Investigating the monolayer structure of caffeine molecules on Au(111)*

Polymorphism is an important property of pharmaceutical ingredients in terms of their bioavailability. As a bottom-up approach we investigated the formation of monolayers of caffeine molecules on a Au(111) surface to understand caffeine's polymorph behavior.
In this work we analyzed the structure formation of caffeine molecules on Au(111) by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. All investigations were conducted at monolayer coverage at room temperature.
By combining these powerful surface science techniques, we demonstrate successful formation of caffeine monolayers including molecular alignment in two mirrored directions of the Au(111) surface. We propose a structure model for the two found surface domains in a three-molecular unit cell. This contributes to an understanding of the driving forces for surface structure formation which is essential for potential applications.