Tailoring The Self-assembly Of Melamine On Gold Surfaces Via Doping With Cu Atoms

Molecular self-assembly has been regarded as an efficient bottom-up method for fabricating nanostructures considering the advantages of spontaneity,highly ordering,and designability.Large number of studies about self-assembly up to days have been conducted on various pure metal surfaces,which is however severely deviated from the multiple-component metallic surfaces applied in various practical processes.As a matter of fact,alloyed surfaces usually hold significantly improved properties against the pure metals,thus find wide applications in heterogeneous catalysis.However,detailed surface science studies of the alloy surfaces are still rather limited.Particularly,the researches about molecular adsorption and reactions are frequently related to small molecules such as CO and O2.Those about the more practically important organic molecules are still severely lacking.With this in mind,we have focused our researches on the techniquely important Au-Cu alloy system.Based on our previous studies of the Cu/Au(111)films,we have turned to a reversed system and prepared Au/Cu(111)films of one to a few atomic layers thickness.With the help of low-temperature scanning tunneling microcopy(STM)in combination with photoemission electron spectroscopies(PES),we have studied the atomic structures of the Au films and investigated the adsorption and assembly behavior of melamine molecules.Our experimental results demonstrated that the atomic lattice of the Au films experience a stepwise increment from one to three layers and finally turn to the same as pure Au(111)since the fourth layer.Similar trend was observed for the electronic properties of the Au films.The PES data clearly revealed the segregation of Cu atoms at the vaious Au layers whose concentration displays a naturally downgoing trend along with the thickening of the Au films.Accordingly,the adsorption and assembling of melamine molecules on these Au films were found largely distinct from that on the Au(111)surface.The melamine molecules preferentially adsorbed on the exposed Cu(111)substrate surfaces until saturation.And then they start to porpulate the Au films and form variously new assembly structures varing with the thickness of the gold films.Until rather thick Au films such as 7 layers,the assembly structures of melamine roughly returned back to that on pure Au(111)surface.Such phenomena can be reasonably attributed to the involvements of the Cu atoms in the assemblies changing along with the varied concentrations in the differently thicked Au films.As a comparison system to melamine on the Au/Cu films,we also investigated the effect of the deposited Cu atoms on the self-assembly films of melamine on the Au(111)surface.The experimental results showed that with the increasing amount of the deposited copper,the self-assembly structure has changed stepwise from a well-defined honeycomb into various new structures which duplicate those found on the Au/Cu(111)films.Such findings further strengthen our proposition of the co-assemblies of Cu and melamine molecules.In the last part of this thesis,we turned to study another interesting organic molecule,1,3,5-benzenetriazole,which is applied to construct two-dimensional covalent network in solution.On both Cu(111)and Au(111)surface,we found 1,3,5-benzenetriazole can form highly ordered assembly structures on the basis of intermolecular hydrogen bonds.These preliminary results should laid a foundation for further investigating its assembly behaviors on the Au-Cu films as well as successful construction of on-surface two-dimensional covalent organic framework materials in the near future.