Oxygen Vacancy Defect Migration in Titanate Perovskite Surfaces: Effect of the A‑Site Cations

Oxygen vacancy formation energies and migration barriers in (001) surfaces of CaTiO₃, SrTiO₃, and BaTiO₃ have been investigated using first principles density functional theory. The degree of distortion within the TiO₂ sublattice in the presence of defects and consequently the defect formation energies in these titanate surfaces are determined by the size of the A-site cation (Ca²⁺ < Sr²⁺ < Ba²⁺). This is notably the case for CaTiO₃, in which the presence of a vacancy defect leads to a heavily distorted local orthorhombic structure within the (001) slab depending on the defect position, despite the overall cubic symmetry of the material modelled. This effectively leads to the TiO₂ sublattice acting as a thermodynamic trap for oxygen vacancy defects in CaTiO₃. By contrast, calculated vacancy diffusion pathways in SrTiO₃ and BaTiO₃ indicate that vacancy diffusion with these larger A-site cations is kinetically and not thermodynamically controlled

Dynamics of Local Chirality during SWCNT Growth: Armchair versus Zigzag Nanotubes

We present an analysis of the dynamics of single-walled carbon nanotube (SWCNT) chirality during growth, using the recently developed local chirality index (LOCI) method [Kim et al. Phys. Rev. Lett. 2011, 107, 175505] in conjunction with quantum chemical molecular dynamics (QM/MD) simulations. Using (5,5) and (8,0) SWCNT fragments attached to an Fe₃₈ catalyst nanoparticle, growth was induced by periodically placing carbon atoms at the edge of the SWCNT. For both armchair and zigzag SWCNTs, QM/MD simulations indicate that defect healingthe process of defect removal during growthis a necessary, but not sufficient, condition for chirality-controlled SWCNT growth. Time-evolution LOCI analysis shows that healing, while restoring the pristine hexagon structure of the growing SWCNT, also leads to changes in the local chirality of the SWCNT edge region and thus of the entire SWCNT itself. In this respect, we show that zigzag SWCNTs are significantly inferior in maintaining their chirality during growth compared to armchair SWCNTs

Temperature Dependence of Catalyst-Free Chirality-Controlled Single-Walled Carbon Nanotube Growth from Organic Templates

The temperature dependence of catalyst-free single-walled carbon nanotube (SWCNT) growth from organic molecular precursors is investigated using DFTB quantum chemical molecular dynamics simulations and DFT calculations. Growth of (6,6)-SWCNTs from [6]­cycloparaphenylene ([6]­CPP) template molecules was simulated at 300, 500, and 800 K using acetylene (C₂H₂) and ethynyl radicals (C₂H) as growth agents. The highest growth rates were observed with C₂H at 500 K. Higher temperatures lead to increased defect formation in the SWCNT structure during growth. Such defects, which cause the loss of SWCNT chirality control, were driven by radical addition reactions with inherently low kinetic barriers. We therefore propose that lower temperature is optimal for the C₂H radical mechanism of SWCNT growth, and predict the existence of an optimum SWCNT growth temperature that balances the rates of growth and defect formation at a given C₂H/C₂H₂ ratio

Chiral-Selective Carbon Nanotube Etching with Ammonia: A Quantum Chemical Investigation

Density functional theory is employed to demonstrate how ammonia-derived etchant radicals (H, NH, and NH₂) can be used to promote particular (<i>n</i>,<i>m</i>) chirality single-walled carbon nanotube (SWCNT) caps during chemical vapor deposition (CVD) growth. We reveal that the chemical reactivity of these etchant radical species with SWCNTs depends on the SWCNT chirality. This reactivity is determined by the extent of disruption to the π-conjugation of the cap structure caused by reaction with the etchant species. H and NH₂ attack single carbon atoms and preferentially react with near-zigzag SWCNT caps, whereas NH prefers to attack across C–C bonds and selectively etches near-armchair SWCNT caps. We derive a model for predicting abundances of (<i>n</i>,<i>m</i>) SWCNTs in the presence of ammonia-derived radicals, which is consistent with (<i>n</i>,<i>m</i>) distributions observed in recent CVD experiments with ferrocene and ammonia. This model also demonstrates that chiral-selective etching of SWCNTs during CVD growth can be potentially exploited for achieving chirality control in standard CVD synthesis

Revealing the Dual Role of Hydrogen for Growth Inhibition and Defect Healing in Polycyclic Aromatic Hydrocarbon Formation: QM/MD Simulations

Quantum mechanical molecular dynamics simulations are employed to reveal the influence of hydrogen on polycyclic aromatic hydrocarbon (PAH) formation in oxygen-lean combustion. While higher hydrogen concentration leads to the inhibition of PAH growth, it simultaneously facilitates pentagon and heptagon defect healing, leading to thermodynamically more stable PAH fragments with more hexagons. We therefore propose the existence of an optimal H/C ratio that facilitates the growth of all-hexagon-containing PAH species. Analysis of the PAH edge reconstruction in our simulations shows a near-equal ratio of armchair and zigzag edge structures. As armchair edge structures are thermodynamically considerably more stable than zigzag edge structures, the present simulations show that both kinetic and thermodynamic factors are needed to understand PAH/graphene edge reconstruction

Single-walled Carbon Nanotube Growth from Chiral Carbon Nanorings: Prediction of Chirality and Diameter Influence on Growth Rates

Catalyst-free, chirality-controlled growth of chiral and zigzag single-walled carbon nanotubes (SWCNTs) from organic precursors is demonstrated using quantum chemical simulations. Growth of (4,3), (6,5), (6,1), (10,1) and (8,0) SWCNTs was induced by ethynyl radical (C₂H) addition to organic precursors. These simulations show a strong dependence of the SWCNT growth rate on the chiral angle, θ. The SWCNT diameter however does not influence the SWCNT growth rate under these conditions. This agreement with a previously proposed screw-dislocation-like model of transition metal-catalyzed SWCNT growth rates [Ding, F.; Proc. Natl. Acad. Sci. 2009, 106, 2506] indicates that the SWCNT growth rate is an intrinsic property of the SWCNT edge itself. Conversely, we predict that the rate of SWCNT growth <i>via</i> Diels–Alder cycloaddition of C₂H₂ is strongly influenced by the diameter of the SWCNT. We therefore predict the existence of a maximum growth rate for an optimum diameter/chirality combination at a given C₂H/C₂H₂ ratio. We also find that the ability of a SWCNT to avoid defect formation during growth is an intrinsic quality of the SWCNT edge

Single-walled Carbon Nanotube Growth from Chiral Carbon Nanorings: Prediction of Chirality and Diameter Influence on Growth Rates

Catalyst-free, chirality-controlled growth of chiral and zigzag single-walled carbon nanotubes (SWCNTs) from organic precursors is demonstrated using quantum chemical simulations. Growth of (4,3), (6,5), (6,1), (10,1) and (8,0) SWCNTs was induced by ethynyl radical (C₂H) addition to organic precursors. These simulations show a strong dependence of the SWCNT growth rate on the chiral angle, θ. The SWCNT diameter however does not influence the SWCNT growth rate under these conditions. This agreement with a previously proposed screw-dislocation-like model of transition metal-catalyzed SWCNT growth rates [Ding, F.; Proc. Natl. Acad. Sci. 2009, 106, 2506] indicates that the SWCNT growth rate is an intrinsic property of the SWCNT edge itself. Conversely, we predict that the rate of SWCNT growth <i>via</i> Diels–Alder cycloaddition of C₂H₂ is strongly influenced by the diameter of the SWCNT. We therefore predict the existence of a maximum growth rate for an optimum diameter/chirality combination at a given C₂H/C₂H₂ ratio. We also find that the ability of a SWCNT to avoid defect formation during growth is an intrinsic quality of the SWCNT edge

Single-walled Carbon Nanotube Growth from Chiral Carbon Nanorings: Prediction of Chirality and Diameter Influence on Growth Rates

Catalyst-free, chirality-controlled growth of chiral and zigzag single-walled carbon nanotubes (SWCNTs) from organic precursors is demonstrated using quantum chemical simulations. Growth of (4,3), (6,5), (6,1), (10,1) and (8,0) SWCNTs was induced by ethynyl radical (C₂H) addition to organic precursors. These simulations show a strong dependence of the SWCNT growth rate on the chiral angle, θ. The SWCNT diameter however does not influence the SWCNT growth rate under these conditions. This agreement with a previously proposed screw-dislocation-like model of transition metal-catalyzed SWCNT growth rates [Ding, F.; Proc. Natl. Acad. Sci. 2009, 106, 2506] indicates that the SWCNT growth rate is an intrinsic property of the SWCNT edge itself. Conversely, we predict that the rate of SWCNT growth <i>via</i> Diels–Alder cycloaddition of C₂H₂ is strongly influenced by the diameter of the SWCNT. We therefore predict the existence of a maximum growth rate for an optimum diameter/chirality combination at a given C₂H/C₂H₂ ratio. We also find that the ability of a SWCNT to avoid defect formation during growth is an intrinsic quality of the SWCNT edge

Accurate Thermochemical and Kinetic Stabilities of C₈₄ Isomers

Accurate double-hybrid density functional theory and isodesmic-type reaction schemes are utilized to report accurate estimates of the heats of formation (Δ_f<i>H</i>) for all 24 isolated-pentagon-rule isomers of the third most abundant fullerene, C₈₄. Kinetic stabilities of these C₈₄ isomers are also considered via C–C bond cleavage rates (<i>P</i>_cleav) calculated using density functional theory. Our results show that the relative abundance of C₈₄ fullerene isomers observed in arc discharge synthesis is the result of both thermochemical and kinetic factors. This provides timely insight regarding the characterization of several C₈₄ isomers that have been obtained experimentally to date. For instance, the established assignments of C₈₄ isomers of (using the Fowler–Manolopoulos numbering scheme) 22 [<i>D</i>₂(IV)], 23 [<i>D</i>_2<i>d</i>(II)], 19 [<i>D</i>_3<i>d</i>], 24 [<i>D</i>_6<i>h</i>], 11 [<i>C</i>₂(IV)], and 4 [<i>D</i>_2<i>d</i>(I)] are consistent with the relative Δ_f<i>H</i> and <i>P</i>_cleav values for these structures. However, our thermochemical and kinetic stabilities of <i>C</i>_<i>s</i> isomers 14, 15, and 16 indicate that the two experimentally isolated <i>C</i>_<i>s</i> isomers are 15 and 16, contrary to some previous assignments. Of the remaining isolated isomers of symmetry <i>C</i>₂ and <i>D</i>₂, definitive assignment was not possible with consideration of only Δ_f<i>H</i> and <i>P</i>_cleav

Single-walled Carbon Nanotube Growth from Chiral Carbon Nanorings: Prediction of Chirality and Diameter Influence on Growth Rates

Catalyst-free, chirality-controlled growth of chiral and zigzag single-walled carbon nanotubes (SWCNTs) from organic precursors is demonstrated using quantum chemical simulations. Growth of (4,3), (6,5), (6,1), (10,1) and (8,0) SWCNTs was induced by ethynyl radical (C₂H) addition to organic precursors. These simulations show a strong dependence of the SWCNT growth rate on the chiral angle, θ. The SWCNT diameter however does not influence the SWCNT growth rate under these conditions. This agreement with a previously proposed screw-dislocation-like model of transition metal-catalyzed SWCNT growth rates [Ding, F.; Proc. Natl. Acad. Sci. 2009, 106, 2506] indicates that the SWCNT growth rate is an intrinsic property of the SWCNT edge itself. Conversely, we predict that the rate of SWCNT growth <i>via</i> Diels–Alder cycloaddition of C₂H₂ is strongly influenced by the diameter of the SWCNT. We therefore predict the existence of a maximum growth rate for an optimum diameter/chirality combination at a given C₂H/C₂H₂ ratio. We also find that the ability of a SWCNT to avoid defect formation during growth is an intrinsic quality of the SWCNT edge