Surface-Induced Symmetry Breaking Leads to Unexpected Vibrational Activity of Melem on Cu(111)

Abstract

The adsorption of melem on Cu(111) at 80K has been studied by Scanning Tunnelling Microscopy (STM), Reflection Absorption Infrared Spectroscopy (RAIRS) and Density Functional Theory (DFT) calculations. STM images and DFT calculations confirm a planar adsorption geometry and a triangular footprint at the surface, with the nitrogen atoms positioned above atop surface sites and the carbon atoms in 3-fold fcc sites. RAIRS data confirm the reduction of molecular symmetry from D3h in the gas phase to C3v in the adsorbed state. DFT calculations show that melem is adsorbed at a height of 3 Å above the surface, consistent with molecule-metal distances observed for physisorbed aromatic molecules. However, a small charge transfer of 0.27 e- and small tilt of the NH2 groups towards the surface observed in the DFT calculations suggest an adsorbed state beyond a simple unperturbed physisorption model. This view is further reinforced by the experimental RAIRS data that show intense and broad red-shifted bands arising from the in-plane NH2 scissors + ring stretching vibrational modes, suggesting the presence of additional intermolecular and molecule-metal interactions in the adsorbed state that lead to bond softening and increased anharmonicity in the vibrational modes.