Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session X27: Nanowires |
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Sponsoring Units: DCMP Chair: Dan Finkenstadt, Naval Research Laboratory Room: Colorado Convention Center 301 |
Friday, March 9, 2007 8:00AM - 8:12AM |
X27.00001: Superconductor-Insulator Transition in Epitaxial Niobium Nanowires Timothy McArdle, Kevin Inderhees, Paul Welander, James Eckstein As the dimensions of a superconducting nanowire are reduced, it undergoes a transition from a superconductor to an insulator. Near Tc, thermally activated phase slips cause this insulating state, and it is believed that in extremely narrow wires quantum phase slips appear and become dominant at low temperature. However, the exact nature of the S-I transition, specifically what parameters control it, is not clear. We report on recent studies of nanowires fabricated using electron beam lithography from single-crystal niobium films grown by ultra-high vacuum molecular beam epitaxy. Since the films are single crystal, the role of disorder is reduced. Our films are 100 angstroms thick, have transition temperatures near 7.2 K, and residual resistance ratios of around 5, typical for ultra-thin single-crystal niobium films. The wires are 10 $\mu $m long and range in width from 35 to 200 nm. \newline This work was supported by the DOE BES at the F. Seitz Materials Research Laboratory at the University of Illinois, Urbana. [Preview Abstract] |
Friday, March 9, 2007 8:12AM - 8:24AM |
X27.00002: Imaging of Few-electron InAs Quantum Dots in InAs/InP Nanowires Erin E. Boyd, Halvar J. Trodahl, Ania Bleszynski, Michael Stopa, R.M. Westervelt, Linus E. Froberg, Lars Samuelson InAs quantum dots are promising contenders for nanoelectronics, spintronics and quantum information processing.~ Their large g-factor makes manipulation of electron spins easier at higher temperatures.~ InAs dots, as small as 10 nm long holding only a few electrons, can be formed by InP barriers in InAs/InP nanowire heterostructures grown using chemical beam epitaxy.~ Coulomb blockade transport measurements done using metal contacts and a back gate show excellent results [1].~ Using a liquid-He cooled scanning probe microscope, we imaged an InAs quantum dot that holds only one-electron, with the conducting tip as a movable gate [2].~ Simulations of electron wavefunctions in the dot show the effect of the back gate and the moveable tip. [1] M. Bj\"{o}rk\textit{ et al.}, Nano Letters\textbf{ 4}, 1621 (2004) [2] A. Bleszynski\textit{ et al.}, 28th Int. Conf. Physics of Semiconductors, 2006 [Preview Abstract] |
Friday, March 9, 2007 8:24AM - 8:36AM |
X27.00003: Imaging Few-Electron Double Quantum Dots in InAs/InP Nanowires Halvar J. Trodahl, Erin E. Boyd, Ania Bleszynski, R. M. Westervelt, Linus E. Froberg, Lars Samuelson InAs quantum dots formed in InAs/InP nanowire heterostructures are attractive candidates for nanoelectronics, spintronics and quantum information processing.~Tunnel-coupled double InAs dots defined by InP barriers can be grown using chemical beam epitaxy; each dot can be small enough to hold just a few electrons. It is difficult to lithographically define gates small enough to individually address each dot. With use of a liquid-He cooled scanning probe microscope (SPM), the Coulomb blockade conductance of a single InAs quantum dot in an InAs/InP nanowire has been imaged, using the SPM tip as a movable gate [1]. This approach can individually tune the charge on each InAs dot in an InAs/InP nanowire. We plan to use this technique to investigate tunnel-coupled InAs double dots. \newline [1] A. Bleszynski et al., 28th Int. Conf. on the Physics of Semiconductors, 2006. [Preview Abstract] |
Friday, March 9, 2007 8:36AM - 8:48AM |
X27.00004: Elastodynamics of GaN nanowires at microwave frequencies Colm Flannery, Kristine Bertness, Sudook Kim, Ward Johnson Single-crystal III-nitride nanowires have been the subject of extensive research because of their potential usefulness in innovative electronic, optoelectronics, and sensing applications. They also have been recognized as having several characteristics, including a relatively high surface-to-volume ratio, that are attractive for nanoelectromechanical systems (NEMS) applications, such as mass sensing. In this report, we present Brillouin-light-scattering (BLS) measurements and theoretical modeling of thermally excited vibrational modes in free-standing GaN nanowires with hexagonal cross sections ($\sim $70 nm to $\sim $200 nm in diameter) grown from a substrate by catalyst-free molecular beam epitaxy to a length of $\sim $8 $\mu $m. A series of modes with frequencies between 8 GHz and 50 GHz are seen in the BLS spectra with axial wavelengths between 270 nm and 1600 nm. Modeling of the normal modes was performed under the approximations of infinite length and circular cross section. The lowest-frequency BLS peak is identified as the lowest-order flexural mode. Stronger higher-frequency modes include ones with greater phase variation around the circumference. [Preview Abstract] |
Friday, March 9, 2007 8:48AM - 9:00AM |
X27.00005: Surround Gate Germanium Nanowire Field Effect Transistors Li Zhang, Ryan Tu, Hongjie Dai Surround gate (SG) GeNW field effect transistors (FETs) were successfully constructed using a novel self-aligned fabrication approach based on the core-shell GeNWs formed with a single-crystalline Ge core and concentric shells of nitride and silicon passivation layer by chemical vapor deposition (CVD), an Al$_{2}$O$_{3}$ gate dielectric layer by atomic layer deposition (ALD) and an Al metal SG shell by isotropic magnetron sputter deposition. Individual SG GeNW FETs show improved switching over GeNW FETs with planar gate stacks owing to improved electrostatics. FET devices comprised of multiple quasi-aligned SG GeNWs in parallel afford on-currents exceeding 0.1mA at low source-drain bias voltages. Capacitance-voltage characteristics of individual GeNW FETs with topgate or SG were directly measured for the first time using a novel method to reduce background capacitance to less than 50aF via device design, ground plane shielding, interprobe shielding, and use of a capacitance bridge circuit. In addition, the mobility of the GeNW FETs was extracted according to the direct electrical and capacitance measurements. [Preview Abstract] |
Friday, March 9, 2007 9:00AM - 9:12AM |
X27.00006: Low temperature electron transport studies of Bi nanowires Zuxin Ye, Hong Zhang, Haidong Liu, Wenhao Wu We report the electron transport studies of Bi nanowires at sub-Kelvin temperatures. Bi nanowiers with a nominal diameter $\sim $ 80nm and a length $\sim $ 10 $\mu $m were electrochemically deposited into the pores of ion track etched polycarbonate membranes. Electric contacts of single Bi nanowires were \textit{in-situ} formed on the on-membrane macroscopic electrodes during the electrochemical deposition. Electron transport properties were measured at temperatures from 60mK to 20K. The temperature dependence of resistance showed a quick drop when the samples were cooled below 0.6K, resembling a superconducting transition. The samples had a finite resistance at low temperature far below the transition, instead of having a zero resistance as regular superconductors. I-V curves showed a zero-bias resistance valley and multiple non-zero-bias peaks symmetrically distributed on both sides of the central valley at T $<$ 0.6K. The magnetic field dependence of resistance showed an unusual hysteresis loop with a butterfly shape when the magnetic field was swept along a close cycle between -0.6T and +0.6T. All these features gradually vanished when an applied magnetic field was increased to 0.6T. We will discuss the origin of this unusual low temperature behavior of Bi nanowires and its relation with the microscopic structures. [Preview Abstract] |
Friday, March 9, 2007 9:12AM - 9:24AM |
X27.00007: Nanoscale Thermal Imaging Kamal Baloch, Todd Brintlinger, Yi Qi, David Goldhaber-Gordon, John Cumings We present real time, in-situ, high resolution thermal imaging of metallic nanowires. The nanowires are grown on the front-side of silicon nitride membranes. Resistive heating along the wires produces thermal gradients which melt/freeze 20-200nm diameter indium islands deposited by thermal evaporation on the back-side of the membrane. These transitions can be imaged using a transmission electron microscope operating in dark-field mode such that contrast corresponds to the phase of an individual island. Global changes in temperature can be used to calibrate the melting point of individual islands and to account for the presence of the $\sim $100nm thick silicon nitride membrane. Thermal modeling confirms the imaged thermal behavior. This technique could be generally employed for thermal imaging of nanowires and nanotubes, wherein the nanoscale systems are imaged in-situ and under electrical bias. Results of local resistive heating in a carbon nanotube device will also be shown [Preview Abstract] |
Friday, March 9, 2007 9:24AM - 9:36AM |
X27.00008: X-ray Radiation Damage Studies of Individual Nanotubes and Nanowires H.D. Mo, C. Nelson, C.-C. Kao, M. Sfeir, A. Bollinger, I. Bozovic, J. Misewich, A. Stein, W. Liu, P. Zschack, N. Bozovic The development of techniques for x-ray studies of individual nanomaterials is motivated by the spectroscopic, structural, and dynamic information that x-rays provide. In combination with other probes (e.g., STM), x-ray techniques promise the complete characterization of nanomaterial properties and functionality, which can be used as feedback for the synthesis of useful nanomaterials. The feasibility of x-ray studies of individual nanomaterials is approaching due to ongoing improvements in x-ray focusing optics and synchrotron radiation sources that together lead to increasing flux densities. However one possible barrier concerns the effects of high intensity x-ray beams on hard nanomaterials, about which little is currently known. Therefore here we report on x-ray damage studies of individual carbon nanotubes and SrRuO3 nanowires. Samples of the two systems were exposed to microfocused x-rays on APS beamline 34-ID for variable amounts of time. Pre-and post-exposure SEM imaging was used to qualitatively study the effects on carbon nanotubes, and real-time monitoring of sample integrity was provided by measuring a current passing through the SrRuO3 nanowires during the exposure. This research is supported by the DOE, under contracts DE-AC02-98CH10886 (BNL) and W-31-109-ENG-38 (ANL). [Preview Abstract] |
Friday, March 9, 2007 9:36AM - 9:48AM |
X27.00009: In-situ and ex-situ analysis of terminal lengths and growth rates of vertically aligned carbon nanotubes grown by acetylene and alcohol catalytic chemical vapor deposition J. Jackson, K. Belay, A. Puretzky, D. Geohegan, H. Christen, H. Cui Controlled synthesis of carbon nanotubes (CNTs) at a fast growth rate which produced long, vertically aligned, and containing required number of walls, from MWNT to double- and single-walled nanotubes by varying processing parameters have been achieved in a thermal chemical vapor deposition (CVD) process. Both in-situ optical monitoring and ex-situ characterization methods have been used to determine the terminal lengths and growth rates of CNTs for acetylene, methanol and ethanol feedstocks grown on silicon substrates coated with an aluminum underlayer ($\sim $10 nm), molybdenum ($\sim $ 0.2 nm), and iron ($\sim $ 1nm). Maximum growth rates of CNTs were 3-8 times and 50-125 times slower for ethanol and methanol, respectively, compared when using an acetylene feedstock. The terminal lengths of CNT arrays were estimated as: from 200 mm to a few mm for acetylene, $\sim $ 20 mm for methanol, and $\sim $ 8 mm for methanol. The terminal lengths, growth rates, the number of walls, and quality of the carbon nanotubes for all three feedstocks show similar temperature dependences. This indicates the existence of a common mechanism responsible for the activation and deactivation of catalyst nanoparticles at different growth temperatures, probably related to catalytic activity and/or the oxidation state of the catalyst nanoparticles. The restart of growth was also observed while controlling subsequent methanol and hydrogen gas flows. [Preview Abstract] |
Friday, March 9, 2007 9:48AM - 10:00AM |
X27.00010: Electrical Properties of Individual Semiconducting Oxide Nanobelt and Their Applications* Yi Cheng, P. Xiong, L. Fields, J.P. Zheng, R. Yang, Z.L. Wang Field-effect transistors (FETs) with multi-terminal electrical contacts were fabricated on individual oxide (SnO$_{2}$ and ZnO) nanobelts. Simultaneous two-terminal and four-terminal measurements enable direct correlation of the FET characteristics with the nature of the contacts. Nanobelt FETs with Schottky contacts were found to exhibit n-channel, p-channel or ambipolar characteristics transistors depending on the properties of the contacts. In contrast, low-resistance ohmic contacts on the nanobelts lead to high-performance n-channel depletion mode FETs with well-defined linear and saturation regimes, ``on/off'' ratio as high as 10$^{7}$ at ambient conditions$^{[1]}$. The electron concentration and effective carrier mobility of the nanobelts in different gases at various temperatures were determined from FET measurements on the channel-limited devices. Sensitive electrical response of the SnO$_{2}$ nanobelt FETs to gas flow containing 0.2-2{\%} H$_{2}$ was observed at room temperature$^{[2]}$. The effort to utilize the channel-limited nanobelt FETs for protein detection will also be reported. *Supported by NSF NIRT grant ECS-0210332. $^{[1] }$Y. Cheng \textit{et al.}, Appl. Phys. Lett. \textbf{89}, 093114 (2006). $^{[2] }$L.L. Fields \textit{et al}., Appl. Phys. Lett. \textbf{88}, 263102 (2006). [Preview Abstract] |
Friday, March 9, 2007 10:00AM - 10:12AM |
X27.00011: Nonequilibrium electromechanical noise in a nanomechanical resonator Patrick Truitt, Jared Hertzberg, Keith Schwab Current carrying electrons passing through a diffusive conductor can undergo elastic collisions with defects or surface boundaries and thus impart momentum to the lattice. At sufficiently low temperatures, where the electron-phonon scattering length is longer than the mean free path, this electromechanical noise can be driven out of equilibrium with the conductor's thermal noise (Joule heating). The resulting force from elastic collisions on a doubly-clamped beam was predicted by Shytov et al [1]. We will discuss our low temperature measurements of a gold-coated, radio-frequency nanomechanical resonator. We current bias the conducting layer and monitor the position of the resonator with an RF-SET. From mechanical noise thermometry, we then compare the observed electromechanical noise force with the theory.{\\}{\\}[1] A.V. Shytov, L.S. Levitov, and C.W.J. Beenakker, Phys. Rev. Lett. \textbf{88}, 228303 (2002) [Preview Abstract] |
Friday, March 9, 2007 10:12AM - 10:24AM |
X27.00012: Electrical-transport properties of individual single-crystalline IrO$_2$ nanorods Y. H. Lin, T. C. Lee, Y. C. Sun, W. B. Jian, H. M. Chang, Y. S. Huang, J. J. Lin We have studied the electrical-transport properties of individual single-crystalline IrO$_2$ nanorods (NRs) prepared by MOCVD. With the help of e-beam lithography, individual NRs are contacted by Cr/Au submicron electrodes from above. Utilizing different probe configurations, not only the intrinsic properties of the NRs but also the temperature dependence of the contact resistance, $R_{\rm{contact}}$, has been determined down to liquid-helium temperatures. Our measured resistivity behavior of the NRs is in close agreement with the current theoretical understanding of this material. On the other hand, we found that the temperature behavior of the $R_{\rm{contact}}$ obeys the law log$R_{\rm{contact}}\propto T^{-1/2}$ over a wide temperature range from 100 K down to liquid-helium temperatures. This later conduction process is ascribed to the hopping of electrons through nanoscale metal granules accidentally formed at the contact region during the thermal evaporation of the submicron electrodes. [Preview Abstract] |
Friday, March 9, 2007 10:24AM - 10:36AM |
X27.00013: Silicon nanoporous pillar array: template for fabricating silicon-based nanocomposites with enhanced physical properties Xin Jian Li, Xiao Nan Fu, Hai Jun Xu, Wei Fen Jiang A triple hierarchical structure, silicon nanoporous pillar array (Si-NPA), was formed on silicon wafers by a hydrothermal method. The structure of Si-NPA is characterized by the regular array of micron-sized silicon pillars, quasi-identical nanopores densely distributing over each pillar, and silicon nanocrystalllites composing the walls of the nanopores. Utilizing the excellent structural regularity and high chemical reactivity of Si-NPA, patterned nanocomposites of CdS, carbon nanotubes (CNTs), Au, and Fe$_{3}$O$_{4}$/Si-NPA were fabricated. Their elemental compositions, morphologies and microstructures were characterized. Ideal physical properties of I-V curve in CdS/Si-NPA heterojunction, field emission in Si-NPA, CNTs/Si-NPA, Au/Si-NPA, and humidity/gas sensitivity in Fe$_{3}$O$_{4}$/Si-NPA were observed and the corresponding mechanisms were analyzed. These results indicate that Si-NPA could be employed as an ideal template to assembly silicon-based functional nanosystems, and might find multiple applications in fabricating novel electronic devices. [Preview Abstract] |
Friday, March 9, 2007 10:36AM - 10:48AM |
X27.00014: Growth and Characterization of ZnSe Nanowires on Au-catalyzed Ge substrates Tina Lin, Benjamin Cooley, Nitin Samarth Semiconductor nanowires derived from ZnSe are of interest for semiconductor spintronics because of the relatively long spin lifetimes in bulk n-ZnSe. The metal-catalyzed growth of ZnSe nanowires has already been demonstrated on a variety of substrates, including GaP and Si.[1,2]. Here we exploit the formation of a low melting point Au-Ge eutectic alloy to initiate the growth of ZnSe nanowires on Au-covered Ge substrates. ZnSe is deposited under ultrahigh vacuum conditions using solid source molecular beam epitaxy. Scanning electron microscopy reveals the formation of dense random arrays of ZnSe nanowires with typical lengths in the range $\sim 1 - 3 \mu$m and diameters of $\sim 10 - 30$ nm. We report a systematic study of the effects of growth temperature, thickness of the gold layer, and ZnSe deposition thickness on the characteristics of the resulting ZnSe nanowires. We also report characterization of these nanowires using high resolution scanned probe and electron microscopies, as well as low temperature optical spectroscopy.\\ 1. Y. F. Chan {\it et al.}, Appl. Phys. Lett. 83, 2665 (2003).\\ 2. A. Colli {\it et al.}, Nanotechnology 16, S139 (2005). [Preview Abstract] |
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