Crystal lattice optimization and new forms of silicon
Abstract (Summary)iii In Chapter 1 a basic outline of the two main methods used in this thesis is given. A genetic algorithm optimization method based on the concept of natural selection is given. The important factors to consider in creating an effective genetic algorithm search are described. I then give a brief overview of Density Functional Theory (DFT) which is the technique most commonly used to do ab-inito calculations on solid-state systems. The basis for its formulation along with how it is applied to a practical system with some approximations is discussed. In Chapter 2 a description of a genetic search algorithm for optimizing the crystal structure of an infinite crystal is given. This method is applied to a system of colloidal spheres, where the packing density is the figure of merit for structure selection. Our examination of self-assembled multi-component crystals of nanoparticles predicts several new structures with stoichiometries of AB (fused spheres), ABC2, ABC3, ABC4 and AB2C2. These new structures have hierarchical layered or linear arrangements that could be useful for functional self-assembled systems. For example, the fused-sphere binary crystal assembles with zig-zag rows of parallel nanowires. The genetic search suceeds while a comparable stochastic algorithm fails to find any structures better than the well-known unary or binary phase-separated systems. Here we describe the algorithm and the results it produces: several new classes of binary and ternary crystals of spherical nanoparticles, including a family of layered perovskite-like systems and an unusual threedimensional array of parallel zig-zag nanowires. iv In Chapter 3, We discuss the possibility of constructing new forms of silicon by building in multiple bonds consistent with molecules that have been produced experimentally. We find a dilated diamond crystal lattice containing a silicon-silicon triple bond that is metastable. This structure has very soft vibrational modes that are common in similar structures with buckled bonds (similar to quarts). The crystal may be stabilized by synthesizing in a rare gas environment, where a mixture of krypton and neon or helium may work best. Constructing a crystal lattice out of the trisilaallene central region leads to an unstable crystal structure. The main reason for this instability is due to the flexibility in the location of the sp-hybridized atom. Relaxing this structure leads to a new sp2-sp3 hybrid structure. There are a family of structures with ribbons of graphitic silicon held together by sp3 silicon atoms. The ribbon width can be increased to lead towards more graphitic silicon-like results. Graphitic silicon is predicted to be stabilized under negative pressure. Due to the similarity to graphitic silicon, we expect this family of structures may be stabilized in an environment of negative pressure as well..
School Location:USA - Pennsylvania
Source Type:Master's Thesis
Date of Publication: