Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session J26: Focus Session: Computational Nanoscience IV: Carbon- and Silicon-based Nanostructures |
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Sponsoring Units: DMP DCOMP Chair: Sohrab Ismail-Beigi, Yale University Room: 328 |
Tuesday, March 17, 2009 11:15AM - 11:51AM |
J26.00001: New Routes Toward Nanotube Synthesis: Computation and Experiment Invited Speaker: The nanotechnology of the future demands controlled and consistent fabrication of different classes of nanostructures. Computational nanoscience can play an important role in the development of novel nanofabrication techniques. By revealing the fundamental differences in the mechanical bending behavior of nanofilms from that of micro- and macro-films, we have carried out atomistic simulations making significant contributions to advance a novel nanofabrication approach, the so-called ``nanomechanical architecture'' of thin films. This approach allows fabrication of different types of nanostructures, with a high level of control over their size and shape based on a priori theoretical/computational designs. The simulations have revealed a self-bending mechanism of Si (Ge) nanofilms leading to formation of pure Si (Ge) nanotubes, which greatly broadens the repertoire of nanotubes that can be made from multilayer films. Furthermore, applying the principle of nanomechanical architecture to the extreme case of the thinnest film possible, a single atomic layer of patterned graphene sheet, a new method for synthesizing carbon nanotubes with an unprecedented control over their size and chirality was proposed. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J26.00002: Thermal Properties of Vertically Aligned Carbon Nanotube-Nanocomposites Boundary Resistance and Inter- Carbon Nanotube Contact: Experiments and Modeling Hai Duong, N. Yamamoto, M. Panzer, A. Marconnet, K. Goodson, D. Papavassiliou, S. Maruyama, B. Wardle It is very significant to experimentally and numerically examine thermal properties of aligned CNT polymeric nanocomposites (PNCs) with variable volume fraction (vol{\%}) and controlled morphology. MWNTs having 200-1000um length synthesized by CVD are densified mechanically to achieve 1-20vol{\%}~Thermal conductivities of MWNT-epoxy along the MWNT axis with different vol{\%} are measured by the temperature gradient with an infrared microscope. The developed random walk model with taking into account the thermal boundary resistance (TBR) at the CNT-epoxy and/or CNT-CNT interface is validated by experimental results The different vol{\%} and CNT aligned in PNCs allows extracting the TBR values between MWNTs and epoxy or even between MWNTs by combining the developed model and experiment results. Further numerical investigation is conducted to compare systematically the thermal conductivity of both MWNT- and SWNT-epoxy with different CNT orientations under the wide range of the CNT vol{\%} with different TBR between the CNTs and the CNT-matrix and various inter-CNT contact degrees. [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J26.00003: Thermal activation of interlayer bonding and its effect on properties of multiwalled carbon nanotubes Chun Tang, Wanlin Guo, Changfeng Chen We report molecular dynamics simulations of multiwalled carbon nanotubes (MWCNTs) at high temperatures. Our results show that thermally activated interlayer bonding have significant influence on structural, mechanical and electronic properties of MWCNTs and lead to new behaviors with implications for their applications. We examine the effect of strain and temperature conditions on the formation of interlayer bonding in MWCNTs and unveil the underlying atomistic mechanisms. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J26.00004: Effective Hamiltonian approach to bright and dark excitons in single-walled carbon nanotubes Sangkook Choi, Jack Deslippe, Steven G. Louie Recently, excitons in single-walled carbon nanotubes (SWCNTs) have generated great research interest due to the large binding energies and unique screening properties associated with one-dimensional (1D) materials. Considerable progress in their theoretical understanding has been achieved by studies employing the ab initio GW-Bethe-Salpeter equation methodology. For example, the presence of bright and dark excitons with binding energies of a large fraction of an eV has been predicted and subsequently verified by experiment. Some of these results have also been quantitatively reproduced by recent model calculations using a spatially dependent screened Coulomb interaction between the excited electron and hole, an approach that would be useful for studying large diameter and chiral nanotubes with many atoms per unit cell. However, this previous model neglects the degeneracy of the band states and hence the dark excitons. We present an extension of this exciton model for the SWCNT, incorporating the screened Coulomb interaction as well as state degeneracy, to understand and compute the characteristics of the bright and dark excitons, such as the bright and dark level splittings. Supported by NSF \#DMR07-05941, DOE \#De-AC02-05CH11231 and computational resources from Teragrid and NERSC. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J26.00005: First-principles studies of the optical properties of carbon nanohoops Joydeep Bhattacharjee, Jeffrey B. Neaton First proposed 70 years ago, cycloparaphenylenes -- cyclic aromatic molecules that are the shortest possible segment of an armchair nanotube -- have been only recently synthesized [1]. Using first-principles density functional theory and a Bethe-Salpter equation approach, we study structural, electronic, and optical properties of this novel class of materials, coined ``carbon nanohoops.'' Remarkably, we find, in agreement with experiments, that smaller hoops have smaller optical absorption gaps. This counterintuitive trend, opposite to that expected from ordinary quantum confinement, reflects a large increase in electron-hole interaction strength with decreasing hoop diameter. The diameter dependence of this interaction is thoroughly explored for several nanohoops, compared with an acyclic series, and discussed in the context of possible applications. [1] R. Jasti, J. Bhattacharjee, J. B. Neaton, and C. R.Bertozzi, submitted (2008). [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J26.00006: First-principles study of methane adsorption on defective graphitic nanostructures Brandon Wood, Debosruti Dutta, Ganapathy Ayappa, Shobhana Narasimhan Efficient storage of methane represents a significant challenge to large-scale implementation of natural gas-based consumer transportation. Activated carbons and related carbon-based nanoporous structures have garnered tremendous interest as storage media due to their unusually high absorptive capacities. However, systematic improvement of these materials relies on a fundamental understanding of the physical and chemical processes involved. We present here extensive energetic calculations of methane adsorption in model carbonaceous systems using density-functional techniques. As exact microstructures of activated carbons are difficult to obtain, we have attempted to isolate likely model nanostructures and defects, including surfaces, edges, point defects, and chemical functionalization. For each of these cases, we analyze changes in the structural, magnetic, and electronic properties upon adsorption. The defect structures exhibiting strongest methane adsorption are isolated, and the relevant mechanisms dominant in binding are identified. The impact of our results in terms of increasing methane absorptive capacity in activated carbons is discussed. [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J26.00007: Effects of hydrogen chemisorption on the structure of carbon nanotubes Andre Muniz, Tejinder Singh, Dimitrios Maroudas We report results of a computational atomic-scale analysis of the effects of atomic hydrogen chemisorption on the structure of single-walled and multi-walled carbon nanotubes (SWCNTs and MWCNTs). The analysis combines classical molecular-dynamics simulations with first-principles density functional theory calculations. We find that H chemisorption induces structural changes in SWCNTs associated with \textit{sp}$^{2}$-to-\textit{sp}$^{3}$ bonding transitions; increasing the H coverage beyond a critical level leads to axial and radial expansion of the SWCNTs that increases monotonically with H coverage. We also investigated the possibility of H-induced inter-shell \textit{sp}$^{3}$ C--C bond formation in MWCNTs. We find several pathways that lead to stable inter-shell bonded structures, which can act as seeds for nucleation of various crystalline carbon phases embedded into the MWCNTs. Finally, we show how the chiralities and relative alignments of adjacent graphene walls in MWCNTs determine the resulting crystalline structures. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J26.00008: First principles study of NH$_{3}$ adsorption on carbon nanowires Jorge-Alejandro Tapia, Alvaro-Daniel Sanchez, Cesar Acosta, Gabriel Canto Recently has been reported a new type of one-dimensional carbon structures. Carbon nanowires formed by a linear carbon-atom chain inside an armchair (5,5) carbon nanotube has been observed using high-resolution transmission electron microscopy. Theoretical and experimental studies of the NH3 adsorption in the carbon nanotubes report changes in the electronic properties of the carbon nanotubes. In the present work we have studied the electronic and structure properties of carbon nanowires (chain@SWCNT) when NH3 atoms are adsorbed. We used the Density Functional Theory and the calculations where performed by the pseudopotentials LCAO method (SIESTA code) and the Generalized Gradient Approximation (GGA) for the exchange-correlation potential. We have analyzed the changes in the atomic structure and density of states (DOS). We found that the electronic character of the carbon chain of the chain@SWCNT system, can be modulate by NH3 adsorption. This research was supported by SEP under Grant No. PROMEP/103.5/07/2595 and the Consejo Nacional de Ciencia y Tecnolog\'{\i}a (Conacyt) under Grants No. 82497 and 60534. [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J26.00009: First-Principles Studies of Octacyclopropylcubane: A Novel High-Energy Density Material Steven L. Richardson, Reeshemah N. Allen, Daniel Finkenstadt, Michael J. Mehl, Mark R. Pederson The ongoing quest for synthesizing novel high-energy density materials (HEDMs) is clearly motivated by a search for new propellants and explosives. Recently de Meijere {\it et al.} have synthesized a new HEDM, octacyclopropylcubane ($C_{32}H_{40}$), in which the eight hydrogen atoms of cubane were replaced by cyclopropyl groups. In this work we report the results of a first-principles density-functional theory (DFT) calculation using the suite of codes known as NRLMOL (Naval Research Laboratory Molecular Orbital Library) to compute the structural, electronic, and vibrational properties of octacyclopropylcubane. We have calculated the vibrational properties of $C_{32}H_{40}$ and compare our results with experiment. We have also employed a DFT-based tight-binding scheme to compute the vibrational density of states for octacyclopropylcubane and compare our results with our full DFT-based results. Interesting enough, the geometry of the cyclopropyl groups in $C_{32}H_{40}$ does not allow for the quartic- concerted torsional mode (QCTM) that we and other workers have previously studied in octanitrocubane. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J26.00010: Efficient first-principles simulation of non-contact atomic force microscopy for structural analysis James Chelikowsky, Tzu-Liang Chan, Cai-Zhuang Wang, Kai-Ming Ho Non-contact atomic force microscopy (nc-AFM) has made significant advances that have allowed one to image a surface at atomic resolution. However, first-principles simulations of nc-AFM images remain a challenge because they involve calculations of the sample together with an atomic model of the AFM tip. We propose an efficient scheme to simulate nc-AFM images by using a first-principles self-consistent potential from the sample as input and without explicit modeling of the AFM tip. Our method is applied to various types of semiconductor surfaces including Si(111) $(7\times7)$, TiO$_{2}$(110) (1$\times1)$, Ag/Si(111)-$(\sqrt{3}\times\sqrt{3})R30^{\circ}$ and Ge/Si(105) $(1\times2)$ surfaces. Our method takes into account electronic effects of the tip-sample interaction, which are important for predicting the bright spot positions and the contrast change with AFM tip height. In addition, we obtain good agreement with experimental results and previous theoretical studies. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J26.00011: Ab initio calculation of Stokes shifts of hydrogenated silicon clusters Marie Lopez del Puerto, Manish Jain, James R. Chelikowsky There is experimental evidence that hydrogenated silicon clusters may have large Stokes shifts. The absorption and emission processes in these clusters are not symmetric because the clusters may undergo structural changes while in an excited state. Several theoretical methods have been used to study this problem, resulting in an array of predicted Stokes shifts that differ in energy by several eVs, and different predicted minimum-energy structures with either relaxed cores or relaxed outer shells. We calculate Stokes shifts using three different methods: density functional theory within the local density approximation (LDA), density functional theory within the generalized gradient approximation (GGA), and time-dependent density functional theory within the local density approximation (TDLDA). We find that these three different methods give similar results both for magnitude of Stokes shift and excited-state structures. The Stokes shift of hydrogenated silicon clusters of 5 to 35 silicon atoms range from 5.5eV to 0.8eV, decreasing with increasing cluster size. [Preview Abstract] |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J26.00012: Calculated Polarizabilities of Diamond and Silicon Nanoclusters Sudha Srinivas, Koblar Jackson, Mingli Yang, Julius Jellinek A scheme for decomposing the electric polarizability of a system into site-specific contributions is applied to hydrogenated nanoclusters of carbon and silicon. Site-specific dipole moments and polarizabilities are obtained from the response of charge densities to external electric fields, and decomposed into local and charge transfer components. We study changes in the polarizabilities of the C and Si atoms as the clusters grow in size. We find that exterior atoms have larger polarizabilities than interior atoms and that the charge transfer contribution to the total cluster polarizability increases with cluster size. We examine the relationship between the atomic polarizabilities in these clusters and bulk polarizability in carbon and silicon. [Preview Abstract] |
Tuesday, March 17, 2009 2:03PM - 2:15PM |
J26.00013: The Role of Vacancies on the Doping in Silicon Nanocrystals Jae-Hyeon Eom, Tzu-Liang Chan, James Chelikowsky We will present results for the effect of intrinsic defects (vacancies) on the doping of silicon nanocrystals by using first-principles calculations, {\it i.e.}, pseudopotentials in real space. We calculated the total energy of a B doped silicon nanocrystal as a function of the vacancy position and the nanocrystal size. We found that the most stable B site strongly depends not only on the cluster size, but also on the position of the vacancy. We also explored the evolution of the interaction between the vacancy and the B dopant by comparing the total energy for several nanocrystal sizes and configurations. [Preview Abstract] |
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