DFT Simulation of XPS Reveals Cu/Epoxy Polymer Interfacial Bonding

Abstract

Experiments and computations are performed to assess the interfacial bonding between Cu and a poly-epoxy surface relevant to many applications. The surface of the poly-epoxy is characterized by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy before and after ultrahigh vacuum Cu deposition. Modifications of the XPS spectra are observed, suggesting a strong interaction between specific C and O atoms of the surface with Cu. Density functional theory (DFT) calculations are then performed to simulate XPS spectra and to better understand bonding. DFT computations are performed in the framework of the uGTS methodology, which takes initial and final state effects into account, and allows to calculate chemical shifts between the different C 1s and O 1s molecular orbitals with good accuracy, for the pristine surface. DFT calculations are then set to determine the preferential adsorption sites of Cu on different sites of the polymer surface. Finally, XPS simulation of the C 1s and O 1s spectra with Cu adsorbed at these sites matches very well with the experimental spectra, indicating that Cu atoms interact preferentially with hydroxyls to form Cu−O−C bonds, stabilized by a transfer of 0.5 electrons from Cu to O; hence, Cu is partially oxidized.