Theoretical Studies On Silicon Binary Clusters

With the development of electronic and nanomaterials industries,silicon element has been occupied dominant status of the processing ofdeveloping the novel electronic device, nanomatrials and photosensitivityapparatus, etc. The fabrication of silicon-based luminescent devices istoday considered of great mainly for the possible of optical andmicroelectronic devices. And photoluminescent materials compatible withexisting silicon technology have reveived great attention from the researchcommunity. In particular, nanometer-sized silicon has been activelystudied in the last years because of its efficient light emission. Thus boththeoretical and experimental studies on silicon based materials have beenof great significance for the development of industry and technology.On the other hand, emerging as an intermediate phase betweenisolated atoms and crystalline solid, silicon and silicon-based clusters haveattracted continuous interest in the last twenty years. The small clusters areexpected to have the physical and chemical properties very different fromthose of the bulk material because of their small physical size. Particularly,if current miniaturization trends continue, the minimum structural size ofindustrial instruments will approach the scale of the silicon clusters in acouple of decades. Several spectroscopic studies viz., photoelectronspectroscopy, Raman and infrared spectroscopy, etc., have been performed tounderstand the atomic structure of small silicon clusters. On the other hand,the computational studies provide an alternative to understand the geometriesof relatively large clusters. For Sin cluster (n≤7), the global minima are firmlyestablished by both ab initio calculations and Raman/infrared spectroscopymeasurements;whereas for n≤12, the global minima based on ab initiocalculations are well accepted. Extensive investigations have been carried outnot only on the homoatomic silicon clusters, but also on the heteroatomicsystems. The problem of the metal-silicon bond has been the topic of anumber of experimental and theoretical studies for bulk metal-silicon systems.Based on the stable structures of small silicon clusters, we have carriedout theoretical investigation on binary silicon clusters with different elements—lithium, carbon and oxygen. The main research results are listed asfollowing:(i) Ab initio calculations are carried out to study the adsorption ofLithium atom on the Sin cluster with n ranging from 2 to 7. At theMP2/6-31G(d) level, the structures of the neutral Sin clusters and the SinLiclusters (n=2-7) are optimized. The single-point energy atQCISD/6-311+G(d,p) level for the optimized isomers are further performed.Harmonic vibrational frequency analysis at the MP2/6-31G(d) level is alsoundertaken to confirm that the optimize geometries are stable. Based on ourresults, the most favorable sites for Li adsorption on the Si2-7 clusters are thebridge sites. In addition, the vertical ionization energies of the SinLi clustersand the electron affinities of the Sin clusters are also calculated. The clearparallelism between the vertical ionization energies of SinLi and the electronaffinities of Sin is found. This is consistent with the fact that the framework ofthe Sin in the SinLi cluster is similar to the structure of the correspondingnegative ion Sinˉ .(ii) Structures and energies of SimCn clusters (m = 2 ? 6, n = 1 ? m)have been investigated systematically using the MP2 method with a basis setof 6-311G(d). And the energy order of the structures obtained is confirmed atthe higher CCSD(T)/6-311G(d) level. The lowest-energy structures of somesmall silicon-carbon hetero-clusters we obtained are in good agreement withthose proposed in literatures. For the series of Si6Cn (n = 1 ? 6) clusters, mostof the stable structures are predicted for the first time. All the structures showa tendency that carbon atoms prefer forming kernels at the center offramework, surrounded by silicon atoms with small coordination number. Thestrong C-C bond is favored over Si-C bond and determines the primarystructure of the SimCn clusters. According to the binding energies of SimCnclusters (m = 2 ? 6, n = 1 ? m), the systems with the ratio of Si:C = 1:1 havethe larger stability compared to the Si-rich hetero systems.(iii) The structures, binding energies, and electronic properties of oneoxygen atom (O) and two oxygen atoms (2O) adsorption on silicon clustersSin with n ranging from 5 to 10 are studied systematically by ab initiocalculations. Twelve stable structures are obtained, two of which are inagreement with those reported in previous literature and the others are newstructures that have not been proposed before. Further investigations on thefragmentations of SinO and SinO2 (n=5-10) clusters indicate that the pathwaysSinO →Sin-1 + SiO and SinO2 → Sin-2 + Si2O2 are most favorable fromthermodynamic viewpoint. Among the studied silicon oxide clusters, Si8O,Si9O, Si5O2 and Si8O2 correspond to large adsorption energies of siliconclusters with respect to O or 2O, while Si8O, with the smallest dissociationenergy, has a tendency to separate into Si7 + SiO. Using the recentlydeveloped quasi-atomic minimal-basis-orbital method, we have alsocalculated the unsaturated valences of the neutral Sin clusters. Our calculationresults show that the Si atoms which have the largest unsaturated valences aremore attractive to O atom. Placing O atom right around the Si atoms with thelargest unsaturated valences usually leads to stable structures of the siliconoxide clusters.(iv) The structures of SinOn clusters with n ranging from 10 to 18 areinvestigated systematically by first-principles calculations based on densityfunctional theory. A new motif for the geometries of large silicon monoxideclusters is proposed. The new structures of SinOn clusters obtained from ourcalculations are energetically more favorable than those proposed in theliterature. Our studies show that the basic motif of (SiO)12-16 clusters is thestable structure of Si8O12 wheel attached with a polygonal bipyramid Sicluster of size 5 ? 7. While for Si18O18, the main framework is changed toanother large wheel structure of Si12O18 consisting of six Si3O3 rings. TheSinOn structural types with a pure small Si cluster attached to a stable siliconoxide fragment such as Si8O12, Si12O18 wheel, etc., might serve as a carrier forfurther growth of Si nanostructures, since the small silicon clusters producedfrom the SinOn fragmentations may aggregate to form larger and larger Siclusters.The calculated results are fully compared with the previous theoreticaland experimental data available in literatures. The series of stable structures ofbinary silicon clusters obtained in our research make us have a furtherunderstanding about the properties of silicon clusters, as well as helping usperforming further exploration on silicon clusters in larger size scale.