Theory and computational studies of magnetic carbon nanotubes and of depletion effects in colloid-polymer systems
Vo, Trinh Thi My
Robert, Marc A.
Doctor of Philosophy
Part I. ab initio Molecular Dynamics of Interaction of Fe Atoms with Single-Wall Carbon Nanotubes. The interaction of Fe atoms with a single-wall carbon nanotube is investigated using the ab initio molecular dynamics method of Car and Parrinello. The variations in stability, band gap, Fermi energy, and total magnetic moment of the Fe-single wall carbon nanotube systems are found to depend on the location of the Fe atoms relative to the carbon nanotube surface. Noteworthy is that the Fe atoms in the Fe-carbon nanotubes systems are coupled ferromagnetically. The curvature effects on the interaction of Fe atoms with carbon nanotubes are also studied by comparing with the Fe-graphite systems. Part II. Phase Transitions and Long-Range Order of Magnetic Carbon Nanotubes. The magnetic coupling between single-wall carbon nanotubes filled with magnetic transition metals, which is assumed to be of the indirect type, is shown to lead to long-ranged ferromagnetic order for arrays of both metallic and semiconducting carbon nanotubes. The critical temperature and spontaneous magnetization are determined. It is found that metallic and semiconducting carbon nanotubes filled with magnetic elements can be turned into magnetic materials. Part III. Computer Simulation of Depletion Effects in Three-Dimensional Colloid-Polymer Systems. The phase behavior of three-dimensional colloid-polymer systems with purely depletion-induced attractions (hard chain polymer and hard sphere colloid) is studied using finite-size scaling and histogram-reweighting Monte Carlo simulations. The nature of the coexisting phases and the phase diagrams are found to depend on the polymer-to-colloid size ratio q. The threshold values of q where liquid-liquid coexistence disappears are found to differ significantly from the value predicted by mean-field theories. Phase separation is found to occur at the "protein limit" of very large polymer and small colloids, in contrast to de Gennes' prediction. Part IV. Depletion Interaction in One Dimension: Short-Range Order. For one-dimensional systems in the continuum in which the particles interact with nearest-neighbor forces, the pair correlation function at short distances can be expressed exactly in a simple form. Results are given for the hard-core interaction potential in which the attractive potential is linear, which represents the depletion interaction potential in one dimension.