Theoretical Investigation Of Transition Metal Encapsulated Silicon Clusters And Some New Monolayer Transition Metal Carbides

Since the discovery of fullerene and graphene,self-assembled nanoscale materials and two dimensional(2D)materials have drawn huge attentions in recent decades.The low dimensional materials could present exciting physical and chemical properties,which the common three dimensional materials don't possess.The extraordinary properties are very promising in the fields of nanodevices,spintronic and catalysis.The technolo-gy continuously improves in needs of smaller size devices,for which the investigations of self-assembled nanoscale materials become more and more important.On the other hand,searching for new 2D materials also draws lots of attentions and has achieved a great progress.Inspired by the investigations of graphene,new 2D materials have been extended from IV group to multi-element 2D materials such as transition metal carbides(TmC),transition metal borides(TmB)and transition metal nitrides(TmN).More and more new 2D materials are synthesized and found to be stable and some could be effectively functionalized.In this paper we focus on the follows:(1)The investigation of geometry and electronic structures of double transition metal encapsulated silicon clusters Ti2Sin(n=3-24).We adopted density functional theory and genetic algorithm to search the ground and metastable states of Ti2Sin(n=3-24),and we focus on the three lowest energy structures to analyze their geometry and electronic structures.The results show that,Ti2Si4 is the most stable structure among the Ti2Sin(n=3-24)because it possesses the highest embedding energy of Si atom,the highest second difference of the cluster energy and the highest energy gap.The relative stable structures are Ti2Si13,Ti2Si21,Ti2Si15,and Ti2Si7.The dominant factor on the stability of clusters are the geometry in small size and electronic shell in large clusters,respectively.When the number n of Ti2Sin is larger than 21,the stability is weakened,which means that the clusters of Ti2Sin(n=3-24)get saturated when n>21.The stability of the double transitional metal Ti encapsulated Si clusters is effectively enhanced.(2)The investigation of the geometry and electronic structures as well as mag-netic properties of three transition metal encapsulated anti-hexagonal prism clusters.We adopted density functional theory and genetic algorithm to search for the ground and metastable states of Tm3Si12,and we focus on the five lowest energy structures to analyze their geometry and electronic structures.The results show that,all the ground states of Tm3Si12 clusters hold the anti-hexagonal prism structures except Sc3Si12.The three transition metals stay on the axis of C6v.Ti3Si12 is antiferromagnetic,while V3Si12,Cr3Si12,and Mn3Si12 are ferrimagnetic,among which the structure of Mn3Si12 is distorted.By investigating anion Mn3S12-and cation Mn3Si12+,we found that their magnetic properties change as the electron transfers.We also investigated the con-dition of two types of transition metal elements encapsulated in the anti-prism Si12 clusters.The structures basically stay the anti-hexagonal prism wheel structure as men-tioned above and most of them are ferrimagnetic except the VTi2Si12,CrTi2Si12,and MnCr2Si12,whose magnetic properties change as electron transfers.What's more,by investigating the energy gap of dozens of clusters,we found that when the valence electron number is 60,the energy gap is obviously larger than others,we believe this provides a new physical picture for superatom theory.(3)In the third work we predicted some new 2D transition metal carbides VC2,MnC2,and V1/2Mn1/2C2,which belong to orthorhombic phase.The transition metal is six coordinated which bonds with six carbon atoms,and the carbon is four coordinated which bonds with four transition metal atoms.The phonon dispersion proved that the predicted 2D TmC nanosheets are stable and the ab initio molecular dynamics(AIMD)results show that the MnC2 is stable below 300 K,VC2 and V2/2Mn1/2C2 remain sta-ble below 1000 K.We also investigated their magnetic properties.The ground state of MnC2 is antiferomagnetic,the Fermi velocity is about 5.37 × 105 m/s showing favor-able electronic transport property.The in-plane Young's modulus of VC2,MnC2,and V1/2Mn1/2C2 are 70.8 N/m,73.6 N/m and 83.7 N/m,respectively.Li adsorption results reveal that the Li adsorption capacity and diffusion barrier for VC2 and V1/2Mn1/2C2 are favorable for application in Li ion battery.And the magnetic property could be easily tuned by surface functionalization.