Research On The Migration, Cyclization And Defects Of Carbon-containing Groups In Graphene-like Material

On-surface synthesis is an emergent synthetic methodology which developed quickly in the last decade.By depositing pre-designed precursor molecules on the substrate surface and inducing the surface chemical reactions via thermal heating and other applicable methods,researchers can synthesize atomically precise nanostructures that are difficult to obtain by traditional in-solution synthesis.Owing to the clean ultra-high vacuum environment,the interference of impurities is remarkably reduced in on-surface synthesis,which allows for direct identification of react intermediates,thus promotes the understanding of chemical reaction mechanisms.At present,surface chemistry is still in a period of vigorous development.Compared with traditional in-solution synthesis,only a few known chemical reactions have been realized by on-surface synthesis.The exploration of reaction and the mechanism behind of functional groups are critical for new chemical design.In this dissertation,the chemical reactions of several chemical functional groups on the metal surface were studied in detail by using the state-of-the-art combined ultra-high vacuum-molecular beam epitaxy-scanning probe microscopy and several carbon nanomaterials with atomically precise structures were synthesized.The mechanism of chemical reaction of these functional groups on the surface and the electronic structure of the product were further explored with combined first-principles calculations.The details are as follows:(1)Identification of phenyl migration reactions on noble metal surface.By studying the intramolecular reactions of DMTPB molecules on three substrates with different reactivities and different symmetries,i.e.,Au(111),Cu(111)and Ag(110),the migration of the phenyl group to the methyl group was observed.The subsequent intramolecular dehydrocyclization of this migrated intermediate leads to the formation of various intermediate structures and the final five monomer nanographene.The step-by-step transformations of molecular products were triggered by varying the annealing temperatures,and the structures of the products were identified by carbon monoxide-modified tips.By means of first-principles calculations,we calculated the reaction barrier of the observed phenyl migration reaction,which explain the experimental observations.(2)On-surface synthesis of graphene nanoribbons with different edge structures using molecular precursor DBDPDPTP containing short alkyl chains.By inducing the Ullmann reaction and subsequent dehydrocyclization of DBDPDPTP on the Au(111)surface,we synthesized ultralong chevron type graphene nanoribbons with periodically bonded methyl groups and extend benzene rings at the edges.The transition process of the two nanoribbons involves the migration and homocoupling of methyl groups.Scanning tunneling spectroscopy show that the transition of the nanoribbons is accompanied by a decrease in the band gap,which is consistent with theoretical calculations.(3)On-surface synthesis of nanographene with pentagon-heptagons.By inducing the Ullmann reaction and intramolecular dehydrocyclization of BPYBA molecules on the surface of Au(111),we obtained a variety of nano-graphene products containing two pairs of pentagonheptagons.The electronic properties of these products are characterized by scanning tunneling spectroscopy combined with theoretical studies.(4)Probing Intrinsic defects in highly oriented pyrolytic graphite and monolayer tungsten disulfide.Two kinds of superstructures induced by various defects on the surface of highly oriented pyrolytic graphite were observed.Single layer tungsten disulfide on highly oriented pyrolytic graphite was synthesized,four types of substitutional defects and charged defects in single-layer tungsten disulfide were identified,and the structure and origin of the charged defects were discussed.The above results illustrated the reaction mechanism of a variety of carbon-containing functional groups on metal substrate,and have successfully obtained several carbon nanomaterials with different electronic properties,which is helpful to promote the understanding of the chemical reaction of functional groups in on-surface synthesis and the development of emerging functional materials.In addition,the identification and characterization of defects in highly oriented pyrolytic graphite and single-layer tungsten disulfide helps to understand the source of defects in materials and to optimize material synthesis processes.