The directed self-assembly of nanostructures: Electric pressure, dipole, double layer and cracking mechanisms.

摘要

This dissertation presents investigations into the self-assembly of a wide number of material systems. This research introduces the concept that electric fields oriented in the plane of a thin polymer film will induce surface instabilities which grow to form structures. This experimental result indicates that large scale fabrication of nano- and micro-structures in a polymer film can be affectively achieved in an easy to construct and maintain setup. Electrostatic interactions have also been presented as a possible controlling mechanism in forming nanometer sized structure in molecularly thin film. A theoretical and computational investigation presented here indicates that the use of dipolar multilayer system combined with a substrate containing embedded electrodes is capable of fabricating patterns which would be difficult to obtain otherwise. It was observed that subsequent layers in a multilayer system will undergo layer-by-layer alignment and feature size reduction. The result has been applied to the ''molecular car'' concept, by which molecules are able to migrate on a surface due to externally controllable electric field patterns. The scaling and ordering of metallic nanoclusters on semiconductor substrates has been explored computationally. Numerical techniques were implemented to calculate the total electrostatic and van der Waals energies for systems containing multiple disks. Observations show that interactions in charge clouds beneath the metallic disks led to an electrostatic repulsive force. Attraction was observed by the van der Waals energy. An energy barrier past which disk coalescence will not occur was observed. This has applications in the development of large-scale nanodot systems which require precise control over both dot location and size. Finally, this dissertation presents results of systematic investigation of nanocracking in thin films using numerical techniques. Using a level-set method to describe an arbitrary crack in a heterogeneous film the competition between elastic energy release and surface energy gain was obtained. The elastic field was computed using an efficient Fourier space based method. Simulations investigating multiple cracks in heterogeneous films indicate that cracks will preferentially propagate towards regions of low stiffness while avoiding regions of high stiffness. Applications towards the fabrication of nanowires by nanocrack patterns have also been presented.