From decoupling to charge transfer: Tuning CuPc electronic states with MgO/Ag(100)

MgO films grown on Ag(100) provide a model platform for controlling molecule–substrate interactions through interface engineering. [1] In this work we report on the growth of MgO ultra-thin layers on Ag(100) with a controlled electronic structure. The goal is to realize a platform for molecular QUBIT implementation. Indeed, the dielectric MgO layer enables precise control of charge transfer between the substrate and adsorbed molecules through interface engineering. Combining scanning tunneling microscopy/spectroscopy (STM/STS), local work function and photoemission measurements complemented by first principal calculations (DFT), we demonstrate the possibility to control the magnetic state of phthalocyanine molecules. MgO films were prepared using reactive oxidation and molecular beam epitaxy (MBE), enabling access to different interface configurations. Different Mg flux and oxygen partial pressure lead to distinct growth regimes, characterized by different bandgap and work function values. A further knob to control the electronic structure is obtained by specific post-growth thermal treatment allowing to reversibly tune the work function. Vacuum annealing increases the barrier contrast between MgO and Ag(100), while subsequent oxygen annealing restores the initial values, demonstrating that oxygen-related interface processes govern the local electronic structure and work function. [2] The tunability of the MgO/Ag(100) system is directly reflected in the electronic structure of adsorbed copper phthalocyanine (CuPc). For MgO films with higher work function, CuPc exhibits electronic states in STM/STS that match our density functional theory calculations for the free-standing molecules, indicating near-complete electronic decoupling from the metal substrate. In contrast, for lower work-function MgO, an additional occupied state emerges near the Fermi level in STS (Figure 1), consistent with charge transfer across the oxide–metal interface, again in agreement with DFT calculations. The results are further supported by measurements on MgO films grown by MBE, which show similar charge-transfer signatures, indicating that these films correspond to a low work function regime with reduced interfacial oxygen. Together, these results demonstrate that MgO/Ag(100) interfaces enable controlled switching between fully decoupled and charge-transfer regimes, providing a platform for stabilizing and tuning molecular electronic states with potential relevance for molecular-scale quantum functionalities.