ICMCTF2006 Session D3-2: Special Forum on Carbon Nanotubes and Related Nano-Structures

Thursday, May 4, 2006 8:30 AM in Room Sunset

Thursday Morning

Start	Invited?	Item
8:30 AM	Invited	D3-2-1 Electronic Applications of Carbon Nanotubes S. Roth (Stuttgart and SINEUROP Nanotech GmbH, Germany) The presentation will give an overview of the state of the art of applications of carbon nanotubes in electronics. Two aspects will be emphasized: Highly conductive tubes or bundles of tubes will be discussed as interconnects with high current density and low risk for electromigration, and semiconducting tubes as nano-sized field effect transistors. As examples a very small transistor (Infineon), the "vertical tube-FET" (Infineon and Samsung) and the "all-carbon transistor" (Max Planck Institute) will be presented and compared to the requirements of the Roadmap for Semiconductor Industry. In addition, transparent conductive electrodes and a transparent nanotube transistor will e described.
9:10 AM	Invited	D3-2-3 The Origin of Hysteresis in Field-Modulated Transport in Carbon Nanotube Field-Effect Transistors: Experiments, a Model, and Implications S. Kar (Rensselaer Polytechnic Institute) With the advancements in the controlled growth of single-wall carbon nanotubes and modern lithographic techniques, gated 3-terminal carbon nanotube field-effect transistors have added to the possibilities of ultra large-scale integration in nano-electronics. It has also opened up the scope for very exciting fundamental research in the field of low dimensional electronic transport properties. However, gate-modulated transport in single-wall carbon nanotubes are known to show significant hysteresis in their transfer characteristics, which stand in the way of accurate estimates of device properties, as well as the predictable performance of the devices. The origin of this hysteresis is generally attributed to the movement of mobile charges/ions in the presence of a trapping/de-trapping mechanism inherently present in the surrounding dielectric layer, as in conventional silicon MOSFETs. From an extensive study of the temperature dependence of the hysteresis behavior, we establish an alternate mechanism, where the screening charges are injected from the nanotube itself into the surrounding dielectric. Any detailed trapping/de-trapping mechanism does not appear to play a significant role, and the experimental results can be simply explained in terms of a capacitive charging of the surrounding dielectric due to the charge injection. On the basis of these results, we present a simple yet effective model to understand and analyze this phenomenon, where the charge injection and its subsequent redistribution into the surrounding dielectric have been modeled as a series RC circuit. We next present a series of experiments with gated three-terminal s-SWNT devices, and also use this model to fit previously published data by other groups. The combination of theory and experiment serve to provide an in-depth picture of the parameters, which play a crucial role in modifying the transfer characteristics in s-SWNT devices under different experimental conditions. In particular, it helps to understand the changes in the position, magnitude and shape of the transfer characteristic curves as commonly seen in literature.
9:50 AM	Invited	D3-2-5 Negative Differential Conductance in Single Wall Carbon Nanotubes at Room Temperature Q. Zhang (Electronics Centre, Singapore); J. Li, T. Zhong (Nanyang Technological University, Singapore) In single-wall carbon nanotubes (SWNTs), motion of electrons is confined in the 1D-like system so that many unique physical phenomena can occur even at room temperature. Negative differential conductance (NDC) is observed in a bundle of SWNTs at room temperature. We find that the peak-to-valley current ratio is about 2 and the peak voltage gradually shifts from 9.2V to 7.8V with sweeping a gate voltage from 6V to -6V. It is noticed that the significant NDC is associated with a "burning-off" T process, in which the metallic SWNTs in the bundle were burnt off and the semiconducting SWNTs are left behind. We suggest that two energy barriers are created on the SWNT bundle during the burning-off process. A quantum well is formed if the barriers are very close to each other so that hole energy in the well is quantized. The observed NDC is believed to be associated with hole resonance tunneling through the double barriers once the hole energy level is aligned with the discrete electronic energy levels between the barriers.
10:30 AM		D3-2-7 Electronic Structure and Thermal Stability of Nitrogenated MWCNTs Probed by NEXAFS Spectroscopy S. Sinha Roy, P. Papakonstantinou, T.I.T. Okpalugo, H. Murphy, J. McLaughlin (University of Ulster, United Kingdom) A dielectric barrier discharge (DBD) system has been used to incorporate nitrogen in Multi-walled carbon nanotubes (MWCNTs). Near edge X-ray absorption fine structure (NEXAFS) have been employed to obtain erudition on the electronic structures of nitrogenated MWCNTs. We observed several fine structures at the spectra of C K, N K, and O K edges. Regardless of N concentration, the C K edge NEXAFS spectra contained a relatively large π* peak, indicative of the graphite like bonding of the tubes. The N K edge spectra showed three main N bonding environments in the energy region 398-403 eV. In situ thermal annealing decisively indicated the origin of those peaks. The peaks N1, N2 and N₃ were assigned to pyridine-like, NO and graphite-like structure respectively. The C K and O K edges spectrum strongly suggested the presences of oxygen containing functional groups on the surface of MWCNTs. Annealing at 800°C removed the CO (π* related) functional groups. In contrast the nitrogen bonded graphite-like structure showed improved thermal stability. Angular dependent investigation of the N K edge π* resonances did not show a notable polarization dependent effect. Real time X-ray photoelectron spectroscopic results displayed the indication of a weak interaction of O₂ with the surface of nitrogenated MWCNTs compared to the pristine ones.
10:50 AM		D3-2-8 Finger Prints in Raman Spectra of Structurally Modified Carbon Nanotubes E. Titus, N. Ali, G. Cabral, J.C. Madaleno, J. Gracio (University of Aveiro, Portugal); D.S. Misra (Indian Institute of Technology, India); B.P. Ramesh, W.J. Blau (Trinity College, Ireland) Raman spectroscopy has proven to be an effective tool for the characterization of various carbon structures. This includes their most famous nano-structured form, namely carbon nanotubes (CNTs) (both singlewalled and multiwalled in isolated and bundle form). The Raman spectra of CNTs are unique, distinctive and it significantly varies from those of other forms of carbon due to its reduced dimensionality. This technique is specifically attractive to the nano-materialists due to its outstanding performance in detailed characterization (structural, vibrational and electronic) of CNTs, compared to most other available characterization techniques. In the present work, we have adopted different procedures to modify and functionalize CNT structures. Raman spectra have been used to probe and monitor structural changes among CNTs, synthesized and modified by various techniques. The spectra revealed the origin of certain new bands and identified the same as defects and structural changes in the CNT geometry.
11:10 AM		D3-2-9 Structural, Compositional, Mechanical Properties of Carbon Nanotube Reinforced Zirconia Matrix Nanocomposite Films by Sol-Gel Process R. Phani, M. Passacantando (University of L’Aquila, Italy); S. Cantucci (University of L'Aquila, Italy) Carbon nanotube reinforced zirconium oxide matrix nanocomposite has been synthesized by simple and cost effective sol-gel process on 440°C steel and polished quartz substrates. Homogeneous distribution of carbon nanotubes within the zirconia matrix have been obtained by mixing carbon nanotubes with zirconia sol and followed by condensation into gel formation. The mixed gel, consisting of zirconia and carbon nanotubes has been dried and subjected to conventional calcinations process at different temperatures ranging from 200°C to 1000°C for 1h in argon atmosphere. The deposited and calcined nanocomposites have been characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), energy dispersive X-ray analyzer, X-ray photoelectron spectroscopy techniques for their structural, morphological and elemental composition of the nanocomposites. Nanoindentation tests of the nanocomposites have shown higher hardness compared to the pristine carbon nanotubes and zirconia films due to an enhanced load sharing of the homogeneously distributed carbon nanotubes in the matrix. At the same time, the fracture toughness of carbon reinforced zirconia matrix nanocomposite has also enhanced due to bridging effect of carbon nanotubes during crack propagation.
11:30 AM		D3-2-10 Fullerene-Like CP_x; a First-Principles Study of the Relative Stability of Precursors and Defect Energetics During Synthetic Growth A. Furlan, G.K. Gueorguiev, H.H. Högberg, S. Stafström, L. Hultman (Linköping University, Sweden) Thin solid films of fullerene-like (FL) compounds attract a growing interest due to their mechanical resiliency. The incorporation of N atoms at carbon sites in graphene planes is known to promote pentagon formation and cross-linking in FL-CNx (0x as an interesting material for thin film applications. Being of higher reactivity than N and having a larger covalent radius, P causes stronger bending and higher linkage of the basal planes than for CN_x. This would imply a material with high compliance and low plasticity, eventually stronger than CN_x. Here, we present results from a first-principles calculation study to show that tetragons in layers are energetically favoured, which is not the case for CN_x. Lower content of P in CP_x compared to N in CN_x causes higher layer curvature and more cross- and inter-linkage sites. Significantly larger diversity of stable precursor species also exist for CP_x growth from the vapour phase compared to the CN_x case. Precursors can be both pure P clusters and CnPm (n,m≤3) structures, dominated by tetragons and hexagons. In CP_x pentagons are less favourable than in CNx. At high group 15 -element concentrations in the deposition flux, surface segregation will limit the amount of structurally incorporated P or N. Results from initial CP_x thin film deposition experiments using DC magnetron sputtering in a HV system is also presented. Structural and tribological characteristics of CP_x and CN_x thin films will be compared.