Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session S27: Focus Session: Carbon Nanotubes: Theory |
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Sponsoring Units: DMP Chair: Mark Pederson, NRL Room: LACC 501C |
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S27.00001: Curvature effects in carbon nanofibers Mina Yoon, Jane Howe, Zhenyu Zhang We apply {\em ab initio} density density functional formalism and also use empirical van der Waals potential to study equilibrium interlayer spacings in multi-wall carbon nanotubes and carbon nanofibers. We find that the generally-accepted empirical potential describing van der Waals interaction cannot explain the experimental observation of the strong curvature effects on interlayer spacings between neighboring graphite sheets. A modified van der Waals potential including curvature effects is constructed based on the experimental observations and theoretical calculations. Furthermore, the possible mechanism on increase or decrease of interlayer spacing in carbon nanofibers will be compared with experimental data. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S27.00002: Bundling up carbon nanotubes through Wigner defects Ant\^onio J. R. da Silva, Adalberto Fazzio, Alex Antonelli We show, using {\it ab initio} total energy density functional theory, that the so-called Wigner defects, an interstitial carbon atom right besides a vacancy, which are present in irradiated graphite can also exist in bundles of carbon nanotubes. Due to the geometrical structure of a nanotube, however, this defect has a rather low formation energy, lower than the vacancy itself, suggesting that it may be one of the most important defects that are created after electron or ion irradiation. Moreover, they form a strong link between the nanotubes in bundles, increasing their shear modulus by a sizeable amount, clearly indicating its importance for the mechanical properties of nanotube bundles. [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S27.00003: Molecular Dynamics Simulation of Nucleation Process of SWNT from a Metal Particle on a Substrate Yasushi Shibuta, Shigeo Maruyama Nucleation process of single-walled carbon nanotubes (SWNTs) from a transition metal cluster on a substrate is studied using classical molecular dynamics (MD) simulations. For describing the effect of the substrate, averaged one-dimensional Lennard-Jones potential is employed between the metal cluster and the bottom boundary of the simulation cell. As the initial condition, a Ni500 cluster is placed on the bottom boundary of the cubic cell of 20 nm. The number of carbon atoms is adjusted to achieve the constant density by adding a new carbon atom to the cell when the metal cluster dissolves a carbon atom. As the metal cluster dissolves carbon atoms, the cluster becomes more wetting to the substrate. This may be due to the different wettability between pure metal and metal-carbide. Graphite structure gradually precipitates from the edge of the cluster. Nucleation mechanism of SWNTs will be discussed by comparing with the simulation using the floated catalyst. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S27.00004: Self-Healing of Divacancies in Carbon Nanotubes. Savas Berber, Atsushi Oshiyama We investigate the occurrence and reconstruction of divacancies in carbon nanotubes, and the electronic structure modification by defects using {\em ab initio} Density Functional calculations. Structure optimization calculations on both arm-chair and zig-zag defective nanotubes with diameters ranging from $4$~{\AA} to $9$~{\AA} reveals self-healing ability of defective nanotubes. Nanotube with ideal divacancy transforms into more stable structure by concerted formation of $2$ new bonds, leaving no unsaturated bonds. Diameter dependence of reconstruction energy and formation energy of divacancies depend on the mechanism of strain distribution for particular orientation of the defect. Divacancy formation is a possible way to stabilize the tubes with high concentration of monovacancies. Band structure of relaxed defective tubes indicate that metallic tubes mostly keep their metallic character while semiconducting tubes may acquire metallic character due to appearance of additional energy levels inside band gap of perfect tube due to new bonds formed during healing process. Five membered rings in relaxed structure can be recognized by a raised profile in simulated Scanning Tunneling Microscope (STM) images, providing a detection tool. In addition, we explore thermal stability of defective nanotubes by elevating the temperature in Nos\'e-Hoover molecular dynamics simulations using a parametrized electronic Hamiltonian. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S27.00005: Effect of intertube bond formation on mechanical and vibrational properties of nanotube ropes Andriy H. Nevidomskyy, G\'abor Cs\'anyi, Mike C. Payne Based on first-principles electronic structure calculations, we predict the formation of chemical bonds between boron nitride (BN) nanotubes, if doped with carbon. Similarly, intertube bonds are predicted to form in ropes of Nitrogen-doped carbon nanotubes. The effect of such bonds on the mechanical properties of carbon and BN nanotube ropes has been analysed, from which we find that the shear modulus of a nanotube rope appears to be greatly enhanced by the presence of intertube bonds. The analysis of phonon vibration spectra of BN nanotube ropes shows the up-shift in the frequency of the radial breathing mode due to the intertube bridging. These findings open prospects both for experimental detection of intertube bonds and possible technological applications. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 4:06PM |
S27.00006: Phonon Scattering and Excitons in Carbon Nanotubes. Invited Speaker: Due to their low dimensionality, carbon nanotubes (CNTs) have striking properties, quite different from these of traditional semiconductors, with important implications for technology. The performance of electronic devices relies on carrier mobility, which is extraordinary high in CNTs at low fields. However, at high fields the mobility is dramatically reduced due to inelastic optical phonon scattering. Optical properties of CNTs, essential for electro-optical devices, are dominated by excitons with binding energies and oscillator strengths orders of magnitude larger than those in conventional semiconductors. We calculate the electron-phonon scattering and binding in CNTs, within a tight binding model [1]. We derive the mobility as a function of temperature, electric field, and nanotube chirality using a multi-band Boltzmann treatment. We find the drift velocity saturates at approximately half the graphene Fermi velocity. Polaronic binding give a band-gap renormalization of 70 meV, an order of magnitude larger than previously suggested. We calculate the properties of excitons in CNTs embedded in a dielectric, for a wide range of tube radii and dielectric environments, by solving the Bethe-Salpeter Equation in a tight binding basis. We find that simple scaling relationships give a good description of the binding energy, exciton size, and oscillator strength as a function the tube radius, the dielectric constant of the embedding material, and the chirality [2]. In addition we calculate optical absorption including the exciton-phonon interaction. We find a phonon sideband at 200 meV above the zero phonon line, due to the creation of exciton plus one optical phonon [3]. [1] V. Perebeinos, J. Tersoff, and Ph. Avouris, cond-mat/0411021. [2] V. Perebeinos, J. Tersoff, and Ph. Avouris, Phys. Rev. Lett. 92, 257402 (2004). [3] V. Perebeinos, J. Tersoff, and Ph. Avouris, cond-mat/0411618. [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S27.00007: Thermal conductivity of single-walled carbon nanotubes with $^{13}$C isotopes Junichiro Shiomi, Shigeo Maruyama Molecular dynamics simulations were performed to investigate the influence of impurities on thermal conductivity (k) of (5,5)-SWNTs. The impurities were represented by mixing $^{13}$C isotopes to a $^ {12}$C-SWNT. Random mixing of $^{13}$C isotopes to $^{12}$C- SWNTs results in decrease of k. The results show not only that k decreases against the fraction of mixed $^{13}$C, but also that k is dependent on the structure of $^{13}$C clusters, seemingly on their sizes. In order to highlight the influence of axial scales of the impurities, we consider SWNTs which consist of $^ {12}$C and $^{13}$C periodically connected with certain interval thickness. The result shows that there is a critical interval thickness which gives the minimum value of k. Spectral analyses reveal the role of phonon modes. Adopting the phantom heat bath model to each end of a SWNT, k can be computed through the Fourier's law. Non-Fourier aspects of the heat transfer in the non-equilibrium SWNTs are also examined by applying a local heat pulse with duration ranging from 40 fs to 4 ps. The results of the simulations exhibit the heat waves of selected phonons traveling from the heated end of the SWNT towards the other. The characteristic properties of the heat flow will be discussed. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S27.00008: Structural, vibrational and thermodynamical properties of carbon allotropes from first-principles: from graphite to nanotubes Nicolas Mounet, Nicola Marzari We studied the ground state and finite-temperature properties of carbon allotropes using a combination of density-functional theory and density-functional perturbation theory. Equilibrium structures, elastic constants, and phonon dispersions of bulk diamond, graphene, graphite, and zigzag and armchair nanotubes are first obtained at the DFT-PBE level, showing on average an excellent agreement with experiments, with the caveat that for the case of graphite the experimental c/a ratio must be used. Thermal expansion coefficients are then determined from the minimization of the vibrational free energy in the quasi-harmonic approximation. Other thermodynamical properties such as heat capacities and the temperature dependence of the elastic constants are also obtained. Finally, the role of different phonon modes on the graphite, graphene, and nanotube thermal expansion or contraction is discussed, together with a full determination of their Gr\"{u}neisen parameters. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S27.00009: Quantum transport through short semiconducting nanotubes: A complex band structure analysis Pawel Pomorski, Christopher Martin Roland, Hong Guo Within an ab initio nonequilibrium Green's function formalism, we have examined the problem of quantum transport through short, semiconducting nanotube devices contacted with Al electrodes. Metallic behavior is predicted for very short nanotubes, which crosses over to semiconducting behavior as the tube length is increased. This behavior finds its origins in the evanescent modes that are present in these finite-sized systems, which cannot be ignored. A complex band structure analysis makes the contributions of these modes particularly transparent. Our calculation also allowed us to study the Schottky barrier formed between the nanotubes and Al contacts. We were also able to study the configuration where the whole system is in close proximity to a metal gate with some gate voltage, as is usually the case in experiment. Our computational method was able to handle metal gate boundary conditions and also implemented a numerical acceleration based on taking advantage of symmetry. References: Pawel Pomorski {\em et al.}, {\em Phys. Rev. B} {\bf 70}, 115408 (2004), Pawel Pomorski {\em et al.}, {\em Phys. Rev. B} {\bf 69}, 115418 (2004). [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S27.00010: Structural ordering in nanotube polymer composites Chenyu Wei, Deepak Srivastava, K.J. Cho The structural and mechanical properties of polymeric carbon nanotube (CNT) composites have been studied through molecular dynamics simulations. Polymer molecules (polyethylene in this study) have been found to form layer structures around the nanotube with oscillating features and with orientations aligned with the tube axis. The increase of the structural order parameter of orientations {\$}$<$S{\_}Z$>${\$} is shown to contribute to the enhancement of mechanical modulus of CNT based composites. {\$}$<$S{\_}Z$>${\$} is found to increase with applied strains, and the corresponding structural change of the composite is shown to be inelastic, which is not fully recovered upon removal of the applied strains. [Preview Abstract] |
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S27.00011: Atomistic simulation of the laser fragmentation of single wall carbon nanotubes Harald Jeschke, Aldo Romero, Martin Garcia, Angel Rubio Femtosecond laser induced structural changes in single wall carbon nanotubes (SWNTs) are investigated by extensive molecular dynamics simulations on time dependent potential energy surfaces. The damage threshold of the SWNT is shown to depend on the chirality of the tube, on its diameter as well as on the laser pulse parameters. For the studied laser parameters, zigzag SWNTs are shown to be more stable with respect to laser excitation than armchair SWNTs. The diameter dependence of the thresholds for structural modification turns out to be nonmonotonic, suggesting the possibility of selectively evaporating SWNTs from nonhomogeneous tube bundles. [Preview Abstract] |
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S27.00012: C60 Polymerization in BN and Carbon Nanopeapods Andrea Trave, Filipe J. Ribeiro, Steven G. Louie, Marvin H. Cohen A variety of C$_{60}$ structures, polymerized and non-polymerized, isolated or encapsulated in BN or carbon nanotubes, have been analysed, to gain insight into the atomic and electronic structure of nanopeapods (NPP). Isolated double-bonded C$_{60}$ chains and dimers are found to be more stable than non-polymerized C$_{60}$. NPP geometries and energetics are unaffected by encapsulation, which gives an energy gain larger than the activation energy for C$_{60}$ polymerization, supporting the hypothesis of partial polymerization of C$_{60}$ molecules in NPP's. Upon encapsulation, BN NPP's remain wide-gap semiconductors, while in metallic carbon NPP's the lowest unoccupied C$_{60}$ states lie just above the Fermi level and charge transfert can take place, stabilizing single-bonded C$_{60}$ chains with wider spacing than double-bonded polymers, closely corresponding to the experimental structural observations. This work was supported by NSF (Grant No.DMR-0087088), and by the Office of Energy Research, Office of Basic Energy Sources, Materials Sciences Division of the US Department of Energy (Contract No. DE-AC03-76SF00098). Part of this work was performed under the auspices of the US Department of Energy by the University of California at the LLNL (Contract No.W-7405-Eng-48). Computational resources at NERSC, NCSA, and NPACI are acknowledged. [Preview Abstract] |
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S27.00013: Gate-controlled Tomonaga-Luttinger liquid in peapods Junji Haruyama, Jun Mizubayashi, Toshiya Okazaki, Takeshi Nakanishi, Hisanori Shinohara, Yuji Awano, Naoki Harada We report the charge transport properties of peapods encapsulating a chain of C60 within a field-effect transistor (FET) structure. We find they are very sensitive to the back gate voltage applied and power laws with large power exponents (between 12 and 1), findings not observed before in carbon nanotubes. Based on atomic-like behaviors observed in the single charging effect, we reveal that part of the power laws (between 3 and 1) arises from a Tomonaga-Luttinger liquid with strong electron-electron interaction, which originates from the large number of occupied subbands due to our peapod's unique electronic states. [Preview Abstract] |
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S27.00014: Analysis of band-gap formation in squashed arm-chair CNT H. Mehrez, A. Svizhenko, M.P. Anantram, M. Elstner, T. Frauenheim The electronic properties of squashed arm-chair carbon nanotubes are modeled using constraint free density functional tight binding molecular dynamics simulations. Independent from CNT diameter, squashing path can be divided into three phases. In the first phase of squashing, the nanotube deforms with negligible force. In the second phase, there is significantly more resistance to squashing with the force being ~40-100nN/per CNT unit cell, and in the last phase the nanotube loses its hexagonal configuration. We compute the change in band- gap as a function of squashing and our main results are: (i) A band-gap initially opens due to interaction between atoms at the top and bottom sides of CNT. The Pi-orbital approximation is successful in modeling the band-gap opening at this stage. (ii) In the second phase of squashing, large Pi-Pi* interaction at the edges becomes important, which can lead to band-gap oscillation. (iii) Contrary to a common perception, nanotubes with broken mirror symmetry can have zero band-gap. (iv) All arm-chair nanotubes become metallic in the third phase of squashing. Finally, we discuss both differences and similarities obtained from the tight binding and density functional approaches. [Preview Abstract] |
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