Investigation of copper and silver surfaces functionalised by N-heterocyclic molecules

Abstract

To bestow particular properties on a metal surface, organic molecules can be used, amongst them N-heterocyclic molecules. In this study, N-heterocyclic molecules were deposited onto Ag(111) and Cu(111) surfaces in an ultra-high vacuum environment and analysed using complementary surface sensitive techniques and computational methods. Scanning tunnelling microscopy (STM) was utilized to observe the topography of the self-assembled monolayers, high-resolution electron energy loss spectroscopy (HREELS) was used to measure the vibrational frequencies of the adsorbed layers, temperature-programmed desorption (TPD) mass spectrometry was used to study the desorption trends of the overlayers; additionally, density functional theory calculations were implemented for deriving the adsorption geometry of the film. This study shows that, by varying the preparation conditions, the self-assembled monolayers of N-heterocyclic molecules can form different structures. For the initial adsorption of benzotriazole (BTAH) on Cu(111) new structures were observed dissimilar to the previously seen hexagonal phase. Additional structures were observed upon exposing an oxidised Cu(111) surface to BTAH resulting in hexagonal structures referred to as the “daisies” and “roses” phase. Through HREELS and STM analysis it was suggested that the BTAH reduces the oxidised surface and passivates the surface forming a protective film. Similar observations were made for a benzannulated N-heterocyclic carbene (NHCDBZ) on an oxidised Ag(111) surface through TPD and HREELS measurements with implication of a urea type by-product. In the case of the initial adsorption of NHCDBZ, the change in substituent geometry upon adsorption to Cu(111) and Ag(111) further supports the intrinsic role of the benzyl substituents in the structure of the self-assembled monolayer. The NHCDBZ film on Ag(111) shows promise as a corrosion inhibitor and also as a chemical etchant. The porous nature of the NHCDBZ on Cu(111) Kagome-like lattice offers possibilities in other fields such as catalyst research and nano electronic applications.