The Theoretical Studies On The Geometric Stuctures And Novel Properties Of Monolayer CuSe

Two-dimensional(2D)materials have attracted much attention for both fundamental interest and their potential applicaitions.So far,a suite of different classes of 2D materials have been discovered,ranging,e.g.,from inorganic Dirac semimetal,semiconductor,topological insulator(TIs),superconductor and ferromagnet,to their organic counterparts.With the progress in the synthesis methods of 2D materials,in addition to the 2D materials which are naturally layered in their bulk phase,such as graphene,black phosphorus and transition metal dichalcogenides,few 2D materials which bulk phase is non-layered are synthesized,such as silicene.Further research is much needed to enrich and understand 2D materials which bulk phase is non-layered via both theory and experiment.The thesis mainly focuses on the geometric stuctures and novel properties of monolayer CuSe which bulk phase is non-layered.We studied the intrinsically pattered CuSe on Cu(111),the Dirac nodal-line fermions(DNLFs)of planer honeycomb CuSe monolayer and the orbital design of TIs from 2D semiconductors.The thesis contains three parts as following:Firstly,we studied the atomic configuration,formation mechanism and functionalization of the intrinsically patterned CuSe on Cu(111)surface.A CuSe(4?3x4?3)-30~o/Cu(111)(11x11)model is proposed,using first principles calculations combined with low energy electron diffraction(LEED),scanning tunneling microscope(STM)and scanning transmission electron microscopy(STEM).To understand the formation mechanism of the intrinsically 13-atoms-nanoviods,the formation energies of nonoviods with deffirent sizes are calculated.With 13-atoms-nanoviods,the formation energy is negative and the smallest,which ensures the experimentally formed13-atoms-nanoviods.The intrinsically patterned CuSe on Cu(111)is an ideal substrate for selective adsorption which we also further studied.Secondly,we predicted that free-standing hole-free monolayer CuSe is endowed with two DNLFs which are protected by mirror reflection symmetry.This very rare DNLF state is evidenced by topologically nontrivial edge states situated inside the spin-orbit coupling gaps.Motivated by the promising properties of hole-free honeycomb CuSe,monolayer CuSe is fabricated on Cu(111)surfaces by molecular beam epitaxy and confirmed success with high resolution STM.The ARPES results are in good agreement with the calculated band structures of CuSe/Cu(111).However,the DNLFs did not observed because of the strong coupling between out-of-plane orbitals of CuSe and the Cu(111)which annihilates the?band.We then calculate the band structure of monolayer CuSe on weakly coupled graphene,showing that DNLFs remains.These results suggest that the honeycomb monolayer transition metal monochalcogenide can be a new platform to study 2D DNLFs.Finally,we proposed a generic approach to convert 2D semiconductors,which are amply abundant,to 2D TIs,which are less available,via selective atomic adsorption and strain engineering.The approach is underlined by an orbital design principle that involves introducing an extrinsic-orbital state into the intrinsic s-bands of a 2D semiconductor,so as to induce-band inversion for a TI phase,as demonstrated by tight-binding model analyses.Remarkably,based on first-principles calculations,we apply this approach to convert the semiconducting monolayer CuS and CuTe into a TI by adsorbing Na and K respectively with a proper-level energy,and CuSe into a TI by adsorbing a mixture of Na and K with a tuned-level energy or by adsorbing either Na or K on a strained CuSe with a tuned-level valence band edge.Our findings open a new door to the discovery of TIs by a predictive materials design,beyond finding a preexisting 2D TI.