Studies On The On-surface Reaction Mechanisms Of Halogenated Aromatics By Photoemission Spectroscopy And Scanning Tunneling Microscopy

On-surface synthesis has been a hot topic in recent years because of its potential application in molecular electronics.Compared with traditional solution chemistry,on-surface synthesis shows a great advantage in the preparation of low-dimensional covalent nanostructures due to its two-dimensional(2D)confinement effect to on-surface reactions.Moreover,the highly active metal substrate can catalyze the reaction so that it can perform under relatively mild conditions.Based on these,various on-surface reactions,including Ullmann coupling,Glaser coupling,dehydrogenation coupling and Sonogashira cross coupling,have been widely studied in the past decades.Different functionalized nanostructures can be synthesized through those reactions by selecting appropriate organic molecular precursors,metal substrates and experimental conditions.In particular,the fabrication of graphene nanoribbons(GNRs)by surface-assisted Ullmann coupling reaction via halogenated aromatics has been the most popular study.During the past decade,GNRs with different widths and edge structures have been formed on different metal surfaces,and their electrical and magnetic properties have been unambiguously explored,which greatly promotes its development in molecular electronics.However,it is still hard to realize the fabrication of "molecular circuits",and then to prepare the so-called "molecular chips".This is because the molecular circuit requires its internal molecules to be covalently linked in a two-dimensional structure,and to minimize the generation of defects,which poses a great challenge to on-surface synthesis for two reasons.On the one hand,the formation of carbon-carbon covalent bond is irreversible,so the structural defects in the products are difficult to self-heal.Secondly,compared with solution chemistry,only few experimental parameters can be adjusted for on-surface synthesis,which is adverse for the precise control of on-surface reactions.All of these hinder the synthesis of high-quality low-dimensional covalent nanostructures.In this dissertation,different kinetic strategies were used to steer the reaction of halogenated aromatics on metal surfaces,to synthesize well-ordered low-dimensional covalent nanostructures.In addition,by the combination of photoemission spectroscopy(PES),scanning tunneling microscopy(STM)and density functional theory(DFT)calculations,we have revealed the reaction mechanism of the on-surface synthesis process.The main achievements of this paper are listed as follows:1.Halogenated nanographene-type molecule,hexiodohexabenzobenzene(I6-HBC),is used as the molecular precursor to study its reaction behavior on the Au(111)surface.The effects of molecular adsorption and migration in on-surface synthesis process have been investigated by preparing the samples with two different methods:stepwise annealing procedure and hot deposition.The result shows that desorption of molecules happens during the annealing of the sample prepared at low temperature.Only when the molecular precursors are deposited onto a hot Au(111)surface,the intermolecular carbon-carbon coupling reaction occurs.In addition,we also investigate the influence of different molecular evaporation rate on the morphology of covalent product:relative low evaporation rate is conducive to the formation of one-dimensional chain products,while a high evaporation rate can promote the generation of two-dimensional covalent structures.2.The topological selective synthesis of covalent structure products on the Cu(111)surface has been successfully achieved by using different experimental parameters.At room temperature(RT),1,3,5-tris(2-bromophenyl)benzene(TBPB)molecules have performed the C-Br bond activations on the surface,however the Ullmann coupling reaction is inhibited.DFT calculations show that the special adsorption configuration of TBPB molecules on the metal substrate is the main reason for the C-H bond activation at low temperature.Disordered dendritic structures are obtained by annealing the samples at room temperature.On the contrary,in hot deposition experiment,ordered structures mainly composed of porous graphene nanoribbons(p-GNR)and two-dimensional nanoporous graphene(2D NPG)have been fabricated.By control experiments,it is proved that molecular self-assembly formed at RT has a negative effect on the formation of ordered structures.While for the reaction at the hot deposition experiment,thermodynamic effects dominate the formation of ordered structures.This work for the first time reports the negative effects of molecular self-assembly to the fabrication of high-quality products,which is expected to provide reference on related work in future.3.By exploring the reaction behavior of TBPB on Ag(111)under different annealing processes,it is proved that molecular self-assembly can effectively control the selectivity of on-surface reaction.The results show that debrominated monomers form dense self-assembled islands at 390 K.When annealed to 480 K,dehydrogenative coupling(DHC)reaction,cross coupling(CC)reaction and Ullmann coupling(UC)reaction take place,while dehydrogenative coupling reaction is dominant.In contrast,when TBPB molecules are deposited on a hot Ag(111)substrate,only cross coupling and Ullmann coupling occur between the debrominated molecules.Further comparative experiments show that it is the involvement of molecular self-assembly that lead to the selective synthesis under different experimental conditions.This work reveals that molecular self-assembly plays an important role in steering the selectivity of on-surface reactions.