Theoretical Study Of Double Transition Metal Atoms Doped Silicon Clusters

The cluster is the bridge between microscopic atoms or molecules and macroscopic condensed materials. Clusters exhibit various size-dependent properties, which are differ-ent from those of bulk materials. It is expected that clusters could act as building blocks of various nanoscale function materials and devices. Silicon-based semiconductor occu-pies an important position in microelectronics industry,which makes silicon cluster the focus of extensive research. However,silicon clusters are unsuitable for building blocks due to their sp3 nature. Some people suggested that the dangling bonds on the surface of silicon cluster could be terminated by doping other atoms. Among all the doping atoms, transition metal (TM) atoms were paid more attentions for their unique electromagnetic properties. Restricted by calculating conditions, previous studies were mainly on single TM atoms doped into silicon clusters,and (?),ed silicon were seldom mentioned. In this work,the geometries,stabilities, electronic and magnetic properties of double 3d TM atoms doped biprism cagelike silicon clusters MM'Si18 (M,M'= Ti, V, Cr. Mn, Fe. Co. Ni, Cu, Zn) and W doped silicon clusters WmSin(m=1,2;n=1-18) have been investigated systematically.Homogeneous double 3d TM atoms doped silicon clusters M2Si18(M=Ti,V,Cr. Mn,Fe, Co, Ni, Cu, Zn)have been systematically studied using density functional theory (DFT), and the optimized geometries,charge population and energies have been analyzed. It has been found that the biprism structure could keep only when the electron number of doping atoms is less than half-filled, The spin multiplicity and symmetry are related to the distance between two TM atoms. All the distance between TM atoms in the clusters are greater than that in the corresponding metal dimers, except Mn. Among isomers with similar structure, the distance between TM atom in the cluster is greater in clusters that has higher spin multiplicity. Charge always transfers from Si to TM atoms except Zn. Co2Si18 and Cr2Si18 have ground states of spin triplet, and in other clusters the magnetic moments annihilate,which may be concerned by the symmetry declines. 18 electron counting rule based on TM atoms closed shell fails to predict, stabilities of M2Si18,but the counting rule based on electron gas closed shell combined with spin conservation condition successfully predicts the special stability of Fe2Si18. Additional calculation has been performed on heterogeneous double 3d TM atoms doped MM'Si18 clusters systemically in order to enrich the samples of magnetic moments and electron numbers.MM'Si18 could be divided into 7 classes by structural feature. Most of the distances in MM'Si18 is greater than that of corresponding TM dimer, except some clusters doped by Ti, V combined with magnetic atom such as Cr,Mn, Fe and Co. Some dimers with magnetic moments up to 10?B quench under 6?B after doped into silicon cages,indicating that silicon atoms reduce the magnetic moments of TM atoms. Charge always transfers from silicon atoms to TM atoms in CrZnSi18 and all the clusters with Zn atom. There are some interesting periodic laws between total magnetic moments and the compound mode of TM atoms. Among all the clusters doped by two different TM atoms, only 3 of them are nonmagnetic.The contribution of silicon atoms to the magnetic moments of clusters increased in the heterogeneous cases,and in some clusters the magnetic moments mainlv come from silicon atoms. Clusters with Ti atom doped are often more stable than others, while those containing Zn atom show poor stabilities. 18-electron counting rule based on the TM atoms closed shell also fails to predict stabilities of MM'Si18.The counting rule from electron gas closed shell combined with spin conservation condition is also doubtable, as stabilities of heterogeneous TM atoms doped cluster which meet the requirements of magic number of electron gas are much lower than Fe2Si18 Besides electron counting rule,other factors such as symmetries,heterogeneous doping, averaged coordination numbers and magnetic moments are concerned with stabilities of MM'Si18 clusters.The structures,charge populations and stabilities of WSin(n=1-12) clusters have been studied systematically using density functional theory (DFT).For each cluster size, extensive search of lowest-energy structure has been conducted by considering a number of structural isomers. The equilibrium site of the W atom in the ground state structures of WSin clusters moves from the surface to the interior sites gradually, with the number of silicon atoms increasing from 1 to 12. Starting from WSi9,W atom is fully encapsulated by the silicon cage. At the critical point of embedding, the geometries, charge populations and stabilities change obviously. Charge always transfers from silicon to W atom, and the charge on W atom increases notable after embedding of tungsten. The magnetic moment of W atom completely quenched after n=3. The stability of doped cluster is higher than that of pure silicon at the corresponding size. The most stable cluster is WSi12 with a prim configuration,which is agree with the previous results.Finally we calculated W2Sin(n=1-18),silicon clusters doped by double W atoms, based on the DFT method. With the increase of silicon from 7 to 18. the two doped W atoms embedded into silicon cluster one by one. When n-10, one tungsten atom is encapsulated by the silicon cage while another tungsten atom still stands outside the cage. Starting from n = 14, W atom outside begins to move into the cage,and completely embeds into cage after n = 16. At the critical points of embedding ,the geometries,charge populations and stabilities change obviously. With the increase of silicon atoms,the distance between two W atoms shows a tendency to increase, and the geometry of cluster tends to have a distribution that it forms two silicon cages centered by W atom. There is an obvious evolution path in which prim structure grows into biprim one. Charge always transfers from silicon to W atoms. The charge population of embedded tungsten atom is greater than that outside the cage,and the difference between them decreases gradually with the embedding of the W atom outside. At the size of n =7-18 all the magnetic moments of W2Sin quench. The most stable cluster is W2Si18.At the two critical points of embedding, the averaged binding energies and embedding energies of clusters increase obviously,indicating that embedment is in favor of improving stability. Meanwhile the fragment energies have two peak at critical points.showing the energy mutations.which may be regarded as a structural phase transition criterion of embedding.