A molecular dynamics study of segregation and diffusion in FCC nanocrystals using the sutton-chen potential

English: English: Nanotechnology research has expanded notably, with a wide range of applications from catalysis in fuels, to optics. A key factor in manufacturing these particles is understanding diffusion and segregation of dopants and impurities in the nanocrystals, as segregation of these impurities influences which atom is exposed to the surface of the nano-particle, and able to react. Understanding these processes in terms of the shape and size of the particle, as well as the effects of temperature, are all important factors for nano-material manufacture. Molecular Dynamics software is uniquely able to study the dynamics inside particles of up to several thousand atoms. The Sutton-Chen potential, in particular, is able to simulate the reactions of face-centred cubic (FCC) metals and model bulk modulus, elastic constants, lattice parameters, surface energies, phonon dispersion, cohesion energy and vacancy formation energy. It is ideally suited for studying the diffusion and segregation dynamics of the large clusters of atoms that make up nanocrystals. In this study, a Molecular Dynamics model using the Sutton-Chen potential was built. This model implements the Verlet Velocity scheme to simulate the kinetics of the atoms, and uses the Berendsen thermostat to keep the system at a constant temperature. The model was tested on six FCC metals, namely Al, Ni, Cu, Pd, Ag and Pt, and, making use of periodic boundaries in order to simulate bulk crystals, calculated the cohesion energy to confirm the effectiveness of the model. The model further confirmed surface orientation dependence for low index surfaces. The relationship for vacancy formation energy Ev(111) > Ev(100) > Ev(110) of applied to all the FCC metals studied. The effects of temperature on other diffusion-related energies in the crystals were also studied. It was further found that the diffusion activation energy of FCC metals has the same relationship of Q(110) < Q(100) < Q(111) Equipped with this information, the model was used for in-depth analysis of Cu, and later Ag, nano-cubes, -rhombicuboctahedrons and -octahedrons. A thorough analysis of the surface orientation dependence, size dependence, shape dependence and temperature dependence of key energies involved in diffusion, created a complete picture of nanoparticle stability and surface reactivity. It was found that larger particles are more stable, and that surface reactivity indicates that nano-rhombicuboctahedrons are more reactive than perfect cubes, and that octahedrons are the least surface-reactive. The final part of this study calculated the segregation energy in Ag/Cu systems to confirm the ability of the mixed Sutton-Chen potential to simulate segregation in alloys. In the Ag/Cu system, Ag is known to segregate to the surface, while Cu desegregates, and the model was able to demonstrate this. As this model can successfully reproduce that segregation, it can become a powerful tool for the study of diffusion dynamics in FCC alloy nano-materials.