ICMCTF2002 Session D1-2: Synthesis, Characterization and Applications of Carbon Nitride, Boron Nitride and Nano-Structures

Tuesday, April 23, 2002 8:30 AM in Room Royal Palm Salon 1-3

Tuesday Morning

Start	Invited?	Item
8:30 AM	Invited	D1-2-1 Towards Field Emission Devices Based on Carbon Nanotubes Jean-Marc Bonard (EPFL, Switzerland) Discovered ten years ago, carbon nanotubes have emerged in the last few years as one of the most exciting topics in nanophysics. Industries are also getting more and more interested in this system, as nanotubes are considered to be serious candidates for various applications. Among these, field emission has emerged as one of the most promising. Carbon nanotubes are capable of emitting high currents at low fields and are purported to be ideal candidates for the next generation of field emission flat panel displays. In this talk, I will on one hand outline the progress achieved so far in the realisation of field emission devices, in particular for flat panel displays and lighting elements. For the latter type of application, we have demonstrated a cylindrical field emission diode, where the cathode is a metallic wire on which multiwall carbon nanotubes are grown by CVD, and which shows excellent performances. This allowed us to realize a luminescent, mercury-free, tube, which could provide an interesting alternative to usual fluorescent tubes. On the other hand, I will also address some key issues for the future developement of such devices. It is becoming increasingly apparent that field emission is an extremely selective process, and that the growth of the emitters has to be carefully controlled in order to optimise the emission and to ensure reproducible characteristics. These facts call for a better understanding of the growth process and of the role of the catalyst, as well as for a careful study of the field emission properties of individual tubes. First steps in these directions will be presented. Our work is supported by the European Community and the Federal Office for Education and Science of Switzerland in the frame of the project CANADIS (IST-1999-20590), by the Swiss National Science Foundation, by the TopNANO21 program and by Nanolight International Ltd.
9:10 AM		D1-2-3 Growth of Carbon Nanowires R.M. Liu, J.M. Ting, C.P. Liu, J.C.A. Huang (National Cheng Kung University, Tainan, Taiwan, ROC) Microwave plasma-enhanced chemical vapor deposition (MPCVD) technique has been employed by many research groups to grow carbon nanotubes (CNT). In this paper, we report the growth of both carbon nanotubes and carbon nanowire using the same MPCVD technique. The growth was performed under various gas mixtures of methane and hydrogen, pressures, microwave plasma powers, and growth times. Various catalyst materials prepared using different methods were used. These substrates include sputter deposited nickel thin films, cobalt thin films, and nickel-containing carbon thin films, and molecular beam epitaxy grown iron and cobalt thin films. It was found that the formation of either carbon nanotubes or carbon nanowires depended on the nature of catalysts. Microstructure of carbon nanotubes, carbon nanowires, and catalysts were studied with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Effects of growth parameters on the characteristics of carbon nanowires and carbon nanotubes were investigated.
9:30 AM		D1-2-4 Growth and Characterization of Vertically Aligned Carbon Nanotubes M. Chhowalla, K.B.K. Teo, GAJ Amaratunga (University of Cambridge, United Kingdom) The growth of vertically-aligned carbon nanotubes (VACNTs) using a direct current plasma enhanced chemical vapor deposition (DC PECVD) system is reported. The growth properties are studied as a function of the Ni catalyst layer thickness, bias voltage, deposition temperature, C₂H₂:NH₃ ratio, and pressure. The minimum diameter of the tube was found to be 20nm. The alignment of the nanotubes depends on the electric field only. The catalyst particle was always found to be at the top of the nanotube. The activation energy for carbon diffusion through the Ni catalyst particle was found to be much lower (0.54eV) in the presence of a plasma as compared to thermal deposition (1.5eV) in the absence of plasma. This has important implications for large area low temperature growth of aligned nanotubes. Our results indicate that the growth occurs by diffusion of carbon through the Ni catalyst particle in the case of thermally grown case while surface diffusion plays an important role in the presence of plasma. The main advantage of PECVD is that it is controllable and deterministic growth process so that the location and orientation of the nanotubes can be precisely controlled by lithographically patterning the location of the catalyst. It will be shown that it is possible to grow uniform arrays of vertically aligned nanotubes on lithographically patterned Si substrates. Field emission measurements on forrests of nanotube and uniform arrays measured using a parallel plate configuration as a well as microscopic scanning anodes will be reported. The results show that short and stubby nanotubes are better suited for field emission in terms of yielding the lowest threshold field (2V/µm) and the highest current density (10mA/cm²).
9:50 AM	Invited	D1-2-5 Electron Structure of the Carbon Nanotubes Studied by X-ray-absorption Spectroscopy W.F. Pong (Tamkang University, Taiwan) X-ray absorption near edge structure (XANES) measurement has been performed to investigate the local electronic structure of the Fe-catalyzed and stabilized carbon nanotubes (CNTs) with various diameters. The intensities of the π^- and σ^-band and the interlayer-state features in the C K-edge XANES spectra of these CNTs vary with the diameter of the CNT. The white-line features at the C K- and Fe L₃-edges suggest a strong hybridization between the C 2p and Fe 3d orbitals, which lead to enhancement of the C K- and reduction of the Fe L₃-edge features, respectively, indicative of a charge transfer from C 2p to Fe 3d orbitals. The Fe K-edge spectra reveal a p-d hybridization effect that reduces p-orbital occupation at the Fe site. Furthermore, the C 1s photoelectron spectromicroscopy image of the aligned CNT using the scanning photoelectron microscopy (SPEM) at Synchrotron Radiation Research Center (SRRC), Taiwan will also be presented.
10:30 AM		D1-2-7 Effects of Catalyst Preparation on the Growth of Carbon Nanotubes R.M. Liu, K.H. Liao, W.Y. Wu, J.M. Ting (National Cheng Kung University, Tainan, Taiwan, ROC) Thin films of transition metals have been used as the catalysts for the growth of carbon nanotubes in recent years. However, limited information, if any, is given regarding to how the nature of thin film catalyst influences the growth and characteristics of carbon nanotubes. In this paper, we report a systematic study addressing such an important issue. Three different catalyst materials, i.e., nickel thin films, copper thin film, and chromium thin films, were prepared using the same reactive sputter deposition technique. These thin films were grown on Si wafers under various conditions. Carbon nanotubes were subsequently grown in a microwave plasma-enhanced chemical vapor deposition system. The growth was performed under different pressures, microwave powers, and gas ratios of methane and hydrogen. Microstructures of carbon nanotubes were examined with scanning electron microscopy (SEM) and transition electron microscopy (TEM). Correlation between the natures of thin film catalysts and the growth/characteristics of carbon nanotubes is reported and discussed.
10:50 AM	Invited	D1-2-8 Multiscale Modeling of Polycrystalline Diamond and Other Carbon-based Systems Olga Shenderova, D. Brenner (NCSU) Carbon-based materials are now commercially available for a variety of applications ranging from wear resistance coatings to optoelectronic microdevices. Material properties that still require research efforts for these applications are ultimate strength and toughness of the polycrystalline diamond films, mechanical stability of carbon based nanodevices as well as electronic properties of carbon-based systems. Some aspects of these issues will be discussed in the talk. Particularly, strength properties of different types of grain boundaries in diamond had been calculated using a multiscale modeling approach. The method combines first-principles density functional calculations on selected grain boundary structures with an analytic mesoscale model. The outputs of this method are energies, cleavage fracture energies and toughness for tilt grain boundaries in diamond over the entire misorientation range predicted with first-principles level of accuracy. The feasibility of designing composites of nanodiamond clusters with carbon nanotubes will be also discussed. Depending on a nanotubes size and morphology, some types of open nanotubes can be chemically connected with different diamond surfaces. Mechanical stability and field emission related electronic characteristics of the novel nanostructures will be discussed.
11:30 AM		D1-2-10 The Properties of Carbon Nitride Nanotubes Synthesized by Electron Cyclotron Resonance Plasma S. H. Lai, Y.L. Chen, H.H. Lin, L. H. Chan, H.C. Shih, X. W. Liu (National Tsing Hua University, Taiwan, ROC) One-dimension carbon nitride nanotubes have been synthesized by an electron cyclotron resonance chemical vapor deposition (ECR-CVD) system with a mixture of C₂H₂, and N₂ as precursors. The carbon nitride nanotubes were synthesized in anodic alumina membrane template which consisted of a packed array of parallel, straight, uniform channels with 30 nicron thick and a diameter of about 50 nm. Samples were analyzed by SEM, HRTEM, XPS, and field emission I-V measurements. The preliminary results show that the properties of the deposited carbon nitride nanostructure materials depend on process parameters such as deposited temperature, the ratio of precursors and a microwave power. The aligned and hollow structure have been verified by SEM and HRTEM. Form the XPS results, N is either bonded to two C atoms ( sp² pyridine-like type ) or to three ( sp³ bridgehead-nitrogen type ) in the hexagonal sheets.