Stepwise Activation Of Multi-Substituted Molecules For Synthesis Of Low-dimensional Organic Nanostructures On Metal Surface

On-surface synthesis has been one of the hottest research in surface science in the past decades.The surface-based confinement effects have been utilized for steering on-surface reactions.Compared with solution chemistry,on,surface synthesis is a powerful tool for fabricating functional molecules and nanomaterials.The most outstanding examples are graphene-based nanosheets and nanoribbons.The choice of surfaces and precursors is the heart of constructing desired nanostructures.Besides,various kinetic parameters,such as surface temperature,precursor concentration and deposition rate have also been demonstrated to play important roles in the formation of nanostructures.By controlling specific experimental conditions,diverse low-dimensional nanostructures are successfully synthesized on surface.Stepwise synthesis is an effective method for synthesis of low-dimensional nanostructures.One of the advantage of stepwise synthesis is the fabrication of geometrically stable and larger intermediate structures,which would be too large for thermal deposition.Besides,some defects are frequently observed during the reaction process.To some extent,stepwise synthesis can avoid these defects and form long-range ordered nanostructures by a careful selection of precursors and surface.Up to now,the precursors in stepwise synthesis are often halohydrocarbon,which undergo an Ullmann-type reaction.By gradually altering the surface temperature,different halogen bonds are selectively activated.As a result,various low-dimensional nanostructures with few defects are synthesized on surface.However,the precursors that contained hydroxyl,carboxyl,amino and many others have been rarely reported.Herein,we utilized two different precursors,namely,bis-(4-bromophenyl)amine(BPA)and 4-bromoethyny-biphenyl(BEBP).Each precursor contains two different functional groups.Stepwise activation of these functional groups leads to the formation of diverse nanostructures.The related researches have been summarized as shown below:1.With the aid of scanning tunneling microscopy(STM)and synchrotron radiation photoemission spectroscopy(SRPES),we studied the stepwise activation of N-H and C-Br bonds in BPA molecules on Ag(111),which leads to the formation of N-Ag-N and C-Ag-C organometallic nanostructures,respectively.Two-coexisting self-assembled structures are formed after depositing BPA on Ag(111)held at 150 K.Then annealing at 303 K leads to the formation of large-scale brick-like self-assembled structure.Besides,partial N-H bonds have been activated at 303 K,resulting in the formation of N-Ag-N organometallic structure.Further annealing at 333 K for 1 hour,the N-H bonds have been completely activated and the Ag(111)surface is covered by N-Ag-N organometallic dimers.Annealing at 333 K for 3 hours triggers the cleavage of C-Br and N-Ag-N bonds.Both N-Ag complexes and N-Ag-N dimers are observed on the surface at this temperature.Annealing to 380 K leads to the breakage of N-Ag-N bonds after the complete dissociation of C-Br bonds.Intramolecular dehydrocyclization emerges at 380 K,thus resulting in the formation of a five-membered ring.Further annealing at 403 K leads to the formation of 3-fold C-Ag-C bonds,producing two-dimensional organometallic networks.2.Combining STM,SRPES and density functional theory(DFT)calculations,we reported an on-surface synthesis of five-membered carbon ring via a[4+1]annulation reaction on Ag(110).Depositing BEBP on Ag(110)held at 313 K leads to the cleavage of C-Br bonds and the formation of C-Ag-C organometallic dimers.Further annealing the sample to 473 K leads to the formation of large-scale well-ordered close-packed islands composed of tetramers.Besides,we also observed some intermediates at the edge of the well-ordered islands.Based on these intermediate and DFT calculation,we unraveled the reaction pathway and mechanism.