Papers

ICMCTF2003 Session D4: Nanotubes and Nanostructured Materials

Wednesday, April 30, 2003 8:30 AM in Room Royal Palm 1-3

Wednesday Morning

Start	Invited?	Item
8:30 AM		D4-1 Functionalisation of the Carbon Nanofibres by Plasma Treatment W. Brandl, G. Marginean (University of Applied Sciences Gelsenkirchen, Germany) A plasmachemical treatment is utilised to improve the surface properties of carbon fibres used for fibre-reinforced composites. In this study differerent plasma functionalisations of carbon-nanofibres surface are presented. Depending on the plasma parameters and functionalising gases (O₂, NH₃, N₂), an enhancement of the fibre surface energy occurs. Those surface treatments are doubly important in composites because they optimise the adhesion between the fibre and the polymer matrix which conducts to the improvement of the mechanical and physical properties of the composite. The morphology and the properties of the carbon nanotubes as well as of the fibre/polymer composites (before and after the plasma functionalisation) were characterised by scanning electrom microscopy (SEM), contact angle measurements, electrical conductivity measurements and testing of the tensile strength of the obtained composites.
8:50 AM		D4-2 Nitrogen Effect in Vertically Aligned CNT Growth T.-Y. Kim (Korea Institute of Science and Technology and Seoul National University, South Korea); K.-R. Lee, S.-C. Lee, K.-Y. Eun (Korea Institute of Science and Technology, South Korea); K.-H. Oh (Seoul National University, South Korea) The growth behavior of carbon nanotubes (CNT) deposited from C₂H₂ by thermal CVD method was investigated. Ni particles of diameter ranging from 15 to 90 nm were used as the catalyst. CNTs were deposited in various environments of N₂, H₂, Ar, NH₃ and their mixtures to investigate the effect of the environment on the CNT growth behavior. The growth of CNT was much enhanced when using NH₃ as the environment gas. Vertically aligned CNTs could be deposited in NH₃ environment, whereas the CNT growth could not be obtained in the mixture of N₂ and H₂ environment of the same ratio of N/H. We could show that the enhanced growth is closely related to the existence of activated nitrogen in the growth environment. Activated nitrogen resulted in the nitrogen incorporation into the CNT wall and cap during growth, which significantly improve the CNT growth behavior. This result is consistent with theoretical calculations of CN_x thin film, showing that nitrogen incorporation to the graphitic basal plane reduces the elastic strain energy for curving the graphitic layer.
9:10 AM		D4-3 Implications of Carbon Nanotubes Synthesis by using High and Low Pressure Plasma Enhanced Chemical Vapor Deposition C. H. Lin (National Chiao Tung University, Taiwan, ROC); C.M. Hsu (Material Research Lab., ITRI, Taiwan, ROC); H.L. Chang, C.T. Kuo (National Chiao Tung University, Taiwan, ROC) The structures and properties of carbon nanotubes (CNTs) are highly process dependent. The arc discharge and laser ablation methods can grow single-walled CNTs bundles, and chemical vapor deposition (CVD) methods are generally more favor to synthesize the well-aligned or spaghetti-like multi-walled CNTs. This study focused on the differences in CNTs properties and their growth mechanisms, grown by high and low pressure plasma enhanced CVD methods, such as microwave plasma chemical vapor deposition (MPCVD) versus microwave plasma electron cyclotron resonance chemical vapor deposition (ECR-CVD). The typical working pressures for both systems are in the range of 5 ~ 25 Torr and 2 ~ 5x10^-3 Torr, respectively. The Co- or Ni-catalysts-coated Si substrates and CH₄/N₂/H₂ source gases were used for CNTs deposition. The CNTs structure and properties were characterized by SEM, TEM, HRTEM, Raman spectra and field emission I-V measurements. It is interesting to note that the CNTs grown by MPCVD have over ten times more in growth rate (typically 1 ~ 3 µm/min) but smaller in tube diameter (about 5 ~ 30 nm), as comparing with the CNTs grown by ECR-CVD (about 30 ~ 80 nm). By comparing the structures and properties of CNTs, the preliminary results indicate that the CNTs grown by MPCVD possess greater verities of nanostructures and better field emission properties. Furthermore, presence of nitrogen in plasma often causes CNTs formation from hollow to bamboo-like CNTs in MPCVD system. In contrast, the well-aligned bamboo-like CNTs are often found in ECR-CVD system with or without the presence of nitrogen. The differences in growth mechanisms between two systems would be stressed. Keywords: Carbon nanotubes, microwave plasma CVD, electron cyclotron resonance CVD, field emission, growth mechanism.
9:30 AM		D4-4 Field Emission Properties of Carbon Nanotubes Prepared Using Vaious Catalysts K.H. Liao, J.M. Ting (National Cheng Kung University, Taiwan, ROC) Carbon nanotube (CNT) has attracted considerable interest since its discovery. Due to its high aspect ratio, small tip radius of curvature, and high conductivity, CNT is an excellent candidate material for use as for field emitters. In this paper, we will present our investigation on the field emission characteristic of CNT prepared using a microwave plasma-enhanced chemical vapor deposition technique. Effect of several unique catalysts on the field emission properties is addressed. These catalysts include single crystal thin Co films prepared using either an ion-beam sputter deposition technique or a molecular beam epitaxy growth technique, polycrystalline Co films with either fcc or hcp structure prepared using a sputter deposition technique, and various metal-containing diamond like carbon films prepared using a reactive sputter deposition technique. Both the catalyst films and CNT were examined using scanning electron microscopy and transmission electron microscopy. The field emission properties of CNT were investigated by means of a vacuum field emission measurement system. The adhesion between CNT and the substrates were also evaluated.
9:50 AM		D4-5 Synthesis, Integration and Application of Carbon Nanotube Based Electron Field Emission Cathodes O. Zhou (University of North Carolina)
10:30 AM		D4-7 Thermally Activated Electron Emission from Nano-tips of Amorphous Diamond and Carbon Nano-tubes J. Sung (Kinik Company, and National Taipei University, Taiwan, ROC); M.-C. Kan, J.-L. Huang (National Cheng-Kung University, Taiwan, ROC); K.-H. Chen (Institute of Atomic and Molecular Sciences, Academia Sinica, Taiwan, ROC) The sp² character of graphite can carry electricity like a metal and the sp³ character of diamond can emit electrons in vacuum like an insulator. In this research, we have studied two ways of combining the electrical conductance and electron emission properties in carbon materials. In one case, the two characters are mingled atomistically to form a rather uniform mixture of amorphous diamond. Alternatively, graphite basal planes are wrapped around to form nano-tubes that exhibit a slight diamond character. Both amorphous diamond and carbon nano-tubes contain emission tips of nano meter sizes. When they are connected to a negative bias, they can emit electrons in vacuum toward an anode at very low turn-on field. However, when the cathode material is heated up, the responses of electron emission in vacuum are dramatically different between the two types of carbon materials. At a temperature of 200^oC, amorphous diamond can emit 30-folds more electrons than it can at room temperature; but carbon nano-tubes (CNTs) show little change of electron emission behavior. The thermally sensitive emission of amorphous diamond indicates that the energies of electrons are discrete but their gaps are small so electrons can climb up the energy ladder to reach the vacuum level for efficient emission. Hence, amorphous diamond contains ample defect bands available for electrons to gain energy momentarily without losing it. On the other hand, the CNTs are substantially graphitic so the energy gap between its conduction band and valence band are still continuous. In this case, electrons cannot acquire energy to reach the vacuum energy unless the temperature is sufficiently high. Hence, it confirms that CNTs emit electrons primarily by enhancing the applied field on nano-tips. This is in contrast to amorphous diamond that emits electrons by the collaboration of graphitic carbon atoms and diamond-like carbon atoms of various states.
10:50 AM		D4-8 Plasma Torch Production of Nanostructured Materials C.K. Chen, J. Phillips, W.L. Perry (Los Alamos National Laboratory) Using a low power (<1000W) atmospheric pressure plasma torch we have been able to generate a variety of unique nanomaterials including metals, oxides, nitrides and carbon nanotubes at relatively high rates. In all cases precursor species, generally micron scale solids, are passed through the plasma torch as an aerosol. In a residence time of the order 0.1 seconds these precursor materials are transformed to nanomaterial products. In particular, metal and oxide nanoparticles are made by vaporizing precursor metals to create a metal atom vapor. In the cool afterglow of the plasma the atoms condense to form nanoparticles. The phase, average size, etc. are a function of the plasma operating conditions. Most recently, we have shown that highly twisted carbon 'filaments' up to 6mm long and several hundred microns in diameter can be produced. These filaments are made of 'ropes' of nanotubes, each rope containing many single wall nanotubes. Each nanotube is apparently thousands of angstroms or longer in length. These filaments are of a size appropriate for use as fiber fillers of polymer matrix, thus this process of production has the potential for creating nanotubes in quantity and quality for 'mechanical' applications.
11:10 AM		D4-9 Controlled Synthesis and Directed Assembly of Vertically Aligned Carbon Nanofibers for Functional Nanoscale Devices V.I. Merkulov (Oak Ridge National Laboratory); A.V. Melechko, M.A. Guillorn (Oak Ridge National Laboratory and The University of Tennessee); T.E. McKnight, D.K. Hensley, M.J. Doktycz, D.H. Lowndes (Oak Ridge National Laboratory); M.L. Simpson (Oak Ridge National Laboratory and The University of Tennessee) The controlled synthesis and directed assembly of materials at the nanoscale remain two of the most difficult challenges for the realization of functional nanoscale devices. For example, while there has been a great deal of significant nanoscale science accomplished with carbon nanotubes (CNT), difficulties in the control of CNT placement, orientation, and chirality have limited the development of technology based on this science. In contrast, the synthesis and assembly of vertically aligned carbon nanofibers (VACNFs) (Fig. 1) is highly controllable, allowing the definition of VACNF location, length, tip diameter, shape, orientation and chemical composition. This control capability enables numerous potential applications in scanning microscopy, field emission devices, nanoelectronics, and nanobiotechnology (Fig. 2). We will present details of VACNF growth by dc PECVD focusing on how growth parameters may be manipulated to control VACNF properties. We will present results that demonstrate significant progress toward the ability to deterministically synthesize VACNF-based carbon nanostructures in a wafer-scale synthesis process and the control of this process to produce a variety of functional nanoscale devices. In addition, phenomenological models that explain important aspects of VACNF growth will be presented and validated with experimental results. Several functional VACNF-based device structures will be presented to illustrate the utility of these nanostructures in a practical nanotechnology.
11:50 AM		D4-11 Structure and Field Emission Properties of SiC Nanotip Arrays Fabricated by One-step and Self-masked ECR-Plasma Etching L.-C. Chen (National Taiwan University, Taiwan); J.S. Hsu, C.-F. Chen (National Chiao Tung University, Taiwan, ROC); H.C. Lo, J.S. Hwang, K.-H. Chen (Academia Sinica, Taiwan, ROC) We report here arrays of SiC/Si nanotips with high aspect ratios (~ 100) and sharp apexes (~ 1 nm) by direct etching from Si substrates. Well-aligned nanotip arrays with a nanotip density as high as 10¹² cm^-2 were achieved by single-step electron cyclotron resonance plasma process using gas mixtures of silane, methane, argon and hydrogen. Detailed structure analyses using high resolution TEM revealed that the process is a self-mask etching process in that, accompanying the etching of Si, deposition of SiC nanoclusters occurred simultaneously, therefore, forming protecting caps on the tips. The nanotip arrays so produced showed magnificent field emission property with typical field emission current of 0.5 mAcm^-2 at an applied field as low as 0.8 V/µm. Furthermore, the nanotip arrays also exhibited excellent stability, as evident by temporal evolution of the emission current at a constant applied voltage measurement which showed less than 3% fluctuation in one hour. The SiC nanotip array produced by ECR-plasma process of monolithic Si wafer offers a reliable, economic field emission electron source alternative to carbon nanotubes. Formation of the SiC nanotip on top of a Si cantilever has also been demonstrated. Such geometry provides potential applications in ultrahigh resolution SPM and field emission microscopy.
12:10 PM		D4-12 Computations of Local Electric Field and Electric Forces Acting on Carbon Nanotubes in DC Plasma Sheath J. Blazek (University of South Bohemia, Czech Republic); P. Spatenka (Technical University Liberec, Czech Republic); F. Pacal, C. Täschner, A. Leonhardt (Institute of Solid State and Materials Research Dresden, Germany) The DC bias has been reported as a necessary condition for aligned growth of carbon nanotubes. To clarify the mechanisms of nanotubes alignment we performed numerical calculations of the electrical field in the collisional sheath and of the electrical force acting on the nanotube tip. The electrical force acting on the catalytic droplet on the nanotube tip has been found relatively huge, e.g. four order of magnitude higher than the gravitational force of a droplet. Based on the model calculations dependences of the force on the form of the nanotube tip and on the distance between the particular nanotubes are discussed. Computational investigations also reveal how this force depends on the DC bias, nanotubes dimensions and the conductivity between the nanotube tip and the biased substrate.