Non-contact Atomic Force Microscope Investigation On Fabrication And Mechanism Of Low Dimensional Functional Organic Nanostructures On Metal Substrates

On-surface synthesis has emerged as a promising way for the fabrication of functional nanostructures with atomic precision.On-surface synthesis adopts bottomup method,i.e.covalent linking of precursor molecules after selective chemical activations to fabricate functional nanostructures,is widely used for synthesis of lowdimensional nanostructures including fullerenes,graphene nanotubes,graphene nanoribbons and covalent organic frameworks.It is deemed as a promising route to realize carbon-based electronic devices.The extraordinary electronic,thermal and mechanical properties of these devices enable its applications in electronics,optical absorption,energy storage and catalysis.The thesis mainly focuses on fabrication of novel functional nanostructures using on-surface synthesis.Combining with in-situ scanning tunneling microscopy,noncontact atomic force microscopy characterizations and density functional theory calculations,we systematically investigated the physical properties and underlying mechanism.The thesis contains three parts as following:1.The first part focuses on controllable synthesis and mechanisms of fabricating sulfur doped graphene nanoribbons.The precursor molecules containing thiophene rings are deposited on surface first.Through controlling annealing temperature.long chain polymers,sulfur doped graphene nanoribbons and finally,pristine chevron type graphene nanoribbons with five-member carbons rings on the edges are successfully fabricated step by step.Combining ultrahigh resolution capability of scanning tunneling microscope and non-contact atomic force microscope,the whole process is thoroughly characterized.Different intermediate states are observed at each annealing stage.We carried out first principle calculations of the ribbon's band structure and transport properties.The ribbon exhibits peculiar negative differential resistance characteristic which means it's possible to be applicated in electron oscillators and AC signal amplifier.2.For the second part we studied selective hierarchical C-H activation of a polycyclic aromatic hydrocarbon molecule on surface.Dehydrogenation process is ubiquitous for almost all on surface processes.However,it's quite challenging to realize selective C-H activation.We designed a model molecule with hetero N atoms as dopants.Two alpha position C-H bonds are activated first both due to the dopant atoms and the alkyl nature.However,of the two equivalent competing dehydrogenation paths,one prevails due to formation of more stable intermediates.Density functional theory(DFT)calculations thoroughly investigated the energetics along the reaction path and justified the reaction selection.Hence the dehydrogenation process at alpha positions also proceeds sequentially.This process is undoubtedly confirmed by ultrahigh resolution scanning tunneling microscope and non-contact atomic force microscope characterization.Finally,N-doped graphene nanoribbons are fabricated through further annealing of the sequentially dehydrogenated multi radical products.The structure is revealed by STM and nc-AFM characterization and atom rearrangements are involved during this chemical transformation.Scanning tunneling spectroscopy reveals the semiconducting property of the ribbon with a bandgap of approximately 2.3 e V.Hence,we successfully demonstrate that this kind of surface mediated selective hierarchical dehydrogenation reaction could possibly be applied in on-surface synthesis field.3.Finally,we studied the influence of different catalytic substrates upon onsurface synthesis.Using 4,13-dibromo-8H,9H-8,9-diaza-8a-borabenzo[fg]tetracene as precursor molecules,we studied formation of different polymerization products on gold,silver and copper.We find that debromination was activated upon moderate annealing on Au(111).The generated radicals then formed one-dimensional organometallic polymer chains through Au bridging.Only a few groups reported observation of C-AuC organometallic structures,and here we confirmed the existence.Further annealing would induce disassemble of organometallic polymer chains indicating low stability of C-Au-C.On Ag(100),debromination was activated at room temperature,and only scramble oligomers were observed on surface.Moderate annealing induces dehydrogenative homocoupling between three-center four-electron functional N-B-N groups,forming dimers bridged by silver atoms.The new N-Ag-N coordination structure is reported for the first time featuring profound chemical significance.On Cu(111)surface,both the one dimensional organometallic polymer chains and metal atoms mediated dimers were observed.These results demonstrate how different catalytic substrate affect the formation of products and enriches our understanding of surface catalyzed on-surface synthesis.