AVS2001 Session NT+EL+NS-ThM: Nanotubes: Growth, Functionalization, and Sensors

Thursday, November 1, 2001 8:20 AM in Room 133

Thursday Morning

Start	Invited?	Item
8:20 AM		NT+EL+NS-ThM-1 Control of Carbon Nanofiber Growth: "Base" versus "Tip" Growth Regimes A.V. Melechko (University of Tennessee); V.I. Merkulov (Oak Ridge National Lab); M.A. Guillorn (University of Tennesse and Oak Ridge National Lab); D.H. Lowndes (Oak Ridge National Lab); M.L. Simpson (University of Tennesse and Oak Ridge National Lab) Carbon nanofibers (CNF) show promise for many applications in such new areas as nanoelectronics and nanobiotechnology. It is very important to have a precise control of the position, orientation, and shape of the CNFs to maximize their utility for these applications. Recently it became possible to achieve such deterministic growth by nanopatterning catalyst and using Plasma Enhanced Chemical Vapor Deposition (PECVD). PECVD is a complex process that involves control of many interdependent parameters such as pressure, mass flow ratio (C2H2/NH3), substrate temperature, substrate material, plasma intensity and bias. Two different CNF growth regimes have been observed. One is when the catalyst particle is detached from the substrate surface and located at the tip of the CNF ("tip-growth" regime). Another regime is when the catalyst particle stays attached to the substrate ("base-growth" regime). We present an experimental study of the parameter space of a DC PECVD process for different regimes of CNF growth: "base-growth", "tip-growth", and intermediate regimes, where both types of CNFs were observed simultaneously. The mechanisms, which are responsible for the competition of these different growth phases, such as catalyst-substrate interaction and interdiffusion, formation of amorphous carbon film, and kinetics of catalysis and carbon diffusion through catalyst particle will be discussed.
8:40 AM		NT+EL+NS-ThM-2 Scanning Tunneling Microscopy and Spectroscopy of GdC₈₂-filled Single-walled Carbon Nanotubes J. Lee (Seoul National University, Korea); J.-Y. Park (Cornell University); H.J. Kim, H. Suh, Y. Kuk (Seoul National University, Korea); H. Kato, T. Okazaki, H. Shinohara (Nagoya University, Japan) In this presentation we will show the atomic resolution STM images of single-walled carbon nanotubes filled with Gdc₈₂ metallofullerenes at ~7K. Atomic resolution images of GdC₈₂-filled SWNTs with small tip bias voltages show nanometer-scale variations in topographic height and in atomic-scale corrugation pattern along the longitudinal axis of the nanotube. In the image with larger tip bias voltage of -1V, we could clearly observe the randomly oriented protrusions spaced roughly by integer multiples of 1.1nm which corresponds to the spacing between metallofullerenes in the nanotube. It is thought that the protrusions are caused by localized band-bending of the SWNT due to each Gd ion's field which is partially-screened by electrons in the encaging fullerene. Comparing images with different tip bias voltages, large DOS participating in the tunneling process, i.e. a large tip bias, might be needed to show enough spatial resolution. The scanning tunneling spectroscopy data of these nanotubes, which also show Gd-atom-induced local field-effect on the characteristic DOS features of SWNTs, will be presented. The possibility of 1-D Kondo effect due to the encapsulated Gd atoms will also be discussed.
9:00 AM		NT+EL+NS-ThM-3 Using Carbon Nanotube Materials to Separate Molecular Mixtures: Predictions from Molecular Dynamics Simulations S.B. Sinnott (The University of Florida); Z. Mao (The University of Kentucky); K.-H. Lee (The University of Florida); R. Andrews, E.A. Grulke (The University of Kentucky) Carbon nanotubes have been proposed as good materials for separation membranes because of their hollow, cylindrical shape and growth in ordered, close-packed bundles. We have therefore studied the manner in which molecular mixtures separate after diffusion into individual carbon nanotubes or nanotube bundles. The mixed systems considered in our study are methane/n-butane, methane/isobutane, methane/ethane, nitrogen/oxygen, nitrogen/carbon dioxide and oxygen/carbon dioxide. The computational approach used is classical molecular dynamics simulations where the forces on the atoms are calculated using empirical potentials that vary with distance.¹ Short-range interactions are calculated using a many-body, reactive empirical bond-order hydrocarbon potential and the long-range interactions are characterized with Lennard-Jones potentials. Some of these molecular mixtures separate within individual nanotubes while others do not. The mechanisms by which the molecules diffuse through the nanotubes are found to play an important role in the separation of some mixtures. Molecular structure also has a large effect on the separation of the molecular mixtures. The helical structure of the nanotube walls is predicted to have no effect on results while the nanotube diameter has a large effect. As the diameter of the nanotubes increases, the amount of separation between the molecules decreases. In nanotube bundles, the diffusion behavior and coefficients of binary molecular systems change relative to the diffusion behavior in individual nanotubes. This research is sponsored by the NASA Ames Research Center and the Advanced Carbon Materials Center through the NSF (DMR-9809686). ¹S.B. Sinnott, L. Qi, O.A. Shenderova, D.W. Molecular Dynamics of Clusters, Surfaces, Liquids, and Interfaces, Ed. W. Hase (JAI Press, Inc., Stamford, CT, 1999, pp. 1-26.
9:20 AM		NT+EL+NS-ThM-4 Chemical Disentanglement of Single-Walled Carbon Nanotube Bundles N. Choi (Joint Research Center for Atom Technology (JRCAT), Japan); H. Tokumoto (National Institute of Advanced Industrial Science and Technology (AIST), Japan); Y. Maeda, T. Wakahara (Niigata Univ., Japan); T. Akasaka (Univ. of Tsukuba, Japan); H. Kataura, M. Kimura, S. Suzuki, Y. Achiba (Tokyo Metropolitan Univ., Japan) Carbon nanotubes (CNTs) exhibit unique electronic and mechanical properties and chemical stability that cannot be realized in other materials, and therefore can be an important material in nanotechnology. Many applications have been demonstrated in the fi elds of materials science and technology, molecular electronic devices, and reliable probe tips for scanning probe microscopy. However, as-grown CNTs contain various contaminants such as catalysts and amorphous carbons, and have various lengths from a fe w nano-meters to milli-meters, and various sizes of bundles. These prevent us to use CNTs for various applications. Then, we have to develop several important key techniques such as how to purify, how to control length, how to disentangle the bundles, and how to disperse individuals in solvents. An important and essential technique to realize these keys is believed to be the chemical modification of CNTs combined with their sonication and centrifugation. In this paper, we will show the chemical process an d their characterization techniques. After purifying single-walled CNTs, we put them into N,N-dimethylformamide (DMF) at a concentration of 0.4 mg/10 ml with the small amount of amine. At the same time, we sonicated and centrifuged them under optimized ti m e, frequency and rotational speed. At each step, we measured a transmission electron microscopy (TEM), a Raman scattering spectroscopy, and an atomic force microscopy (AFM). These three techniques have proved that our technique has indeed worked out properly. Especially, the chemical modification of the CNT ends was confirmed by the AFM observation of CNTs covalently attached to gold colloidal particles.
9:40 AM	Invited	NT+EL+NS-ThM-5 Electromechanical Properties of Carbon and Boron-Nitride Nanotubes A. Zettl (University of California, Berkeley) The electronic properties of single- and multi-walled carbon nanotubes have been investigated via transport measurements under controlled environmental conditions, and in-situ electro-mechanical measurements inside a high resolution transmission electron microscope (TEM) and scanning tunneling microscope (STM). Our transport measurements show that the electronic structure of nanotubes is exceedingly sensitive to adsorbed gases. For example, the thermoelectric power for pure vacuum annealed tubes is negative, while that for oxygen-dosed samples is positive. Similarly, the electrical resistivity for individual tubes is sensitive to chemical environment, and nanotubes form robust oxygen and other chemical sensors. The theoretical basis for these sensitivities are explored via quantum transport models. For electromechanical studies, a special nanotube manipulator has been constructed for insertion into a TEM. Individual multi-walled nanotubes have been variously manipulated. We have discovered ways to peel and sharpen individual nanotubes (much like the sharpening process of a china marker pencil), pull the central core tubes out from (and reinsert them into) the outer nanotube shells of multi-walled tubes (nanotube "telescoping"), and induce nanotube collapse of cylindrical tubes into nanotube ribbons. Similarly, boron nitride nanotubes have been synthesized and the electromechanical response characterized in bulk and individually inside the TEM.
10:20 AM		NT+EL+NS-ThM-7 Magnetic "Smart-Wires": Magnetic and Electronic Properties of Nickel and Iron Nanotubes Grown on Polypeptide Templates H. Matsui, S. Pan, E. Goun, M. Klimov, B.P. Tonner (University of Central Florida) We describe a new architecture for spin-tronic magnetic devices, using a biologically modified, metal coated, peptide nanotube process which results in tubular, magnetic nanowires.^1,2 The magnetic nano-wires are formed from a polypeptide backbone, coated with nickel, or iron, or with multilayers. The morphology of the tubes is that of a hollow, cylindrical metal pipe, with widths from 20-500nm, and lengths of up to a few microns. By functionalizing the ends of the tubes with special molecules, the nano-tubes can be "wired" to specific attachment sites on a substrate by molecular recognition. We call this a "smart-wire" concept, since the instructions for "wiring" the circuits are built into the molecular nanostructures themselves. In this paper, we describe magnetic and electronic transport measurements on aligned Nickel nanotube arrays and individual nanotubes, using both conventional and scanned-probe techniques. ¹ Matsui, H.; Gologan, B., J. Phys. Chem. B. 2000, 104, 3383. ² Matsui, H.; Pan, S.; Gologan, B.; Jonas, S., J. Phys. Chem. B. 2000, 104, 9576.
10:40 AM		NT+EL+NS-ThM-8 Chemical Reactivity of Carbon Nanotubes and Fullerenes S. Park (Stanford University); D. Srivastava (NASA Ames Research Center); K. Cho (Stanford University) In most applications of carbon nanotubes and fullerenes, molecules are attached to either external or internal surface of carbon nanotubes and fullerenes to functionalize them. Therefore, the chemical reactions on nanotubes or fullerenes play an important role in understanding how to functionalize nanotubes and fullerenes. We have analyzed and compared the chemical reactivity of both external and internal surfaces of carbon nanotubes and fullerenes. The chemical reactivity analysis can be used to control the localized functionalization process as well as to predict energies and configurations of chemical reactions. Also this analysis can be applied to examine the storage capacity of carbon nanotubes and fullerenes. The chemical reactivity of carbon nanotubes and fullerenes can be characterized by a pyramidal angle, which is defined as the angle between σ bond and π orbital minus 90 degree. We analyze the chemical reactivity in terms of pyramidal angle. All analyses have been done by total energy density functional theory pseudo-potential method, and we have used a hydrogen atom as a point probe to investigate the chemical reactivity. We have developed a way to express the chemical relativity as function of pyramidal angle. We have found that the external chemical reactivity depends strongly on the initial pyramidal angle but the internal chemical reactivity is less sensitive to it. And we have also found that the internal chemical reactivity has more complex behavior than external chemical reactivity.
11:00 AM		NT+EL+NS-ThM-9 Atomic Resolution Imaging of WS2 Nanotubes L. Scheffer, S.R. Cohen, R. Rosentsveig, R. Popovitz-Biro, R. Tenne (Weizmann Institute of Science, Israel) Recent improvements in synthetic yields of inorganic nanotubes of metal dichalcogenides have enhanced the possibility of their technological applications.¹ The unique optical and wear characteristics of these nanotubes make them ideal candidates for electromechanical systems.² Correlation of the structural and electro-optical properties is a first step in this direction. Theory predicts delicate interplay between size, structure, and electrical properties of these nanotubes.³ Until now, correlation of the nanotube chirality with electrical properties has been indirect and scarce. In this work we present high resolution transmission electron microscopy (TEM) and atomic-resolution scanning tunneling microscopy (STM) images of nanotubes of WS₂. By relating the atomic registery to the tube-axis direction, the chirality of the tubes is determined. Current-voltage (I/V) spectroscopy in the STM was then applied to individual nanotubes to examine correlation between bandgap, density of states, and the nanotube chirality and size. ¹ A. Rothschild, G.L. Frey, M. Homyonfer, R. Tenne, M. Rappaport, Mat. Res. Innovat. 3, 145 (1999). ² L. Rapoport, Y. Feldman, M. Homyonfer, H. Cohen, J. Sloan, J.L. Hutchison, and R. Tenne, Wear 229, 975 (1999). ³G Siefert, H. Terrones, M. Terrones, G. Jungnickel, T. Frauenheim, Phys. Rev. Lett. 85, 146 (2000).
11:20 AM		NT+EL+NS-ThM-10 In-situ Observed Atomic Structures at Carbon Nanotube Tips under Applied Electric Field T. Kuzumaki, Y. Horiike (The University of Tokyo, Japan); T. Kizuka (Nagoya University, Japan); T. kona, C. Oshima (Waseda University, Japan) Carbon nanotubes show characteristics that are of particular interest as electron sources for field emission displays. In this study, tip structures of the nanotubes were in-situ observed under applied electric field by using a high-resolution transmission electron microscopy (HRTEM), field ion microscopy (FIM) and also field emission microscopy (FEM). HRTEM used in this experiment equipped with newly designed two specimen holders system. HRTEM observation successfully revealed that the tip of the nanotube bent during the field emission and protrudent structure was formed along the normal to the electric field. The results demonstrate that the electric field exerted mechanical stress on the surface structure. The in-situ observations lead us to development of nano-processing technology of the nanotubes. We found that burst and evaporation, and bonding of individual nanotube tips can be performed by the contact with another nanotube, amorphous carbon, or metals at the applied voltage. The tip of the nanotube was burst and evaporated at the contact at the applied voltage of more than 2V. At the burst tip, each carbon layer was often connected with the neighbor layers. After the tip burst, however, two nanotubes were bonded at the contact when the applied voltage was less than 2V. A rod specimen in which the nanotubes are sticking out of the tip was fixed on the tungsten wire with carbon binder, and was introduced into the ultra-high vacuum chamber of 3X10^-8 Pa for FIM and FEM experiments. Prior to the FEM observation, we evaluated the cap structure of the nanotubes by FIM and confirmed that several bright areas as emission sites, and they were either deformed honeycomb structures composed of hexagonal or pentagonal carbon rings. The bright area sites of the honeycomb structures observed in FIM moved with increment of the applied voltage.
11:40 AM		NT+EL+NS-ThM-11 Characterization of CVD Grown Carbon Nanotubes and Field Emission Properties C. Dong, M. Gupta (Old Dominion University); G.R. Myneni (Jefferson Lab) Carbon nanotubes(CNT) were synthesized by the thermal chemical vapor deposition with the decomposition of acetylene gas in the argon carrier gas under temperature of 700 ºC on Ni or hastelloy substrates. Carbon nanotubes were characterized by X-ray photoelectron spectroscopy (XPS), Raman Scattering, Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM). Field emission properties of well characterized carbon nanotubes was extensively studied. Low turn-on electric field of ~ 1V/um was achieved. Long-term stability of emission was studied. Field emission profile was examined by imaging using phosphor screen. Carbon nanotubes appear to have better field emission properties and stability comparing to Spindt type emitters.