Theoretical Studies On The Conducting Properties Of Two-dimensional Material And The Precious Metal Clusters Supported On Them

In this thesis, we have studied graphene superlattice and two-dimensional transitionmetal compound materials to modulate their band-gaps. We discussed the effects of thedegenerate perturbation, inversion symmetry, magnetic coupling and defect concentration onthe band-gap opening. Besides, we have also searched for the stable Pt clusters supported onthe graphene and graphene oxide.Firstly, we have studied the graphene band-gap engineering by introducing differentnonmagnetic defects or substitutional doping. Comparing to the primitive unit cell ofgraphene, the pseudo graphene superlattice, referred to the pristine graphene supercell,modulates the boundary condition accordingly. According to the energy band-folding picture,these superlattices can be categorized into two groups on the basis of the Dirac cone position.In some cases, the Dirac points K and K′in primitive cell are folded to the Γ point of pseudosuperlattices. The coincidence of Dirac points with Γ point results in the four-fold degeneracy.In these systems, band-baps at Γ point can be opened by introducing periodically arrangeddefects such as the antidots, which could be easily utilized in experiment for example bymaking the graphene nanomesh through lithography technique. In the other cases, the twofolddegenerate Dirac points remain non-equivalent with Г point in pseudo superlattice and themethod to break the inversion symmetry could open their band-gaps.Secondly, the electronic and magnetic properties of antidot patterned graphenesuperlattices have been carried out. the studied superlattices could be sorted into threecategories depending on the structural characters of Y-junction and the magnetism distribution.The antiferromagnetic coupling between the neighboring magnetic clusters is favorable forone magnetic superlattice group while it is between the neighboring single magnetic atoms forthe other magnetic superlattices, which adds a quantum parameter to the structural geometryto open the band gap. The nonmagnetic superlattices preserve the equivalence between theAB sublattices of graphene. Accordingly, only some of them open their band-gaps because ofthe degenerate perturbation, most of them are gapless semimetal. Furthermore, the effects ofneck width on band gap width, the magnetic coupling effects, and the antidot edge induceddispersionless band are also carefully discussed. These results could benefit further studies on graphene-based nanostructures toward their applications in next generation high performancenanoelectronics.In the third part, we have studied in detail the Pt clusters supported on graphene andgraphene oxide. In graphene, the adsorption energy of Pt atoms is rather small. Therefore Ptclusters are difficultly to adsorb on the graphene. According to the recent discovery ofgraphene oxide, their surface contains the oxygen atom (-O-), hydroxy (-OH) and sp2covalentbonding C atoms. Because of the presence of these functional groups, graphene oxide may bea kind of good supporting material for precious metal clusters. Wang et al. of ChineseAcademy of Sciences found in experiment that Pt clusters could be synthesized on grapheneoxide sheet with sparse distribution. Our first-principles results confirmed that Pt atoms caneasily adsorb on the graphene oxide. Due to the presence of the oxygen atom (-O-) and sp2covalent bonding C atoms in graphene oxide, the Pt clusters could be anchored on grapheneoxide by the enhanced interaction between Pt and substrate. According to the calculationresults, we found that Pt atoms on the graphene oxide tend to form Pt3, Pt6, Pt9and Pt13clusters. These results could benefit further studies of the catalytic oxidation properties andhydrogen storage properties of the graphene oxide supported Pt clusters.In the last part, the two-dimensional material of transition metal compounds such as VS2are also examined in our studies, which have been successfully synthesized in experiment.Similar to the graphene, these two-dimensional materials also have nanoribbons, nanotubesand other nano-structures. We have carried out detailed first-principle studies on theconducting properties. In two different kinds of structures named H and T, only armchair edgenanoribbons or nanotubes with narrower width could open band-gaps, which can make thematerials to be semiconductor or half metal. However, for the zigzag nanoribbons ornanotubes, the good conducting properties remain. Therefore, the conductivity of somecorresponding VS2nanostructures can keep unaffected by their structural confinements. Thesematerials are possible to be used for the quasi-one-dimensional conductive material.