Investigating The Steering Effects Of Substrate On The Surface Self-Assembly And Reactions Of Halogenated Aromatic Via Synchrotron Radiation Photoelectron Spectroscopy And Scanning Tunneling Microscopy

Organic molecular materials have promising applications in developing new information functional devices.They are inexpensive to produce,environmentally friendly,and offer tunability of the structure and properties.However,traditional molecular materials often face challenges with low electron transport efficiency,poor ordering,low solubility,etc.These challenges limit the application potential of molecular materials in high-precision devices.To overcome these difficulties,onsurface synthesis(OSS)has emerged in recent years.The so-called OSS allows the bottom-up production of functional covalent nanostructures with atomic precision in ultrahigh vacuum(UHV)using organic chemical reactions.Unlike classical solution phase chemistry,OSS does not require a solvent,resulting in minimal byproducts and no limitations on solubility.Additionally,the use of solid surfaces effectively helps to stabilize reactive products,thereby promoting the synthesis of various low-dimensional functional carbon nanostructures with novel physical properties.To date,numerous organic chemical reactions have been successfully carried out on surfaces that form various one-dimensional or two-dimensional covalent nanostructures.Although OSS presents many advantages over classical solution phase chemistry,challenges arise when multiple reaction pathways are involved in an on-surface reaction.The construction of complex nanostructures typically requires multiple reaction pathways.Because of the restriction of extrinsic catalysts in clean UHV environments,the reaction selectivity of such multipath reactions is relatively low.As a result,it ischallenging to achieve accurate characterization of the desired structures,and the resulting low quality and yield of the products limit their potential applications.Therefore,one of the major challenges in OSS is how to efficiently control the surface reaction pathways and improve the yield of desired products.In this study,we prepared samples using the molecular beam epitaxy(MBE)growth technique and investigated the reactions of different precursor molecules on solid metal surfaces using experimental and theoretical methods,including synchrotron radiation photoelectron spectroscopy(SRPES),scanning tunneling microscopy/spectroscopy(STM/STS),and density functional theory(DFT).We fabricated various low-dimensional covalent nanostructures and explored the influence of the substrate on the surface reaction pathway.The main findings of the study are as follows:(1)The reaction pathways and resulting chiral nanostructures of the prochiral 2,6dibromo-anthracene(DBA)precursor molecules on Ag(111)and Ag(100)substrates were investigated.Depositing prochiral DBA on Ag(111)and Ag(100)held at 120 K leads to the formation of homochiral self-assembled and racemic self-assembled structures,respectively.Thermal annealing induces the stepwise formation of organometallic and covalent structures on both surfaces.Similar to the low-temperature experiment,high chiral selectivity was observed on Ag(111),where organometallic chains and covalent polymers are composed of homochiral DBA molecules.In contrast,poor chiral selectivity is found on Ag(100).This difference is presumably attributed to the lattice symmetry and substrate reactivity,as supported by the comparison experiments carried out on Au(111)and Cu(111).(2)Two biphenyl-based molecules with two and four bromine substituents,i.e.,2,2’-dibromobiphenyl(DBBP)and 2,2’,6,6’-tetrabromo-biphenyl(TBBP),show completely different reaction pathways on the Ag(111)surface,leading to the selective formation of different covalent products.By combining STM,SRPES,and DFT calculations,we unravel the underlying reaction mechanism.After debromination,a biradical biphenyl can be stabilized by surface Ag adatoms,while a four-radical biphenyl undergoes spontaneous intramolecular annulation due to its extreme instability on Ag(111).Such different chemisorption-induced precursor states between DBBP and TBBP consequently lead to different reaction pathways after further annealing.In addition,using STS and bond-resolving STM,we determine the bondlength alternation of the biphenylene dimer product,which contains 4-,6-,and 8membered rings with atomic precision.The 4-membered ring units turn out to be radialene structures.(3)The reaction pathways and mechanisms of the precursor molecule 2,2’,6tribromo-1,1’-biphenyl(TriBBP)on Ag(111)and Cu(111)substrates were investigated.TriBBP exhibits two distinct reaction pathways on the Ag(111)surface:(1)debromination of TriBBP results in spontaneous intramolecular cyclization,ultimately leading to the formation of a covalent dimer embedded with a 4-membered ring.(2)Alternatively,TriBBP can form a planar rhombic nanographene through intermolecular selective aryl-aryl coupling and dehydrogenation reactions.On the Cu(111)surface,the strong molecule-substrate interaction leads to ortho C-H bond activation of TriBBP,resulting in the formation of disordered polyphenylene covalent products after further annealing treatment.