Self-assembly And Reactions Of Ph-PET On The (111) Surface Of Noble Metal

The development of high precision electronic devices is an important frontier of international scientific research competition.The process and integration of siliconbased electronic devices can no longer meet the needs of micro and nano-level electronic components.Therefore,the development of new high-performance semiconductor materials that can be applied to micro-and nanoelectronic/optoelectronic devices is a key issue.Graphene is a monatomic layer material with high chemical stability and thermal stability.However,the band gap of intrinsic graphene is zero,and it can not be used as a semiconductor in electronic devices.Hence,it is still a key problem to obtain two-dimensional semiconductor materials with good stability,long-range order and high electron transport performance.Two-dimensional polymers with single atomic layer show considerable prospects in the research and development of new electronic devices in recent years due to their diversity,tunable band gap and environmental stability.However,there are still many problems to be solved in the surface synthesis of monolayer two-dimensional polymers,including low yield,low coverage due to easy molecular desorption,and difficult impurity removal.In particular,the use of single-step reaction or multi-step reaction of the same type makes the reaction type with relatively high reaction energy barrier unable to be applied.In order to solve the above problems,the self-assembly of Ph-PET molecules and reactions on Au(111),Ag(111)and Cu(111)surfaces were systematically studied by ultra-high vacuum scanning tunneling microscopy(STM)and density functional theory(DFT)through using Ph-PET with special functional groups as the precursor.Different from the previous single-step reaction or single type of multi-step reaction,this work initiated multi type of step-by-step reaction with improved the yield and controllability of the reaction,and obtained monatomic layer,atom level ordered two-dimensional polymer containing triazine ring.By comparing the rules and characteristics of assembly and reaction of Ph-PET molecules on different metal substrates,and combining with theoretical calculation,the effects of electron localization on different metal surfaces and their electron aggregation on their catalytic molecular reactions were discussed.The specific research contents of this paper are as follows:The self-assembly and controlled stepwise reactions of Ph-PET molecules on the Au(111)surface were investigated using STM combined with X-ray photoelectron spectroscopy and DFT theoretical calculations.Three types of stepwise reactions corresponding to the terminal functional groups of Ph-PET molecules were triggered by thermal excitation,including alkyne ring trimerization,C –O bond cleavage reaction and C–H bond activation reaction.The reaction process was also simulated by CI-NEB method to clarify that the substrate played a catalytic role in the reaction process.The final products obtained in the study were triazine ring-containing,monoatomic layer-ordered two-dimensional macromolecules.The stepwise reaction reduced the energy barrier of the total reaction and improved the yield and controllability of the reaction.The theoretical band gap of the obtained two-dimensional polymer layer is 3.41 e V,which is a wide band gap organic semiconductor.Considering the good stability of the covalent organic framework containing a triazine ring and the high electron mobility in the two-dimensional plane,this type of graphene 2D polymer is expected to have promising applications in the field of wide-bandgap semiconductor-related devices.Considering that the catalytic activity of Ag(111)and Cu(111)surfaces is higher than that of Au(111)surface,the self-assembly and reactions of Ph-PET molecules on Ag(111)and Cu(111)surfaces were investigated by combining experimental studies with theoretical calculations,respectively.Ph-PET molecules formed a selfassembled structure on Ag(111)surface were similar to that obtained on Au(111)surface,but their self-assembled structures were similar to that of Au(111)surface.The self-assembled structure obtained on the Ag(111)surface,but their orderliness was lower than that of the assembled structure formed on the Au(111)surface.By studying the effect of annealing on the Ag(111)surface assembly structure,it was found that annealing caused a transformation of the Ag(111)surfac e assembly structure.On the other hand,Ph-PET molecules deposited onto the Cu(111)surface at room temperature formed disordered structures,while annealing treatment leaded to the formation of dense rows of structures.DFT calculations of the reaction p rocess of Ph-PET molecules on both substrates showed that metal atoms on the surface of Ag(111)and Cu(111)substrates participated in the reaction process,acting as catalysts that resulting in the coupling between molecules and the eventual formation of covalent structures.The self-assembly and reactions of Ph-PET molecules on the surfaces of Au(111),Ag(111)and Cu(111)precious metals were comparatively analyzed and studied.The surface electronic structures of Ph-PET molecules on the three substrates were studied by DFT theoretical calculations,and it was found that the delocalized electrons of the substrates affect the degrees of freedom of Ph-PET molecules: the degrees of freedom of Ph-PET molecules were relatively high on the Au(111)surface and low on the Ag(111)and Cu(111)surfaces.This difference in molecular degrees of freedom allows the Ph-PET molecules to form a single type self-assembled structure on the Au(111)surface and to undergo a multi-step structural transition on the Ag(111)surface.The relatively weaker electron delocalization of the Cu(111)surface makes the molecular movement restricted,leading to the difficulty of spontaneous formation of self-assembled structures of Ph-PET molecules on the Cu(111)surface.And the surface electron localization and the electron aggregation situation affect the reactivity of Ph-PET molecules,leading to selective C–H bond activation reactions of Ph-PET molecules on the three substrate surfaces.This study helps to deepen the understanding of surface molecular reactions and surface synthesis of 2D polymeric materials,which in turn promotes the development of new 2D organic semiconductor materials and their related devices.