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Please use this identifier to cite or link to this item: http://hdl.handle.net/123456789/8833

Title: Structural and stability of some carbon and silicon clusters: an ab initio study
Authors: El-Azhary, Adel A.
Al Mogren, Muneerah Mogren Abdulaziz
Keywords: Silicon clusters
Carbon
AB initio
Chemistry method
Stoichiometry
Chainlike structures
HOMO–LUMO
Energy gaps
Neutral
Ring structure
Issue Date: 2008
Abstract: Optimized geometries, energies, vibrational frequencies, HOMO-LUMO energy gap and binding energy of the different possible structures of pure carbon Cn (n = 2–5), pure silicon Sin (n = 2–5), pure nitrogen Nn (n = 5), silicon-carbon SimCn (m = 1–4, n = 1–4, n + m = 2–5), silicon-nitrogen SimNn (m = 1–4, n = 1–4, n + m = 5), carbon-nitrogen CmNn (m = 1–4, n = 1–4, n + m = 5) nanoclusters have been studied within the G3 model. The G3 theory is a compound ab initio chemistry method that encompasses a sequence of calculations at different levels of theory. The calculations were also performed at the HF, B3LYP and MP2 levels of theory using the 6-311+G* basis set. The results have been compared with other theoretical and experimental results available from literature. One of the primary goals of this work is to determine how the structure, bonding and relative stabilities of the clusters are affected by size and stoichiometry. Detailed results are presented on the isomeric energy level differences and binding energies. We summarize the major trends observed for the lowest energy isomers. One of the most important trends seen is the strong tendency for the carbon rich clusters to form a chainlike structures, whereas the silicon rich clusters are either planar or three Dimensional. Mostly, the Si–C bonds are favored over the C–C bonds, in a way that the carbon atoms tend to separate more from each other to form maximum Si–C bonds. The binding energy has a decreasing tendency with the decreasing of the stability of the cluster. This suggests that the energy gain is larger when adding a carbon atom than when adding a silicon atom. On the other hand, the Si–Si bonds in the silicon–nitrogen SimNn clusters are a critical factor governing their stability, more than in the silicon–carbon SimCn clusters. For the species investigated so far, the most stable structures have the least possible number of Si-Si bonds. For the binding energy, the energy gain is larger when adding a nitrogen atom than when adding a silicon atom. The last studied cluster is the mixed is carbon–nitrogen CmNn clusters. The binary cluster ions with a linear chain configuration are those containing a single nitrogen atom. The nitrogen atom is usually located at the end of the carbon chain. The binding energy has a decreasing tendency with the decreasing of the stability of the cluster. This suggests that the energy gain is larger when adding a nitrogen atom than when adding a carbon atom. The C3 cation and N5 neutral, anion and cation have the highest HOMO–LUMO energy gaps. The study concluded that the SiN4 neutral cluster has a high band gap of 13.26229 eV, high binding energy of 5.534287 eV, high ionization potential of 11.9910 eV ,low electron affinity of 0.25116 eV, and high symmetry with electronic state, that assure SiN4 structure has high stability of this cluster. 11A Finally, we found only the C5h of the ring, C5v of the bowl and C2of the cage structures for C20 had no imaginary vibrational frequencies. Geometry optimization was done at the HF, B3LYP and MP2 levels of theory using the 6-31+G* basis set. hC5 vC5 2C Harmonic vibrational frequencies were calculated at the HF and B3LYP levels. We concluded that the neutral ring structure is the most stable. The ionization potential for C20 are 8.080969 eV at HF/6-31+G* and HOMO-LUMO energy gap is 8.52152 eV.
Description: This study has been conducted & Submitted in partial fulfillment of the requirements for the degree of PhD in Quantum Chemistry, Chemistry Department, College of Science, King Saud University Jamadil-Awwal 1429H – May 2008G
URI: http://hdl.handle.net/123456789/8833
Appears in Collections:College of Science

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