Theoretical Studies On The Growth Pattern And Optical Property Of Silicon Nanoclusters And Sensing Mechanism Of SnO₂

As an intermediate class of structures between single atom and a bulk solid,atomic clusters have attracted continuous interests over decades. One of the most interesting outstanding questions, despite of intensive studies, is how big does a cluster take the corresponding bulk bonding geometry? This question is particularly important for silicon clusters due to the dominate role silicon plays in the modern advanced technologies. Silicon nanoclusters（NCs） with bulk diamond bonding geometry exhibit unique optical and electronic properties due to the quantum confinement effects related to the size of the NCs. These unique optical and electronic properties are significantly different from that of bulk crystalline Si and make Si NCs promising for applications in many advanced nanotechnologies.In this thesis, we have investigated semiconductor silicon nanoclusters using the TB（tight-binding） and density functional theory（DFT） calculations. We try to summary the growth patterns of Si clusters, and predict transition scale of bulk-like.On this basis, the optical absorption spectra of hydrogen passivated Si NCs are computed using TDDFT method. At the same time, the optical properties of Si NCs by coating fullerene carbon cage were calculated using TDDFT method. On the other hand, the CO adsorption on the In-doped SnO₂（110） surface has been studied by the first-principles calculations based on density functional theory, we try to understand the reaction pathways of CO and Sn O₂（110） surface, and thorough understand the reaction mechanism of CO on In-doped SnO₂（110） surface.The main results are lists as follows:（1） We performed a comprehensive study for Si₂₂₀ NCs using the TB and DFT calculations. Several structural motifs including the bulk-like, icosahedral,bucky-diamond, and onion-like structures have been investigated. The bulk-like structures are found to be most stable than other motifs in Si₂₂₀, and the structures exhibit a dominate portion of diamond-like structures passivated by the reconstructed Si（111） facets. We also showed that the bulk-like structures with no symmetry but a large portion of Si（111） facets are more stable than the high symmetry bulk-like geometries. And the icosahedral Si₂₂₀ is energetically competitive with the bulk-like structures at magic size of n=220. We also calculated the energies of the Sin naonoclusters as the function of cluster size（16 ≤ n ≤ 224）, the calculated results suggest that the cohesive energy varies almost linearly versus cluster size within a given class of structures. However, the slope of the energy curve changes when there is a transition from one to another motif. Our study also showed that a structural transition from the bucky-diamond to the bulk-like motif is likely to occur at the size rang of 173-215 atoms, which make Si NCs promising for applications in many advanced nanotechnologies.（2） The optical properties of hydrogen passivated Si NCs up to 370 atoms have been investigated using TDDFT calculations. Calculation results show that UV absorption spectra of the bare Si NCs exhibit a broad band, while those of the hydrogen passivated Si NCs show relatively sharp peaks. Calculation results show that an absorption peak in the visible light range appears for the hydrogen passivated NCs except for icosahedral isomer, and there is significant blue shift in optical spectra of hydrogenated Si NCs compared to those of bare Si NCs. In addition, the double peaks also appear for the hydrogen passivated onion-like and bucky-diamond isomers.（3） The optical absorption spectra of the Si_m and Si_m@C₂n（m=7-13, 2n=60, 70,76 and 84） NCs are computed using TDDFT method. The results show that the absorption peaks in the visible light range appear for the Si_m@C₂n clusters and there is significant red shift in optical spectra of Si_m@C₂n compared to those of Si_m cluster,which appear in the UV range. It is interesting that total absotption spectrum of Si_m@C₂n clusters appears a broad absorption band. The whole absorption spectrum is similar to the solar radiation spectrum. We thought that this may provide a new direction for the design of solar cells.（4） We have studied the structures of the In-doped SnO₂（110） surface by the DFT calculations. It was found that the In-doping can lead to local modification for the surface structure. We have also investigated the CO adsorption mechanism on the In-doped SnO₂（110） surface with an Ob vacancy. It was found that CO physically adsorbed on the undoped SnO₂（110） surface and can be oxidized to CO₂ on the In-doped SnO₂（110） surface. In the transition state on the favorable one-step pathway, the bridge oxygen vacancy site is closer to CO than the In site, showing that oxygen vacancy is more important than In for activating the CO oxidation. Thus,we could consider that the role of the In-doping is to generate oxygen vacancies,which can activate the CO oxidization, and the In-doping is very important to improve the gas sensing properties of the SnO₂-based sensors for detecting CO. This will provide the theoretical guidance for searching for better performance of gas sensitive materials.