Session Z17 - Studies in Carbon Nanotubes.
ORAL session, Friday midday, March 26
511F, Palais des Congres

[Z17.001] Ab initio study of field emission currents of graphitic ribbons

Using first-principles calculations based on the density functional theory, we have investigated the field emission properties of graphitic ribbons with zigzag and armchair edges. For the armchair and H-terminated armchair ribbons, most of emission currents are from the states near Fermi level, but they are on Łk* band and thus the strongest current component do not come from the zone center. For an array of nano-ribbons, we find that the emission current dependence on lateral space between ribbons for the clean and H-terminated ribbons are rather unconventional, which is traced to their different surface dipoles and local fields. Our study also shows that for these nano-structures, Fowler-Nordheim formula does not always apply, and conventional analysis using Fowler-Nordheim equations can give work functions that deviate substantially from the real work function of the system.

[Z17.002] Phonon Overlaps in Molecular Quantum Dot Systems

We model the amplitudes and frequencies of the vibrational sidebands for the new molecular quantum dot systems. We calculate the Franck-Condon phonon overlaps in the 3N-dimensional configuration sapce. We solve the general case where the vibrational frequencies and eigenmodes change during the transition. We perform PM3 and DFT calculations for the case of the dumb bell-shaped C140 molecule. We find that the strongest amplitudes are associated with the 11 meV stretch mode, in agreement with experiment. The experimental amplitudes vary from molecule to molecule; indicating that the molecular overlaps are environment dependent. We explore overlaps in the presence of external electric fields from image charges and counter ions.

[Z17.003] Linear and nonlinear optical properties of carbon and BN nanotubes from first-principles

Because of their apparent one-dimensional charactor and chirality, nanotubes are expected to exhibit a number of unusual optical properties such as optical activity, circular dichroism and second harmonic generation and to have applications in optical and electro-optical devices. In this contribution, I will present the salient results of our recent systematic ab initio studies of both linear and non-linear optical properties of all three types (armchair, zigzag and chiral) of carbon and BN nanotubes [1]. Specifically, the dielectric function and electron energy loss function as well as the structural and electronic properties of the carbon and BN nanotubes have been calculated and analysized in terms of the characteristic optical transitions in graphite and hexagonal BN, respectively. For the chiral carbon nanotubes, the second-order optical susceptibility functions have also been calculated and are found to be very large, suggesting that the chiral carbon nanotubes could be good materials for optical and electro-optical applications such as second-harmonic generation and sum frequency generation. Both achiral and chiral BN nanotubes can also exhibit nonlinear optical behavior, and this, together with a wide band gap, indicates that the BN nanotubes would be very promising for optical and electro-optical applications.

[1] G.Y. Guo, K.C. Chu, D.-S. Wang and C.-G. Duan, Phys. Rev. B (submitted).

[Z17.004] Hydrogen Storage in Single Walled Carbon Nanotubes: Effects of Nanotube Size and Chirality

There are a number of fundamental questions concerning H2 storage in single walled carbon nanotubes. First, do carbon nanotubes differ significantly from other carbon materials, such as graphite and graphite intercalation compounds, and if they do, in what aspects? If carbon nanotubes are indeed different, what is their adsorption capacity under ambient conditions? Finally, does the adsorption depend on the nanotube architecture and size? Using first-principles based molecular simulation methods, we have performed extensive studies to address these questions. The computational results for several selected systems were first validated by comparison with the available experimental reports in the literature. Subsequently, we systematically examined the effects of nanotube chirality and size on the heat of adsorption for H2. We will present the detailed results of our simulation studies and summarize our findings in this presentation.

[Z17.005] Electronic properties of carbon nanotubes with covalent sidewall functionalization

Covalent sidewall functionalization of SWNT leads to drastic changes of nanotube electronic states near the Fermi level. Using computational methods at different levels (ab initio density functional, extended Huckel, and empirical force field), we have investigated the binding energy, the charge transfer, and the band structure of the functionalized carbon nanotube with different functionalization molecules. The sp3 hybridization between the functional group and nanotube induces a half filled impurity state near the Fermi level. The localization length associated with the impurity state is found to be of few nm, and insensitive to the functional group. While the binding energy and local structural distortion is functional group specific. Systematic difference between metallic and semiconducting tubes will be also discussed. Our finding may have important implications for nanotube-based band structure engineering, nanoelectric devices, and sensor applications.

[Z17.006] Optimization of the electric arc discharge synthesis of single-walled carbon nanotubes on the basis of near-IR spectroscopy

The metal catalyst plays an important role in the synthesis of single-walled carbon nanotubes (SWNT). It was originally reported that nickel and yttrium provide the optimum yield for bulk production of SWNTs by the electric arc discharge technique, at a catalyst composition of 4.0/ at. percent of Ni and 1.0 atomic percent of Y.[1] More recent work suggested a much lower concentration of Ni and Y for optimum SWNT generation: 0.6 atomic percent of Ni and 0.1 atomic percent of Y.[2] Both results were based on TGA, SEM and Raman analysis of milligrams quantities of SWNT soot. Recently we developed a quantitative technique to reliably assess the carbonaceous purity of bulk (10g scale) quantities of SWNTs, based on near-IR spectroscopy.[3] In this presentation we report the utilization of this technique for the determination of the purity of SWNT soot as a function of the catalyst composition. We present the range of optimum concentrations of Ni and Y for the bulk synthesis of the SWNTs.

[1] C.Journet et al., Nature 388 (1997) 756; [2] M.Takizawa et al., Chem. Phys. Lett. 326 (2000) 351; [3] M.E. Itkis et al., Nano Lett. 3 (2003) 309.

[Z17.007] In situ Optical Monitoring of Vertically-Aligned Multiwall Carbon Nanotube Array Growth During Chemical Vapor Deposition

A detailed experimental study of vertically aligned arrays of multi-wall carbon nanotubes (VAA-MWNT) growth by chemical vapor deposition (CVD) based on time-resolved reflectivity (TRR) as a diagnostic to measure and control the length of VAA-MWNTs in situ is performed. Attenuation of a reflected HeNe laser beam and Fabry-Perot fringes are used to measure the length of VAA-MWNT arrays throughout the first 10 microns of growth, providing in situ growth rates and permitting the kinetics and termination of growth to be studied. VAA-MWNT growth was investigated between 530 C and 900 C on Si substrates with evaporated Al/Fe/Mo multiplayer catalysts and acetylene feedstock. It was demonstrated that the growth terminates rapidly at a relatively low (535-600 C) and high (800-900 C) temperatures, showing a relatively narrow temperature window for optimal growth of long VAA-MWNTs (up to 2-4 mm) around 700 C, at a growth rate of about 0.2 - 0.3 microns/s. Nanotube lengths were controlled by rapid evacuation of the chamber. The extinction coefficients of the VAA-MWNTs were studied and correlated with nanotube wall structures. It was demonstrated that decreasing the partial pressure of the C2H2 gas could increase the growth termination length. A simple kinetic model was considered to explain the observed growth kinetics and to discuss the main processes responsible for the growth of VAA-MWNTs.

The authors gratefully acknowledge the funding support from DARPA-DSO, NASA-Langley Research Center, and the U.S. Department of Energy under contract DE-AC05-00OR22725 with the Oak Ridge National Laboratory, managed by UT-Battelle, LLC and the Laboratory-Directed Research and Development Program at ORNL.

[Z17.008] Double-Walled carbon Nanotubes under Compression

We report high pressure Raman scattering studies on double-walled carbon nanotubes produced by thermal annealing of "peapod ropes". The radial breathing modes from both the inner and outer tubes were studied under pressure up to about 7 GPa using 541.5 nm and 647.1 nm excitations. We show that the logarithmic pressure derivative of the radial mode frequency (\Phi) due to the outer tubes follows D2 dependence (D = diameter of the tube) as was previously reported for single-walled nanotube bundles. The value of \Phi measured for the inner tubes, on the other hand, agrees with the theoretical predictions for isolated tubes. These observations indicate that only the outer tube walls deform under compression and the inner tubes are well shielded. Furthermore, for pressures above 3 GPa, a well-defined sharp multiplet structure is observed for the radial modes of inner tubes with D ~ 0.64 nm when excited with 647.1 nm laser. The origin and the pressure dependence of the peaks comprising the multiplet will be discussed. This work is supported by DMR-0104706 and U. Penn. MRSEC grants from NSF.

[Z17.009] Carbon Nanotubes Embedded in Oriented Polymer Nanofibers by Electrospinning

The electrospinning process was used successfully to fabricate nanofibers of poly(ethylene oxide) [PEO] in which carbon nanotubes, either multi-walled (MWCNT) or single-walled (SWCNT) are embedded. MWCNTs were dispersed in water using SDS or Gum Arabic - a highly branched polyelectrolyte. Aqueous dispersion of SWCNT's was achieved using an alternating copolymer of styrene and maleic anhydride, hydrolyzed with NaOH. The focus of this work is on the development of axial orientations in the multi-component nanofibers. The degree of orientation of polymers, surfactants and nanotubes was studied using X-ray diffraction and transmission electron microscopy. Individual nanotubes were successfully embedded in the polymer nanofibers with good axial alignment. A high degree of alignment of PEO crystals and SDS layers was also found in the electrospun nanofibers containing SWCNT's. Oriented ropes of the nanofibers were fabricated in a converging electric field by a rotating disc with a tapered edge. These results can lead to further usage of the nanofibers with embedded carbon nanotubes in applications such as nano-scale energy storage devices.

[Z17.010] Dispersion Dependent conductivity of Single Wall Carbon Nanotubes in Epoxy Composites

Aggregation and rope formation of single wall carbon nanotubes (SWNTs) play a key role in determining the electrical properties of SWNT-composite systems. We have developed a procedure for incorporating SWNTs into an epoxy matrix that allows us to control the dispersion of nanotubes from homogeneous to diffuse agglomerates to dense aggregates. Direct current conductivity measurements and images from optical and scanning electron microscopes show that if the nanotubes are allowed to form agglomerates/aggregates in the epoxy matrix, then the nanotubes form conductive network at a very low concentration (~0.006 vol%). On the other hand, a homogeneous dispersion of nanotubes in the epoxy matrix leads to a higher percolation threshold (0.01 vol%). We will discuss the percolation behavior of nanotubes with existing models of non-interacting and interacting rods. This work has been partially supported by the NSF through the MRSEC grant DMR 00-79909, DMR-0203378, by DARPA/ONR through N00014-01-1-0831, and by NASA through grant NAG8-2172.

[Z17.011] Experimental Evidence for a New Understanding of the Tunneling Conductance in Multi-Wall Nanotubes

We report on experiments that provide motivation for a new description of the tunneling conductance (G) in multiwall nanotubes (MWNT). Previous experiments suggest that the zero-bias conductance has a power law dependence on temperature and bias, in agreement with the Luttinger liquid (LL) model. While our results generally concur with those in the literature, they also reveal a number of unexplained features: the conductance minimum, Gmin, does not necessarily occur at zero bias, G is asymmetric with bias, and there is a deviation from the power law dependence at low temperatures. In addition, Gmin has an unusual B-field dependence. For B parallel to the MWNT axis, Gmin remains constant, while for B perpendicular, Gmin shifts with increasing B. These features, as well as those described previously, can be explained in terms of inter-shell interaction in the MWNT, without the need for incorporating electron-electron interactions. Supported by the NSF (DMR-0112824 and ECS-0224114), the DoE (DE-FG02-00ER45832), and NASA (NCC5-571).

[Z17.012] Photoconductivity of Single-Wall Carbon Nanotube Films

To date, there have been few reported measurements of the photoconductivity of carbon nantoube films due to the difficulty in obtaining a measurable signal using standard excitation sources. This is despite the fact that the absorption spectrum of carbon nanotubes films is widely available. Here, we describe measurements of the photoconductivity (PC) of single wall nanotube (SWNT) films using a tunable fs-pulsed laser system. Three clear peaks are observed in the PC near 0.6, 1,2 and 1.9 eV corresponding to the SWNT S_11, S_22, and M_11 transitions. These peaks are demonstrated to correlate directly with the three commonly observed peaks in the SWNT absorption spectrum. The PC is observed to increase step-wise as a function of increasing power, while the temperature dependence is non-monotonic, with a minimum in PC observed at around 75K. These behaviors are thought to be due to the presence of non-conductive trap states in the nanotube energy spectrum.

[Z17.013] Modelling carbon nanotubes with balance equations

We use the balance equation approach together with the boosted statistical operator for describing steady-state and time-dependent (transient) quantum transport in mesoscopic structures to model carbon nanotubes. The balance equations for momentum and energy are derived within the framework of the tight-binding approximation and we show that the quantum mechanical balance equations for momentum and energy are suited for analyzing the local and global transport properties of carbon nanotubes.

Part Z of program listing