Researchers Database

OHTA Yasuhito

    Faculty Division of Natural Sciences Research Group of Chemistry Associate Professor
Last Updated :2021/10/20

researchmap

Degree

  • Kanazawa University, Mar. 2001

Research Interests

  • 分子動力学 量子化学 量子波束 

Research Areas

  • Nanotechnology/Materials, Basic physical chemistry

Published Papers

  • Temperature-dependent BN cluster formation dynamics from a boron cluster: Density-functional tight-binding molecular dynamics simulations

    OHTA Yasuhito

    Jul. 2017, Computational Materials Science, 139, 16?25, doi

  • Possible Mechanism of BN Fullerene Formation from a Boron Cluster: Density-Functional Tight-Binding Molecular Dynamics Simulations

    OHTA Yasuhito; Yasuhito Ohta

    Jan. 2016, Journal of Computational Chemistry, 37, 886-895, doi

  • Quantum chemical simulations reveal acetylene-based growth mechanisms in the chemical vapor deposition synthesis of carbon nanotubes

    Ying Wang; Xingfa Gao; Hu-Jun Qian; Yasuhito Ohta; Xiaona Wu; Gyula Eres; Keiji Morokuma; Stephan Irle

    Nonequilibrium quantum chemical molecular dynamics (QM/MDs) simulation of early stages in the nucleation process of carbon nanotubes from acetylene feedstock on an Fe-38 cluster was performed based on the density-functional tight-binding (DFTB) potential. Representative chemical reactions were studied by complimentary static DFTB and density functional theory (DFT) calculations. Oligomerization and cross-linking reactions between carbon chains were found as the main reaction pathways similar to that suggested in previous experimental work. The calculations highlight the inhibiting effect of hydrogen for the condensation of carbon ring networks, and a propensity for hydrogen disproportionation, thus enriching the hydrogen content in already hydrogen-rich species and abstracting hydrogen content in already hydrogen-deficient clusters. The ethynyl radical C2H was found as a reactive, yet continually regenerated species, facilitating hydrogen transfer reactions across the hydrocarbon clusters. The nonequilibrium QM/MD simulations show the prevalence of a pentagon-first nucleation mechanism where hydrogen may take the role of one "arm" of an sp(2) carbon Y-junction. The results challenge the importance of the metal carbide formation for SWCNT cap nucleation in the VLS model and suggest possible alternative routes following hydrogen-abstraction acetylene addition (HACA)-like mechanisms commonly discussed in combustion synthesis. (C) 2014 Elsevier Ltd. All rights reserved., PERGAMON-ELSEVIER SCIENCE LTD, Jun. 2014, CARBON, 72, 22 - 37, doi;web_of_science

    Scientific journal

  • Self-Consistent-Charge Density-Functional Tight-Binding/MD Simulation of Transition Metal Catalyst Particle Melting and Carbide Formation

    Yoshiko Okamoto; Fuyuko Kawamura; Yasuhito Ohta; Alister J. Page; Stephan Irle; Keiji Morokuma

    Iron, nickel, and cobalt are commonly employed catalyst transition metals in catalytic chemical vapor deposition (CCVD) growth of SWCNTs. Quantum chemical molecular dynamics simulations (QM/MD) of transition metal particle melting and carbide formation during the early stage of single-walled carbon nanotube (SWCNT) growth are presented here. The self-consistent-charge density-functional tight-binding (SCC-DFTB) method was employed as the potential for MD simulations over timescales of several hundreds of picoseconds at temperatures ranging from 400 K to 2000 K. Model systems consisting of 24C(2) molecules and a single C-30 'SWCNT cap-fragment' chemisorbed on the surface of transition metal clusters (Fe-38, Co-38, Ni-38) were employed. The melting behavior is compared with those of corresponding pristine transition metal clusters. Co displayed a larger tendency towards melting and 'bulk' carbide formation over the entire temperature range, whereas Fe and Ni clusters exhibited only surface carbide formation. Carbon surface diffusion was found for all metals, and the growth of carbon clusters by additional ring formation was observed in Fe/Ni trajectories at high temperatures, especially when high electronic temperature was employed. Although no sudden increase in Lindemann indices was observed clearly, we conclude that melting of Fe/Ni clusters starts at lower temperatures when carbon is present on the cluster, whereas the opposite trend is observed for Co clusters., AMER SCIENTIFIC PUBLISHERS, Sep. 2011, JOURNAL OF COMPUTATIONAL AND THEORETICAL NANOSCIENCE, 8 (9), 1755 - 1763, doi;web_of_science

    Scientific journal

  • QM/MD simulation of SWNT nucleation on transition-metal carbide nanoparticles

    OHTA Yasuhito; A. J. Page; Honami Yamane; Y. Ohta; S. Irle; K. Morokuma

    Oct. 2010, Journal of the American Chemical Society, 132, 15699-15707

  • Mechanisms of single-walled carbon nanotube nucleation, growth, and healing determined using QM/MD methods

    OHTA Yasuhito; A. J. Page; Y. Ohta; S. Irle; K. Morokuma

    Jul. 2010, Accounts of Chemical Research, 43, 1375-1385

  • Comparison of single-walled carbon nanotube growth from Fe and Ni nanoparticles using quantum chemical molecular dynamics methods

    Alister J. Page; Soma Minami; Yasuhito Ohta; Stephan Irle; Keiji Morokuma

    Metal-catalyzed SWCNT growth has been modeled using quantum chemical molecular dynamics (QM/MD) in conjunction with feeding of carbon atoms to C 40-Fe55 and C40-Ni55 model complexes at 1500 K. The rate of Fe55-catalyzed SWCNT growth determined in this work was 19% slower than the Fe38-catalyzed growth rate. Conversely, Ni55-catalyzed SWCNT growth exhibited a growth rate 69% larger than of Fe55-catalyzed SWCNT growth, a fact consistent with excellent performance of Ni in laser evaporation and carbon-arc experiments. Ni55-catalyzed growth was preceded by the formation of extended polyyne chains at the base of the SWCNT, and so differed fundamentally from Fe55-catalyzed growth. These polyyne chains usually persisted for 10-30 ps. Subsequent polyyne ring condensation resulted in carbon polygon addition at the SWCNT base. The relative stabilities of the Cn carbon cluster moieties on the Fe55 and Ni55 surfaces were consistent with the relative strengths of the Fe-C, Ni-C and C-C interactions. The presence of smaller carbon moieties on the Fe55 surface led to the dissemination of surface iron atoms, and subsequent iffusion of short C n units through the subsurface region of the catalyst particle. Conversely, the Ni55 catalyst particle was observed to be more stable, remaining intact to a greater extent. © 2010 Elsevier Ltd. All rights reserved., Elsevier Ltd, 2010, Carbon, 48 (11), 3014 - 3026, doi

    Scientific journal

  • Quantum Chemical Molecular Dynamics Simulation of Single-Walled Carbon Nanotube Cap Nucleation on an Iron Particle

    OHTA Yasuhito; Y. Ohta; Y. Okamoto; A. J. Page; S. Irle; K. Morokuma

    Oct. 2009, ACS NANO, 3, 3413-3420

  • Defect healing during single-walled carbon nanotube growth: A density-functional tight-binding molecular dynamics investigation

    Alister J. Page; Yasuhito Ohta; Yoshiko Okamoto; Stephan Irle; Keiji Morokuma

    Quantum chemical molecular dynamics have been employed to investigate the healing of single-walled carbon nanotubes (SWNTs) during growth. In trajectories based on self-consistent-charge density-functional tightbinding (SCC-DFTB) energies and gradients, gas-phase carbon atoms were supplied to the carbon-iron boundary of a model C40-Fe38 complex at two different rates (1 C/0.5 ps and 1 C/10 ps). The lower rate of carbon supply was observed to promote SWNT growth, compared to the higher rate, for the same number of carbon atoms supplied. This promotion of growth was ascribed to the suppression of pentagon and heptagon incorporation in the sp2 carbon network observed at lower carbon supply rates. The most successful example of growth occurred when the respective periods of hexagon and pentagon formation were out of phase and heptagon formation was limited. Higher carbon supply rates tended to result in the encapsulation of the Fe38 cluster by the extended sp2 carbon cap, due to a saturation of pentagon and heptagon defects in the latter. The greater tendency toward hexagon formation found using a lower carbon supply rate was attributed to the relative rates of defect removal and addition from the sp2 carbon cap during the growth process. The defect removal (i.e., healing) process of the sp2 carbon cap occurred via ring isomerization, which resulted in the removal of 5-7, adatom, and monovacancy defects. These healing mechanisms generally occurred over time scales of several picoseconds and depended largely on the presence of the catalyst surface. The healing mechanisms observed in this work represent a possible pathway by which control over the (n, m) chirality of a nascent SWNT is obtained during the growth process. © 2009 American Chemical Society., 2009, Journal of Physical Chemistry C, 113 (47), 20198 - 20207, doi

    Scientific journal

  • Single-walled carbon nanotube growth from a cap fragment on an iron nanoparticle: Density-functional tight-binding molecular dynamics simulations

    Yasuhito Ohta; Yoshiko Okamoto; Stephan Irle; Keiji Morokuma

    Growth of a single-walled carbon nanotube (SWNT) from a corannulene cap fragment on an iron cluster is demonstrated using density-functional tight-binding molecular dynamics simulations of laser synthesis. In order to explore multiple reaction pathways for the cap fragment to evolve into tubular form on the iron surface, reaction dynamics between the metal-bound cap fragment and gas-phase carbon atoms were investigated. It is found that rapid growth of the cap fragment can take place when carbon atoms are supplied in the vicinity of the cap fragment. In this reaction process, a high-density supply of add atoms leads also to rearrangements of existing s p2 -carbon-cap structures involving the formation of pentagons and heptagons as long-lived defects, while short-lived four- and eight-membered rings and short polyyne chains appear during the dynamics as important intermediate structures, facilitating growth. © 2009 The American Physical Society., 01 May 2009, Physical Review B - Condensed Matter and Materials Physics, 79 (19), doi

    Scientific journal

  • Single-walled carbon nanotube growth from a cap fragment on an iron nanoparticle: Density-functional tight-binding molecular dynamics simulations

    Yasuhito Ohta; Yoshiko Okamoto; Stephan Irle; Keiji Morokuma

    Growth of a single-walled carbon nanotube (SWNT) from a corannulene cap fragment on an iron cluster is demonstrated using density-functional tight-binding molecular dynamics simulations of laser synthesis. In order to explore multiple reaction pathways for the cap fragment to evolve into tubular form on the iron surface, reaction dynamics between the metal-bound cap fragment and gas-phase carbon atoms were investigated. It is found that rapid growth of the cap fragment can take place when carbon atoms are supplied in the vicinity of the cap fragment. In this reaction process, a high-density supply of add atoms leads also to rearrangements of existing sp(2)-carbon-cap structures involving the formation of pentagons and heptagons as long-lived defects, while short-lived four- and eight-membered rings and short polyyne chains appear during the dynamics as important intermediate structures, facilitating growth., AMER PHYSICAL SOC, May 2009, PHYSICAL REVIEW B, 79 (19), 195415/1-7, doi;web_of_science

    Scientific journal

  • Milestones in Molecular Dynamics Simulations of Single-Walled Carbon Nanotube Formation: A Brief Critical Review

    OHTA Yasuhito; S. Irle; Y. Ohta; Y. Okamoto; A. J. Page; Y. Wang; K. Morokuma

    Jan. 2009, Nano Research, 2, 755-767

  • Density-functional tight-binding molecular dynamics simulations of SWCNT growth by surface carbon diffusion on an iron cluster

    OHTA Yasuhito; Y. Ohta; Y. Okamoto; S. Irle; K. Morokuma

    Jan. 2009, Carbon, 47, 1270-1275

  • Temperature dependence of iron-catalyzed continued single-walled carbon nanotube growth rates: Density functional tight-binding molecular dynamics simulations

    OHTA Yasuhito; Y. Ohta; Y. Okamoto; S. Irle; K. Morokuma

    Dec. 2008, The Journal of Chemical Physics C, 113, 159-169

  • Rapid Growth of a Single-Walled Carbon Nanotube on an Iron Cluster: Density-Functional Tight-Binding Molecular Dynamics Simulations

    OHTA Yasuhito; Y. Ohta; Y. Okamoto; S. Irle; K. Morokuma

    Jun. 2008, ACS NANO, 2, 1437-1444

  • Extended spin-boson model for nonadiabatic hydrogen tunneling in the condensed phase

    Yasuhito Ohta; Alexander V. Soudackov; Sharon Hammes-Schiffer

    A nonadiabatic rate expression for hydrogen tunneling reactions in the condensed phase is derived for a model system described by a modified spin-boson Hamiltonian with a tunneling matrix element exponentially dependent on the hydrogen donor-acceptor distance. In this model, the two-level system representing the localized hydrogen vibrational states is linearly coupled to the donor-acceptor vibrational mode and the harmonic bath. The Hamiltonian also includes bilinear coupling between the donor-acceptor mode and the bath oscillators. This coupling provides a mechanism for energy exchange between the two-level system and the bath through the donor-acceptor mode, thereby facilitating convergence of the time integral of the probability flux correlation function for the case of weak coupling between the two-level system and the bath. The dependence of the rate constant on the model parameters and the temperature is analyzed in various regimes. Anomalous behavior of the rate constant is observed in the weak solvation regime for model systems that lack an effective mechanism for energy exchange between the two-level system and the bath. This theoretical formulation is applicable to a wide range of chemical and biological processes, including neutral hydrogen transfer reactions with small solvent reorganization energies. (c) 2006 American Institute of Physics., AMER INST PHYSICS, Oct. 2006, JOURNAL OF CHEMICAL PHYSICS, 125 (14), 144522/1-16, doi;web_of_science

    Scientific journal

MISC

  • Temperature-dependent BN cluster formation dynamics from a boron cluster: Density-functional tight-binding molecular dynamics simulations Yasuhito Ohta

    Yasuhito Ohta

    The time-evolution dynamics of a boron nitride (BN) cluster from an amorphous B cluster is simulated by quantum chemical molecular dynamics based on the density-functional tight-binding method. In the simulations, N atoms are sequentially supplied around the B cluster in conjunction with the arcmelting BN fullerene synthesis from B-rich compounds. The simulations are performed at 1000, 1500, 2000, 2500, and 3000 K, and we run 30 trajectories for 200 ps at each temperature. At low temperature (1000 K), the BN clusters tend to form stuffed cage structures, characterized by internal BN branched units. As the temperature increases, the proportion of stuffed cage structures decreases, whereas that of cage-like structure increases. At intermediate temperatures (2000 and 2500 K), most of the BN clusters develop into hollow structures, exhibiting a strong cage forming preference. At 3000 K, the BN clusters tend to form sparse branched chain structures, without forming cage-like structures. Mobility analysis of the cluster atoms at all the temperatures reveals that the transition from a liquid-like state to a solid-like state occurs at 2000 and 2500 K, whereas the cluster remains in a solid-like state at 1000 K and a liquid-like state at 3000 K. The N-2 dissociation reactions from the BN cluster proceed through various N-2 unit formation processes in the BN cluster. We describe details of the representative N-2 unit formation processes classified by the bonding of the cluster N atoms. (C) 2017 Elsevier B.V. All rights reserved., ELSEVIER SCIENCE BV, Nov. 2017, COMPUTATIONAL MATERIALS SCIENCE, 139, 16 - 25, doi;web_of_science

  • Temperature-dependent BN cluster formation dynamics from a boron cluster: Density-functional tight-binding molecular dynamics simulations

    OHTA Yasuhito

    2017, Computational Materials Science, 139, 16–25, doi

  • Possible mechanism of BN fullerene formation from a boron cluster: Density-functional tight-binding molecular dynamics simulations

    Y. Ohta

    We simulate the formation of a BN fullerene from an amorphous B cluster at 2000 K by quantum mechanical molecular dynamics based on the density-functional tight-binding method. We run 30 trajectories 200 ps in length, where N atoms are supplied around the target cluster, which is initially an amorphous B36 cluster. Most of the incident N atoms are promptly incorporated into the target cluster to form B-N-B bridges or NB3 pyramidal local substructures. BN fullerene formation is initiated by alternating BN ring condensation. Spontaneous atomic rearrangement and N2 dissociation lead to the construction of an sp2 single-shelled structure, during which the BN cluster undergoes a transition from a liquid-like to a solid-like state. Continual atomic rearrangement and sporadic N2 dissociation decrease the number of defective rings in the BN cluster and increase the number of six-membered rings, forming a more regular shell structure. The number of four-membered rings tends to remain constant, and contributes to more ordered isolated-tetragon-rule ring placement., John Wiley and Sons Inc., 15 Apr. 2016, Journal of Computational Chemistry, 37 (10), 886 - 895, doi

  • Possible Mechanism of BN Fullerene Formation from a Boron Cluster: Density-Functional Tight-Binding Molecular Dynamics Simulations

    Y. Ohta

    We simulate the formation of a BN fullerene from an amorphous B cluster at 2000 K by quantum mechanical molecular dynamics based on the density-functional tight-binding method. We run 30 trajectories 200 ps in length, where N atoms are supplied around the target cluster, which is initially an amorphous B-36 cluster. Most of the incident N atoms are promptly incorporated into the target cluster to form B-N-B bridges or NB3 pyramidal local substructures. BN fullerene formation is initiated by alternating BN ring condensation. Spontaneous atomic rearrangement and N-2 dissociation lead to the construction of an sp(2) single-shelled structure, during which the BN cluster undergoes a transition from a liquid-like to a solid-like state. Continual atomic rearrangement and sporadic N-2 dissociation decrease the number of defective rings in the BN cluster and increase the number of six-membered rings, forming a more regular shell structure. The number of four-membered rings tends to remain constant, and contributes to more ordered isolated-tetragon-rule ring placement. (C) 2016 Wiley Periodicals, Inc., WILEY-BLACKWELL, Apr. 2016, JOURNAL OF COMPUTATIONAL CHEMISTRY, 37 (10), 886 - 895, doi;web_of_science

  • Quantum chemical simulations reveal acetylene-based growth mechanisms in the chemical vapor deposition synthesis of carbon nanotubes

    Ying Wang; Xingfa Gao; Hu-Jun Qian; Yasuhito Ohta; Xiaona Wu; Gyula Eres; Keiji Morokuma; Stephan Irle

    Nonequilibrium quantum chemical molecular dynamics (QM/MDs) simulation of early stages in the nucleation process of carbon nanotubes from acetylene feedstock on an Fe-38 cluster was performed based on the density-functional tight-binding (DFTB) potential. Representative chemical reactions were studied by complimentary static DFTB and density functional theory (DFT) calculations. Oligomerization and cross-linking reactions between carbon chains were found as the main reaction pathways similar to that suggested in previous experimental work. The calculations highlight the inhibiting effect of hydrogen for the condensation of carbon ring networks, and a propensity for hydrogen disproportionation, thus enriching the hydrogen content in already hydrogen-rich species and abstracting hydrogen content in already hydrogen-deficient clusters. The ethynyl radical C2H was found as a reactive, yet continually regenerated species, facilitating hydrogen transfer reactions across the hydrocarbon clusters. The nonequilibrium QM/MD simulations show the prevalence of a pentagon-first nucleation mechanism where hydrogen may take the role of one "arm" of an sp(2) carbon Y-junction. The results challenge the importance of the metal carbide formation for SWCNT cap nucleation in the VLS model and suggest possible alternative routes following hydrogen-abstraction acetylene addition (HACA)-like mechanisms commonly discussed in combustion synthesis. (C) 2014 Elsevier Ltd. All rights reserved., PERGAMON-ELSEVIER SCIENCE LTD, Jun. 2014, CARBON, 72, 22 - 37, doi;web_of_science

  • Quantum chemical simulations reveal acetylene-based growth mechanisms in the chemical vapor deposition synthesis of carbon nanotubes

    Ying Wang; Xingfa Gao; Hu-Jun Qian; Yasuhito Ohta; Xiaona Wu; Gyula Eres; Keiji Morokuma; Stephan Irle

    Nonequilibrium quantum chemical molecular dynamics (QM/MDs) simulation of early stages in the nucleation process of carbon nanotubes from acetylene feedstock on an Fe-38 cluster was performed based on the density-functional tight-binding (DFTB) potential. Representative chemical reactions were studied by complimentary static DFTB and density functional theory (DFT) calculations. Oligomerization and cross-linking reactions between carbon chains were found as the main reaction pathways similar to that suggested in previous experimental work. The calculations highlight the inhibiting effect of hydrogen for the condensation of carbon ring networks, and a propensity for hydrogen disproportionation, thus enriching the hydrogen content in already hydrogen-rich species and abstracting hydrogen content in already hydrogen-deficient clusters. The ethynyl radical C2H was found as a reactive, yet continually regenerated species, facilitating hydrogen transfer reactions across the hydrocarbon clusters. The nonequilibrium QM/MD simulations show the prevalence of a pentagon-first nucleation mechanism where hydrogen may take the role of one "arm" of an sp(2) carbon Y-junction. The results challenge the importance of the metal carbide formation for SWCNT cap nucleation in the VLS model and suggest possible alternative routes following hydrogen-abstraction acetylene addition (HACA)-like mechanisms commonly discussed in combustion synthesis. (C) 2014 Elsevier Ltd. All rights reserved., PERGAMON-ELSEVIER SCIENCE LTD, Jun. 2014, CARBON, 72, 22 - 37, doi;web_of_science

  • Acetylene oligomerization and oligoyne crosslinking on an iron particle: Quantum chemical molecular dynamics simulations inspired by experiment

    Ying Wang; Yasuhito Ohta; HuJun Qian; Alister J. Page; Keiji Morokuma; Stephan Irle

    AMER CHEMICAL SOC, Mar. 2011, ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY, 241, web_of_science

    Summary international conference

  • QM/MD Simulation of SWNT Nucleation on Transition-Metal Carbide Nanoparticles

    Alister J. Page; Honami Yamane; Yasuhito Ohta; Stephan Irle; Keiji Morokuma

    The mechanism and kinetics of single-walled carbon nanotube (SWNT) nucleation from Fe- and Ni-carbide nanoparticle precursors have been investigated using quantum chemical molecular dynamics (QM/MD) methods. The dependence of the nucleation mechanism and its kinetics on environmental factors, including temperature and metal-carbide carbon concentration, has also been elucidated. It was observed that SWNT nucleation occurred via three distinct stages, viz, the precipitation of the carbon from the metal-carbide, the formation of a "surface/subsurface" carbide intermediate species, and finally the formation of a nascent sp(2)-hybidrized carbon structure supported by the metal catalyst. The SWNT cap nucleation mechanism itself was unaffected by carbon concentration and/or temperature. However, the kinetics of SWNT nucleation exhibited distinct dependences on these same factors. In particular, SWNT nucleation from NixCy nanoparticles proceeded more favorably compared to nucleation from FexCy nanoparticles. Although SWNT nucleation from FexCy and NixCy nanoparticle precursors occurred via an identical route, the ultimate outcomes of these processes also differed substantially. Explicitly, the Ni-x-supported sp(2)-hybridized carbon structures tended to encapsulate the catalyst particle itself, whereas the Fe-x-supported structures tended to form isolated SWNT cap structures on the catalyst surface. These differences in SWNT nucleation kinetics were attributed directly to the relative strengths of the metal-carbon interaction, which also dictates the precipitation of carbon from the nanoparticle bulk and the longevity of the resultant surface/subsurface carbide species. The stability of the surface/subsurface carbide was also influenced by the phase of the nanoparticle itself. The observations agree well with experimentally available data for SWNT growth on iron and nickel catalyst particles., AMER CHEMICAL SOC, Nov. 2010, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 132 (44), 15699 - 15707, doi;web_of_science

  • QM/MD Simulation of SWNT Nucleation on Transition-Metal Carbide Nanoparticles

    Alister J. Page; Honami Yamane; Yasuhito Ohta; Stephan Irle; Keiji Morokuma

    The mechanism and kinetics of single-walled carbon nanotube (SWNT) nucleation from Fe- and Ni-carbide nanoparticle precursors have been investigated using quantum chemical molecular dynamics (QM/MD) methods. The dependence of the nucleation mechanism and its kinetics on environmental factors, including temperature and metal-carbide carbon concentration, has also been elucidated. It was observed that SWNT nucleation occurred via three distinct stages, viz, the precipitation of the carbon from the metal-carbide, the formation of a "surface/subsurface" carbide intermediate species, and finally the formation of a nascent sp(2)-hybidrized carbon structure supported by the metal catalyst. The SWNT cap nucleation mechanism itself was unaffected by carbon concentration and/or temperature. However, the kinetics of SWNT nucleation exhibited distinct dependences on these same factors. In particular, SWNT nucleation from NixCy nanoparticles proceeded more favorably compared to nucleation from FexCy nanoparticles. Although SWNT nucleation from FexCy and NixCy nanoparticle precursors occurred via an identical route, the ultimate outcomes of these processes also differed substantially. Explicitly, the Ni-x-supported sp(2)-hybridized carbon structures tended to encapsulate the catalyst particle itself, whereas the Fe-x-supported structures tended to form isolated SWNT cap structures on the catalyst surface. These differences in SWNT nucleation kinetics were attributed directly to the relative strengths of the metal-carbon interaction, which also dictates the precipitation of carbon from the nanoparticle bulk and the longevity of the resultant surface/subsurface carbide species. The stability of the surface/subsurface carbide was also influenced by the phase of the nanoparticle itself. The observations agree well with experimentally available data for SWNT growth on iron and nickel catalyst particles., AMER CHEMICAL SOC, Nov. 2010, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 132 (44), 15699 - 15707, doi;web_of_science

  • Mechanisms of Single-Walled Carbon Nanotube Nucleation, Growth, and Healing Determined Using QM/MD Methods

    Alister J. Page; Yasuhito Ohta; Stephan Irle; Keiji Morokuma

    Since their discovery in the early 1990s, single-walled carbon nanotubes (SWNTs) have spawned previously unimaginable commercial and industrial technologies. Their versatility stems from their unique electronic, physical/chemical, and mechanical properties, which set them apart from traditional materials. Many researchers have investigated SWNT growth mechanisms in the years since their discovery. The most prevalent of these is the vapor-liquid-solid (VLS) mechanism, which is based on experimental observations. Within the VLS mechanism, researchers assume that the formation of a SWNT starts with co-condensation of carbon and metal atoms from vapor to form liquid metal carbide. Once the liquid reaches supersaturation, the solid phase nanotubes begin to grow. The growth process is partitioned into three distinct stages: nucleation of a carbon "cap-precursor," "cap-to-tube" transformation, and continued SWNT growth. In recent years, molecular dynamics (MD) simulations have come to the fore with respect to SWNT growth. MD simulations lead to spatial and temporal resolutions of these processes that are superior to those possible using current experimental techniques, and so provide valuable information regarding the growth process that researchers cannot obtain experimentally. In this Account, we review our own recent efforts to simulate SWNT nucleation, growth, and healing phenomena on transition-metal catalysts using quantum mechanical molecular dynamics (QM/MD) methods. In particular, we have validated each stage of the SWNT condensation mechanism using a self-consistent-charge density-functional tight-binding (SCC-DFTB) methodology. With respect to the nucleation of a SWNT cap-precursor (stage 1), we have shown that the presence of a transition-metal carbide particle is not a necessary prerequisite for SWNT nucleation, contrary to conventional experimental presumptions. The formation and coalescence of polyyne chains on the metal surface occur first, followed by the formation of the SWNT cap-precursor, "ring condensation", and the creation of an sp(2)-hybridized carbon structure. In our simulations, the nucleation process takes approximately 400 ps. This first step occurs over a much longer time scale than the second stage of SWNT condensation (approximately 50 ps). We therefore observe SWNT nucleation to be akin to the rate-limiting step of the SWNT formation process. In addition to the QM/MD simulation of various stages of SWNT nucleation, growth, and healing processes, we have determined the effects of temperature, catalyst composition, and catalyst size on the kinetics and mechanism of SWNT growth. With respect to temperature dependence, we observe a "sweet-spot" with respect to the efficiency of SWNT growth. In addition, Ni-catalyzed SWNT growth is observed to be 70-100% faster compared to Fe-catalyzed SWNT growth, depending on the catalyst particle size. We also observe a noticeable increase in SWNT growth rates using smaller catalyst particles. Finally, we review our recent QM/MD investigation of SWNT healing. In particular, we recount mechanisms by which adatom defects, monovacancy defects, and a "5-7 defect" are removed from a nascent SWNT. The effectiveness of these healing mechanisms depends on the rate at which carbon moieties are incorporated into the growing SWNT. Explicitly, we observe that healing is promoted using a slower carbon supply rate. From this rudimentary control of SWNT healing, we propose a route towards chirality-controlled SWNT growth., AMER CHEMICAL SOC, Oct. 2010, ACCOUNTS OF CHEMICAL RESEARCH, 43 (10), 1375 - 1385, doi;web_of_science

    Book review

  • Mechanisms of Single-Walled Carbon Nanotube Nucleation, Growth, and Healing Determined Using QM/MD Methods

    Alister J. Page; Yasuhito Ohta; Stephan Irle; Keiji Morokuma

    Since their discovery in the early 1990s, single-walled carbon nanotubes (SWNTs) have spawned previously unimaginable commercial and industrial technologies. Their versatility stems from their unique electronic, physical/chemical, and mechanical properties, which set them apart from traditional materials. Many researchers have investigated SWNT growth mechanisms in the years since their discovery. The most prevalent of these is the vapor-liquid-solid (VLS) mechanism, which is based on experimental observations. Within the VLS mechanism, researchers assume that the formation of a SWNT starts with co-condensation of carbon and metal atoms from vapor to form liquid metal carbide. Once the liquid reaches supersaturation, the solid phase nanotubes begin to grow. The growth process is partitioned into three distinct stages: nucleation of a carbon "cap-precursor," "cap-to-tube" transformation, and continued SWNT growth. In recent years, molecular dynamics (MD) simulations have come to the fore with respect to SWNT growth. MD simulations lead to spatial and temporal resolutions of these processes that are superior to those possible using current experimental techniques, and so provide valuable information regarding the growth process that researchers cannot obtain experimentally. In this Account, we review our own recent efforts to simulate SWNT nucleation, growth, and healing phenomena on transition-metal catalysts using quantum mechanical molecular dynamics (QM/MD) methods. In particular, we have validated each stage of the SWNT condensation mechanism using a self-consistent-charge density-functional tight-binding (SCC-DFTB) methodology. With respect to the nucleation of a SWNT cap-precursor (stage 1), we have shown that the presence of a transition-metal carbide particle is not a necessary prerequisite for SWNT nucleation, contrary to conventional experimental presumptions. The formation and coalescence of polyyne chains on the metal surface occur first, followed by the formation of the SWNT cap-precursor, "ring condensation", and the creation of an sp(2)-hybridized carbon structure. In our simulations, the nucleation process takes approximately 400 ps. This first step occurs over a much longer time scale than the second stage of SWNT condensation (approximately 50 ps). We therefore observe SWNT nucleation to be akin to the rate-limiting step of the SWNT formation process. In addition to the QM/MD simulation of various stages of SWNT nucleation, growth, and healing processes, we have determined the effects of temperature, catalyst composition, and catalyst size on the kinetics and mechanism of SWNT growth. With respect to temperature dependence, we observe a "sweet-spot" with respect to the efficiency of SWNT growth. In addition, Ni-catalyzed SWNT growth is observed to be 70-100% faster compared to Fe-catalyzed SWNT growth, depending on the catalyst particle size. We also observe a noticeable increase in SWNT growth rates using smaller catalyst particles. Finally, we review our recent QM/MD investigation of SWNT healing. In particular, we recount mechanisms by which adatom defects, monovacancy defects, and a "5-7 defect" are removed from a nascent SWNT. The effectiveness of these healing mechanisms depends on the rate at which carbon moieties are incorporated into the growing SWNT. Explicitly, we observe that healing is promoted using a slower carbon supply rate. From this rudimentary control of SWNT healing, we propose a route towards chirality-controlled SWNT growth., AMER CHEMICAL SOC, Oct. 2010, ACCOUNTS OF CHEMICAL RESEARCH, 43 (10), 1375 - 1385, doi;web_of_science

    Book review

  • Comparison of single-walled carbon nanotube growth from Fe and Ni nanoparticles using quantum chemical molecular dynamics methods

    Alister J. Page; Sonia Minami; Yasuhito Ohta; Stephan Irle; Keiji Morokuma

    Metal-catalyzed SWCNT growth has been modeled using quantum chemical molecular dynamics (QM/MD) in conjunction with feeding of carbon atoms to C-40-Fe-55 and C-40-Ni-55 model complexes at 1500 K. The rate of Fe-55-catalyzed SWCNT growth determined in this work was 19% slower than the Fe-38-catalyzed growth rate. Conversely, Ni-55-catalyzed SWCNT growth exhibited a growth rate 69% larger than of Fe-55-catalyzed SWCNT growth, a fact consistent with excellent performance of Ni in laser evaporation and carbon-arc experiments. Ni-55-catalyzed growth was preceded by the formation of extended polyyne chains at the base of the SWCNT, and so differed fundamentally from Fe-55-catalyzed growth. These polyyne chains usually persisted for 10-30 ps. Subsequent polyyne ring condensation resulted in carbon polygon addition at the SWCNT base. The relative stabilities of the C carbon cluster moieties on the Fe-55 and Ni-55 surfaces were consistent with the relative strengths of the Fe-C, Ni-C and C-C interactions. The presence of smaller carbon moieties on the Fe-55 surface led to the dissemination of surface iron atoms, and subsequent diffusion of short C-n units through the subsurface region of the catalyst particle. Conversely, the Ni-55 catalyst particle was observed to be more stable, remaining intact to a greater extent. (C) 2010 Elsevier Ltd. All rights reserved., PERGAMON-ELSEVIER SCIENCE LTD, Sep. 2010, CARBON, 48 (11), 3014 - 3026, doi;web_of_science

  • Comparison of single-walled carbon nanotube growth from Fe and Ni nanoparticles using quantum chemical molecular dynamics methods

    Alister J. Page; Sonia Minami; Yasuhito Ohta; Stephan Irle; Keiji Morokuma

    Metal-catalyzed SWCNT growth has been modeled using quantum chemical molecular dynamics (QM/MD) in conjunction with feeding of carbon atoms to C-40-Fe-55 and C-40-Ni-55 model complexes at 1500 K. The rate of Fe-55-catalyzed SWCNT growth determined in this work was 19% slower than the Fe-38-catalyzed growth rate. Conversely, Ni-55-catalyzed SWCNT growth exhibited a growth rate 69% larger than of Fe-55-catalyzed SWCNT growth, a fact consistent with excellent performance of Ni in laser evaporation and carbon-arc experiments. Ni-55-catalyzed growth was preceded by the formation of extended polyyne chains at the base of the SWCNT, and so differed fundamentally from Fe-55-catalyzed growth. These polyyne chains usually persisted for 10-30 ps. Subsequent polyyne ring condensation resulted in carbon polygon addition at the SWCNT base. The relative stabilities of the C carbon cluster moieties on the Fe-55 and Ni-55 surfaces were consistent with the relative strengths of the Fe-C, Ni-C and C-C interactions. The presence of smaller carbon moieties on the Fe-55 surface led to the dissemination of surface iron atoms, and subsequent diffusion of short C-n units through the subsurface region of the catalyst particle. Conversely, the Ni-55 catalyst particle was observed to be more stable, remaining intact to a greater extent. (C) 2010 Elsevier Ltd. All rights reserved., PERGAMON-ELSEVIER SCIENCE LTD, Sep. 2010, CARBON, 48 (11), 3014 - 3026, doi;web_of_science

  • Defect Healing during Single-Walled Carbon Nanotube Growth: A Density-Functional Tight-Binding Molecular Dynamics Investigation

    Alister J. Page; Yasuhito Ohta; Yoshiko Okamoto; Stephan Irle; Keiji Morokuma

    Quantum chemical molecular dynamics have been employed to investigate the healing of single-walled carbon nanotubes (SWNTs) during growth. In trajectories based on self-consistent-charge density-functional tight-binding (SCC-DFTB) energies and gradients, gas-phase carbon atoms were supplied to the carbon-iron boundary of a model C-40-Fe-38 complex at two different rates (1 C/0.5 ps and 1 C/10 ps). The lower rate of carbon supply was observed to promote SWNT growth, compared to the higher rate, for the same number of carbon atoms Supplied. This promotion of growth was ascribed to the suppression of pentagon and heptagon incorporation in the Sp(2) carbon network observed at lower carbon supply rates. The most successful example of growth occurred when the respective periods of hexagon and pentagon formation were out of phase and heptagon formation was limited. Higher carbon supply rates tended to result in the encapsulation of the Fe-38 cluster by the extended Sp(2) carbon cap, due to a saturation of pentagon and heptagon defects in the latter. The greater tendency toward hexagon formation found using a lower carbon supply rate was attributed to the relative rates of defect removal and addition from the sp(2) carbon cap during the growth process. The defect removal (i.e., healing) process of the sp(2) carbon cap occurred via ring isomerization, which resulted in the removal of 5-7, adatom, and monovacancy defects. These healing mechanisms generally occurred over time scales of several picoseconds and depended largely on the presence of the catalyst surface. The healing mechanisms observed in this work represent a possible pathway by which control over the (n, m) chirality of a nascent SWNT is obtained during the growth process., AMER CHEMICAL SOC, Nov. 2009, JOURNAL OF PHYSICAL CHEMISTRY C, 113 (47), 20198 - 20207, doi;web_of_science

  • Quantum Chemical Molecular Dynamics Simulation of Single-Walled Carbon Nanotube Cap Nucleation on an Iron Particle

    Yasuhito Ohta; Yoshiko Okamoto; Alister J. Page; Stephan Irle; Keiji Morokuma

    The atomic scale details of single-walled carbon nanotube (SWNT) nucleation on metal catalyst particles are elusive to experimental observations. Computer simulation of metal-catalyzed SWNT nucleation is a challenging topic but potentially of great importance to understand the factors affecting SWNT diameters, chirality, and growth efficiency. In this work, we use nonequilibrium density functional tight-binding molecular dynamics simulations and report nucleation of sp(2)-carbon cap structures on an iron particle consisting of 38 atoms. One C(2) molecule was placed every 1.0 ps around an Fe(38) cluster for 30 ps, after which a further 410 ps of annealing simulation without carbon supply was performed. We find that sp2-carbon network nucleation and annealing processes occur in three sequential and repetitive stages: (A) polyyne chains on the metal surface react with each other to evolve into a Y-shaped polyyne junction, which preferentially form a five-membered ring as a nucleus; (8) polyyne chains on the first five-membered ring form an additional fused five- or six-membered ring; and (C) pentagon-to-hexagon self-healing rearrangement takes place with the help of short-lived polyyne chains, stabilized by the mobile metal atoms. The observed nucleation process resembles the formation of a fullerene cage. However, the metal particle plays a key role in differentiating the nucleation process from fullerene cage formation, most importantly by keeping the growing cap structure from closing into a fullerene cage and by keeping the carbon edge "alive" for the addition of new carbon material., AMER CHEMICAL SOC, Nov. 2009, ACS NANO, 3 (11), 3413 - 3420, doi;web_of_science

  • Defect Healing during Single-Walled Carbon Nanotube Growth: A Density-Functional Tight-Binding Molecular Dynamics Investigation

    Alister J. Page; Yasuhito Ohta; Yoshiko Okamoto; Stephan Irle; Keiji Morokuma

    Quantum chemical molecular dynamics have been employed to investigate the healing of single-walled carbon nanotubes (SWNTs) during growth. In trajectories based on self-consistent-charge density-functional tight-binding (SCC-DFTB) energies and gradients, gas-phase carbon atoms were supplied to the carbon-iron boundary of a model C-40-Fe-38 complex at two different rates (1 C/0.5 ps and 1 C/10 ps). The lower rate of carbon supply was observed to promote SWNT growth, compared to the higher rate, for the same number of carbon atoms Supplied. This promotion of growth was ascribed to the suppression of pentagon and heptagon incorporation in the Sp(2) carbon network observed at lower carbon supply rates. The most successful example of growth occurred when the respective periods of hexagon and pentagon formation were out of phase and heptagon formation was limited. Higher carbon supply rates tended to result in the encapsulation of the Fe-38 cluster by the extended Sp(2) carbon cap, due to a saturation of pentagon and heptagon defects in the latter. The greater tendency toward hexagon formation found using a lower carbon supply rate was attributed to the relative rates of defect removal and addition from the sp(2) carbon cap during the growth process. The defect removal (i.e., healing) process of the sp(2) carbon cap occurred via ring isomerization, which resulted in the removal of 5-7, adatom, and monovacancy defects. These healing mechanisms generally occurred over time scales of several picoseconds and depended largely on the presence of the catalyst surface. The healing mechanisms observed in this work represent a possible pathway by which control over the (n, m) chirality of a nascent SWNT is obtained during the growth process., AMER CHEMICAL SOC, Nov. 2009, JOURNAL OF PHYSICAL CHEMISTRY C, 113 (47), 20198 - 20207, doi;web_of_science

  • Quantum Chemical Molecular Dynamics Simulation of Single-Walled Carbon Nanotube Cap Nucleation on an Iron Particle

    Yasuhito Ohta; Yoshiko Okamoto; Alister J. Page; Stephan Irle; Keiji Morokuma

    The atomic scale details of single-walled carbon nanotube (SWNT) nucleation on metal catalyst particles are elusive to experimental observations. Computer simulation of metal-catalyzed SWNT nucleation is a challenging topic but potentially of great importance to understand the factors affecting SWNT diameters, chirality, and growth efficiency. In this work, we use nonequilibrium density functional tight-binding molecular dynamics simulations and report nucleation of sp(2)-carbon cap structures on an iron particle consisting of 38 atoms. One C(2) molecule was placed every 1.0 ps around an Fe(38) cluster for 30 ps, after which a further 410 ps of annealing simulation without carbon supply was performed. We find that sp2-carbon network nucleation and annealing processes occur in three sequential and repetitive stages: (A) polyyne chains on the metal surface react with each other to evolve into a Y-shaped polyyne junction, which preferentially form a five-membered ring as a nucleus; (8) polyyne chains on the first five-membered ring form an additional fused five- or six-membered ring; and (C) pentagon-to-hexagon self-healing rearrangement takes place with the help of short-lived polyyne chains, stabilized by the mobile metal atoms. The observed nucleation process resembles the formation of a fullerene cage. However, the metal particle plays a key role in differentiating the nucleation process from fullerene cage formation, most importantly by keeping the growing cap structure from closing into a fullerene cage and by keeping the carbon edge "alive" for the addition of new carbon material., AMER CHEMICAL SOC, Nov. 2009, ACS NANO, 3 (11), 3413 - 3420, doi;web_of_science

  • Milestones in Molecular Dynamics Simulations of Single-Walled Carbon Nanotube Formation: A Brief Critical Review

    Stephan Irle; Yasuhito Ohta; Yoshiko Okamoto; Alister J. Page; Ying Wang; Keiji Morokuma

    We present a brief review of the most important efforts aimed at simulating single-walled carbon nanotube (SWNT) nucleation and growth processes using molecular dynamics (MD) techniques reported in the literature. MD simulations allow the spatio-temporal movement of atoms during nonequilibrium growth to be followed. Thus, it is hoped that a successful MD simulation of the entire SWNT formation process will assist in the design of chirality-specific SWNT synthesis techniques. We give special consideration to the role of the metal catalyst particles assumed in standard theories of SWNT formation, and describe the actual metal behavior observed in the reported MD simulations, including our own recent quantum chemical MD simulations. It is concluded that the use of a quantum potential is essential for a qualitatively correct description of the catalytic behavior of the metal cluster, and that carbide formation does not seem to be a necessary requirement for nucleation and growth of SWNTs according to our most recent quantum chemical MD simulations., TSINGHUA UNIV PRESS, Oct. 2009, NANO RESEARCH, 2 (10), 755 - 767, doi;web_of_science

  • Milestones in Molecular Dynamics Simulations of Single-Walled Carbon Nanotube Formation: A Brief Critical Review

    Stephan Irle; Yasuhito Ohta; Yoshiko Okamoto; Alister J. Page; Ying Wang; Keiji Morokuma

    We present a brief review of the most important efforts aimed at simulating single-walled carbon nanotube (SWNT) nucleation and growth processes using molecular dynamics (MD) techniques reported in the literature. MD simulations allow the spatio-temporal movement of atoms during nonequilibrium growth to be followed. Thus, it is hoped that a successful MD simulation of the entire SWNT formation process will assist in the design of chirality-specific SWNT synthesis techniques. We give special consideration to the role of the metal catalyst particles assumed in standard theories of SWNT formation, and describe the actual metal behavior observed in the reported MD simulations, including our own recent quantum chemical MD simulations. It is concluded that the use of a quantum potential is essential for a qualitatively correct description of the catalytic behavior of the metal cluster, and that carbide formation does not seem to be a necessary requirement for nucleation and growth of SWNTs according to our most recent quantum chemical MD simulations., TSINGHUA UNIV PRESS, Oct. 2009, NANO RESEARCH, 2 (10), 755 - 767, doi;web_of_science

  • Single-walled carbon nanotube growth from a cap fragment on an iron nanoparticle: Density-functional tight-binding molecular dynamics simulations

    Yasuhito Ohta; Yoshiko Okamoto; Stephan Irle; Keiji Morokuma

    Growth of a single-walled carbon nanotube (SWNT) from a corannulene cap fragment on an iron cluster is demonstrated using density-functional tight-binding molecular dynamics simulations of laser synthesis. In order to explore multiple reaction pathways for the cap fragment to evolve into tubular form on the iron surface, reaction dynamics between the metal-bound cap fragment and gas-phase carbon atoms were investigated. It is found that rapid growth of the cap fragment can take place when carbon atoms are supplied in the vicinity of the cap fragment. In this reaction process, a high-density supply of add atoms leads also to rearrangements of existing sp(2)-carbon-cap structures involving the formation of pentagons and heptagons as long-lived defects, while short-lived four- and eight-membered rings and short polyyne chains appear during the dynamics as important intermediate structures, facilitating growth., AMER PHYSICAL SOC, May 2009, PHYSICAL REVIEW B, 79 (19), 195415/1-7, doi;web_of_science

  • Single-walled carbon nanotube growth from a cap fragment on an iron nanoparticle: Density-functional tight-binding molecular dynamics simulations

    Yasuhito Ohta; Yoshiko Okamoto; Stephan Irle; Keiji Morokuma

    Growth of a single-walled carbon nanotube (SWNT) from a corannulene cap fragment on an iron cluster is demonstrated using density-functional tight-binding molecular dynamics simulations of laser synthesis. In order to explore multiple reaction pathways for the cap fragment to evolve into tubular form on the iron surface, reaction dynamics between the metal-bound cap fragment and gas-phase carbon atoms were investigated. It is found that rapid growth of the cap fragment can take place when carbon atoms are supplied in the vicinity of the cap fragment. In this reaction process, a high-density supply of add atoms leads also to rearrangements of existing sp(2)-carbon-cap structures involving the formation of pentagons and heptagons as long-lived defects, while short-lived four- and eight-membered rings and short polyyne chains appear during the dynamics as important intermediate structures, facilitating growth., AMER PHYSICAL SOC, May 2009, PHYSICAL REVIEW B, 79 (19), 195415/1-7, doi;web_of_science

  • Density-functional tight-binding molecular dynamics simulations of SWCNT growth by surface carbon diffusion on an iron cluster

    Yasuhito Ohta; Yoshiko Okamoto; Stephan Irle; Keiji Morokuma

    Iron-catalyzed SWCNT growth by carbon diffusion starting from a carbon cap has been demonstrated in density-functional tight-binding molecular dynamics simulations. A C(40) (5,5) SWCNT cap attached to an Fe(38) cluster was employed as initial model system. After 40 carbon atoms were supplied onto the iron surface for 20 ps, dynamics were continued for 160 ps without supply of further carbon feedstock. Growth of the SWCNT sidewall is mainly due to surface-diffusion of short carbon chains, and to a lesser degree due to sub-surface diffusion. Newly created rings consist only of pentagons and hexagons, while heptagons are infrequent and short-lived, which seems to be caused by the slower, more ordered sidewall growth due to the diffusion process. (C) 2009 Elsevier Ltd. All rights reserved., PERGAMON-ELSEVIER SCIENCE LTD, Apr. 2009, CARBON, 47 (5), 1270 - 1275, doi;web_of_science

  • Density-functional tight-binding molecular dynamics simulations of SWCNT growth by surface carbon diffusion on an iron cluster

    Yasuhito Ohta; Yoshiko Okamoto; Stephan Irle; Keiji Morokuma

    Iron-catalyzed SWCNT growth by carbon diffusion starting from a carbon cap has been demonstrated in density-functional tight-binding molecular dynamics simulations. A C(40) (5,5) SWCNT cap attached to an Fe(38) cluster was employed as initial model system. After 40 carbon atoms were supplied onto the iron surface for 20 ps, dynamics were continued for 160 ps without supply of further carbon feedstock. Growth of the SWCNT sidewall is mainly due to surface-diffusion of short carbon chains, and to a lesser degree due to sub-surface diffusion. Newly created rings consist only of pentagons and hexagons, while heptagons are infrequent and short-lived, which seems to be caused by the slower, more ordered sidewall growth due to the diffusion process. (C) 2009 Elsevier Ltd. All rights reserved., PERGAMON-ELSEVIER SCIENCE LTD, Apr. 2009, CARBON, 47 (5), 1270 - 1275, doi;web_of_science

  • Temperature Dependence of Iron-Catalyzed Continued Single-Walled Carbon Nanotube Growth Rates: Density Functional Tight-Binding Molecular Dynamics Simulations

    Yasuhito Ohta; Yoshiko Okamoto; Stephan Irle; Keiji Morokuma

    The temperature dependence of continued single-walled carbon nanotube (SWNT) growth on an iron cluster is investigated using quantum chemical molecular dynamics simulations based on the density functional tight-binding method. As a model system for continued SWNT growth, a (5,5) armchair-type SWNT seed attached to an iron Fe-38 cluster was used. Continuous and rapid supply of C atoms was provided in the vicinity of the nanotube-metal interface area. The simulations were performed at temperatures of 1000, 1500, and 2000 K. The simulations reveal fastest growth at 1500 K, although the differences are moderate. In the observed growth process, formation of polyyne chains at the rim of the nanotube-metal interface efficiently initiates pentagon/hexagon/heptagon ring formations in the carbon sidewall, leading to "lift-off" of the nanotube from the metal cluster. At 1000 K, the SWNT lift-off is suppressed despite the fact that the total number of created rings in the nanotube is comparable to that at 1500 K. In addition, relatively long polyyne chains tend to form extensions from the carbon sidewall to the metal cluster at 1000 K, whereas at 2000 K, deformation of the nanotube becomes more pronounced and diameter narrowing sets in, and polyyne chains at the rim of the nanotube easily dissociate at this high temperature. These physical and chemical events at 1000 and 2000 K can be considered inhibiting factors preventing efficient growth of the nanotube., AMER CHEMICAL SOC, Jan. 2009, JOURNAL OF PHYSICAL CHEMISTRY C, 113 (1), 159 - 169, doi;web_of_science

  • Temperature Dependence of Iron-Catalyzed Continued Single-Walled Carbon Nanotube Growth Rates: Density Functional Tight-Binding Molecular Dynamics Simulations

    Yasuhito Ohta; Yoshiko Okamoto; Stephan Irle; Keiji Morokuma

    The temperature dependence of continued single-walled carbon nanotube (SWNT) growth on an iron cluster is investigated using quantum chemical molecular dynamics simulations based on the density functional tight-binding method. As a model system for continued SWNT growth, a (5,5) armchair-type SWNT seed attached to an iron Fe-38 cluster was used. Continuous and rapid supply of C atoms was provided in the vicinity of the nanotube-metal interface area. The simulations were performed at temperatures of 1000, 1500, and 2000 K. The simulations reveal fastest growth at 1500 K, although the differences are moderate. In the observed growth process, formation of polyyne chains at the rim of the nanotube-metal interface efficiently initiates pentagon/hexagon/heptagon ring formations in the carbon sidewall, leading to "lift-off" of the nanotube from the metal cluster. At 1000 K, the SWNT lift-off is suppressed despite the fact that the total number of created rings in the nanotube is comparable to that at 1500 K. In addition, relatively long polyyne chains tend to form extensions from the carbon sidewall to the metal cluster at 1000 K, whereas at 2000 K, deformation of the nanotube becomes more pronounced and diameter narrowing sets in, and polyyne chains at the rim of the nanotube easily dissociate at this high temperature. These physical and chemical events at 1000 and 2000 K can be considered inhibiting factors preventing efficient growth of the nanotube., AMER CHEMICAL SOC, Jan. 2009, JOURNAL OF PHYSICAL CHEMISTRY C, 113 (1), 159 - 169, doi;web_of_science

  • Rapid growth of a single-walled carbon nanotube on an iron cluster: Density-functional tight-binding molecular dynamics simulations

    Yasuhito Ohta; Yoshiko Okamoto; Stephan Irle; Keiji Morokuma

    Continued growth of a single-walled carbon nanotube (SWNT) on an Fe cluster at 1500 K is demonstrated using quantum chemical molecular dynamics simulations based on the self-consistent-charge density-functional tight-binding (SCC-DFTB) method. In order to deal with charge transfer between carbon and metal particles and the multitude of electronic states, a finite electronic temperature approach is applied. We present trajectories of 45 ps length, where a continuous supply of carbon atoms is directed toward the C-Fe boundary between a 7.2 angstrom long armchair (5,5) SWNT fragment and an attached Fe-38 cluster. The incident carbon atoms react readily at the C-Fe interface to form C- and C-2-extensions on the tube rim that attach to the Fe cluster. These bridging sp-hybridized carbon fragments are vibrationally excited and highly mobile and, therefore, become engaged in frequent bond formation and breaking processes between their constituent C and the Fe atoms. The sp-hybridized carbon bridge dynamics and their reactions with the Fe-attached nanotube end bring about formations of new five-, six-, and seven-membered carbon rings extending the tube sidewall, resulting in overall continued growth of the nanotube on the Fe cluster up to nearly twice its length. Due to the random nature of new polygon formation, sidewall growth is observed as an irregular process without clear SWNT chirality preference. Compared to fullerene formation, heptagon formation is considerably promoted., AMER CHEMICAL SOC, Jul. 2008, ACS NANO, 2 (7), 1437 - 1444, doi;web_of_science

  • Rapid growth of a single-walled carbon nanotube on an iron cluster: Density-functional tight-binding molecular dynamics simulations

    Yasuhito Ohta; Yoshiko Okamoto; Stephan Irle; Keiji Morokuma

    Continued growth of a single-walled carbon nanotube (SWNT) on an Fe cluster at 1500 K is demonstrated using quantum chemical molecular dynamics simulations based on the self-consistent-charge density-functional tight-binding (SCC-DFTB) method. In order to deal with charge transfer between carbon and metal particles and the multitude of electronic states, a finite electronic temperature approach is applied. We present trajectories of 45 ps length, where a continuous supply of carbon atoms is directed toward the C-Fe boundary between a 7.2 angstrom long armchair (5,5) SWNT fragment and an attached Fe-38 cluster. The incident carbon atoms react readily at the C-Fe interface to form C- and C-2-extensions on the tube rim that attach to the Fe cluster. These bridging sp-hybridized carbon fragments are vibrationally excited and highly mobile and, therefore, become engaged in frequent bond formation and breaking processes between their constituent C and the Fe atoms. The sp-hybridized carbon bridge dynamics and their reactions with the Fe-attached nanotube end bring about formations of new five-, six-, and seven-membered carbon rings extending the tube sidewall, resulting in overall continued growth of the nanotube on the Fe cluster up to nearly twice its length. Due to the random nature of new polygon formation, sidewall growth is observed as an irregular process without clear SWNT chirality preference. Compared to fullerene formation, heptagon formation is considerably promoted., AMER CHEMICAL SOC, Jul. 2008, ACS NANO, 2 (7), 1437 - 1444, doi;web_of_science

  • PHYS 479-Growth process of single walled carbon nanotubes from metal cluster: Density functional tight-binding molecular dynamics simulation

    Yasuhito Ohta; Yoshiko Okamoto; Stephan Irle; Keiji Morokuma

    AMER CHEMICAL SOC, Apr. 2008, ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY, 235, web_of_science

    Summary international conference

  • COMP 52-Quantum chemical molecular dynamics simulations of carbon nanotube self-assembly on transition metal catalysts

    Stephan Irle; Yasuhito Ohta; Yoshiko Okamoto; Zhi Wang; Guishan Zheng; Keiji Morokuma

    AMER CHEMICAL SOC, Aug. 2007, ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY, 234, web_of_science

    Summary international conference

  • COMP 115-Quantum chemical molecular dynamics simulations of growth of fullerenes, metallofullerenes and carbon nanotubes

    Keiji Morokuma; Stephan Irle; Zhi Wang; Guishan Zheng; Benjamin Y. Finck; Biswajit Saha; Yasuhito Ohta; Yoshiko Okamoto

    AMER CHEMICAL SOC, Aug. 2007, ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY, 234, web_of_science

    Summary international conference

  • Extended spin-boson model for nonadiabatic hydrogen tunneling in the condensed phase

    Yasuhito Ohta; Alexander V. Soudackov; Sharon Hammes-Schiffer

    A nonadiabatic rate expression for hydrogen tunneling reactions in the condensed phase is derived for a model system described by a modified spin-boson Hamiltonian with a tunneling matrix element exponentially dependent on the hydrogen donor-acceptor distance. In this model, the two-level system representing the localized hydrogen vibrational states is linearly coupled to the donor-acceptor vibrational mode and the harmonic bath. The Hamiltonian also includes bilinear coupling between the donor-acceptor mode and the bath oscillators. This coupling provides a mechanism for energy exchange between the two-level system and the bath through the donor-acceptor mode, thereby facilitating convergence of the time integral of the probability flux correlation function for the case of weak coupling between the two-level system and the bath. The dependence of the rate constant on the model parameters and the temperature is analyzed in various regimes. Anomalous behavior of the rate constant is observed in the weak solvation regime for model systems that lack an effective mechanism for energy exchange between the two-level system and the bath. This theoretical formulation is applicable to a wide range of chemical and biological processes, including neutral hydrogen transfer reactions with small solvent reorganization energies. (c) 2006 American Institute of Physics., AMER INST PHYSICS, Oct. 2006, JOURNAL OF CHEMICAL PHYSICS, 125 (14), 144522/1-16, doi;web_of_science

  • Extended spin-boson model for nonadiabatic hydrogen tunneling in the condensed phase

    Yasuhito Ohta; Alexander V. Soudackov; Sharon Hammes-Schiffer

    A nonadiabatic rate expression for hydrogen tunneling reactions in the condensed phase is derived for a model system described by a modified spin-boson Hamiltonian with a tunneling matrix element exponentially dependent on the hydrogen donor-acceptor distance. In this model, the two-level system representing the localized hydrogen vibrational states is linearly coupled to the donor-acceptor vibrational mode and the harmonic bath. The Hamiltonian also includes bilinear coupling between the donor-acceptor mode and the bath oscillators. This coupling provides a mechanism for energy exchange between the two-level system and the bath through the donor-acceptor mode, thereby facilitating convergence of the time integral of the probability flux correlation function for the case of weak coupling between the two-level system and the bath. The dependence of the rate constant on the model parameters and the temperature is analyzed in various regimes. Anomalous behavior of the rate constant is observed in the weak solvation regime for model systems that lack an effective mechanism for energy exchange between the two-level system and the bath. This theoretical formulation is applicable to a wide range of chemical and biological processes, including neutral hydrogen transfer reactions with small solvent reorganization energies. (c) 2006 American Institute of Physics., AMER INST PHYSICS, Oct. 2006, JOURNAL OF CHEMICAL PHYSICS, 125 (14), 144522/1-16, doi;web_of_science

Teaching Experience

  • 計算科学 (Nara Women's University)



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