Professor Alister Page

The defect chemistry of heteroanionic semiconductors can have a large effect on ionic conductivity and optoelectronic properties. Of particular interest are oxynitrides, which have found broad interest recently in sunlight driven water splitting due to their favourable band edge positions while being more stable compared to pure nitrides. The mixed-anion content however creates challenges in terms of retaining nitrogen stoichiometry under the oxidizing conditions of the oxygen evolution reaction (OER), either through formation of a passivating oxide layer or bulk nitrogen loss. In comparison to ternary and perovskite oxynitrides, layered perovskites have potential for improved stability against critical performance limiting defects and present an avenue for improving theoretical water splitting efficiency. In this work, the layer and anion ordering specific O2- and N3- defect properties of Ruddlesden-Popper Sr2TaO3N perovskite oxynitride were extensively investigated using first-principles calculations. We screen anion orderings, then compare anion defect formation and model the e- redistribution across the Sr2+ and Ta5+ sublattices with neutral VO and VN defects. Following this we map the vacancy-mediated anion diffusion pathway barriers with the nudged elastic band (NEB) of O2-/VO and N3-/VN both in the plane of and perpendicular to TaON layers of Sr2TaO3N. The findings of this study suggest that cis- to trans- shifts in the local N3--Ta5+-N3- anion ordering modulate the anisotropy of the vacancy-mediated N3- and O2- diffusion in and out of plane relative to the TaON layer. This work points to the potential of a novel avenue for defect engineering in layered mixed-anion materials and oxynitride photocatalysts.

Electrolytes are central to life and technology but lack complete understanding. Recent experiments with highly concentrated electrolytes have revealed electrostatic decay lengths orders of magnitude larger than those predicted by theory and simulation. This phenomenon, dubbed 'anomalous underscreening' and its origin is still lack a comprehensive understanding. Herein we provide a perspective over recent developments in this field and discuss phenomena that, while potentially pertinent to electrolyte underscreening, are yet to be fully explored - i.e. the 'known-unknowns' of electrostatic underscreening in concentrated electrolytes.

Metal halide perovskites (MHPs) demonstrate an exceptional combination of properties. Rapid progress has extended their application beyond solar cells, light-emitting diodes, photodetectors, and lasers to include memristors, artificial synapse devices, and pressure induced emission. In particular, the vacancy-ordered double perovskite Cs2TiBr6 has been identified as a promising material. The effective characterization of MHPs requires accurate and efficient methods for the calculation of electronic structure. Koopmans compliant (KC) functionals are an accurate and computationally efficient alternative to many-body perturbation theory using the GW approximation but have yet only been validated on a small number of simple materials. In this work, KC functionals were applied to the more complex case of Cs2TiBr6 and gave a zero-temperature fundamental gap of 4.28 eV, in close agreement with the value of 4.44 eV obtained using the accurate, but more computationally expensive, evGW0 approach. The temperature-dependent renormalization of the bandgap has also been investigated and found to be significant. Agreement with the experimental optical bandgaps of 1.76-2.0 eV would also require the inclusion of exciton binding energy.

S-4-methylbenzyl-ß-N-(2-methoxybenzylmethylene)dithiocarbazate ligand, 1, prepared from S-(4-methylbenzyl)dithiocarbazate, was used to produce a novel series of transition metal complexes of the type, [M (L)2] [M = Cu(II) (2), Ni(II) (3), and Zn(II) (4), L = 1]. The ligand and its complexes were investigated by elemental analysis, FTIR, 1H and 13C-NMR, MS spectrometry, and molar conductivity. In addition, single X-ray crystallography was also performed for ligand, 1, and complex 3. The Hirshfeld surface analyses were also performed to know about various bonding interactions in the ligand, 1, and complex 3. The investigated compounds were also tested to evaluate their cytotoxic behaviour. However, complex 2 showed promising results against MCF-7 and MDA-MB-213 cancer cell lines. Furthermore, the interaction of CT-DNA with ligand, 1, and complex 2 was also studied using the electronic absorption method, revealing that the compounds have potential DNA-binding ability via hydrogen bonding and hydrophobic and van der Waals interactions. A molecular docking study of complex 2 was also carried out, which revealed that free binding free energy value was -7.39 kcal mol-1.

Ferrocene (Fc) is an effective precursor for the direct synthesis of high quality single-walled carbon nanotubes (SWCNTs) via floating catalyst chemical vapor deposition (FCCVD). However, the formation mechanism of the Fe floating catalyst and the SWNCT growth precursors, such as carbon chains, during Fc decomposition are not well understood. Here, we report first principles nonequilibrium quantum chemical molecular dynamics simulations that investigate the decomposition of Fc during FCCVD. We examine the influence of additional growth precursors including ethylene, methane, CO, and CO2 on the Fc decomposition mechanism and show that the dissociation of these species into C2Hx radicals and C atoms provides the key growth agents for the nucleation of carbon chains from Fc-derived species such as cyclopentadienyl rings. Without an additional growth precursor, Fc decomposes via the spontaneous cleavage of Fe-C and C-H bonds, thereby enabling Fe atoms to cluster and form the floating catalyst. On the basis of these simulations, we detail the two competing chemical pathways present during the initial stages of FCCVD: Fe catalyst nanoparticle growth and carbon chain growth. The latter is accelerated in the presence of the additional growth precursors, with the identity of the precursor determining the nature of the balance between these competing pathways.

The objective of this review is to summarise the achievements of tin-based compounds as potential anticancer agents in the last 10 years. Tin-based compounds attracted growing attention in many important scientific areas, especially in drug discovery. The motivation of tin-based as anticancer agents arises from the limitations of platinum-based drugs. The major challenge in drug discovery has become to find out the alternatives with enhanced selectivity and specificity towards target biomacromolecules. In this review, we have focussed our attention on tin-based compounds and cytotoxicity studies of tin-based compounds against cancer cells. This mini review also briefly discusses the possible mechanism of action of the tin-based compounds.

Pt is a key high-performing catalyst for important chemical conversions, such as biomass conversion and water splitting. Limited Pt reserves, however, demand that we identify more sustainable alternative catalyst materials for these processes. Here, we combine state-of-the-art graph neural networks and crystal graph machine learning representations with active learning to discover new, low-cost Pt alloy catalysts for biomass reforming and hydrogen evolution reactions. We identify 12 Pt-based alloys which have comparable catalytic activity to that of the exemplar Pt(111) surface. Notably, Cu3Pt and FeCuPt2 exhibit near identical catalytic performance as that of Pt(111). These results demonstrate the potential of machine learning for predicting new catalytic materials without recourse to expensive DFT geometry optimizations, the current bottleneck impeding high-throughput materials discovery. We also examine the performance of d-band theory in elucidating trends in binary and ternary Pt alloys.

Niobium perovskite oxynitrides are emerging as promising semiconductor materials for solar energy conversion processes, due to their physical properties and amenability to defect engineering. However, defect engineering in mixed-anion semiconductors such as perovskite oxynitrides is generally hindered by the absence of long-range order in the crystal lattice, a phenomenon known as anion-ordering. We demonstrate how anion ordering influences the stability and mobility of point defects in two exemplar perovskite oxynitrides, BaNbO2N and LaNbON2. Accurate first-principles calculations show that fully cis-anion orderings in BaNbO2N are more stable than fully trans-anion orderings, whereas anion orderings with mixed dimensionality may be more prevalent in the lower-symmetry LaNbON2. Anion ordering in LaNbON2 is also influenced by a pronounced A-site coordination sphere effect not observed in BaNbO2N, whereby local La-(O,N)12 coordination environments give rise to alternating LaO and LaN layers in the bulk material. Anion order was predicted to effect the redistribution of electrons upon anion vacancy creation to the cation sublattice. Diffusion barriers for O2- vacancies in trans-ordered BaNbO2N were found to be lower than those for N3- vacancies, suggesting that stabilising trans-ordered phases of BaNbO2N will yield more effective retention of nitrogen content in this material. The reverse is the case for LaNbON2, with N3- vacancy defects exhibiting more facile diffusion than O2- vacancy defects. We believe these insights will aid the emergent understanding of defect engineering in mixed-anion perovskite oxynitride semiconductors, and specifically help facilitate strategies for stabilising their nitrogen content.

We present a systematic assessment of the density functional tight binding (DFTB) method for calculating heats of formation of fullerenes with isodesmic-type reaction schemes. We show that DFTB3-D/3ob can accurately predict ¿fH values of the 1812 structural isomers of C60, reproduce subtle trends in ¿fH values for 24 isolated pentagon rule (IPR) isomers of C84, and predict ¿fH values of giant fullerenes that are in effectively exact agreement with benchmark DSD-PBEP86/def2-QZVPP calculations. For fullerenes up to C320, DFTB ¿fH values are within 1.0 kJ mol-1 of DSD-PBEP86/def2-QZVPP values per carbon atom, and on a per carbon atom basis DFTB3-D/3ob yields exactly the same numerical trend of (¿fH [per carbon] = 722n-0.72 + 5.2 kJ mol-1). DFTB3-D/3ob is therefore an accurate replacement for high-level DHDFT and composite thermochemical methods in predicting of thermochemical stabilities of giant fullerenes and analogous nanocarbon architectures.

Life as we know it is dependent upon water, or more specifically salty water. Without dissolved ions, the interactions between biological molecules are insufficiently complex to support life. This complexity is intimately tied to the variation in properties induced by the presence of different ions. These specific ion effects, widely known as Hofmeister effects, have been known for more than 100 years. They are ubiquitous throughout the chemical, biological and physical sciences. The origin of these effects and their relative strengths is still hotly debated. Here we reconsider the origins of specific ion effects through the lens of Coulomb interactions and establish a foundation for anion effects in aqueous and non-aqueous environments. We show that, for anions, the Hofmeister series can be explained and quantified by consideration of site-specific electrostatic interactions. This can simply be approximated by the radial charge density of the anion, which we have calculated for commonly reported ions. This broadly quantifies previously unpredictable specific ion effects, including those known to influence solution properties, virus activities and reaction rates. Furthermore, in non-aqueous solvents, the relative magnitude of the anion series is dependent on the Lewis acidity of the solvent, as measured by the Gutmann Acceptor Number. Analogous SIEs for cations bear limited correlation with their radial charge density, highlighting a fundamental asymmetry in the origins of specific ion effects for anions and cations, due to competing non-Coulombic phenomena. This journal is

Graphene, carbon nanotubes (CNTs) and fullerenes are the basic set of low-dimensional carbon allotropes. The latter two arise from the former by selective removal and addition of carbon atoms. Nevertheless, given their morphological disparities, the production of each is typically devised from entirely different starting points. Here, it is demonstrated that all three allotropes can nucleate from (pseudo-)spherical, nanometer-sized transition metal clusters in a gas-suspension when the chemical conditions are favorable. The experimental results indicate that graphitic carbon embryos nucleate on the catalyst particles and sometimes transform into 2D graphene flakes through chain polymerization of carbon fragments forming in the surround gas atmosphere. It is further shown that hydrogenation reactions play an essential role by stabilizing the emerging flakes by mitigating the pentagon and heptagon defects that lead into evolution of fulleroids. Ab initio molecular dynamics simulations show that the ratio of hydrogen to carbon in the reaction is a key growth parameter. Since structural formation takes place in a gas-suspension, graphene accompanied by fullerenes and single-walled CNTs can be deposited on any surface at ambient temperature with arbitrary layer thicknesses. This provides a direct route for the production and deposition of graphene-based hybrid thin films for various applications.

Covalent organic frameworks (COFs) have attracted much interest due to their utility as functional materials. Unfortunately, experimental synthesis struggles with low single crystallinity of COFs. We have theoretically investigated isomer structures of a representative two-dimensional COF for both monolayer and three-dimensional stacking orders. We show that rotations of p-phenylene rings are common in monolayers, however, affect the global stacking order substantially. We also discuss the discrepancy between powder X-ray diffraction patterns corresponding to the structures predicted by our calculations and those experimentally observed. The discrepancy demonstrates the importance of dynamics in the self-assembly process of COF organic components.

We demonstrate the impact of magnetic moment on single-walled carbon nanotube (SWCNT) nucleation, using quantum chemical molecular dynamics simulations. The mechanism of SWCNT cap formation on Fe38 nanoparticle catalysts can be manipulated by altering the total magnetic moment. When the magnetic moment is low, large cap structures dominated by pentagons are formed. Increasing the magnetic moment leads to smaller, "flatter" sp2 carbon networks with a higher proportion of hexagons, which are incapable of lifting away from the catalyst surface. These results demonstrate that SWCNT (n,m) chirality distributions can be potentially manipulated via altering the magnetic moment of the catalyst-carbon interface. We also show that higher magnetic moments leads to slower SWCNT nucleation. This is a consequence of the extent of Fe ¿ C charge transfer, which is proportional to the total magnetic moment. As the magnetic moment is increased, the Fe38 nanoparticle has higher catalytic activity and is able to stabilise carbon in its subsurface region, thereby impeding the nucleation process.

Polychlorinated dibenzothiophene (PCDT) and polychlorinated thianthrene (PCTA) are sulfur analogues of dioxins, such as polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F). In this work, we present a detailed mechanistic and kinetic analysis of PCDT and PCTA formation from the combustion of 2,4,5-trichlorothiophenol. It is shown that the formation of these persistent organic pollutants is more favourable, both kinetically and thermodynamically, than their analogous dioxin counterparts. This is rationalised in terms of the different influences of the S¿H and O¿H moieties in the 2,4,5-trichlorothiophenol and 2,4,5-trichlorophenol precursors. Kinetic parameters also indicate that the yield of PCDT should exceed that of PCDD. Finally, we demonstrate here that the degree and pattern of chlorination on the 2,4,5-trichlorothiophenol precursor leads to subtle thermodynamic and kinetic changes to the PCDT/PCTA formation mechanisms. [Figure not available: see fulltext.]

Graphene is a 2-dimensional allotrope of carbon with remarkable physicochemical properties. Currently, the most promising route for commercial synthesis of graphene for technological application is chemical vapor deposition (CVD). The optimization of this chemical process will potentially enable control over crucial properties, such as graphene quality and domain size. Such optimization requires a detailed atomistic understanding of how graphene nucleation and growth take place during CVD. This mechanism depends on a multitude of synthetic parameters: temperature, CVD pressure, catalyst type, facet and phase, feedstock type, and the presence of chemical etchants, to name only a few. In this feature article, we highlight recent quantum chemical simulations of chemical vapor deposition (CVD) graphene nucleation and growth. These simulations aim to systematically span this complex CVD "parameter space" toward providing the necessary understanding of graphene nucleation, to assist the optimization of CVD graphene growth.

Atomic force microscopy (AFM), density functional theory (DFT) calculations, and contact angle measurements have been used to investigate the liquid-highly ordered pyrolytic graphite (HOPG) electrode interface for three deep eutectic solvents (DESs) as a function of applied potential. The DESs examined are 1:2 mixtures of choline chloride and urea (ChCl:urea), choline chloride and ethylene glycol (ChCl:ethylene glycol), and choline chloride and glycerol (ChCl:glycerol). DFT calculations reveal that in all cases the molecular component is excluded from the graphite interface at all potentials, while chloride and choline are attracted into the Stern layer at positive and negative potentials, respectively. AFM force curves confirm these trends and also show that the first near surface liquid layer in contact with the Stern layer is rich in the molecular component. The extent of near surface layering increases with potential and the hydrogen bonding capacity of the molecular component. The variation in the macroscopic contact angle with potential is consistent with changes in the Stern layer composition.

Density functional theory is employed to demonstrate how ammonia-derived etchant radicals (H, NH, and NH2) can be used to promote particular (n,m) 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 p-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 (n,m) SWCNTs in the presence of ammonia-derived radicals, which is consistent with (n,m) 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.

We combine combustion experiments and density functional theory (DFT) calculations to investigate the formation of chlorobenzenes from oxidative thermal decomposition of 1,3-dichloropropene. Mono- to hexa-chlorobenzenes are observed between 800 and 1150. K, and the extent of chlorination was proportional to the combustion temperature. Higher chlorinated congeners of chlorobenzene (tetra-, penta-, hexa-chlorobenzene) are only observed in trace amounts between 950 and 1050. K. DFT calculations indicate that cyclisation of chlorinated hexatrienes proceeds via open-shell radical pathways. These species represent key components in the formation mechanism of chlorinated polyaromatic hydrocarbons. Results presented herein should provide better understanding of the evolution of soot from combustion/pyrolysis of short chlorinated alkenes.

© 2014 Elsevier B.V. All rights reserved. We investigate structure and photo-excitation in C60-zinc tetraphenylporphyrin (ZnTPP) and C60F48-ZnTPP complexes, which are promising candidates for organic photovoltaic devices. The C60-ZnTPP complex results from p-p stacking between the fullerene and porphyrin structures, and has a binding energy of 76.0 kJ/mol. Fluorination of the C60 cage leads to decrease in ZnTPP binding, due to reduced p-p stacking interaction. C60-ZnTPP photo-excitation results largely from internal ZnTPP p ¿ p* transitions, although delocalised ZnTPP p ¿ C60 p* transitions are also observed below 300 nm. The more intense photo-excitations of C60F48-ZnTPP arise solely from localised ZnTPP p ¿ p* transitions.

The inability to synthesize single-wall carbon nanotubes (SWCNTs) possessing uniform electronic properties and chirality represents the major impediment to their widespread applications. Recently, there is growing interest to explore and synthesize well-defined carbon nanostructures, including fullerenes, short nanotubes, and sidewalls of nanotubes, aiming for controlled synthesis of SWCNTs. One noticeable advantage of such processes is that no metal catalysts are used, and the produced nanotubes will be free of metal contamination. Many of these methods, however, suffer shortcomings of either low yield or poor controllability of nanotube uniformity. Here, we report a brand new approach to achieve high-efficiency metal-free growth of nearly pure SWCNT semiconductors, as supported by extensive spectroscopic characterization, electrical transport measurements, and density functional theory calculations. Our strategy combines bottom-up organic chemistry synthesis with vapor phase epitaxy elongation. We identify a strong correlation between the electronic properties of SWCNTs and their diameters in nanotube growth. This study not only provides material platforms for electronic applications of semiconducting SWCNTs but also contributes to fundamental understanding of the growth mechanism and controlled synthesis of SWCNTs.

We present quantum chemical simulations demonstrating how single-walled carbon nanotubes (SWCNTs) form, or "nucleate", on the surface of Al2O3 nanoparticles during chemical vapor deposition (CVD) using CH4. SWCNT nucleation proceeds via the formation of extended polyyne chains that only interact with the catalyst surface at one or both ends. Consequently, SWCNT nucleation is not a surface-mediated process. We demonstrate that this unusual nucleation sequence is due to two factors. First, the p interaction between graphitic carbon and Al2O3 is extremely weak, such that graphitic carbon is expected to desorb at typical CVD temperatures. Second, hydrogen present at the catalyst surface actively passivates dangling carbon bonds, preventing a surface-mediated nucleation mechanism. The simulations reveal hydrogen's reactive chemical pathways during SWCNT nucleation and that the manner in which SWCNTs form on Al2O3 is fundamentally different from that observed using "traditional" transition metal catalysts. (Chemical Equation Presented).

We present a quantum chemical investigation of benzofuran and cholorobenzofuran formation mechanisms during the combustion of 1,3-dichloropropene. Density functional theory and Gaussian-n thermochemical methods are used to propose detailed mechanistic reaction pathways. These calculations indicate that oxidation of phenylvinyl radical intermediates and subsequent ring closure are key mechanistic pathways in the formation of benzofuran and chlorobenzofuran. Thermochemical and kinetic parameters presented herein will assist in further elucidation of dioxin formation mechanisms from thermolyses of hydrocarbon moieties.

The discovery of carbon nanotubes (CNTs) and graphene over the last two decades has heralded a new era in physics, chemistry and nanotechnology. During this time, intense efforts have been made towards understanding the atomic-scale mechanisms by which these remarkable nanostructures grow. Molecular simulations have made significant contributions in this regard; indeed, they are responsible for many of the key discoveries and advancements towards this goal. Here we review molecular simulations of CNT and graphene growth, and in doing so we highlight the many invaluable insights gained from molecular simulations into these complex nanoscale self-assembly processes. This review highlights an often-overlooked aspect of CNT and graphene formation - that the two processes, although seldom discussed in the same terms, are in fact remarkably similar. Both can be viewed as a 0D¿1D¿2D transformation, which converts carbon atoms (0D) to polyyne chains (1D) to a complete sp2-carbon network (2D). The difference in the final structure (CNT or graphene) is determined only by the curvature of the catalyst and the strength of the carbon-metal interaction. We conclude our review by summarizing the present shortcomings of CNT/graphene growth simulations, and future challenges to this important area.

Density functional tight binding (DFTB), which is ~100-1000 times faster than full density functional theory (DFT), has been used to simulate the structure and properties of protic ionic liquid (IL) ions, clusters of ions and the bulk liquid. Proton affinities for a wide range of IL cations and anions determined using DFTB generally reproduce G3B3 values to within 5-10 kcal/mol. The structures and thermodynamic stabilities of n-alkyl ammonium nitrate clusters (up to 450 quantum chemical atoms) predicted with DFTB are in excellent agreement with those determined using DFT. The IL bulk structure simulated using DFTB with periodic boundary conditions is in excellent agreement with published neutron diffraction data.

The highly ordered pyrolytic graphite (HOPG)/1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([OMIm]Tf2N) interface is examined by ultrahigh vacuum scanning tunneling microscopy (UHV-STM), atomic force microscopy (UHV-AFM) (and as a function of potential by in situ scanning tunneling microscopy (STM)), in situ atomic force microscopy (AFM), and density functional theory (DFT) calculations. In situ STM and AFM results reveal that multiple ionic liquid (IL) layers are present at the HOPG/electrode interface at all potentials. At open-circuit potential (OCP), attractions between the cation alkyl chain and the HOPG surface result in the ion layer bound to the surface being cation rich. As the potential is varied, the relative concentrations of cations and anions in the surface layer change: as the potential is made more positive, anions are preferentially adsorbed at the surface, while at negative potentials the surface layer is cation rich. At -2 V an unusual overstructure forms. STM images and AFM friction force microscopy measurements both confirm that the roughness of this overstructure increases with time. DFT calculations reveal that [OMIm]+ is attracted to the graphite surface at OCP; however, adsorption is enhanced at negative potentials due to favorable electrostatic interactions, and at -2 V the surface layer is cation rich and strongly bound. The energetically most favorable orientation within this layer is with the [OMIm]+ octyl chains aligned "epitaxially" along the graphitic lattice. This induces quasi-crystallization of cations on the graphite surface and formation of the overstructure. An alternative explanation may be that, because of the bulkiness of the cation sitting along the surface, a single layer of cations is unable to quench the surface potential, so a second layer forms. The most energetically favorable way to do this might be in a quasi-crystalline/multilayered fashion. It could also be a combination of strong surface binding/orientations and the need for multilayers to quench the charge. © 2014 American Chemical Society.

Chemical vapor deposition (CVD) growth of graphene on Cu(111) has been modeled with quantum chemical molecular dynamics (QM/MD) simulations. These simulations demonstrate at the atomic level how graphene forms on copper surfaces. In contrast to other popular catalysts, such as nickel and iron, copper is in a surface molten state throughout graphene growth at CVD-relevant temperatures, and graphene growth takes place without subsurface diffusion of carbon. Surface Cu atoms have remarkably high mobilities on the Cu(111) surface, both before and after graphene nucleation. This surface mobility drives "defect healing" processes in the nucleating graphene structure that convert defects such as pentagons and heptagons into carbon hexagons. Consequently, the graphene defects that become "kinetically trapped" using other catalysts, such as Ni and Fe, are less commonly observed in the case of Cu. We propose this mechanism to be the basis of copper's ability to form high-quality, large-domain graphene flakes. © 2014 the Partner Organisations.

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. © 2013 American Chemical Society.

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 singlewalled carbon nanotube (SWCNT) growth are presented here. The self-consistent-charge densityfunctional 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. Copyright © 2011 American Scientific Publishers.