ICMCTF2004 Session D4: Nanotubes and Nanostructured Materials

Thursday, April 22, 2004 1:30 PM in Room Royal Palm 4-6

Thursday Afternoon

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1:30 PM D4-1 Novel One Dimensional Nanostructures
Meyya Meyyappan (NASA Ames Research Center)

Carbon nanotubes (CNTs) are elongated fullerenes with diameter as small as 0.7nm and several microns long. They exhibit a combination of remarkable mechanical properties and unique electronic properties and thus offer significant potential for revolutionary applications in electronics devices, computing and data storage technology, sensors, composites, storage of hydrogen or lithium for battery development, nanoelectromechanical systems (NEMS), and as tip in scanning probe microscopy (SPM) for imaging and nanolithography. Thus the CNT synthesis, characterization and applications touch upon all disciplines of science and engineering and this talk will provide an overview of the CNT field.

It is possible now to grow vertically oriented, well-aligned nanowires of semiconducting materials such as silicon, GaAS, GaN and high temperature oxide such as ZnO, InO etc. These inorganic nanowires exhibit very interesting electronic and optical properties compared to their two dimensional thin film cousins and are being investigated for device, laser and sensor applications. An overview of growth, characterization and applications of these nanowires will also be covered.

2:10 PM D4-3 Growth and Morphology of Aligned Carbon Nanotube Layers
K. Bartsch, A. Leonhardt (Leibniz-Institute of Solid State and Materials Research Dresden, Germany)
Aligned grown multi-walled carbon nanotubes (MWNT) have become increasingly important for potential applications such as in nanoelectronics, field emission devices and biological probes. Various kinds of thermally or plasma activated chemical vapour deposition (CVD) are promising as deposition process using substrates coated with thin catalyst layers of some nanometer thickness from Fe, Ni, or Co. Especially, bias assisted plasma CVD favours an excellent alignment of the tubes. Layers of aligned multi-walled carbon nanotubes were deposited on thin Co layers by bias assisted microwave CVD in the temperature range 973 - 1273 K and characterized with respect to their structure. The tubular and the bamboo-like structure occurred simultaneously in the entire temperature range. In order to figure out the reasons for the observed structural instabilities, the catalyst particle size distribution on the substrate and in the nanotube tips was investigated. The particle size distribution in the nanotube tips was found to be time dependent. Clearing up this, it could be shown that the different growth rates of the tubular and bamboo-like grown tubes are causing this effect. Furthermore, it is stated and experimentally proved that the metastable state of the catalyst particles is responsible for the limited length growth of the tubes and for structure changes occurring during the growth. Possible growth mechanisms and possibilities to overcome stopping the growth will be discussed.
2:30 PM D4-4 Growth Kinetics of Vertically Aligned Carbon Nanostructures at Low Temperatures
G.Y. Chen, C.H.P. Poa, S.R.P. Silva (University of Surrey, United Kingdom); S. Haq (BAE Systems, United Kingdom)
The temperature and time dependence of the growth of vertically aligned carbon nanostructures using DC plasma Chemical Vapour Deposition (CVD) below 550°C are studied. The nanostructures are synthesised using a C2H2/N2 plasma and nickel as the catalyst. We found that as the synthesis temperature is lowered below 450°C, a thin film of amorphous carbon is preferentially deposited. The nucleation of nanofiber-like structures may be possible through the reduction of the carbon feedstock. This method allows the synthesis of vertically aligned structures at temperatures without intentional heating. The synthesised conical nanostructures have base dimensions of the order of approximately 100nm with a tip size of 20nm. Based on the observations of experimental process variations and the morphology of the synthesised structures, we propose a growth mechanism for such low temperature growth. This process allows for the deployment of low temperature substrates, alleviates the need for diffusion barrier layers and potentially will allow the design and manufacture of device structures that has to date been hindered by the high temperatures associated with conventional carbon nanotube/fiber growth.
2:50 PM D4-5 Attachment of Nickel-nano Particles on Multiwalled Carbon Nanotubes.
P.K. Tyagi, E. Titus, M.K. Singh, A. Misra, D.S. Misra, N. Kumar (Indian Institute of Technology, India); M. Roy, A.K. Dua (BARC-Bombay, India)
Carbon nanotubes have recently caught the imagination of the scientists the world over. Filling of the tubes with variety of materials uniformly in the core section of the tubes is a challenging task and attempts have been made to achieve the same. Similarly the attachment of the nanosized clusters to the tube ends will open tremendous possibility of applications. In this paper, we report the application of the pulse laser deposition (PLD) for attachment of the nickel nanoclusters with the surface of the tubes. Our motivation is to study the interface between the clusters and the tubes and to fabricate the novel structures using PLD. Carbon nanotubes were prepared by Microwave Plasma Chemical Vapor Deposition (MPCVD) technique using methane and hydrogen as precursors gases. The tubes were purified by acid treatment and oxidation and were dispersed in isopropyl alcohol. A Ni target placed inside the suspension was ablated with a Nd:YAG laser (2nd harmonic of 532nm) in argon atmosphere. The energy density of the laser at the target was 3.2 J/cm2 . The Ni nanoclusters generated with this technique were of the size 20-40 nm and self arranged on nanotubes. The characterization of the nanoparticles attached with the tubes was carried out with Raman spectroscopy and Transmission electron microscopy (TEM). The particle size and distribution of the sizes was measured with image analyses of the electron microscope. The attachment of Ni particles was evident from TEM studies.
3:10 PM D4-6 Effects of Alloy Catalyst Composition on the Microstructure of Carbon Nanotubes and Nanowires
K.H. Liao (National Cheng Kung University, Taiwan, R.O.C.); J.M. Ting (National Cheng Kung University, Taiwan)
Growth of carbon nanotubes (CNT) has been a subject that receives considerable research for more than 10 years. One of the important issues addressed is the preparation, and therefore the characteristics, of catalysts. Several methods have been developed for the preparation of catalysts with various characteristics. However, understanding the relationship between the nature of a catalyst and CNT's characteristics desires additional studies. Furthermore, new potential catalysts remain to be explored. In this study, we have investigated the use of new iron alloys as the catalysts for the growth of CNT and carbon nanowire (CNW). The preparation of the thin film alloy catalysts and its effect on the microstructure of CNT are addressed. The thin film alloy catalysts were prepared using either a sputter deposition technique or a sol-gel process. The resulting alloy catalysts exhibit different compositions, thickness, roughness, grain size, and crystallinity. The growth of CNT/CNW was carried out using a microwave plasma enhanced chemical vapor deposition method. The reaction gases were C2H2 and NH3 with a fixed C2H2/NH3 ratio of 1/9. The growth time was 3 min. CNT/CNW obtained were characterized for microstructure using micro-Raman and high resolution transmission electron microscopy (HRTEM). The relationship between catalyst particle composition and the microstructure of CNT/CNW is discussed.
3:30 PM D4-7 Synthesis of Ultrathin Carbon Nanosheets by Inductively Coupled Radio Frequency Plasma Enhanced Chemical Vapor Deposition and Field Emission Test
J.J. Wang, D.M. Manos, B.C. Holloway (College of William and Mary); C. Park, T. Anderson (University of Florida); V.P. Mammana (International Technology Center)
Nanometer edged carbon structures, carbon nanoflakes (CNFs), have been synthesized on various substrates by radio-frequency (RF) plasma enhanced chemical vapor deposition (CVD) method. Different gas systems are involved in generating CNFs, such as CH4-H2, C2H2-H2 and C2H2-NH3. Typical deposition conditions are: substrate temperature of 680°C, overall gas flow rate of 10 sccm, chamber pressure of 25~120 mTorr, and hydrocarbon concentration in the range of 5~100 %. Scanning electron microscopy shows that the flake-like structure has a large smooth surface and grows almost vertically to the substrate. The flake edges are several nanometers in thickness and hundreds nanometers in width, and very stable under small electron spots. Raman spectra show a typical carbon feature with D and G peaks at 1350 and 1580 cm-1, respectively. Intensity ratio of these two peaks, I(D)/I(G), varies with hydrocarbon concentration, indicating a change in crystallinity of the CNFs based on deposition conditions. Field emission spectroscopy and I-E measurements show a good electron emission property, indicating that this structure could act as a conductive, robust, edge emitter.
3:50 PM D4-8 Carbon Nanotubes and ZnO Nanowires
Zhifeng Ren (Boston College)

The carbon nanotubes aligned periodically have many potential applications in electronics, optics, etc. In this talk, I will discuss the aligned carbon nanotubes grown by plasma enhanced hot filament chemical vapor deposition on Ni films by magnetron sputtering, Ni dots by e-beam lithography, Ni dots by electrochemical deposition, and two-dimensional periodical nickel dot array by nanosphere self-assembly. The size of the nickel dot and spacing between them and location are tunable by the preparation methods. It has been found that well-aligned carbon nanotubes with site density of 105 -1012 / cm2 have been successfully obtained. With these arrays, a variety of applications have been studied including field emission, nanoelectrode, photonic band gap crystals, etc.

In the second part, I will talk about ZnO nanowires. A variety of novel indium affected ZnO hierarchical nanostructures, including bristle-like ZnO nanostructures with 2-, 4- and 6-fold symmetries, nanobridges, nanopins and nanorods on nanobelt have been successfully grown by a vapor transport and condensation technique. For the 4-fold and 6-fold symmetries, it was found that the major core nanowires are single crystal In2O3 with 6, 4, and 2 facets, and the secondary nanorods are single crystal hexagonal ZnO and grow either perpendicular on or slanted to all the facets of the core In2O3 nanowires. The core In2O3 nanowires have diameters of about 50 - 500 nm, whereas the secondary ZnO nanorods have diameters of about 20 - 200 nm. Depending on the diameter of the core In2O3 nanowires, the secondary ZnO nanorods grow either as a single row or multiple rows. In contrast, the 2-fold symmetry, nanorods on nanobelt and nanobridges are homo-epitaxial growth, but with the morphology affected by the indium distribution in the nanobelt. These hierarchical nanostructures may find applications in a variety of fields such as field emission, photovoltaics, transparent EMI shielding, supercapacitors, fuel cells, high strength and multifunctional nanocomposites, etc. that require not only high surface area but also structural integrity.

4:30 PM D4-10 RF-sputtered Lithium Cobalt Oxide Thin Films and Nanowires Prepared using AAO Template
C.L. Liao, Y.H. Lee, K.Z. Fung (National Cheng Kung University, Taiwan, R.O.C.)
Lithium cobalt oxide has been widely studied as a cathode material in lithium batteries. In lithium microbatteries, it is well known that the electrochemical properties of the thin-film cathode can be enhanced by increasing its surface area and minimizing the diffusion paths for lithium ion. In this study, LiCoO2 films with LiCoO2 nanowires on top were prepared using RF sputtering technique on an anodic aluminum oxide (AAO) template. We conjectured that the LiCoO2 nanowires and the structure of LiCoO2 film with LiCoO2 nanowires on top would enhance the electrochemical properties, such as retention, of LiCoO2 cathode. In this work, LiCoO2 powders were first synthesized at 700°C for 12 hours from the mixture of Li2CO3 and CoCO3. To prepare the sputtering target, the powders were pressed into 2-inch pallet and sintered at 700°C for 3 hours by hot-pressing. The LiCoO2 films were deposited under the following conditions: RF power of 100W, working pressure of 20mTorr, flow rate of 12sccm, and distance between target and substrate of 40mm. The structure of the film was analyzed by X-ray diffraction analysis, Raman spectroscopy, and TEM analysis. The morphologies of the films were observed by SEM. The electrochemical measurements including charge-discharge test and cyclic voltammetery were conducted.
4:50 PM D4-11 Characterization and Properties of Inorganic Fullerene-like Nanoparticles for the Application of Tribological Coatings
J.J. Hu, J.E. Bultman, J.S. Zabinski (Air Force Research Laboratory)
Nano-structured materials have some unique merits in lubrication through various syntheses. Simple arc-methods utilizing water or liquid nitrogen mediums permit continuous synthesis of nanoparticles at relatively low-cost and the process is environmentally benign. There have been recent reports of growing carbon nanotubes in liquid nitrogen, and carbon nano-onions in water. Although graphite and MoS2 are both common solid lubricants, nano-structured carbons are less effective than MoS2 as lubricants. In this study, the arc method utilizing water was employed to grow some known, as well as new structures. We have observed IF-MoS2 nanoparticles from the arc discharge of MoS2 rods in water and have found these particles to have different structures. The tribological properties of inorganic fullerene-like MoS2 nanoparticles were investigated by means of lateral force microscopy in the environments of dry nitrogen and ambient air, respectively. For comparison, the investigation was also taken on sputtered MoS2 films. According to those measurements, the inorganic fullerene-like MoS2 nanoparticles have significantly lower friction response to the load than the sputtered MoS2 film. The lubrication mechanism of inorganic fullerene-like nanoparticles will be discussed in detail.
5:10 PM D4-12 Fabrication and Characterization of Au2Si-filled Silicon Nanotubes and Self-organized Au2Si Pea-podded Silicon Nanowires
M.S. Hu (National Taiwan University of Science and Engineering, Taiwan, R.O.C.); L.S. Hong (National Taiwan University of Science and Technology, Taiwan, R.O.C.); K.H. Chen (Academia Sinica, Taiwan, R.O.C.); C.H. Shen, L.C. Chen (National Taiwan University of Science and Technology, Taiwan, R.O.C.)
For realizing applications of one-dimensional (1D) nanostructures in nanoelectronics, formation and understanding of metal-semiconductor hetero-junction is a crucial part. We report here two typical forms of novel Au2Si-Si composite 1D nanostructures. Specifically, a core-shell type Au2Si-filled Si nanotubes and a peapod type Si nanowires containing self-organized Au2Si nanospheres. Both types of 1D nanostructures were fabricated on Si substrates using Au as catalyst by microwave plasma chemical vapor deposition (MWCVD) under identical conditions except the process time. The MWCVD process is a highly energized and non-equilibrium process which allows formation of metastable Au2Si core enclosed by Si shell to occur rapidly. The self-organized Au2Si nanosphere-chains embedded in Si nanowires were observed at a later stage. Owing to the continuous supply of Si source, the initially continuous Au2Si core enclosed by Si nanotubes evolved into separated Au2Si rods, followed by the formation of periodical Au2Si nanoshperes. The interface between metal silicide and Si is sharp, as observed by transmission electron microscopy, for both type of composite nanostructures. The optical properties of the as-prepared products were characterized by cathodoluminescence measurement, which showed room temperature luminescence in visible range. Relationship between the composite structure and the luminescence property will be discussed.
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