Surface-supported Alkoxylated Benzene Molecular Self-assembly Investigated By STM

Surface-supported molecular self-assembly has been widely applied in nanoscience and technology, biological pharmacy and surface physics and chemistry. Controllable formation of self-assembled monolayers(SAMs) plays an important role in the fabrication of molecular devices. Generally speaking, self-assembled structure relies on a subtle balance between intermolecular(molecule-molecule and molecule-solvent) and interfacial(molecule-substrate and solvent-substrate) interactions. As a consequence,besides the foremost important factors such as chemical species and the overall geometry of molecular building blocks, the self-organization behavior of molecules on surfaces can also be strongly affected by other parameters, such as the topography of a supporting surface, the temperature of the solution, the type of solvent, and the concentration of solute. The complexity of the self-assembly process may result in the co-existence of different phases, thus reduces the degree of control for molecular patterning. Therefore, an essential question in supramolecular chemistry and nanoscience is how to prepare desirable uniform structures over large scale in a controllable manner.Using scanning tunneling microscope(STM) as the main research mean, we have achieved the following results:1. We systematically study the molecular packing of alkoxylated benzene(B-OCn)( n = 12, 14, 16 and 18) on HOPG surfaces. Three different phases are obtained and the phases can be controlled by changing the concentrations of the solution. Our studies,particularly the in situ diluting and concentrating experiments, clearly demonstrate that the phase transition of B-OCn on HOPG surface is driven thermodynamically, leading to high controllability and reversibility. Finally, we have studied the phase transition behavior with different alkoxy chain lengths, and gave the phase diagram of the self-assembly of B-OCn( n = 12, 14, 16 and 18) on HOPG surfaces.2. We prepared the B-OC16 SAMs on HOPG using high vacuum deposition. Thesample was investigated by STM under ambient condition. We have successfully obtained the large-scale uniform structure of SAMs on HOPG by adjusting the evaporating temperature to 180℃. More importantly, we found that the orientation of the self-assembly structure is unified over large scale. In future work, these large-scale,uniform orientated SAMs will be used as inducing-layer for the high quality crystallization of functional molecules.3. We performed a systematical study about the self-assembly behaviour of B-OC16 molecules on different substrates under ultra high vacuum(UHV). On Au(111), we got a special honeycomb structure that is composed of six B-OC16 dimers. On Cu(100)surface, on the other hand, a modulated stripe structure is observed. The different self-assembly behaviors on various substrate indicate that the type and the symmetry of the substrate play an important role in the molecular self-assembly.