Structural investigation of caffeine monolayers on Au(111)

The asymmetric and achiral character of caffeine (C8H10N4O2) leads to two on-surface chiralities which has an impact on its on-surface formation. An analysis of its on-surface behavior reveals new insights of its crystallite growth. In this study the structural formation of caffeine monolayers on a Au(111) surface was analyzed by scanning tunneling microscopy (STM), low energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. The monolayers were prepared by molecular beam epitaxy (MBE) and analyzed at room temperature. Caffeine molecules self-assemble in a quasihexagonal phase on Au(111) similar to the high-temperature α phase. Two mirrored hexagonal domains are present with respect to the surface. Within the XPS measurements, no strong surface interaction was found. Therefore, a theoretical analysis of a hypothetical free-standing caffeine monolayer structure was performed by ab initio simulations. We found that a caffeine monolayer with three molecules per unit cell is preferable to one with just a single molecule, as could be expected from the LEED pattern.