and F.Mauri, Phys. Rev. B 73, 165419 (2006). [3] M. Oron-Carl and R. Krupke, Phys. Rev. Lett. 100, 127401 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P24.00004: Electrical Transport in Long Bundles of Carbon Nanotube-Metal Hybrids Saikat Talapatra, Rakesh Shah, Clayton Schenk, Xianfeng Zhang, Swastik Kar Although a number of works have proposed that bundles of carbon nanotubes can withstand high current densities at low resistances for high-performance applications, such structures have been demonstrated to fall short of proposed expectations. This is chiefly due to limited access to all nanotubes in a bundle in conventional two-terminal device configurations, with low number of effective conducting channels. By depositing a small quantity of high-conductance metal alloys that wet the nanotube surface, our CNT-Au/Pd alloy hybrid conductors show improved performance in terms of failure current density and resistivity. Low temperature transport measurements show that the nanotube bundles with metal coating and especially after the high-bias treatment show more and more metallic nature, with decreased negative temperature coefficient of resistance. The results obtained will be discussed in the framework of transport theories of quasi-one dimensional systems. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P24.00005: Phonon populations in a biased carbon nanotube transistor Mathias Steiner, Marcus Freitag, Vasili Perebeinos, James Tsang, Joshua Small, Megumi Kinoshita, Dongning Yuan, Jie Liu, Phaedon Avouris We present a comprehensive picture of the phonon populations in an electrically-driven carbon nanotube transistor, including the Raman-active G and radial breathing modes (RBM), and also the Raman-inactive zone boundary mode (K), and intermediate-frequency mode (IFP), populated by anharmonic decay. The effective temperature of the RBM is considerably lower than the intermediate- and high-frequency mode temperatures, which we explain by a phonon-decay bottleneck. We include substrate polar phonon scattering to fully account for the device electronic characteristics. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P24.00006: Electronic and thermal transport in carbon nanostructures: the role of low-frequency modes Nicola Bonini, Nicola Marzari Low-frequency phonon modes play an important role in the electronic and thermal transport properties of carbon nanotubes and ultrathin graphitic films. Not only they determine the very high thermal conductivity of these materials, but they also affect the electrical transport: at low bias they weakly scatter electrons, while at high bias they concur to determine the population of those optical phonon modes that most strongly limit the electrical conductivity. Quite interestingly, these low frequency phonons are also expected to couple to the vibrational modes of a surrounding medium more efficiently than high frequency phonons, providing an effective channel for the exchange of vibrational energy between the nanostructure and the environment. Here we use density functional theory and density functional perturbation theory to characterize the inelastic relaxation mechanisms---phonon-phonon and electron-phonon interactions---that determine the lifetime of these phonon modes. We will discuss the relevance of these results to estimate the transport properties of carbon nanomaterials. [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P24.00007: Temperature-dependant current saturation in double-wall carbon nanotubes Delphine Bouilly, Matthieu Paillet, Richard Martel Current saturation is known to occur at high voltage bias in carbon nanotubes, for single-wall as well as multi-wall configurations. This saturation is generally attributed to the backscattering of carriers by optical phonons. Here we report transport measurements performed on single double-walled carbon nanotubes as a function of temperature between 77K and 400K. The good quality of the contacts between the nanotubes and the electrodes allows to observe a temperature dependence in the I-V curves. At high temperature, the saturation current shows a value around 25$\mu $A, as expected from the energy of optical phonons, but then increases non-linearly with decreasing temperature. The low-bias conductance is also measured to increase with decreasing temperature. Phenomenological models are investigated in order to explain the observed trends. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P24.00008: Conductance Switching in Gated Graphene Nanoribbons Jesse M. Kinder, Jonathon J. Dorando, Garnet K.L. Chan We have investigated transport through locally gated metallic graphene nanoribbons using a numerical tight-binding method. We consider a device in which the orientation of the gate with respect to the axis of the ribbon is variable. We find that the conductance, as calculated within the nonequilibrium Green function formalism, depends strongly on the gate voltage and the orientation of the gate. In particular, we identify specific angles at which a small change in gate voltage results in a large change in the probability of transmission. This response occurs in ribbons with zigzag or armchair edges and could provide a mechanism for a nanometer-scale electronic switch. Using the effective Dirac Hamiltonian for electrons in graphene, we provide a qualitative explanation for the ON-OFF response at particular angles and characterize its dependence on the geometry of the ribbon. [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P24.00009: Variable Range Hopping Conductivity in Carbon Nanotube Threads. Chaminda Jayashinghe, David Mast, Mark Schulz, Vesselin Shanov We have measured the low temperature, dc electrical transport in threads spun from long multi-wall carbon nanotubes (MWCNT). The electrical transport in these threads shows variable range hopping (VRH) behavior at low temperatures, as well as non-linear IV characteristics at high applied electric fields. The MWCNT used to make the threads have an outer diameter from about 6 nm to 30 nm; these MWCNT's have been grown in lengths up to 18mm. The diameter of the CNT threads in this study have diameters of 15 and 25 microns; the threads were spun using 2mm long MWCNT's. The room temperature (RT) resistivity of the threads is on the order of 5 mOhm cm and can be changed by post-spinning treatment strategies such as high temperature annealing. From 300K down to 4.2K, the resistivities show an exponential dependence with temperature consistent with VRH conduction. As the RT resistivity decreases, the low temperature transport shifts from being dominated, at low temperatures, by Coulomb-Gap VRH described by Efros and Shklovskii [1] to 3D-VRH as first formulated by Mott [2]. Analysis will be given of how the VRH behavior changes with intrinsic and post-treatment thread resistivity. [1] A.L. Efros and B.I. Shklovskii, J. Phys. C: Solid State Physics. \textbf{8}, L49 (1975). [2] N.F. Mott, J. Non-Cryst. Solids\textbf{ 1}, 1 (1968). [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P24.00010: Experimental and computational study of 1/f noise scaling in single-walled carbon nanotube percolation films Ashkan Behnam, Gijs Bosman, Ant Ural We study the scaling of 1/f noise in single-walled carbon nanotube percolation films as a function of device parameters and film resistivity both experimentally and computationally. The results suggest that the noise generated by tube-tube junctions dominates the total 1/f noise in nanotube films and that the noise amplitude depends strongly on device dimensions, nanotube degree of alignment, and the film resistivity, following a power-law relationship with resistivity near the percolation threshold after properly removing the effect of device dimensions. We also find that the critical exponents associated with the noise-resistivity and noise-device dimension relationships are not universal invariants, but rather depend on the specific parameter that causes the change in the resistivity and 1/f noise, and the values of the other device parameters. Since 1/f noise is a more sensitive measure of percolation than resistivity, these results not only provide important fundamental physical insights into the complex interdependencies associated with percolation transport in nanotube films, but also help understand and improve the performance of these nanomaterials in potential device applications, such as nanoscale sensors, where noise is an important figure of merit. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P24.00011: Anisotropic Terahertz Response of Highly Aligned Single-Walled Carbon Nanotubes L. Ren, X. Wang, L. Booshehri, D. Hilton, J. Kono, C. Pint, R. Hauge, D. Rana, K. Takeya, I. Kawayama, M. Tonouchi Dynamic conductivities of degenerate 1-D electrons are expected to provide a wealth of information on quantum confinement, electron interactions, and disorder. Here, we use terahertz time-domain spectroscopy (THz-TDS) to determine the complex dielectric function of a thin film of highly aligned single-walled carbon nanotubes (SWNTs) on sapphire. The THz electromagnetic wave used was linearly polarized, and the measured dielectric function was very anisotropic. As the angle between the nanotube axis and the THz electric filed changed, anisotropy was clearly observed for both the real and imaginary parts of the dielectric function. The absorption of the THz wave decreased monotonically with increasing angle from 0 to 90 degrees, with maximum and minimum absorption at 0 and 90 degrees, respectively. Through a proper model, the complex dynamic conductivity was extracted and showed a non-Drude-like frequency dependence, with the real part increasing monotonically with increasing frequency between 0.2 and 1.8 THz. [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P24.00012: Carbon nanotube diode performance and photovoltaic response Daner Abdula, Moonsub Shim Due to their unique electronic properties, carbon nanotubes have been at the forefront in the development of next generation electronic devices. The p-n diode is arguably the most pivotal electronic and photovoltaic device. Up to now, nanotube diodes have had major drawbacks including complex quad-terminal device geometries to achieve electrostatic doping, large series resistances from the inclusion of an intrinsic region at the junction, unstable n-type doping, and Zener breakdown. We have developed a method to create two terminal abrupt junction diodes from single semiconducting carbon nanotubes with simple photo-patterned polymer layers defining air-stable p- and n-regions. These intratube diodes show nearly ideal behavior with relatively low series resistance and no sign of Zener breakdown at room temperature. Spatial doping profiles measured by micro-Raman spectroscopy and selective electrochemical gating of the n-region indicate that diode performance depends strongly on relative doping levels. A short circuit current of 1.4 nA with an open circuit voltage of 205 mV are measured when illuminated to saturation. [Preview Abstract] |
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