A Computational Investigation Of Transition Metal Doped Silicon Clusters By Using Density Functional Theory

In this thesis, we investigate the geometric, electronic and optical properties ofMBen(M=Fe,Co,Ni) clusters using density functional theory (DFT). The maincontents are presented as the followings:First, we give a brief introduction to clusters. With a size between those ofatoms and macroscopical systems, clusters have many unique properties, and at-tract much experimental and theoretical research attentions. At first, we introducethe basic properties of clusters. Then, some commonmethods in experimental andtheoretical studies on clusters are discussed, including the preparation and detectionof clusters, and theoretical studies. Then, research on clusters is reviewed and pre-viewed. At last, we simply describe the purpose and results of our work on clusters.Second, we introduce the basic concept and progress of DFT. Development ofquantum chemistry promotes the establishment of DFT. Thomas-Fermi model is thefirst theory using density of electrons as the main variable.Theorem of Hohenberg-Kohn is the fundament of DFT and is developed to Kohn-Sham equation, whichcan be used to perform real calculations. Now, new corrections and extensions,together with developed exchange-correlation functionals, have made DFT moreaccurate and suitable for more systems. At the end of this chapter, we introduce theGaussian and DMOL3 Software.In Chapter 3, The growth behaviors, stabilities, electronic propertiesand charge transfer of TiSin(n=2-15) clusters are investigated theoretically atB3LYP\LanL2DZ level with spin configuration considered. All the calculated re-sults are summarized as follows. (1) For each cluster size, an extensive search ofthe lowest-energy structures has been conducted by considering a number of struc-tural isomers. In the ground-state structures of TiSin clusters, the Ti atom graduallymoves from convex, surface, to interior sites as the number of Si atom varying from2 to 15. Starting from n=12, Ti atom completely falls into the center of the Siouter frame, forming metal-encapsulated Si cages, which can be explained by using16-electron rule. (2) According to the average binding energy analysis of the Sinand TiSin clusters, the average binding energies of TiSin clusters are larger than those of pure Sin clusters, it can be concluded that the doped Ti in the Sin clus-ters improves the stability of silicon clusters. (3) According to the calculated [D(n,n-1)] and [?2E(n)] of the most stable TiSin clusters, theoretical results show thatTiSin clusters at n=6, 8, 12 posses relatively higher stability. It can be exceptedthat the encapsulated TiSi12 cluster can be acted as building block of the Ti-dopedcluster-assembled materials because of its higher relative stability. (4) The HOMO-LUMO gap also exhibits sized and geometric dependencies. As compared to puresilicon clusters, a universal narrowing of HOMO-LUMO gap in TiSin cluster isfound, which means that the chemical reactivity of TiSin clusters are enhanced .The charge in the TiSin clusters always transfers from the Si atoms to the Ti atom.In chapter 4,Using first-principles DFT-GGA calculations, the geometries,stabilities, and magnetic properties of the transition metal(TM) encapsulatedTMSin(TM=Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, n=8-16) clusters have beensystematically studied. The results are summarized as follows: (1)Every transi-tion metal atom(TMA) will fall into the center of Si outer frame and form a metal-encapsulated Si cage at a certain size (ScSi14, TiSi12, VSi12, CrSi12, MnSi11, FeSi10,CoSi10, CuSi12, ZnSi14) . The size of the smallest cage-like structures is not de-termined by the radius of the TMA, the bonding properties and the orbital hy-bridization between TMA and Si atoms also perform an important role. (2)Theelectron shell filling(18 or 20-electron rule) rule has a limited applicability sincewe found that stability depends on other factors as well (d state shell of TMA, pdhybridization, the atom size etc.) (3)The charge in the smallest TMSin cage-likeclusters always transfers from Si atoms to the TMA, there is also internal electrontransfer among 3d, 4s, and 4p states in TMA. (4)The total magnetic moments ofTMSin clusters and the magnetic moments of TMA in TMSin clusters are not al-ways quenched when the TMA falls into the center of the Si outer frame. Themagnetic moments of CrSi12, NiSi10, ZnSi14 are completely quenched. The totalmagnetic moments are not only contributed by TMA but also by Si atoms. Thereason why the magnetic moments are quenched for certain TMA at certain sizes isstill an uncertain question.