The Theoretical Studies Of Structures And Topological Properties Of Several Carbon-based Low-dimensional Materials

Carbon-based materials have a variety of physical and chemical properties,and are widely used in life,industry,aerospace,and other fields.With the rapid development of the synthetic chemistry,a series of low-dimensional carbon-based materials have been discovered and studied.These low-dimensional materials have attracted attention due to their novel physical properties.For example,graphene has a Dirac state at fermi level,magic-angle twisted bilayer graphene is a Mott insulator,zigzag graphene nanoribbons(ZGNRs)have semiconductor energy gaps and antiferromagnetic ground state,and two-dimensional(2D)organic topological insulators have non-trivial topological edge states.Due to the diversity of the existing forms of carbon-based materials,it provides lots of possibilities for low-dimensional carbon-based materials with excellent properties.This thesis studies the structures and physical properties of several low-dimensional carbon-based materials by using theoretical calculations.The main research content includes the quantum anomalous Hall effect(QAHE)in 2D Cu-dicyanobenzene(Cu-DCB)coloring-triangle(CT)metal-organic lattice,tuning the band gaps and topological properties of graphene nanoribbons(GNRs)by nitrogen and boron atoms substitution,recovery of the Dirac states of graphene by intercalating 2D traditional semiconductors at the graphene-metal interface,and selective activation and polymerization of several carbon-based functional nanostructures on the metal surfaces.The main results of this study are as follows:1.Prediction of QAHE in 2D Cu-DCB CT lattice using first-principles calculations combined with the maximum localized Wannier functions.The 2D Cu-DCB lattice is formed by Cu atoms coordinated with DCB molecules.Molecular dynamics simulation shows that the 2D Cu-DCB lattice is stable at room temperature.The ground state of the 2D Cu-DCB lattice is ferromagnetic(FM).The FM ordering is studied with the Ising model,which yields a Curie temperature of about 100 K.The calculated Chen numbers and the semi-infinite chiral edge states both confirm that the lattice has non-trivial topological properties,which makes the 2D Cu-DCB lattice an ideal material for realizing QAHE.If the metal atom Cu in the lattice is replaced with Au,a flat band with a smaller bandwidth and a larger band gap can be obtained around the Fermi level,which suggests that 2D Au-DCB CT lattice is promising for the fractional quantum Hall effect.This work enriched the family of QAH insulators and provided new ideas for the subsequent search for QAH insulators.2.Tuning the band gaps and topological properties of GNRs through nitrogen and boron atom substitution.Density functional theory(DFT)calculations reveal that the introduction of NBN atoms on the specific cove-edge will transform the GNR from an antiferromagnetic ground state to a non-magnetic ground state,and the substitution of NBN atoms will increase the band gap of GNR.It is also found that the magnetic properties of the GNRs rely on the shape of the cove edge.After introducing NBN atom on the zigzag edge of GNRs,the system maintains the antiferromagnetic ground state.The calculation results show that the band gaps of the zigzag GNRs will decrease with the increase of the width and doping concentration of NBN atoms.In addition,we designed a method to tuning the topological properties of chevron-shaped GNRs through nitrogen atom doping and then constructed a topological non-trivial GNR heterostructure.This work provides a valuable guide for subsequent experimental realization.3.Recovery of the Dirac state of graphene on Ni(111)by intercalating 2D traditional semiconductors.By using DFT calculations,various types of 2D semiconductors were considered,including theoretically predicted 2D double-layer honeycomb,single-layer honeycomb materials and 2D silicon.Calculation results show that these 2D semiconductors decouple the graphene from the Ni(111)surface and restore the Dirac state of graphene.The shift of the Dirac point,which is an indicator of the doping level,is proportional to the work function difference between graphene and the 2D semiconductors on Ni(111).It is found that the multilayer semiconductor intercalation can also effectively protect the intrinsic band gaps of the 2D semiconductors on the metal substrate.In addition,the doping of graphene can be tuned by changing the thickness of the 2D semiconductors.This work provides a way for tailoring the electronic properties of graphene and further fabrication of graphene-based devices.4.Using first-principles calculations combined with scanning tunneling microscopy(STM)and non-contact atomic force microscopy(nc-AFM),the selective activation of carbon-based functional nanostructures was studied.The first work is about symmetric molecule 4,4?-diamino-p-terphenyl(DATP)molecule with two identical amino end groups.It has asymmetric adsorption geometries after the DATP adsorbed on the Cu(111)surface,and the amino groups of the DATP molecule are located at different adsorption sites.DFT calculations find that asymmetric adsorption results in a change of the binding affinity of the amino groups,therefore lead to different activities of the two amino groups in the DATP molecule.The experimental results are consistent with DFT calculations.The second work is about the hierarchical control of selective carbon-hydrogen(C-H)bonds activation in a nitrogen-doped polycyclic aromatic molecule TPPIP on Ag(100).DFT calculations reveal that four ortho C(sp~3)-H bonds in two N-heterocycles have different activation energies and may result in selective activation of C-H bonds.Then hierarchical activation of two pairs of ortho C(sp~3)-H bonds at different annealing temperatures was observed by using high-resolution STM and nc-AFM technique.Further annealing the sample at increasing temperature results in the formation of nitrogen-doped W-shaped GNR through intermolecular coupling.These works provide a new route towards surface-induced asymmetric activation of asymmetric compounds.