Ambipolar charge transfer of larger fullerenes enabled by the modulated surface potential of h-BN/Rh(111)

A detailed understanding of how molecules interact with two-dimensional materials, particularly concerning energy level alignment and charge transfer processes, is essential to incorporate functional molecular films into next-generation 2D material-organic hybrid devices. One of the major challenges in integrating molecular films in field-effect transistors is facilitating ambipolar charge transport, which is often hindered by the large electronic gap of the organic layers. This work compares the adsorption site-dependent energy level alignment of C-60, C-70, and C-84 fullerenes induced by the spatial variation of the electrostatic surface potential of the h-BN/Rh(111) Moire superstructure. As the size of the fullerenes increases, the HOMO-LUMO gap shrinks. In the case of C-84, we find an intrinsic charge transfer from the substrate to the fullerenes adsorbed in the Moire pore centers, rendering them negatively charged. The electric field effect-induced charging of neutral fullerenes and discharging of intrinsically negatively charged fullerenes are investigated using scanning tunneling spectroscopy, non-contact atomic force microscopy, and Kelvin probe force spectroscopy. Our findings show that on metal-supported h-BN, the LUMO level of C-84 is sufficiently close to the Fermi energy that it can be neutral or 1e(-) negatively charged depending on slight variations of the electrostatic potential. The findings propose a path to make ambipolar charge transfer accessible and efficient by circumventing the need to overcome the fullerenes' electronic gap.