Studying The Molecular Adsorption And Reaction On Surfaces By Imaging Chemical Bonds

Organic molecules can form various low-dimensional nanostructures via different covalent connections on surfaces.These carbon-based nanomaterials have a wide range of potential application in future molecular semiconductor devices.Therefore,it is of great significance to realize the controllable preparation of low-dimensional functional molecular structures with sufficient knowledge about the adsorption and reaction characteristics of organic molecules.Since direct observation of adsorption structures and chemical changes of molecules require sub-nanometer real space resolution,scanning tunneling microscopy(STM)is widely applied to identify molecules/atoms.Unfortunately,STM is not able to characterize chemical bonds.In recent decades,newly developed noncontact atomic force microscopy(nc-AFM)based on specific tip modification enables detailed characterization of molecular chemical structures,making it a powerful tool for chemists to study on-surface reactions.Herein,we systematically investigated the adsorption and reaction pathways of four kinds of organic molecules on metal single crystal surfaces at sub-molecular/chemical-bond resolution using ultra-high-vacuum low-temperature STM and nc-AFM.Additionally,an extension of the bond imaging methodology has been discussed in the last chapter.The main contents consist of the following five parts:1.We studied the adsorption charateristics of symmetric 4,4"-diamino-p-terphenyl(DATP)molecules on a three-fold symmetric Cu(111)surface.Bond-resolution nc-AFM images show that the adsorbed DATP molecules are apparently dissymmetric at 5 K with one end group fuzzy.Further investigations demonstrate that the breakdown of symmetry is caused by the lattice mismatch between the molecules and the substrate.We further realize site-selective modification of the symmetric molecules with a second molecule by exploiting the surface-induced dissymmetry.2.On-surface dehalogenation of a multi-halogen substituted aromatic molecule has been characterized in details on Cu(111).Different halogen atoms are separately cut off from the molecule with thermal-induced or tip-induced methods,generating different intermediates.Bond-resolution nc-AFM images combined with DFT calculations precisely interpret the adsorption structures of the intermediate states.Our findings provide a fundamental aspect for on-surface synthesis based on Ullmann-type reactions.3.We explored the influence of different metals(Au or Ag)on the structures of organometallics.Precursors tetrabromo-tetrachloro-perylene dehalogenated in steps on Au(111)and Ag(111)and formed distinct organometallics.Only quasi-one-dimensional organo-Au nanoribbons are observed on Au(111).The bay regions of the edges are fused with one Au atom forming closed five-membered rings.The Au atoms are passivated by two chlorine atoms.On the other hand,the organo-Ag nanoribbons have different chiral edge structures featuring with alternating open and closed five-membered rings.The special edge structure allows organo-Ag nanoribbons to be more closely packed together.Parallel packed organo-Ag nanoribbons are able to fuse with each other after further annealing.4.Nanographene(NG)molecules with specific structures have been synthesized via selective cyclodehydrogenation on surfaces.Chemists usually synthesize NG molecules with specific geometries and properties via the cyclodehydrogenation of designed molecular precursors.However,it is challenging to provide desired products using precursors with multiple reaction pathways.We have successfully characterized the planarization of complex precursors,distinct from the reaction route in solution,by exploiting the flattened adsorption structures of the precursors and molecule-surface interactions.5.We experimented with higher eigenmodes in nc-AFM for chemical-bond imaging.In principle,nc-AFM can work in multimode to obtain different sample information simultaneously and enhance imaging contrast.We tried imaging molecules using the second eigenmode of a tuning fork sensor and acquired bond resolution in spite of the much higher effective stiffness compared with that of the first eigenmode.Our findings pave the way to chemical-bond imaging with multimode nc-AFM.