Visualizing Interfacial Charge Trapping in a Heterostructure of a Monolayer Metal-Organic Framework on a van der Waals Substrate.

Two-dimensional conjugated metal-organic frameworks (2D c-MOFs) have garnered significant attention for their potential applications in energy devices such as supercapacitors and batteries due to their high electrical conductivity and large surface area. In this study, we synthesize a single-layer 2D c-MOF, Ni3(HAT)2, on a van der Waals substrate of graphite, serving as a model system for a bilayer MOF-based supercapacitor. Employing scanning tunneling microscopy and spectroscopy (STM/S) and density functional theory analysis, we find that monolayer Ni3(HAT)2 retains its intrinsic structural and electronic properties, exhibiting an effective electron mass of approximately 0.18 me and gapless density of states near the Fermi level. Tunneling spectroscopy shows that the Ni3(HAT)2 monolayer undergoes charging and discharging at a voltage as low as 0.1 V in the tunnel junction. The DFT analysis reveals that the Ni3(HAT)2 monolayer transfers 0.18 e-/nm2 to a graphene substrate. We use STM/S to visualize that the substrate charges are trapped underneath the pores of Ni3(HAT)2. Our study sheds light on charge transfer and storage in a model system of MOF-based supercapacitor devices.