Influence of Copper Surface Orientation on the Hydrogen-Bonded Assembly of 2-Methylbenzimidazole Molecules: A Comparative Study

In this study, the self-assembly structures of 2-methylbenzimidazole (MBI) molecules on Cu(100) and Cu(111) surfaces were examined using scanning tunneling microscopy (STM), atomic force microscopy (AFM), and density functional theory (DFT) calculations. It was found that MBI molecules form hydrogen-bonded structures in one-dimensional (1D) or two-dimensional (2D) configurations on both Cu(100) and Cu(111) surfaces. On the Cu(100) surface, the hydrogen-bonded lines are aligned along two equivalent Cu ⟨11̅0⟩ directions, matching the lattice of the underlying surface. This alignment agrees with calculation results, which shows MBI molecules preferentially adsorbing on hollow sites with their molecular backbones perpendicular to these lines. The molecular lines spread out across the surface area, and the distance between them gradually decreases as coverage increases, indicating repulsive interactions between the lines. Conversely, on the Cu(111) surface, the 1D molecular chains are curly, with the molecular backbones oriented in various directions relative to the substrate, indicating a weaker interaction between the molecules and the substrate compared to that on the Cu(100) surface. With further increases in coverage on both surfaces, densely packed 2D molecular structures emerge, albeit with different lattice parameters. The 2D structures on Cu(100) adjust to match the substrate, whereas those on Cu(111) may assume various orientations. The distance between molecules is greater on Cu(111) than on Cu(100), suggesting a restriction of hydrogen bonding structures on the Cu(100) surface. Additionally, a noticeable height difference among the molecules in the 2D structures was observed, possibly due to a slight tilting of the molecular backbone when more molecules are integrated into the lattice. These findings demonstrate how the assembly structures of molecules depend on the interaction between molecule–molecule and molecule–substrate interactions.