Amitesh Maiti - Academia.edu (original) (raw)

Papers by Amitesh Maiti

Physical Review B, 2002

... Jose A. Rodriguez,* Jan Hrbek, Zhipeng Chang, Joseph Dvorak, and Tomas Jirsak Department of C... more ... Jose A. Rodriguez,* Jan Hrbek, Zhipeng Chang, Joseph Dvorak, and Tomas Jirsak Department of Chemistry, Brookhaven National Laboratory, Upton, New ... 5 JA Rodriguez, T. Jirsak, L. Gonzalez, J. Evans, M. Perez, and A. Maiti, J. Chem. ... 6 A. Bogicevic and DR Jennison, Phys ...

Physical Review B, 1997

Molecular-dynamics and total-energy calculations using a realistic three-body potential for carbo... more Molecular-dynamics and total-energy calculations using a realistic three-body potential for carbon reveal the basic atomic processes by which single-shelled nanotubes can grow out of metal-carbide particles by the root growth mechanism. We find that nanometer-sized protrusions on the metal-particle surface lead to the nucleation of very narrow tubes. Wide bumps lead to a strained graphene sheet and no nanotube growth.

A class II atomistic force field with Lennard-Jones 6-9 nonbond interactions is used to investiga... more A class II atomistic force field with Lennard-Jones 6-9 nonbond interactions is used to investigate equations of state (EOS) for important high explosive detonation products N2 and H2O in the temperature range 700-2500 K and pressure range 0.1-10 GPa. A standard 6th order parameter-mixing scheme is then employed to study a 2:1 (molar) H2O:N2 mixture, to investigate in particular the

Physical review. B, Condensed matter, Jan 15, 1995

The kinetics of carbon nanotube growth under arc discharge conditions were investigated over diff... more The kinetics of carbon nanotube growth under arc discharge conditions were investigated over different length and time scales using complementary numerical techniques. Relaxation by ab initio molecular dynamics (Car-Parrinello method) shows that large electric fields present at the tube tips are not the critical factor responsible for the open-ended growth observed experimentally. Classical molecular-dynamics simulations using realistic many-body carbon-carbon potentials

Physical Review B, 2002

... Jose A. Rodriguez,* Jan Hrbek, Zhipeng Chang, Joseph Dvorak, and Tomas Jirsak Department of C... more ... Jose A. Rodriguez,* Jan Hrbek, Zhipeng Chang, Joseph Dvorak, and Tomas Jirsak Department of Chemistry, Brookhaven National Laboratory, Upton, New ... 5 JA Rodriguez, T. Jirsak, L. Gonzalez, J. Evans, M. Perez, and A. Maiti, J. Chem. ... 6 A. Bogicevic and DR Jennison, Phys ...

The Journal of Physical Chemistry C, 2009

ABSTRACT Polyhedral oligomeric silsesquioxane (POSS)-based amine ligands are used as capping agen... more ABSTRACT Polyhedral oligomeric silsesquioxane (POSS)-based amine ligands are used as capping agents in the reductive growth of Pd nanocrystallites, and the effects of the specific form of the ligand on the structure of both the secondary aggregate and the primary metal nanoparticle are explored. Secondary aggregates of the ligand capped Pd particles are seen to form a uniform structure only if (a) the POSS ligand is totally functionalized with amine groups and (b) the amine groups are in the hydrochloride form. By casting these results in the context of density functional theory calculations, we show that the morphology of Pd core−shell nanomaterials is determined by the ratio of self-interaction potentials of the ligands to their interaction with solvent. The structures of the primary metal crystallites are also shown to be sensitive to the amine ligand in the hydrochloride form. Our results further show that the POSS versions of a ligand will frequently form precipitating capped metal nanoparticle structures, where the discrete molecular forms produce no isolable metal nanoparticles at all on the time scale of the experiments. A study of the diffusion rates of the reactive species in the Pd/POSS-ligand system shows a correlation between the growth of observable metal crystallites and sufficiently slow ligand diffusion kinetics. Finally, we illustrate through a TEM time series that Pd nucleation seems to happen without a prior formation of observable self-assembled POSS ligand templates.

The Journal of Physical Chemistry B, 2006

Chemistry of mixed aromatic-alkyne systems on a metal surface is of general interest in many indu... more Chemistry of mixed aromatic-alkyne systems on a metal surface is of general interest in many industrial processes. We use Density Functional Theory (DFT) to investigate the chemistry of one such system, i.e., 1,4diphenyl-butadiyne, or DPB, in contact with Pd(110) and Pd(111) surfaces. Reaction pathways and energetics of important processes are explored, including H 2 adsorption, dissociation and migration on the metal surface, DPBmetal interaction, the energetics of H uptake, and the effects of impurities like CO and CO 2 on H chemistry. We find that: (i) strong aromatic-metal interaction leads to significant binding strength of DPB molecule to both Pd surfaces, especially the (110); (ii) H 2 molecule readily dissociates on the Pd surface into H-radicals, which get taken up by alkyne triple bonds; (iii) CO has strong binding to the metal surface, but interacts weakly with H radicals; (iv) CO 2 binds weakly to the metal surface, but could potentially lead to interesting chemical reactions with H.

The Journal of Physical Chemistry B, 2003

... (28) McCarthy, B.; Coleman, JN; Czerw ... Fabien A. Lemasson, Timo Strunk, Peter Gerstel, Fra... more ... (28) McCarthy, B.; Coleman, JN; Czerw ... Fabien A. Lemasson, Timo Strunk, Peter Gerstel, Frank Hennrich, Sergei Lebedkin, Christopher Barner-Kowollik, Wolfgang Wenzel, Manfred M. Kappes, and Marcel Mayor. Journal of the American Chemical Society 2011 133 (4), 652-655. ...

Physical Review Letters, 2002

Atomistic simulations using a combination of classical forcefield and Density-Functional-Theory (... more Atomistic simulations using a combination of classical forcefield and Density-Functional-Theory (DFT) show that carbon atoms remain essentially sp 2 coordinated in either bent tubes or tubes pushed by an atomically sharp AFM tip. Subsequent Green's-function-based transport calculations reveal that for armchair tubes there is no significant drop in conductance, while for zigzag tubes the conductance can drop by several orders of magnitude in AFM-pushed tubes. The effect can be attributed to simple stretching of the tube under tip deformation, which opens up an energy gap at the Fermi surface.

Physical Review Letters, 1994

The growth energetics of carbon nanotubes during arc discharge conditions are investigated. Ab in... more The growth energetics of carbon nanotubes during arc discharge conditions are investigated. Ab initio molecular dynamics calculations show that the electric field alone cannot stabilize the growth of open metallic tubes. The addition of atoms and small clusters to tubes were studied using realistic atomic potentials. Deposition on tubes narrower than ~3 nm leads to nucleation of curved defects (adjacent

Physical Review Letters, 2001

Recent experiments indicate that water molecules adsorbed on carbon nanotube tips significantly e... more Recent experiments indicate that water molecules adsorbed on carbon nanotube tips significantly enhance field-emission current. Through first-principles density-functional theory calculations we show that the water-nanotube interaction is weak in zero electric field. However, under emission conditions large electric field present at the tube tip: (a) increases the binding energy appreciably, thereby stabilizing the adsorbate; and (b) lowers the ionization potential (IP), thereby making it easier to extract electrons. Lowering of IP is enhanced further through the formation of a water cluster on the nanotube tip.

Physical Review Letters, 1998

Physical Review E, 2010

We have performed molecular-dynamics simulations of dynamic compression waves propagating through... more We have performed molecular-dynamics simulations of dynamic compression waves propagating through amorphous carbon using the Tersoff potential and find that a variety of dynamic compression features appear for two different initial densities. These features include steady elastic shocks, steady chemically reactive shocks, unsteady elastic waves, and unsteady chemically reactive waves. We show how these features can be distinguished by analyzing time-dependent propagation speeds, time-dependent sound speeds, and comparison to multiscale shock technique (MSST) simulations. Understanding such features is a key challenge in quasi-isentropic experiments involving phase transformations. In addition to direct simulations of dynamic compression, we employ the MSST and find agreement with the direct method for this system for the shocks observed. We show how the MSST can be extended to include explicit material viscosity and demonstrate on an amorphous Lennard-Jones system.

physica status solidi (b), 2002

ABSTRACT

Nature Chemistry, 2010

Delivery of prebiotic compounds to early Earth from an impacting comet is thought to be an unlike... more Delivery of prebiotic compounds to early Earth from an impacting comet is thought to be an unlikely mechanism for the origins of life because of unfavourable chemical conditions on the planet and the high heat from impact. In contrast, we find that impact-induced shock compression of cometary ices followed by expansion to ambient conditions can produce complexes that resemble the amino acid glycine. Our ab initio molecular dynamics simulations show that shock waves drive the synthesis of transient C-N bonded oligomers at extreme pressures and temperatures. On post impact quenching to lower pressures, the oligomers break apart to form a metastable glycine-containing complex. We show that impact from cometary ice could possibly yield amino acids by a synthetic route independent of the pre-existing atmospheric conditions and materials on the planet.

Nano Letters, 2003

SnO 2 nanoribbons with exposed (1 0 1 h) and (0 1 0) surfaces have recently been demonstrated to ... more SnO 2 nanoribbons with exposed (1 0 1 h) and (0 1 0) surfaces have recently been demonstrated to be highly effective NO 2 sensors even at room temperature. The sensing mechanism is examined here through first principles density functional theory (DFT) calculations. We show that the most stable adsorbed species involve an unexpected NO 3 group doubly bonded to Sn centers. Significant electron transfer to the adatoms explains an orders-of-magnitude drop in electrical conductance. X-ray absorption spectroscopy indicates predominantly NO 3 species on the surface, and computed binding energies are consistent with adsorbate stability up to 700 K. Nanoribbon responses to O 2 and CO sensing are also investigated.

Molecular Simulation, 2005

Carbon nanotube (CNT)-polymer composites, with potential applications in structural materials, op... more Carbon nanotube (CNT)-polymer composites, with potential applications in structural materials, optoelectronics, sensors, biocatalysis, and thermal and electromagnetic shielding are an important emerging area of nanotechnology. However, progress has been slow due to difficulties in dispersing CNTs into the polymer matrix. We attack the problem from a Flory-Huggins point of view, and present novel simulations of the dispersion process at the mesoscale. The solubility parameter of the CNTs is mapped out as a function of tube diameter, and compared with that of well-known polymers. Parallel alignment of CNTs with the application of shear, and dispersion by attaching organic functional groups are also investigated.

Molecular Simulation, 2000

The growth of carbon nanotubes was investigated using a variety of complementary simulation techn... more The growth of carbon nanotubes was investigated using a variety of complementary simulation techniques. Currently, a number of experimental methods are used to synthesize carbon nanotubes suggesting that different mechanisms play a role in their formation. However, it has been shown that growth of nanotubes takes place primarily at the open-ended tips of nanotubes. Ab initio simulations show that the high electric fields present at the nanotube tips in carbon arc discharges cannot be responsible for keeping the tubes open. Rather, the opening and closing of tubes is controlled by the formation of curvature-inducing defects such as adjacent pentagon pairs. On narrow tubes, the formation of such defects is favored leading to the rapid closure of the tubes. By contrast, the formation of hexagons, which lead to straight open-ended growth is favored on large-diameter tubes, with an estimated crossover radius of about 3 nm. Large-scale molecular dynamics and kinetic Monte Carle simulations have been used to verify these ideas. We have also explored the role of the so-called lip–lip interactions during growth. Such an interaction is important in producing multiwalled nanotubes, where the interaction between two open nanotube tips leads to the formation of a network of bonds. Simulations show that such an interaction is indeed significant, but does not provide the additional stabilization required for straight, open-ended, multiwalled nanotube growth. Finally, we consider the formation of nanotubes in the presence of large and small catalytic particles. In the former case, growth is believed to take place via a root-growth mechanism, while the direct adsorption and extrusion of carbon from the vapor dominates the latter. Both mechanisms lead to the formation of small-diameter, single-wall nanotubes.

Macromolecules, 2007

Atomistic simulations are used to study the adhesion properties of amorphous perfluoro- and fluor... more Atomistic simulations are used to study the adhesion properties of amorphous perfluoro- and fluoro-polymers onto two different crystal surfaces of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). Properties of the bulk amorphous polymer melts are also investigated. The fluoropolymers studied in this article include Kel-F 800, Teflon{reg_sign} AF, Hyflon AD{reg_sign}, and Cytop{reg_sign}. Simulations of the bulk polymer melts were performed over a wide range of

Journal of the American Chemical Society, 2002

Synchrotron-based high-resolution photoemission and first-principles density-functional slab calc... more Synchrotron-based high-resolution photoemission and first-principles density-functional slab calculations were used to study the interaction of gold with titania and the chemistry of SO(2) on Au/TiO(2)(110) surfaces. The deposition of Au nanoparticles on TiO(2)(110) produces a system with an extraordinary ability to adsorb and dissociate SO(2). In this respect, Au/TiO(2) is much more chemically active than metallic gold or stoichiometric titania. On Au(111) and rough polycrystalline surfaces of gold, SO(2) bonds weakly and desorbs intact at temperatures below 200 K. For the adsorption of SO(2) on TiO(2)(110) at 300 K, SO(4) is the only product (SO(2) + O(oxide) --> SO(4,ads)). In contrast, Au/TiO(2)(110) surfaces (theta;(Au) < or = 0.5 ML) fully dissociate the SO(2) molecule under identical reaction conditions. Interactions with titania electronically perturb gold, making it more chemically active. Furthermore, our experimental and theoretical results show quite clearly that not only gold is perturbed when gold and titania interact. The adsorbed gold, on its part, enhances the reactivity of titania by facilitating the migration of O vacancies from the bulk to the surface of the oxide. In general, the complex coupling of these phenomena must be taken into consideration when trying to explain the unusual chemical and catalytic activity of Au/TiO(2). In many situations, the oxide support can be much more than a simple spectator.

Physical Review B, 2002

... Jose A. Rodriguez,* Jan Hrbek, Zhipeng Chang, Joseph Dvorak, and Tomas Jirsak Department of C... more ... Jose A. Rodriguez,* Jan Hrbek, Zhipeng Chang, Joseph Dvorak, and Tomas Jirsak Department of Chemistry, Brookhaven National Laboratory, Upton, New ... 5 JA Rodriguez, T. Jirsak, L. Gonzalez, J. Evans, M. Perez, and A. Maiti, J. Chem. ... 6 A. Bogicevic and DR Jennison, Phys ...

Physical Review B, 1997

Molecular-dynamics and total-energy calculations using a realistic three-body potential for carbo... more Molecular-dynamics and total-energy calculations using a realistic three-body potential for carbon reveal the basic atomic processes by which single-shelled nanotubes can grow out of metal-carbide particles by the root growth mechanism. We find that nanometer-sized protrusions on the metal-particle surface lead to the nucleation of very narrow tubes. Wide bumps lead to a strained graphene sheet and no nanotube growth.

A class II atomistic force field with Lennard-Jones 6-9 nonbond interactions is used to investiga... more A class II atomistic force field with Lennard-Jones 6-9 nonbond interactions is used to investigate equations of state (EOS) for important high explosive detonation products N2 and H2O in the temperature range 700-2500 K and pressure range 0.1-10 GPa. A standard 6th order parameter-mixing scheme is then employed to study a 2:1 (molar) H2O:N2 mixture, to investigate in particular the

Physical review. B, Condensed matter, Jan 15, 1995

The kinetics of carbon nanotube growth under arc discharge conditions were investigated over diff... more The kinetics of carbon nanotube growth under arc discharge conditions were investigated over different length and time scales using complementary numerical techniques. Relaxation by ab initio molecular dynamics (Car-Parrinello method) shows that large electric fields present at the tube tips are not the critical factor responsible for the open-ended growth observed experimentally. Classical molecular-dynamics simulations using realistic many-body carbon-carbon potentials

Physical Review B, 2002

... Jose A. Rodriguez,* Jan Hrbek, Zhipeng Chang, Joseph Dvorak, and Tomas Jirsak Department of C... more ... Jose A. Rodriguez,* Jan Hrbek, Zhipeng Chang, Joseph Dvorak, and Tomas Jirsak Department of Chemistry, Brookhaven National Laboratory, Upton, New ... 5 JA Rodriguez, T. Jirsak, L. Gonzalez, J. Evans, M. Perez, and A. Maiti, J. Chem. ... 6 A. Bogicevic and DR Jennison, Phys ...

The Journal of Physical Chemistry C, 2009

ABSTRACT Polyhedral oligomeric silsesquioxane (POSS)-based amine ligands are used as capping agen... more ABSTRACT Polyhedral oligomeric silsesquioxane (POSS)-based amine ligands are used as capping agents in the reductive growth of Pd nanocrystallites, and the effects of the specific form of the ligand on the structure of both the secondary aggregate and the primary metal nanoparticle are explored. Secondary aggregates of the ligand capped Pd particles are seen to form a uniform structure only if (a) the POSS ligand is totally functionalized with amine groups and (b) the amine groups are in the hydrochloride form. By casting these results in the context of density functional theory calculations, we show that the morphology of Pd core−shell nanomaterials is determined by the ratio of self-interaction potentials of the ligands to their interaction with solvent. The structures of the primary metal crystallites are also shown to be sensitive to the amine ligand in the hydrochloride form. Our results further show that the POSS versions of a ligand will frequently form precipitating capped metal nanoparticle structures, where the discrete molecular forms produce no isolable metal nanoparticles at all on the time scale of the experiments. A study of the diffusion rates of the reactive species in the Pd/POSS-ligand system shows a correlation between the growth of observable metal crystallites and sufficiently slow ligand diffusion kinetics. Finally, we illustrate through a TEM time series that Pd nucleation seems to happen without a prior formation of observable self-assembled POSS ligand templates.

The Journal of Physical Chemistry B, 2006

Chemistry of mixed aromatic-alkyne systems on a metal surface is of general interest in many indu... more Chemistry of mixed aromatic-alkyne systems on a metal surface is of general interest in many industrial processes. We use Density Functional Theory (DFT) to investigate the chemistry of one such system, i.e., 1,4diphenyl-butadiyne, or DPB, in contact with Pd(110) and Pd(111) surfaces. Reaction pathways and energetics of important processes are explored, including H 2 adsorption, dissociation and migration on the metal surface, DPBmetal interaction, the energetics of H uptake, and the effects of impurities like CO and CO 2 on H chemistry. We find that: (i) strong aromatic-metal interaction leads to significant binding strength of DPB molecule to both Pd surfaces, especially the (110); (ii) H 2 molecule readily dissociates on the Pd surface into H-radicals, which get taken up by alkyne triple bonds; (iii) CO has strong binding to the metal surface, but interacts weakly with H radicals; (iv) CO 2 binds weakly to the metal surface, but could potentially lead to interesting chemical reactions with H.

The Journal of Physical Chemistry B, 2003

... (28) McCarthy, B.; Coleman, JN; Czerw ... Fabien A. Lemasson, Timo Strunk, Peter Gerstel, Fra... more ... (28) McCarthy, B.; Coleman, JN; Czerw ... Fabien A. Lemasson, Timo Strunk, Peter Gerstel, Frank Hennrich, Sergei Lebedkin, Christopher Barner-Kowollik, Wolfgang Wenzel, Manfred M. Kappes, and Marcel Mayor. Journal of the American Chemical Society 2011 133 (4), 652-655. ...

Physical Review Letters, 2002

Atomistic simulations using a combination of classical forcefield and Density-Functional-Theory (... more Atomistic simulations using a combination of classical forcefield and Density-Functional-Theory (DFT) show that carbon atoms remain essentially sp 2 coordinated in either bent tubes or tubes pushed by an atomically sharp AFM tip. Subsequent Green's-function-based transport calculations reveal that for armchair tubes there is no significant drop in conductance, while for zigzag tubes the conductance can drop by several orders of magnitude in AFM-pushed tubes. The effect can be attributed to simple stretching of the tube under tip deformation, which opens up an energy gap at the Fermi surface.

Physical Review Letters, 1994

The growth energetics of carbon nanotubes during arc discharge conditions are investigated. Ab in... more The growth energetics of carbon nanotubes during arc discharge conditions are investigated. Ab initio molecular dynamics calculations show that the electric field alone cannot stabilize the growth of open metallic tubes. The addition of atoms and small clusters to tubes were studied using realistic atomic potentials. Deposition on tubes narrower than ~3 nm leads to nucleation of curved defects (adjacent

Physical Review Letters, 2001

Recent experiments indicate that water molecules adsorbed on carbon nanotube tips significantly e... more Recent experiments indicate that water molecules adsorbed on carbon nanotube tips significantly enhance field-emission current. Through first-principles density-functional theory calculations we show that the water-nanotube interaction is weak in zero electric field. However, under emission conditions large electric field present at the tube tip: (a) increases the binding energy appreciably, thereby stabilizing the adsorbate; and (b) lowers the ionization potential (IP), thereby making it easier to extract electrons. Lowering of IP is enhanced further through the formation of a water cluster on the nanotube tip.

Physical Review Letters, 1998

Physical Review E, 2010

We have performed molecular-dynamics simulations of dynamic compression waves propagating through... more We have performed molecular-dynamics simulations of dynamic compression waves propagating through amorphous carbon using the Tersoff potential and find that a variety of dynamic compression features appear for two different initial densities. These features include steady elastic shocks, steady chemically reactive shocks, unsteady elastic waves, and unsteady chemically reactive waves. We show how these features can be distinguished by analyzing time-dependent propagation speeds, time-dependent sound speeds, and comparison to multiscale shock technique (MSST) simulations. Understanding such features is a key challenge in quasi-isentropic experiments involving phase transformations. In addition to direct simulations of dynamic compression, we employ the MSST and find agreement with the direct method for this system for the shocks observed. We show how the MSST can be extended to include explicit material viscosity and demonstrate on an amorphous Lennard-Jones system.

physica status solidi (b), 2002

ABSTRACT

Nature Chemistry, 2010

Delivery of prebiotic compounds to early Earth from an impacting comet is thought to be an unlike... more Delivery of prebiotic compounds to early Earth from an impacting comet is thought to be an unlikely mechanism for the origins of life because of unfavourable chemical conditions on the planet and the high heat from impact. In contrast, we find that impact-induced shock compression of cometary ices followed by expansion to ambient conditions can produce complexes that resemble the amino acid glycine. Our ab initio molecular dynamics simulations show that shock waves drive the synthesis of transient C-N bonded oligomers at extreme pressures and temperatures. On post impact quenching to lower pressures, the oligomers break apart to form a metastable glycine-containing complex. We show that impact from cometary ice could possibly yield amino acids by a synthetic route independent of the pre-existing atmospheric conditions and materials on the planet.

Nano Letters, 2003

SnO 2 nanoribbons with exposed (1 0 1 h) and (0 1 0) surfaces have recently been demonstrated to ... more SnO 2 nanoribbons with exposed (1 0 1 h) and (0 1 0) surfaces have recently been demonstrated to be highly effective NO 2 sensors even at room temperature. The sensing mechanism is examined here through first principles density functional theory (DFT) calculations. We show that the most stable adsorbed species involve an unexpected NO 3 group doubly bonded to Sn centers. Significant electron transfer to the adatoms explains an orders-of-magnitude drop in electrical conductance. X-ray absorption spectroscopy indicates predominantly NO 3 species on the surface, and computed binding energies are consistent with adsorbate stability up to 700 K. Nanoribbon responses to O 2 and CO sensing are also investigated.

Molecular Simulation, 2005

Carbon nanotube (CNT)-polymer composites, with potential applications in structural materials, op... more Carbon nanotube (CNT)-polymer composites, with potential applications in structural materials, optoelectronics, sensors, biocatalysis, and thermal and electromagnetic shielding are an important emerging area of nanotechnology. However, progress has been slow due to difficulties in dispersing CNTs into the polymer matrix. We attack the problem from a Flory-Huggins point of view, and present novel simulations of the dispersion process at the mesoscale. The solubility parameter of the CNTs is mapped out as a function of tube diameter, and compared with that of well-known polymers. Parallel alignment of CNTs with the application of shear, and dispersion by attaching organic functional groups are also investigated.

Molecular Simulation, 2000

The growth of carbon nanotubes was investigated using a variety of complementary simulation techn... more The growth of carbon nanotubes was investigated using a variety of complementary simulation techniques. Currently, a number of experimental methods are used to synthesize carbon nanotubes suggesting that different mechanisms play a role in their formation. However, it has been shown that growth of nanotubes takes place primarily at the open-ended tips of nanotubes. Ab initio simulations show that the high electric fields present at the nanotube tips in carbon arc discharges cannot be responsible for keeping the tubes open. Rather, the opening and closing of tubes is controlled by the formation of curvature-inducing defects such as adjacent pentagon pairs. On narrow tubes, the formation of such defects is favored leading to the rapid closure of the tubes. By contrast, the formation of hexagons, which lead to straight open-ended growth is favored on large-diameter tubes, with an estimated crossover radius of about 3 nm. Large-scale molecular dynamics and kinetic Monte Carle simulations have been used to verify these ideas. We have also explored the role of the so-called lip–lip interactions during growth. Such an interaction is important in producing multiwalled nanotubes, where the interaction between two open nanotube tips leads to the formation of a network of bonds. Simulations show that such an interaction is indeed significant, but does not provide the additional stabilization required for straight, open-ended, multiwalled nanotube growth. Finally, we consider the formation of nanotubes in the presence of large and small catalytic particles. In the former case, growth is believed to take place via a root-growth mechanism, while the direct adsorption and extrusion of carbon from the vapor dominates the latter. Both mechanisms lead to the formation of small-diameter, single-wall nanotubes.

Macromolecules, 2007

Atomistic simulations are used to study the adhesion properties of amorphous perfluoro- and fluor... more Atomistic simulations are used to study the adhesion properties of amorphous perfluoro- and fluoro-polymers onto two different crystal surfaces of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). Properties of the bulk amorphous polymer melts are also investigated. The fluoropolymers studied in this article include Kel-F 800, Teflon{reg_sign} AF, Hyflon AD{reg_sign}, and Cytop{reg_sign}. Simulations of the bulk polymer melts were performed over a wide range of

Journal of the American Chemical Society, 2002

Synchrotron-based high-resolution photoemission and first-principles density-functional slab calc... more Synchrotron-based high-resolution photoemission and first-principles density-functional slab calculations were used to study the interaction of gold with titania and the chemistry of SO(2) on Au/TiO(2)(110) surfaces. The deposition of Au nanoparticles on TiO(2)(110) produces a system with an extraordinary ability to adsorb and dissociate SO(2). In this respect, Au/TiO(2) is much more chemically active than metallic gold or stoichiometric titania. On Au(111) and rough polycrystalline surfaces of gold, SO(2) bonds weakly and desorbs intact at temperatures below 200 K. For the adsorption of SO(2) on TiO(2)(110) at 300 K, SO(4) is the only product (SO(2) + O(oxide) --> SO(4,ads)). In contrast, Au/TiO(2)(110) surfaces (theta;(Au) < or = 0.5 ML) fully dissociate the SO(2) molecule under identical reaction conditions. Interactions with titania electronically perturb gold, making it more chemically active. Furthermore, our experimental and theoretical results show quite clearly that not only gold is perturbed when gold and titania interact. The adsorbed gold, on its part, enhances the reactivity of titania by facilitating the migration of O vacancies from the bulk to the surface of the oxide. In general, the complex coupling of these phenomena must be taken into consideration when trying to explain the unusual chemical and catalytic activity of Au/TiO(2). In many situations, the oxide support can be much more than a simple spectator.