STM Study Of Self-assembly Of {Mo₁₅₄} Molecules On HOPG

Nanoscience and technology is a new field in recent years,which has received the attention of all countries in the world and developed rapidly and widely.Molecular surface self-assembly is a simple and common method to construct 2D nanomaterials.However,the self-assembly of molecules on the surface is a complex process affected by many factors including driving force,solvent,concentration,substrate and so on.Surface self-assembly can easily obtain a wide range of uniform nanostructures,but how to effectively control the direction of molecular self-assembly to construct target functional materials is the focus and difficulty in the field of nanoscience.Based on this,this paper takes scanning tunneling microscope（STM）as the main research means,and does the following two aspects of research:Assembly structures of n-octadecane and stearic acid（octadecanoic acid）on Highly Oriented Pyrolytic Graphite（HOPG）were investigated using octanoic acid as solvent.We found that the assembly structures of n-octadecane and stearic acid are also different due to the different functional groups.Hexadecane depends on the van der Waals force between molecules and molecules and between molecules and graphite to form a"ridge"-like nanostructure,and the molecular axis and"ridge"are arranged in 90°.Due to the repulsive force between molecules,there is a certain distance between"ridge"and"ridge".For stearic acid,due to the formation of O…H-O hydrogen bonds between carboxylic acid groups,produced"head-to-head"and"tail-to-tail"intersecting"ridge"like structures.And the angle between the molecular axis and the"ridge"is（77.6±0.1）°.Hydrogen bonding becomes the main driving force relative to van der waals forces.The assembled structures were investigated using ethanol,ethanol:toluene=1:1 and ethanol:1-phenyloctane=1:1 as solvents using"wheel"-like molybdate{Mo₁₅₄}as molecules.We found that{Mo_154}on HOPG can form a large range,homogeneous,multi-domain,stable assembly structure,and the structure is not affected by solvent.{Mo₁₅₄}molecules are standing,parallel to each other and inclined to form a"ridge"structure.The information of{Mo₁₅₄}molecules in occupied and unoccupied states is detected under positive bias and negative sheet pressure.It is found that there are slight differences STM image presentation structure in both cases.And this assembled structure can be maintained at room temperature for more than one month,which indicates that the two-dimensional molybdate assembled structure has great potential and possibility in practical application.