Energy threshold for electron-induced reactions in surface-grown Cu(II) oxalate layers

Layers of surface-grown coordination polymers can be decomposed or modified by electron irradiation. This offers interesting perspectives for nanofabrication processes. However, a fundamental understanding of the underlying electron-induced chemistry is required to gain full control over such processes. Therefore, this study reinvestigates the electron-induced decomposition of Cu(II) oxalate which yields the volatile product CO2. Previous studies proposed that the decomposition proceeds via a reaction mechanism driven by electron ionization. However, the reaction was only studied at electron energies above 30 eV. This ruled out a more detailed experimental identification of the electron interactions that initiate the decomposition of Cu(II) oxalate. Herein, Cu(II) oxalate samples were grown on the surface of self-assembled monolayers of mercaptoundecanoic acid in a layer-by-layer approach using Cu(II) acetate and oxalic acid. The Cu(II) oxalate samples were characterized by reflection absorption infrared spectroscopy (RAIRS). Electron-stimulated desorption (ESD) of neutral CO2 was monitored to measure the decomposition of the layers as a function of electron energy. The results reveal a threshold for the decomposition of Cu(II) oxalate to CO2 at around 3.5 eV to 4 eV which is ascribed to a reaction driven by ligand-to-metal charge-transfer electronic excitation.