Theoretical Studies On Aluminum And Aluminum Oxide Clusters

Aluminum and aluminum based materials have been of wide application in various fields. With the development of technology, aluminum element has occupied important status in processing and developing the novel electronic device, electronic industries, and nanomaterials, etc. The small clusters are expected to have the physical and chemical properties different from those of the bulk material because of their small physical size and quantum effect. A promising area of research on nanoscale materials is to search for clusters that could serve as the building blocks of new materials. Among various clusters, magic clusters have special stability, therefore, they will play important role for the synthesis of novel materials.The aim of this thesis is to study the rule of geometric structure, stability, and structural dependence on size of aluminum and aluminum based clusters, and find structure and material with novel physical and chemical properties. All the calculations were performed by GAUSSIAN 03 program and MATERIALS STUDIO package. The density functional theory (DFT) is used to search stable structures of aluminum and aluminum oxides. The energy, HOMO-LUMO gap, Mulliken and natural charge, molecular orbital pictures and Wiberg bond index have been caculated in order to analyze stabilities and electronic structures for these optimized equilibrium geometries. The main research results are listed as follows: (1) The stable structures, energies, and electronic properties of neutral, cationic, and anionic clusters of Al_n (n = 2 - 10) are studied systematically at the B3LYP/6-311G(2d) level. We find that our optimized structures of Al₅⁺, Al₉⁺, Al₉^-, Al₁₀, Al₁₀⁺, and Al₁₀^- clusters are more stable than the corresponding ones proposed in previous literatures. For the studied neutral aluminum clusters, our results show that the stability has an odd/even alternation phenomenon. We also find that the Al3, Al7, Al₇⁺, and Al₇^- are more stable than their neighbors according to their binding energies. For Al₇⁺ with a special stability, the nucleus-independent chemical shifts and resonance energies are calculated to evaluate its aromaticity. In addition, we present results on hardness, ionization potential, and electron detachment energy.(2) Based on the stable structures of the neutral Al_n (n = 2 - 10) clusters, the Al_nO (n = 2 - 10) clusters are further investigated at B3LYP/6-311G(2d) level and the lowest-energy structures are searched. The structures show that oxygen tends to either be absorbed at the surface of the aluminum clusters or be inserted between Al atoms to form an Al_n-1OAl motif, of which the Al_n-1 part keeps the stable structure of pure aluminum clusters.(3) The structures, binding energies, and electronic properties for Al7X, Al₇X^-, Al₁₃X-, Al₁₃X₂^-, and Al₁₃X₁₂^- (X = F, Cl, Br) were studied at the B3LYP/6-311+G(2d,p) level. Among the systems studied, Al₇ and Al₁₃ clusters in Al₇X and Al₁₃X^- reveal alkali-like and halogen-like superatom characters, respectively. Al₇ can bind with one halogen atom to form a salt-like compound as Al₇^+δ-X^-δ. Al₁₃^- can combine with one halogen atom to form a diatomic halogen anion Al₁₃X^-. However when adding more halogens, the superatom structure would be destroyed, resulting in low symmetry compounds with the center Al atom moving towards the cluster surface. The structures of Al₁₃X_1,2,12^- (X = F, Cl, Br) are similar to those of X = I, however, their binding energies and electron structures are much different. In addition, the analyses of the calculated NBO charges show that Cl and Br have similar properties, but much different from F, when interacting with the Al clusters. The Al-Cl and Al-Br bonds have more covalent character in Al₇X and Al₁₃X_2,12^- in contrast to the corresponding Al-F bond which has prominent ionic character.(4) Structures and stabilities of (Al₂O₃)_n (n = 1 - 10, 15, and 30) cage nanoclusters as well as their onion-like structure are studied by means of first principles calculations. The new compounds possess high symmetries, in which Al and O atoms are saturated and bonding via single bond. The calculated results reveal that the structures of (Al₂O₃)_n (n = 1 - 9) can stabilized existence. In contrast to C₆₀, the Al₆₀O₉₀ can be more stable by putting small size Al₂O₃ cage into them, consistent with the experimental result. Electronic properties of these new kinds of aluminum oxide fullerenes were also discussed.