Nanostructures of Silicon

The major objective of this Thesis work is to search for optimal structures of small and intermediate sized silicon clusters, and to study the physical and chemical properties of these clusters. Stuffed fullerene model was proposed to generate intermediate sized silicon clusters in a systematical fashion. Ideal structures of chemically unreactive clusters: Si _{33}, Si_{39 } and Si_{45}, were generated by this model. The structures thus generated were characterized by a unique surface structure with dangling bonds all saturated. They were found to resemble certain unreactive structures on the Si(111)-7 x 7 reconstruction surface. And this explains why such magic number clusters are least reactive in the chemisorption experiments. An efficient optimization package (TB-MD) has been developed for the structural optimization of small and intermediate sized silicon clusters. Non-orthogonal Tight-binding Hamiltonian was used, and the algorithm of Simulated Annealing with Molecular Dynamics (MD) was used for the searching along the complex, many dimensional potential surface. Exhaustive TB-MD simulations have been performed for small silicon clusters up to 10 atoms. The resulting lowest energy structures were in excellent agreement with those obtained from exact quantum calculations. Restricted TB-MD simulations were performed for magic number clusters Si_{33}, Si_ {39}, and Si_{45}, and energetically stable structures were obtained for these clusters. Further LDA studies were performed for the original structures obtained from stuffed fullerene model and the final structures obtained from TB-MD simulations. These structures were found to be energetically stable in the LDA studies. Vibrational frequencies of all normal modes have been calculated for small silicon clusters. Reasonable agreement with the exact quantum calculations was achieved. And satisfactory agreement with experimental studies of Raman spectra of Si_4, Si_7 and Si_7 was obtained. Vibrational studies have also been applied to larger silicon clusters. Two different structures of Si_{45} were studied, and two different patterns of vibrational frequencies were obtained. The vibrational studies indicate that signatures other than cohesive energies exist for different cluster structures. Such signatures could be very valuable in identifying ground state structures of silicon clusters, especially when the difference in cohesive energy becomes too small to be distinguished from calculations.