Nigel A. Marks

Hybrid approach for generating realistic amorphous carbon structure using metropolis and reverse Monte Carlo

An improved method for the modelling of carbon structures based on a hybrid reverse Monte Carlo (HRMC) method is presented. This algorithm incorporates an accurate environment dependent interaction potential (EDIP) in conjunction with the commonly used constraints derived from experimental data. In this work, we compare this new method with other modelling results for a small system of 2.9 g/cc amorphous carbon. We find that the new approach greatly improves the structural description, alleviating the common problem in standard reverse Monte Carlo method (RMC) of generating structures with a high proportion of unphysical small rings. The advantage of our method is that larger systems can now be modelled, allowing the incorporation of mesoscopic scale features.

Modelling diamond-like carbon with the environment-dependent interaction potential

The environment-dependent interaction potential is a transferable empirical potential for carbon which is well suited for studying disordered systems. Ab initio data are used to motivate and parametrize the functional form, which includes environment-dependence in the pair and triple terms, and a generalized aspherical coordination describing dihedral rotation and non-bonded π-repulsion. Simulations of liquid carbon compare very favourably with Car-Parrinello calculations, while amorphous networks generated by liquid quench have properties superior to Tersoff, Brenner and orthogonal tight-binding calculations. The efficiency of the method enables the first simulations of tetrahedral amorphous carbon by deposition, and a new model for the formation of diamond-like bonding is presented.

Dehydroxylation of kaolinite to metakaolin—a molecular dynamics study

The thermally induced transformation of kaolinite to metakaolin is simulated using molecular dynamics through a step-wise dehydroxylation approach. The simulation shows that the removal of structural water through dehydroxylation produces a distortion or buckling effect in the 1 : 1 Al–Si layers, which is due to the migration of the aluminium into vacant sites provided by the inter-layer spacing. The structural change is characterized by a loss of crystallinity and a concomitant change in aluminium coordination from octahedral to tetrahedral, with this study confirming the presence of 5-fold aluminium within the metakaolin structure. The degree and probability of Al migration are proportional to the amount of local disorder within the structure, which is governed by the degree of local hydroxyl group loss. This results in the formation of aluminium clusters within the layers. This study proposes that instead of a uniform structure, metakaolin exhibits regions of differing aluminium concentrations, which can have major effects in the reaction chemistry at those sites.

Mechanism for the Amorphisation of Diamond

The breakdown of the diamond lattice is explored by ion implantation and molecular dynamics simulations. We show that lattice breakdown is strain-driven, rather than damage-driven, and that the lattice persists until 16% of the atoms have been removed from their lattice sites. The figure shows the transition between amorphous carbon and diamond, with the interfaces highlighted with dashed lines.

Atomistic simulation of energy and temperature effects in the deposition and implantation of amorphous carbon thin films

Molecular dynamics simulations of carbon deposition and implantation are performed using the newly developed environment dependent interaction potential. Three scenarios are considered: room temperature deposition, post-deposition implantation and high temperature substrate heating. The room temperature depositions exhibit the characteristic energy dependence observed experimentally and shows that tetrahedral amorphous carbon forms at energies well below the subplantation threshold. In agreement with the experiment, implantation results in graphitisation and stress reduction and the critical dose for maximal change are well predicted. Simulations of ex-situ and in-situ heating investigate kinetic effects and thermal stability respectively, with the latter revealing an unexpected epitaxial growth mode.

Simulating temperature effects in the growth of tetrahedral amorphous carbon: The importance of infrequent events

This thin-film deposition study of tetrahedral amorphous carbon shows that including infrequent processes on the millisecond scale substantially improves the accuracy of molecular dynamics simulations. Elevated temperature between energetic impacts is used to activate processes which are typically ignored. In agreement with experiment, the simulations show an abrupt transition in which diamondlike carbon transforms into vertically oriented graphitic sheets. The simulations also highlight the importance of infrequent events in combination with energetic impact. In the absence of the latter, the transition temperature is significantly higher, in good correlation with experiment.

Surface structure and sputtering in amorphous carbon thin films: a tight-binding study of film deposition

A tight-binding simulation of the atom-by-atom deposition of amorphous carbon (a-C) at 100 eV incident energy is presented. More than 500 atoms were deposited. Chains are observed to form on the surface, some of which are sputtered. The good agreement with the experimental sputter frequency data and observation that all such clusters are linear provides strong support for the existence of these chains and the direct emission model of sputtering. The bulk of the grown film is a-C with a tetrahedral bonding fraction of 20%. Experiments have shown that at this incident energy of 100 eV, tetrahedral a-C is the preferred structural form rather than the a-C produced by this simulation. This discrepancy is attributed to the short range of the interatomic potential.

Fluid dynamic lateral slicing of high tensile strength carbon nanotubes.

Lateral slicing of micron length carbon nanotubes (CNTs) is effective on laser irradiation of the materials suspended within dynamic liquid thin films in a microfluidic vortex fluidic device (VFD). The method produces sliced CNTs with minimal defects in the absence of any chemical stabilizers, having broad length distributions centred at ca 190, 160 nm and 171 nm for single, double and multi walled CNTs respectively, as established using atomic force microscopy and supported by small angle neutron scattering solution data. Molecular dynamics simulations on a bent single walled carbon nanotube (SWCNT) with a radius of curvature of order 10 nm results in tearing across the tube upon heating, highlighting the role of shear forces which bend the tube forming strained bonds which are ruptured by the laser irradiation. CNT slicing occurs with the VFD operating in both the confined mode for a finite volume of liquid and continuous flow for scalability purposes.

Molecular dynamics and experimental studies of preferred orientation induced by compressive stress

Abstract It is well established that ion irradiation of glassy carbon with energetic ions leads to the formation of a dense amorphous surface layer. In this work we show using cross-sectional TEM that oriented graphite-like regions are formed within the implanted layer of glassy carbon implanted with 50 keV C ions at high doses. The preferred orientation is such that the sp 2 bonded graphite-like sheets lie normal to the implanted surface. Stress measurements of the implanted material show the presence of a biaxial compressive stress. Molecular dynamics simulations of a two-dimensional analogue of graphite show a similar preferred orientation effect. Thermodynamic calculations predict that a non-hydrostatic stress can result in preferred orientation in anisotropic materials such as graphite. The preferred orientation can be explained in terms of the combined effects of the mobility introduced by the implanted ions and the anisotropic stress field.

Radioparagenesis: The formation of novel compounds and crystalline structures via radioactive decay

When a crystalline material is made with radioactive isotopes, the structure of that material will change as the radioisotope decays. Using density functional theory, we explore the potential structures formed from this decay, a process we term radioparagenesis. Using three systems as examples – CsCl, SrO, and Lu2O3 – we describe how in each case a here-to-fore unobserved crystalline phase of BaCl, ZrO, and Hf2O3 can be formed, resulting in novel crystalline materials. We examine how the formation of these phases depends on the parent structure and the pathways available to the system upon the decay of the radioisotope. We discuss the implications of this phenomenon for the formation of new materials.