X-ray photoelectron spectroscopy study of the interaction of N-heterocyclic carbenes with polycrystalline copper surfaces
Al Rashed, Abrar
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The interaction of organic molecules with metal surfaces to form self-assembled monolayers (SAMs) has been extensively investigated. The formation of SAM on metal surfaces has potential applications in microelectronics, chemical sensing and metal protection in which the SAM quality and stability play critical roles. Thiols are the most widely used in SAM technology. However, the metal-thiolate bond is rapidly oxidized under ambient conditions. Therefore, new ligands were investigated to replace thiols, namely N-heterocyclic carbenes (NHCs). NHCs are well known as ligands for transition metal complexes. Consequently, they are strong candidates for SAM formation on transition metal surfaces. So far, NHCs have been applied to gold surfaces, which produced films with remarkable stability. However, less is understood about the interaction of NHCs with other transition metals. One particularly interesting transition metal is copper, which has significant applications in electronics, heat transfer, etc. Copper is cheaper than gold, but it is easily oxidized and corroded in different media, thus finding a method of preventing this oxidation is very important. Here, we present a study of NHC interactions with clean and oxidized copper surfaces at different temperatures. Detailed X-ray photoelectron spectroscopy (XPS) experiments have been carried out, using both a standard XP spectrometer at fixed X-ray energy, and experiments at variable photoelectron energies carried out at the Canadian Light Source synchrotron. The result shows that exposing the oxidized copper surface to the NHC solution led to a complete removal or reduction of the oxide layer from the surface, followed by a SAM formation on the reduced surface. Evidence from mass spectrometry shows copper and oxygen complexes in the solution, which proves the ability of the NHCs to interact with the oxide layer and remove it from the surface. Further experiments carried out at varying temperature and NHC exposure times have revealed further details on the mechanism of carbene reaction with the surface. These results show promise in gaining a more fulsome understanding of the dynamics of NHC interactions with Cu, with a view to designing surfaces that are passivated towards oxidation.