Regulation Of Self-assembly Structures And Reactions Of Polycyanamide On Metal Surfaces

Self-assembly monolayer (SAM) on surface is not only an important bottom-up method to construct well-defined nanostructures but also an effective approach to modify surface properties. Due to the high symmetry in structure and capability in forming hydrogen bonds, melamine (1,3,5-triazine-2,4,6-triamine) has been frequently applied as building blocks for fabricating various SAMs. Its porous co-assemblies with other molecules were often explored to control the distribution of various guest molecules such as C60 and thiols. Melem is the tripolymer of melamine. It thus has similar symmetry as the latter but with obviously more variables for hydrogen bonding connection. Both melamine and melem hold molecular structures that are similar to the repeating units in graphitic carbon nitride (g-C3N4), a hot material of photocatalytic interests, making them the widely used raw materials for fabricating the latter. Therefore, the explorations of self-assemblies of melamine and melem would not only reveal the active roles of hydrogen bonding in tuning the assembly structures, but also provide useful information for further fabrication of highly ordered surface covalent structures similar to g-C3N4) Here in this thesis, we have conducted a systematic low-temperature scanning tunneling microscopy (LT-STM) study of the self-assemblies and thermal reactions of both melamine and melem on Au(111), Cu(111) single crystals and Cu/Au(111) thin films. The obtained main results are shown in the following.1. At room temperature, melamine tends to form a highly ordered honeycomb structure on Au(111) based on the intermolecular hydrogen bonding network. Upon exposure to CO atmosphere, we found on the surface the single atomic Au adatom species can be induced in combination with equal number of single Au vacancies. Both species are enclosed and stabilized by the nanometer sized pores of the melamine assembly. Density functional theory (DFT) calculations suggest that the synergistic interactions of CO and melamine to the Au adatoms may be the driving force of this phenomenon. This finding provides a potentially a new strategy for preparing monoatomic metal species on surface which can be of catalytic interests.In addition, we have also conducted STM manipulations over both Au adatoms and vacancies. The Au adatoms can be readily moved across the melamine film and even onto the bare surface of Au(111), while the movement of the vacancies was hardly realized without affecting the surrounding molecules. Our scanning tunneling spectroscopy (STS) measurements showed that the gold adatoms have relatively stronger empty states while the vacancies have stronger filled states.2. We examined a number of preparation parameters of melamine film on Cu(111), including coverage, temperature and deposition speed, and systematically investigated their influences on tuning the assembly structures. We particularly focused on low melamine coverage regime, wherein the thermodynamic and kinetic factors can be well separated. By precisely controlling the annealing conditions and the melamine evaporating speeds, the molecular/dissociated states of the adsorbed melamine can be well controlled. Accordingly the assembly structures can be tuned sequentially from honeycomb to the mixed-state, then to triangular, and finally to the most stable double-chain structures. The successful fabrications of the sequence of metastable structures provide deep understandings into the surface assembling process of organic molecules, serving for better control of the assembly structures.3. Besides Cu(111) and Au(111), we further prepared thin Cu films on Au(111) and investigated the adsorption and assembling behavior of melamine. We found the 1-Layer Cu film has a pseudomorphic (1×1) subsurface structure, overlaid with a complete Au layer on top. Whereas the true Cu adlayers with Cu(111) lattice start from the 2-Layer Cu, but the electronic properties restore roughly since the 3-Layer Cu film. On the ultrathin Cu films, the melamine was found to adsorb preferentially on the exposed Au(111) area by forming a new type assembly structure together with the trapped Cu adatoms. Only at high coverages, melamine adsorbed onto the Cu films and form honeycomb structures similar to that on pure Au(111). Melamine chemisorbed in dissociated states on the 3-Layer Cu film, but the highly ordered assembly structure was only found on the 4-Layer and thicker films. This part of work has provided the first case of studying organic assemblies on supported metal films, demonstrating the doping effect and film thickness effect in tuning the molecular adsorption states and the corresponding self-assembly structures.4. As a comparison with melamine, we studied the self-assembly behavior of melem on Au(111) and Cu(111) by investigating similar control parameters such as coverage and temperatures. At room temperature, melem was found to physisorb on Au(111) and form a few ordered structures, some of which were similar to those reported on Ag(111) but one was close to the mixed-state double layer structure of melamine on Cu(111). And an interesting finding was that the geometries of intermolecular hydrogen bonding could be manipulated via thermal annealing or tip interactions. On Cu(111) surface, on the other hand, melem adsorbed dissociatively and assembled into similar double-chain structures as what melamine does on the same surface. High temperature annealing of melem led to only desorption on Au(111). Whereas on Cu(111), annealing led to a disproportionation reaction of melem which yielded molecular melem and a fully-dehydrogenated radical. The molecular melem assembled into the same honeycomb structure as on Au(111) but the radicals dispersed randomly on the surface. At even higher temperatures, polymerization of melem occurred and resulted in a periodic covalent film that is exactly the same as that produced by melamine under the same conditions. This covalent film show the same periodicity as g-C3N4, but the chemical composition still awaits further characterizations.In conclusion, we have systematically investigated the self-assemblies and thermal reactions of both melamine and melem on Au(111), Cu(111) and Cu/Au(111) surfaces in this thesis. We discussed in detail the roles of both thermodynamic and kinetic factors in controlling the adsorption states as well as the assembled structures of such oligomers of cyanimide. The series of novel findings in this thesis are believed of importance in deepening our understandings of the surface assembling process and the influential parameters. The investigated surface reactions may also provide new strategies for effective fabrication of highly ordered surface covalent structures in the future.