Keith Beardmore

Molecular dynamics studies of particle impacts with carbon-based materials

Abstract Molecular dynamics simulations of particle impacts on carbon-based materials are described. A number of different semi-empirical, many-body potentials are used and modified to suit the particular problem involved. The simulations include the surface damage, sputtering, and ion reflection from graphite surfaces. The displacement energy thresholds and interstitial formation in graphite is also described. Simulations involving radiation damage in polymers and C60 film growth on Si surfaces are also considered.

Empirical potentials for C[sbnd]Si[sbnd]H systems with application to C60 interactions with Si crystal surfaces

Abstract A semiempirical potential is developed for modelling both the chemistry and the bulk properties of C[sbnd]Si[sbnd]H systems based on the Tersoff formulation. The potentials are compared with the known energetics of small Si[sbnd]H[sbnd]C clusters with good results. The potential is used to investigate the interaction of Ca with hydrogenated crystal surfaces in the energy range 100-250 eV. The simulations show that a wide variety of interactions is possible. The molecule can stick on the surface either directly or by bouncing across the surface. Reflection from the surface is also possible.

Ion bombardment of polyethylene

Abstract The bombardment of polyethylene by argon atoms is examined by molecular dynamics (MD) simulations. The radiation damage causes chain scission, cross linking and carbonisation of the target. The evolution of gas and loss of volatile species in the form of ejected clusters is analysed and the number and type of ejected molecules is quantified. The process by which these effects occur is discussed. It is found that a single impact trajectory may be divided into two parts, the first having a duration of under 0.4 picoseconds and the second lasting for many picoseconds. The initial impact causes bond breaking and sputtering of atoms and small hydrocarbon radicals. At later times recombination takes place between fragments remaining within the target, producing molecules of varying size that diffuse through the target and escape.

A molecular dynamics study of energetic particle impacts on carbon and silicon

Abstract Molecular dynamics simulations of energetic particle interactions with silicon and graphite crystal surfaces are discussed. The simulations can be used to describe the physical state of the surface after such interactions and the dynamic development of the resulting surface damage can be examined. We demonstrate in the case of atomic impact at energies ⪆ a few hundred eV, that craters can form on silicon while bumps are formed on graphite. Impact of C 60 molecules is also discussed. On graphite, hexagonal surface waves propagate from the impact point and surface bonds remain unbroken at energies ≈ a few hundred eV. The interactions with silicon surfaces at impact energies of ≈ few eV depend crucially on the form of the potential. At keV energies the C 60 molecule disintegrates and a large crater forms near the impact point.

ENERGETIC FULLERENE INTERACTIONS WITH SI CRYSTAL-SURFACES

The interaction of fullerene molecules with silicon crystal surfaces is modelled using molecular dynamics. The results are compared to similar interactions with graphite surfaces. In contrast to the results for graphite, it is found that the molecule rarely reflects intact from the surface. When reflection does occur it is always at near grazing incidence with impact energies less than 300 eV. At normal incidence and similar energies the molecule remains intact, but becomes embedded in the surface layers of the Si lattice. Grazing incidence ( approximately=75-80 degrees to the surface normal) at energies of a few hundred eV results in the C60 molecule becoming trapped in the surface binding potential. The molecule can roll across the surface for up to one revolution before coming to rest. At energies of greater than approximately=500 eV, at grazing incidence, the molecule breaks up on impact with the majority of the constituent atoms reflected. Normal incidence with impact energies in excess of 1 keV leads to disintegration of the C60 molecule and sputtering from the crystal, with the ejection of atoms and larger SixCy molecules. This is especially evident at energies greater than 4 keV where high-energy deposition near the impact point creates a crater surrounded by a hot disordered region from which Si atoms can be thermally ejected for times up to the order of 1 ps.

H–C60 and low energy H impact with fullerite

The interaction of energetic hydrogen (25–50 eV) with fullerite is studied theoretically to determine if endohedral H@C60 is feasible. Ab initio quantum calculations are used to calculate the binding energy of various H–C60 configurations and these are used in the fitting of a classical many-body C–H potential. Molecular-dynamics simulations are carried out of the interaction of individual H atoms with a fullerite crystal at both 25 and 50 eV using this classical potential. It is shown to be feasible to implant H atoms with a good probability within the surface layer fullerene molecules, thus suggesting an experimental procedure for the production of H@C60.

C60 film growth and the interaction of fullerenes with bare and H terminated Si surfaces, studied by molecular dynamics

Abstract The interaction of C 60 molecules with energies from 0.1–250 eV with Si{100} surfaces is examined by molecular dynamics simulation. An empirical many-body potential function has been developed to describe interactions between C, Si and H atoms. This is used to compare C 60 interactions with pure and hydrogen terminated Si surfaces. The simulations of fullerene impact demonstrate the resilience of the C 60 cage. At energies up to 250 eV the fullerene can bounce off the surface, but the C 60 -surface interaction is strongly affected by the angle of incidence and the structure of the target surface. Energies greater than around 150 eV can cause damage to the fullerene and usually result in the molecule becoming bound to the surface. The growth of fullerene films from low energy C 60 deposition on a Si surface is studied using a combination of many-body and long-range potential functions. This enables us to observe how the structural properties of the first layers change during film growth. Movies of several simulations have been generated to show details of typical interactions.

Early stages of bump formation on the surface of ion-bombarded graphite

Abstract We investigate possible mechanisms for the formation of bumps, of atomic dimensions, in the surface layer of graphite under low-energy, single ion bombardment. We perform a Molecular Dynamics simulation and analyse the results in view of recent experimental data obtained with the Scanning Tunnelling Microscope. Only the early stages of bump formation are described. The depth profile of momentum deposited in the cascade is evaluated and used, in conjunction with the atomic density profile, to characterise interlayer distortions. The surface bumps appear under the presence of stresses developed in the near-surface layers of the target, due to defects created by the collision cascade. The bump originates around the positions of vacancies and interstitials, and is stabilised by the presence of interlayer interstitials. We have also produced a video film that helps in the visualisation of the time evolution of topographical features.

The computer simulation of energetic particle-solid interactions

Abstract Over the last decade the use of computer simulation in predicting physical phenomena associated with ion beam processing of materials has increased both in use and reliability. This is partly due to the dramatic increase in computer power and decrease in computer cost, but is also being achieved due to an increased understanding of the physical processes occurring. With the increase in computer power has come not just the ability to perform more complex calculations but also the methods for complex data representation in animated form. By animating the results it is much easier to observe collective effects such as acoustic wave propagation at a surface due to, for example, molecule impact.

The interaction of C60 with hydrogen plasma

Abstract This paper describes the results of molecular-dynamics (MD) simulations of low-energy (14–33 eV) H atom interactions with C60 molecules and compares the results of these calculations with some experiments. The simulations predict that the initial energy of the C60 molecules prevents the H atoms from trapping at the centre of the C60 cage but binding of the H atoms externally to the cage can occur. It is also predicted that, if the interactions were to occur with initially ‘cold’ C60 molecules, then trapping of the H atoms at the centre of the cage should be possible. Experiments involving the interaction of a low-energy (≈ 25 eV) H plasma with C60 have also been carried out, and these show no evidence of hydrogen implanted within the cage but do suggest that hydrogen is bound externally to the C60 cage in confirmation of the theoretical predictions.