Scanning Tunneling Microscopy Study Of Low Dimensional Structures On Cu(111) Surface

Macroscope,mesoscope and microscope scales show the different administrative structure of our material world.Though there is remarkable difference between the macroscopic world and the quantum world,they are intimately correlated at the same time.The investigation of quantum phenomena and laws can greatly stimulate the knowledge about our universe.With the persistent progress of surface science,all kinds of low dimensional quantum phenomena occurred on surfaces catch the eyes of researchers.The invention and application of scanning tunneling microscope(STM)in surface science has vastly promoted the fast development of related fields.In the introduction chapter one,we will focus on the basic knowledge of STM and its typical applications in surface science.First we highlight the huge promotion effect of STM towards human's cognitive competence of nature from the perspective of the evolution history of microscope.After that,basics about the manipulation power of STM tip and the scanning tunneling spectroscopy(STS),as well as the working principle and fundamental theory of STM will be outlined.The surface science will be briefly covered in several aspects,including its birth and development,research contents and methods.Following is the typical applications of STM in several branches of surface science,such as the surface adsorption,surface assembly and two dimensional materials.In the last part of this chapter,our experimental instruments and the main research work will be introduced.In chapter two,we mainly investigate the adsorption behavior of alkali metal potassium(K)on Cu(111)surface at low temperature,especially the surface structure and electronic characteristics at the low coverage limit.As a typical surface chemisorption system,the experimental result indicates that there is obvious charge transfer from K to the substrate after adsorption,which leads to the formation of surface dipole layer.The STM data reveals that the combination effects of surface dipole and surface state electron result in the occurrence of abnormal standing waves.In this process,the defect structures on surfaces act as the scattering centers of standing waves.Basing on the bias-dependent and coverage-dependent experimental results we come up with the interaction model of tip electrical field with the surface dipole,which can well explain the experimental observations.Besides,from the results of dz-dV spectra we find that the coverage variation of local K atoms can obviously affect the local electronic states.At the end of this chapter,we give the primary results of the co-adsorption of K and organic coronene molecule.It is found that the STM tip tends to be well modified by this co-adsorption system and frequently shows ultrahigh spatial resolution.With the help of this kind of "sharp"tip we carry out detailed geometrical characterization of the co-adsorption sample.In chapter three,we pay our attention to the recognition of molecular chirality on materials surface.The main idea is to investigate the low dimensional chiral evolution behavior of dibenzo[g,p]chrysene(DBC),a kind of helicene-like flexible organic molecule,on metal surfaces.By the way of systematically changing the deposition condition,we obtain a serial of samples with different coverages on Cu(111)surfaces.Combining the STM images and density functional theory(DFT)calculations&simulations we carefully inspect the expression of molecular chirality on Cu(111)surfaces in different dimensions,including the zero dimensional single molecule,quasi one dimensional molecular chain and two dimensional racemic film.As the STM images reveal,the molecular conformation will be achiral as the result of strong substrate modulation in the situation of single molecule adsorption.However,all of the DBC molecules turn their conformations into chiral when a monolayer is reached,which is mainly ascribed to the great enhancement of intermolecular interactions.In the mediate coverage,stripe-like molecular chains are observed,which are dominated by achiral DBC molecules but chiral molecules are occasionally seen in these chains.The analysis concerning local chemical surrounding of singe chiral molecules in chain and racemic structures shows that the chiral conformation is stabilized by its asymmetric molecular environment.That's to say the occurrence of molecule chirality in molecule aggregates is the result of self-induction process via helical intermolecular aromatic coupling.After that,we show our experimental results of DBC on Au(111)and Ag(111)as a comparison with results on Cu(111).It is shown that though the substrate modulation can be generalized within the framework of single molecule adsorption;there is no conformation transformation from achiral to chiral in increasing coverage regime.All these results reflect the substrate dependence of DBC molecular chirality.In the last part of this chapter,we demonstrate the very beginning work concerning alkali metal doping of DBC and its chiral expression at the mesoscope.In chapter four,we will discuss our work in the preparation and characterization of new type two dimensional materials—stanene,exactly following the research idea of graphene-like two dimensional materials.We successfully fabricate large area and monolayer stanene on Cu(111)surface through ultra-high vacuum low temperature molecular beam epitaxial(MBE)method.With the combination of STM,STS,DFT and angle-resolved photoemission spectra(ARPES)technique we systematically characterize the geometric and electronic structure of stanene.Through the deposition control we also obtain stanene flakes with quasi hexagonal shapes.STM images reveal that the edge Sn atoms are dominated by zigzag type.What is unexpected is that both the stanene film and flakes show atomically flat honeycomb structures from the high resolution STM images,which is very similar to graphene.The structure optimization and the interface charge distribution calculations verify the stability of the planar stanene.The ARPES investigations show that there is a local gap as large as 0.3 eV at about 1.2 eV below the Fermi energy at ? point of the surface brillouin zone.Such a large gap is assigned to the result of substrate enhanced spin-orbital coupling effect.The DFT band calculations match the ARPES results and give the same size gap.In the end of this chapter we will show more experimental results about the temperature stability of this material system.Room temperature phase and possible buckled stanene is further observed and analyzed.