Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session T37: Focus Session: Carbon Nanotube Transport |
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Sponsoring Units: DMP Chair: Ethan Minot, Oregon State University Room: 705/707 |
Thursday, March 6, 2014 11:15AM - 11:51AM |
T37.00001: High-Performance Gate Dielectric for Carbon-based Nanoelectronics Invited Speaker: Lian Mao Peng Gate dielectric layer with high-quality and high-efficiency is an important component and technological challenge for high-performance top-gated carbon-based field-effected transistors (FETs) including carbon nanotube (CNT) FETs and graphene FETs. We address high-quality yttrium oxide (Y$_{2}$O$_{3})$, which can be grown on CNT/graphene through a simple and cheap process. Top-gate CNT FETs adopting 5 nm Y$_2$O$_3$ layer as its gate dielectric showed excellent device characteristics, especially including an ideal subthreshold swing of 60 mV/decade (up to the theoretical limit of an ideal FET at room temperature). High quality Y$_2$O$_3$ dielectric layer has also been integrated into top-gate G-FETs as gate insulator layer, and its thickness has been reduced continuously down to 3.9 nm with an equivalent oxide thickness (EOT) of 1.5 nm and excellent insulativity. High carrier mobility up to 5400 cm2/V$\cdot$s and high top gate efficiency of up to 120 (relative to that of back gate with 285 nm SiO$_2$) are simultaneously realized in G-FETs with high quality Y$_2$O$_3$ gate oxide at high oxidizing temperature. Moreover, benefitted from large gate capacitance as 2.28 $\mu $F/cm2, quantum capacitance in graphene has been accurately measured and retrieved. [Preview Abstract] |
Thursday, March 6, 2014 11:51AM - 12:03PM |
T37.00002: Elemental charge sensitivity of liquid-gated carbon nanotube transistors Tal Sharf, Neng-ping Wang, Joshua Kevek, Heather Wilson, Stefan Heinze, Ethan Minot Electron transport in carbon nanotubes (CNTs) is extremely sensitive to electrostatic~perturbations, suggesting that CNT field-effect transistors (FETs) are promising candidates for~low-power digital switches and high-performance sensors. In this work,~we show that the perturbation caused by a single elemental charge~strongly affects the room temperature conductance of a CNT FET. We make~use of naturally occurring activated charge traps in SiO$_{\mathrm{2}}$ to observe~random telegraph signals which reach 20{\%} of the baseline signal. Our~measurements are made in a liquid-gated environment where these~telegraph signals are persistent over long time scales and tunable by~gate-voltage. Gate-voltage dependence is compared to non-equilibrium~Greens function calculations. We verify the theoretically predicted~relationship between signal magnitude and gate voltage, and show that~this relationship differs dramatically from predictions based simply on~transconductance. Our measurements confirm the exciting possibility of~detecting elemental charges at room temperature, and verify a~theoretical framework for predicting conductance changes due to motion~of an elemental charge near a CNT FET. [Preview Abstract] |
Thursday, March 6, 2014 12:03PM - 12:15PM |
T37.00003: Direct measurement of mean free paths in single-walled carbon nanotubes by Kelvin probe force microscopy Elliot J. Fuller, Deng Pan, Brad L. Corso, O. Tolga Gul, Philip G. Collins The inelastic mean free path $\lambda $ of a conductor is determined by the scattering mechanisms relevant to its electronic resistance. The behavior of $\lambda $ is of particular interest for single-walled carbon nanotubes (SWNTs) because they are quasi-one-dimensional conductors believed to have minimal acoustic phonon scattering. Previous measurements of $\lambda $ used very long SWNTs contacted by large arrays of electrodes, but this is impractical for studying the device-to-device variability that results from SWNT chirality and environmental effects. Here, we use Kelvin probe force microscopy (KPFM) to directly measure potential gradients in biased SWNT field effect transistors with short channel lengths. The KPFM measurements directly determine $\lambda $ as a function of bias in individual devices and can distinguish contact resistance and disorder from homogeneous inelastic scattering. At 185 K, we observe $\lambda $ decreasing from nearly 1 $\mu $m at low bias to 150 nm at high bias. Fitting $\lambda $ to established models determines the roles of surface plasmon-polariton scattering in one limit and optical phonon emission at the other. We find the optical phonon mean free path for spontaneous emission to be 40 to 60 nm at 300 K, significantly longer than observed in previous experimental studies. The results demonstrate KPFM as a powerful tool for studying SWNT physics and suggest usefulness for studying other nanoscale circuits. [Preview Abstract] |
Thursday, March 6, 2014 12:15PM - 12:27PM |
T37.00004: Testing the pseudospin conjecture in carbon nanotubes: transport measurement to determine the scattering strength of charged impurity as a function of chirality Ryuichi Tsuchikawa, Zhengyi Zhang, Daniel Heligman, James Hone, Masa Ishigami Metallic carbon nanotubes are predicted to be resilient to scattering by charged impurities while semiconducting carbon nanotubes are susceptible to the same impurities as a result of the pseudospin degree of freedom. However, this pseudospin conjecture has never been tested directly. We have measured the resistivity of nanotubes as a function of the density of charged impurities and determined their scattering cross section as a function of chirality to test this conjecture. We found that the charged impurities affect transport properties of both metallic and semiconducting nanotubes. We will discuss the implication of our results on the pseudospin conjecture. [Preview Abstract] |
Thursday, March 6, 2014 12:27PM - 12:39PM |
T37.00005: Photothermoelectric Effect in Suspended Semiconducting Carbon Nanotubes Lee Aspitarte, Tristan DeBorde, Tal Sharf, Josh Kevek, Ethan Minot We have performed scanning photocurrent microscopy measurements of field-effect transistors (FETs) made from individual suspended carbon nanotubes (CNTs).Photocurrent generation in individual carbon nanotube based devices has been previously attributed the photovoltaic effect, in contrast to graphene based devices which are dominated by the photothermoelectric effect. In this work, we present the first measurements of strong photothermoelectric currents in individual suspended carbon nanotube field-effect transistors. In certain electrostatic doping regimes light induced temperature gradients lead to significant thermoelectric currents which oppose and overwhelm the photovoltaic contribution. Our measurements give new insight into the tunable and spatially inhomogeneous Seebeck coefficient of electrostatically-gated CNTs and demonstrate a new mechanism for optimizing CNT-based photodetectors and energy harvesting devices. [Preview Abstract] |
Thursday, March 6, 2014 12:39PM - 12:51PM |
T37.00006: Transport in Suspended Ultraclean Carbon Nanotube Double Dots Cheng Pan, Marc Bockrath Using split gates, we modulate the charge density along the length of suspended ultraclean single-wall carbon nanotubes to produce $pp$, $pn$, $np$ and $nn$ configurations. With pn junctions present, the nanotubes act as a double quantum dot system. We perform transport experiments to investigate Kondo physics in this coupled tunable system. In polarized pp configurations, we observe conductance modulations that we attribute to backscattering induced by a potential step within the nanotube. We estimate the step spatial size from the electron wavelength cutoff of the scattering. We will discuss our latest results. [Preview Abstract] |
Thursday, March 6, 2014 12:51PM - 1:03PM |
T37.00007: Ultraclean single, double, and triple carbon nanotube quantum dots with recessed Re bottom gates Minkyung Jung, Jens Schindele, Stefan Nau, Markus Weiss, Andreas Baumgartner, Christian Schoenenberger Ultraclean carbon nanotubes (CNTs) that are free from disorder provide a promising platform to manipulate single electron or hole spins for quantum information. Here, we demonstrate that ultraclean single, double, and triple quantum dots (QDs) can be formed reliably in a CNT by a straightforward fabrication technique. The QDs are electrostatically defined in the CNT by closely spaced metallic bottom gates deposited in trenches in Silicon dioxide by sputter deposition of Re. The carbon nanotubes are then grown by chemical vapor deposition (CVD) across the trenches and contacted using conventional electron beam lithography. The devices exhibit reproducibly the characteristics of ultraclean QDs behavior even after the subsequent electron beam lithography and chemical processing steps. We demonstrate the high quality using CNT devices with two narrow bottom gates and one global back gate. Tunable by the gate voltages, the device can be operated in four different regimes: i) fully p-type with ballistic transport between the outermost contacts (over a length of ~ 700 nm), ii) clean n-type single QD behavior where a QD can be induced by either the left or the right bottom gate, iii) n-type double QD and iv) triple bipolar QD where the middle QD has opposite doping (p-type). [Preview Abstract] |
Thursday, March 6, 2014 1:03PM - 1:15PM |
T37.00008: Singlet-Triplet Kondo effect in a quantum dot with dissipation Chung-Ting Ke, Henok Mebrahtu, Ivan Borzenets, Yuriy Bomze, Alex Smirnov, Harold Baranger, Gleb Finkelstein We studied the singlet-triplet Kondo effect in a carbon nanotube contacted by resistive leads which form dissipative baths. With dissipation parameter r $\approx $ 0.5, the conventional spin $\mbox{1/2}$ Kondo resonances in odd electron valleys are strongly suppressed. However, the singlet-triplet Kondo effect induced by applying perpendicular magnetic field in a 2-electron valley appears to survive. The resonance demonstrates an unusual dependence on the side gate voltage, being enhanced in a particular part of the phase space. We also report on the peculiar dependence of the resonance on bias voltage. [Preview Abstract] |
Thursday, March 6, 2014 1:15PM - 1:27PM |
T37.00009: Quantitative analysis of the oxidation effects on the electrical characteristics of high-purity, large-diameter semiconducting carbon nanotubes Jia Gao, Yueh-Lin Loo Many attempts have been made to utilize carbon nanotubes for chemical, biological and gas sensing applications. Previous studies show that adsorbed ozone (O3) on carbon nanotubes can drastically influence their electrical characteristics. On the one hand, ozone act as p dopants; exposure thus leads to an increase in electrical conductivity. On the other hand, ozone readily oxidizes carbon nanotubes; this chemical reaction results in a decrease in conductivity. It remains ambiguous which process dominates and quantitative evaluation of these two effects is lacking. In this study, we elucidate the interaction between ozone and carbon nanotubes by evaluating the field-effect mobilities of polymer-sorted large diameter semiconducting carbon nanotubes based transistors. Upon exposure to ozone, we observe a positive shift in the threshold voltage from -0.7 to 11.7 V and a concurrent decrease of hole mobility from 2.5 to 0.5 cm$^{2}$/Vs. Accordingly, the source-drain current exhibits a non-monotonic dependence on ozone exposure time. This dependence reveals that doping dominates the electrical characteristics of carbon nanotube transistors initially. Beyond 3-minutes of ozone exposure, chemical oxidation dominates, resulting in a progressive decrease in source-drain current. [Preview Abstract] |
Thursday, March 6, 2014 1:27PM - 1:39PM |
T37.00010: Copper-coated Nanotubes at the Single Nanotube Scale Deng Pan, Brad Corso, O. Tolga Gul, Philip Collins High conductivity and high ampacity are both essential specifications for next-generation solid-state electronics. Recently, Subramaniam \textit{et. al.} reported remarkable increases in copper conductivity and ampacity using a bulk composite of copper and carbon nanotubes (CNTs) [1]. Here, we describe similar measurements performed with a model system composed of individual single-walled or multi-walled CNTs. Cu electrodeposition upon single CNT devices achieved nanometer-scale coatings that were electrically tested as a function of film thickness and device temperature. We do not observe the same conductivity enhancements reported for bulk Cu-CNT composites, but improvements in ampacity has been observed when compared to pure Cu. The thinnest Cu films have the hightest ampacity, indicating that the CNT core is essential to the enhanced current capacity. \\[4pt] [1] C. Subramaniam \textit{et. al.}, Nat. Comm. \textbf{4} (2013) [Preview Abstract] |
Thursday, March 6, 2014 1:39PM - 1:51PM |
T37.00011: Transport Properties of p-n Junctions Formed in Boron/Nitrogen Doped Carbon Nanotubes and Graphene Nanoribbons Mahmoud Hammouri, Igor Vasiliev We apply {\it ab initio} computational methods based on density functional theory to study the transport properties of p-n junctions made of single-walled carbon nanotubes and graphene nanoribbons. The p-n junctions are formed by doping the opposite ends of carbon nanostructures with boron and nitrogen atoms. Our calculations are carried out using the SIESTA electronic structure code combined with the generalized gradient approximation for the exchange-correlation functional. The transport properties are calculated using a self-consistent nonequilibrium Green's function method implemented in the TranSIESTA package. The modeled nanoscale p-n junctions exhibit linear I-V characteristics in the forward bias and nonlinear I-V characteristics with a negative differential resistance in the reverse bias. The computed transmission spectra and the I-V characteristics of the p-n junctions are compared to the results of other theoretical studies and to the available experimental data. [Preview Abstract] |
Thursday, March 6, 2014 1:51PM - 2:03PM |
T37.00012: Anisotropy in broadband microwave conductance spectra of highly oriented multi-walled carbon nanotube sheets Brian Brown, Julia Bykova, Austin Howard, Mark Lee, Anvar Zakhidov Highly oriented multi-walled carbon nanotube (MWCNT) sheets are drawn from carbon nanotube (CNT) forests grown by chemical vapor deposition (CVD) synthesis on silicon (Si) substrates with iron catalyst. Sheets are assembled on coplanar waveguides (CPWs) with and without densification in isopropyl alcohol, and are aligned either parallel or perpendicular to the electric field polarization of the propagating field. Broadband microwave conductance spectra is measured using capacitive coupling to the MWCNT sheets up to a frequency of 50 GHz over a temperature range of 4 to 300 K. We find a parallel/perpendicular conductance ratio of up to 200/1 with very weak temperature dependence. The behavior of the AC conductance anisotropy will be compared to DC measurements of the resistance anisotropy. [Preview Abstract] |
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