On-Surface Synthesis And STM Study Of Quasi One-Dimensional Carbon-Based Nanomaterials

Quasi one-dimensional carbon-based materials are the basic materials for future molecular devices,which can be used either as functional devices or as conducting wires.Graphene nanoribbon(GNR)is one of quasi one-dimensional carbon-based materials.It was first obtained by cutting graphite flakes or carbon nanotubes.Through the top-down pathway,one can only obtain graphene nanoribbons with a width of more than 10 nm,whose band gaps were too small for the application.More importantly,the edges of these graphene nanoribbons from the top-down methods are very rough and full of defects.In recent years an alternative approach is developed to prepare the GNRs by bottom-up synthesis,catching the attention of people.GNRs can be synthesized through the C-C coupling of small organic molecules on surfaces or in solutions.GNRs through bottom-up synthesis have a narrow width with a few carbon atoms and well-defined edges with atomic precision,which perfectly meet the requirements of molecular devices.The different widths and types graphene nanoribbons can be fabricated via designing suitable precursor molecules with different electronic and optical properties.In this paper,we focus on the on-surface synthesis of armchair graphene nanoribbons with different widths and doped by other atoms.Scanning tunneling microscope and scanning tunneling spectroscope have been used to characterize the morphology and electronic properties of graphene nanoribbons respectively.Another carbon-based material,alkane,is also investigated in terms of its polymerization on Au(110)surfaces.The main research contents are as follows:1.5-AGNRs have been firstly synthesized by using 1,4,5,8-tetrabromonaphthalene as the molecular precursor.The molecules will form metal-organic hybrids firstly and then form 5-AGNRs on Au(111)or Ag(111)surfaces through dehalogenation and subsequent cleavage of C-Au or C-Ag bonds.Another precursor(3,4,9,10-perylenetetracarboxylic dianhydride)is applied to fabricate 5-AGNRs on Cu(111)surfaces.All of the reactions are carried out in an ultrahigh vacuum system and characterized by STM&STS to investigate not only morphology and electronic properties of the products,but also the reaction pathways.2.8,10-AGNRs have been successfully synthesized on Au(111)surfaces.Poly(para-phenylene)(PPP or 3-AGNR)wires were used as precursors for the synthesis of 8,10-GNRs with fusion of 5,7-AGNRs respectively.The synthetic process of target production was characterized by scanning tunneling microscope(STM).For 8-AGNRs,scanning tunneling spectroscope(STS)reveals one occupied state(-0.5 V)and one unoccupied state(+1.8 V)on Au(111)surfaces.Other unoccupied and occupied states with smaller density between the two states overlap of surface state can't be observed.The band gap of 10-AGNR on Au(111)surface is 2.0 eV by STS,in accordance with the results from GW model with image charge(IC)screening.3.5-AGNRs substituted with nitrile(CN)functional groups have been synthesized on Au(111)surfaces.The CN groups modify the edges of 5-AGNR.The gold atoms doped five carbon atoms widths armchair GNRs(Au-5AGNRs)have been successfully fabricated on Au(111)surfaces by using 1,4,5,8-tetrabromonaphthalene(TBN)as molecular precursor.The key point of synthesis is to control the reaction temperature that just makes some of C-Au bonds break.STS reveals one occupied state about 1 eV above Fermi level owing to the gold atoms doping.Doping has played a good role in regulating the energy band structure of graphene nanoribbons.4.The reaction mechanism of linear alkane polymerization on Au(110)surface with one-dimensional constraint has been investigated.The surface of(1×2)-Au(110)pre-covered with molecules can be completely transformed into(1×3)-Au(110)by introducing branched methylidene groups on both sides of the aliphatic chain(18,19-dimethylidene-hexatriacontane)or locally shifted into(1×3)-Au(110)under exposure to low energy electrons(for alkane dotriacontane).STM investigations demonstrate that alkane chains adsorbed on(1×3)-Au(110)are more reactive than on(1×2)-Au(110),presenting a solid experimental proof for structure-reactivity relationships.The experimental results are further confirmed by density functional theory(DFT)simulations.