### Session A30: Focus Session: Carbon Nanotubes: Chirality-Controlled Growth of Carbon Nanotubes and Nanostructures

 Monday, March 15, 2010 8:00AM - 8:36AM A30.00001: The role of composition-dependent catalyst structure on chirality distributions of as-grown SWCNTs Invited Speaker: R. Mohan Sankaran The exemplary physiochemical properties of single-walled carbon nanotubes (SWCNTs) are largely determined by their diameter and chiral angle, specified as chiral indices (n,m). Despite the significant progress that has been made in purifying mixtures of SWCNTs to obtain monodisperse samples (in terms of chirality), large-scale applications require improvements in the homogeneity of as-grown SWCNTs. Recent reports have shown that a potential route to controlling the chirality distribution of SWCNTs during growth is through catalyst design [1,2]. In this talk, we present our approach which is based on tuning the catalyst composition, independent of size, to perturb the catalyst structure and ultimately influence the SWCNT chirality. 1. Harutyunyan et al., Science 326, 116 (2009). 2. Chiang et al., Nature Mater. 8, 882 (2009). Monday, March 15, 2010 8:36AM - 9:12AM A30.00002: On the Preferential Growth of Metallic Single-Walled Carbon Nanotubes Thin Films Invited Speaker: Avetik Harutyunyan The lack of reasonably homogeneous single walled carbon nanotube (SWCNT) materials hinders their ubiquitous applications. There have been significant achievements in separating SWCNTs according to their conductivity and in enriching the distribution of nanotubes with a specific conductivity. Meanwhile, despite studies regarding direct control over carbon nanotube structure during growth, there is only a limited understanding of exactly what determines carbon nanotube chirality during catalytic growth and, thereby, the electronic structure of grown SWCNT. Here we report the results of the studies of growth of SWCNTs thin films from Fe nanocatalysts deposited onto a SiO$_{2}$/Si support and in situ annealed in a He or Ar ambient that contains various ratios of H$_{2}$ and H$_{2}$O. Our investigations reveal that the variation of the noble gas ambient during thermal conditioning of the catalyst, in combination with oxidative and reductive species, alters the fraction of tubes with metallic conductivity from about 20{\%} of the population to a maximum of 91{\%}. The tubes have been identified based on Raman, photoluminescence and electrical (field effect transistor performance) characterizations by using special prepared reference sample. In situ environmental transmission electron microscopy observations of the SiO$_{2}$ supported Fe nanocatalysts in H$_{2}$O, H$_{2}$/H$_{2}$O, Ar/H$_{2}$O and He/H$_{2}$O gaseous environments reveal that presence of Ar in the ambient leads to significant coarsening of nanocatalysts with rounded surface morphology, while under He ambient the nanocatalyst is more faceted. Various scenarios such as adsorption and roughening induced morphology rearrangements of the catalyst particles and their relationships with grown tubes electronic structures will be presented. Monday, March 15, 2010 9:12AM - 9:48AM A30.00003: Chirality selection during Catalytic Nucleation of Carbon Nanotubes Invited Speaker: John Robertson We study the possibility of chirality selection during the catalytic chemical vapor deposition (CVD) of a Single-Walled Carbon Nanotubes (SWNT) by density functional simulations. When nanotubes grow by root growth, the tube's chirality is fixed at nucleation by the chirality of the initial cap that nucleates on the catalyst cluster surface. We previously studied the energies of caps of different chiralities on a flat, fixed Ni(111) layer [1]. We now consider different caps binding to various planes and apices of a 55 atom cluster of Fe and Ni. Firstly, we find that the caps bind more strongly to these nucleation sites than to the flat (111) layer. A second point is that the cap causes considerable distortion of the metal cluster, while the caps themselves are relatively undistorted. This is because C-C bonds are stronger than metal-metal bonds. Of the three high symmetry nucleation sites on the Fe cluster, the binding of (6,5) and (7,5) caps to the pyramidal corner is 2 eV per cap stronger on average than on other sites. Binding energies also depend on the cap diameter, this explains why the pyramid site is less stable for certain caps. For the Ni cluster, there is a strong preference to the zigzag and chiral caps; binding energies for these two caps are generally greater than for armchair caps by 2.5 eV per cap. Lastly, there is strong segregation of carbon atoms to the outside of the cluster, with little dissolution into cluster. \\[4pt] [1] S Reich, L Li, J Robertson, Chem Phys Lett 421 469 (2006) Monday, March 15, 2010 9:48AM - 10:00AM A30.00004: Investigation of Nanotube Growth Kinetics via In-Situ Spectroscopy Rahul Rao , David Liptak , Roberto Acosta , Benji Maruyama Analysis of single-walled carbon nanotubes (SWNTs) during growth via Raman and fluorescence spectroscopy offers a unique approach to understand and control their growth, which remains challenging due to a large variability of synthesis parameters. In our technique individual SWNTs are grown via chemical vapor deposition inside an environmental cell coupled to an automated stage on a light microscope. Laser-induced growth occurs from catalyst nanoparticles on thermally isolated islands within substrates where the micro-Raman excitation laser also serves as the localized heat source. Computer control over substrate temperature, position, feed gas composition, and chamber pressure enable rapid real-time exploration of SWNT growth parameter space. Micro-Raman spectra are collected in-situ from the low frequency radial breathing mode and D/G band regions of the growing SWNTs. The evolution of the G band of individual SWNTs over time is modeled with an exponential growth equation and indicates rapid time constants and consequently short catalyst lifetimes. Comparison of SWNT growth kinetics from various catalyst particles will be presented and implications for controlled growth of SWNTs will be discussed. Monday, March 15, 2010 10:00AM - 10:12AM A30.00005: The effect of gaseous ambients on catalytic nanoparticles and the impact on nanotube chirality selection Seung Min Kim , Dmitri N. Zakharov , Avetik R. Hartyunyan , Eric A. Stach We have shown that single-wall carbon nanotubes (SWNT) with metallic conductivity can be preferentially grown (with 91\% population) by varying the nobel gas environment (He / Ar) during thermal annealing of the Fe catalysts used to mediate nanotube growth, and in combination with residual pressures of hydrogen and water. Real time transmission electron microscopy observations of Fe nanoparticles show that the catalysts are strongly facetted in the presence of He ambient, but rounded in Ar ambients. Additionally, the presence of different ambients and different thermal annealing durations prior to nanotube nucleation leads to different populations of semiconducting and metallic nanotubes. These observations indicate that both the shape and the size distribution of the catalysts strongly affect the resulting nanotube chirality. It is well known that the shape of the particles also affects the coarsening behavior: it is crucial to fully understand how the affect of different ambients modifies coarsening behavior. We will describe in-situ annealing experiments conducted in an environmental-cell transmission electron microscope. These observations will be compared with recent coarsening models that describe how coarsening proceeds differently in particles with different equilibrium shapes. Monday, March 15, 2010 10:12AM - 10:24AM A30.00006: Analysis of Diamond Nanocrystal Formation from Multi-walled Carbon Nanotubes Andre Muniz , Tejinder Singh , Dimitrios Maroudas A systematic analysis is presented of the nanocrystalline structures generated due to the inter-shell C-C bonding between adjacent concentric graphene walls of multi-walled carbon nanotubes (MWCNTs). The analysis combines a comprehensive exploration of the parameter space determined by the geometrical characteristics of the individual graphene walls comprising the MWCNTs with density functional theory calculations of inter-shell C-C bonding and molecular-dynamics relaxation of the resulting nanocrystalline structures. We find that these structures can provide seeds for the nucleation of the cubic-diamond and hexagonal-diamond phase in the form of nanocrystals embedded in the MWCNTs. The resulting lattice structure is determined by the relative alignment of adjacent graphene walls in the MWCNT. These crystalline phases are formed over the broadest range of nanotube diameters and for any possible combination of chirality of graphene walls. The generated nanocrystal size is determined by the chiral-angle difference between adjacent graphene walls in the MWCNT. Monday, March 15, 2010 10:24AM - 10:36AM A30.00007: Extreme synthesis and chemical doping of diamond aerogel Peter J. Pauzauskie , Jonathan C. Crowhurst , Marcus A. Worsley , Ted A. Laurence , Yinmin Wang , A.L.D. Kilcoyne , Peter K. Weber , Trevor M. Willey , Kenneth S. Visbeck , William J. Evans , Joe H. Satcher, Jr. Amorphous carbon aerogels have attracted much interest in recent years due to their low density, large intrinsic surface areas ($>$1000 m$^{2}$/g), large pore volume, low dielectric constant, and high strength. We use high-pressure ($\sim$20 GPa) laser-heating ($>$600\r{ }C) within a diamond anvil cell (DAC) to convert the amorphous network of a low-density (40mg/cc) carbon aerogel into an ultrananocrystalline diamond aerogel. Photoluminescence spectroscopy and confocal time-correlated single-photon counting indicate the recovered material contains both negatively-charged and neutral nitrogen-vacancy (NV) complexes. Synchrotron scanning transmission x-ray microscopy (STXM) is used to compare the carbon electronic density-of-states of the amorphous starting material with the recovered diamond aerogel with $\sim$100 meV energy resolution. Finally, we use nanoscale secondary ion mass spectrometry to investigate doping of the resorcinol-formaldehyde starting material with the aim of chemically tuning heteroatomic point defects within this diamond material system. Monday, March 15, 2010 10:36AM - 10:48AM A30.00008: Designing rigid carbon foams Sora Park , Jeung-Sun Ahn , Young-Kyun Kwon , Kritsada Kittimanapun , David Tom\'anek We use {\em ab initio} density functional calculations to study the stability, elastic properties and electronic structure of contiguous $sp^2$ carbon networks with negative Gaussian curvature, called schwarzites. We find these structures, which may be thought of as dense arrays of inter-connected nanotubes, unusually rigid. We focus on two systems with cubic unit cells containing between 152 and 200 carbon atoms, which we find metallic. The contiguous internal graphitic surface of schwarzites subdivides space into two distinct subspaces. Due to the ease of atomic diffusion, schwarzites appear as a useful alternative to graphite electrodes for a new generation of Li-ion batteries. Since the concentration of electron donors or acceptors can be modified reversibly, the doping level of the schwarzite may be tuned in order to optimize specific reactions, including H$_2$ storage, in the other subspace. The other subspace can alternatively accommodate a duplicate schwarzite structure. The two interpenetrating, but separated structures can be used as electrodes of the ultimate super-capacitor. Monday, March 15, 2010 10:48AM - 11:00AM A30.00009: Oxidized dahlia-like carbon nanohorns: adsorption of argon, methane and nitrogen Vaiva Krungleviciute , Aldo Migone , Masako Yudasaka , Sumio Iijima Different types of nanohorns, and nanohorn aggregates, can be produced depending on the parameters used during their production. We used a sample consisting mainly of dahlia-like nanohorn aggregates. In the dahlia-like aggregates the individual nanohorns are aligned radially, forming a sphere, with their individual ends protruding from the outer surface of the spherule. Our sample was oxidized by flowing pure oxygen. This treatment opened entry ports into the spaces at the interior of the individual nanohorns. We measured volumetric adsorption isotherms at 77.3 K with argon, methane, and nitrogen. For argon and methane two distinct substeps are present in the adsorption data; the steps for each of these two gases are comparable in size. For nitrogen there is a clear step present at lower adsorbent loadings, and a more gradual growth in the amount adsorbed as saturation is approached.