| An adsorbed monolayer on a solid surface may separate into phases that self-assemble into various two-dimensional patterns on the nanoscale. The limited tunability of the feature size, as well as the long-range disorder, impedes the application of these domain patterns. This thesis investigates why the stable patterns form on solid surfaces, and how to manipulate the patterns by guided self-assembly.; Domain patterns form to minimize the combined free energy of mixing, phase boundary, and elasticity. When the concentration modulates, the surface stress becomes nonuniform, causing an elastic field inside the substrate. Experiments on substrates with various crystalline symmetries enlighten us to use a variety of forms of anisotropy to line up meandering patterns and make ordered structures. We develop a thermodynamic theory to account for the anisotropy in surface stress, substrate stiffness, and phase boundary energy. The elastic field in the anisotropic substrate is solved by a method based on the Stroh formalism. We then focus on the pattern of periodic stripes, and determine the orientation of the stripes that minimizes the free energy. As an example, we examine in detail the (110) surface of a cubic crystal. The stripes can orient along either [1¯10], or [001], or certain directions off the two crystalline axes. Stripes can organize into herringbone structures, further relaxing the elastic energy. Phase diagrams are constructed with respect to varying parameters that characterize anisotropy.; On the basis of available experimental data and theories, we suggest that alkanethiol monolayers on gold should form domain patterns under certain conditions. The effect of surface stress is usually weak. Molecules adsorbed on a substrate surface are electric dipoles. Contact potential between dissimilar domains causes an electrostatic field in the dielectric. The competition between the domain boundary energy and the electrostatic energy selects an equilibrium domain size, which can be tuned by varying the alkyl chain length, by incorporating polar groups into the molecules, and by placing a high-permittivity dielectric crystal at a small gap above the monolayer. The domain patterns observed so far are not organized in long-range. We suggest that the electrostatic interaction between molecular dipoles and an external object placed above should guide the molecules to form designed patterns. The guiding object could be a conducting tip, or a mask with a topographic surface, or a mask with a contact potential field. The process transfers the mask pattern to a molecular pattern on the substrate. Numerical results are presented. |
