Adsorption And Self-assembly Of Ethanol Molecules On Bi(111)

Characterization of nanostructures becomes very important with the development of nanoscience and nanotechnology. The metal surface molecular adsorption system, such as single molecular adsorption, diffusion, vibration, the molecule-substrate interaction, and molecular self-assembly have attracted considerable attentions of researchers.Scanning tunneling microscope(STM), capable of atomic-scale resolution in real space has become a very powerful technique in the area of nanoscience and nanotechnology. It can characterize the surface of the material with atomic resolution and the electronic local density of states(LDOS) near Fermi levelIn this paper, we have studied the adsorption and self-assembly of ethanol molecules on Bi(111) surface by means of low-temperature scanning tunneling microscopy(STM). At low coverage, ethanol molecules appeared on the Bi(111) surface in the form of monomers and dimers. The high-resolution STM image of the ethanol monomer is pear-shaped. The ethanol molecules tend to be adsorbed at atop sites and bond to the surfaces through the lone pair electrons of oxygen atom. There are two stable conformers of ethanol molecules: trans-gauche and cis-gauche. ethanol molecule adsorbed on Bi(111) is mainly cis-gauche, with the hydroxyl group pointing away from the ethyl mirror plane. Ethanol molecules may bind with each other by the H-bond through its hydroxyl group. Ethanol molecules are subjected to two forces acting on the Bi(111) surface, one is ethanol intermolecular hydrogen bonding interaction, the other is the attraction of the substrate. At low coverage, interactions between molecules- substrate is stronger and it is difficult for the ethanol molecules to form large molecular clusters.With the coverage increased, massive identical ethanol hexamers, one-dimensional molecular chains and molecular rings were observed on the substrate. On hexagonal metal surfaces intermolecular hydrogen bonding leads to the formation of a hexagonal two-dimensional lattice, the adjacent ethanol molecule in hexamer interconnected by hydrogen bond. The cyclic topologies in increasing average H-bond strength is almost twice as strong as the linear dimer H-bond energy, the ethanol hexamer in which all monomers participate as H-donor and H-acceptor at the same time. We can judge that cooperative effects increase the strength of H-bond, and make the ethanol hexamer stability increased. Besides, ethanol molecules can agglomerate to unstable one-dimensional molecular chains and molecular rings by hydrogen bonds. The increased number of hydrogen bonds weakened the coupling between the structures and the substrate, so these structures are easily destroyed.After the ethanol self-assembly at high coverage was annealed at 310 K for 2 hours, large areas of self-assembled films of ethanol were obtained. The lattice constant is a=630pm, b=380pm, θ = 89°, which is different from that of ethanol crystal. The formation of the self-assembled film can be attributed to the strong intermolecular interaction, which can dominate the molecular-substrate interaction.