Exceptionally stiff two-dimensional molecular crystal by substrate-confinement.

We demonstrated an approach to effectively apply in-plane pressures to molecular layers by utilizing the substrate confinement effect. The compressed crystal structure and mechanical behaviors of carbon monoxide (CO) monolayer subjected to the confinement of Cu(100) substrate were jointly investigated by low temperature scanning tunneling microscopy experiments and first-principles density functional theory calculations. By increasing molecular coverage, an exceptionally large Young's modulus of 33 GPa was derived for the constrained CO monolayer film. This extreme in-plane pressure leads to site-specific tilting geometries, polymeric-like electronic states, and vibrational behaviors of CO molecules in the compressed phases. These results provide an extended understanding of the physical and chemical properties of intermolecular interactions in this fundamental system.