Realizing nearly-free-electron like conduction band in a molecular film through mediating intermolecular van der Waals interactions Min Feng School of Physics and Technology and Key Laboratory of Artificial Micro- and Nano-Structures of Ministry of Education, Wuhan University, Wuhan 430072, China*

The degree of electron delocalization in organic semiconductors is critical for their adoption in electronic and optoelectronic applications.Electron delocalization, however, is usually facile through chemical bonds in conjugated organic materials, but is rarely optimal when the noncovalent intermolecular van der Waals (vdW) forces define the self-assembly and the intermolecular electronic coupling. As a typical vdW material, C₆₀ molecules form solids through the attractive vdW force, thus are characterized with flat electronic bands with inconsequential dispersions and hopping transport, which is unfavourable for their practical applications.
To enhance intermolecular electronic hybridization, we introduce a method to mediate the vdW interactions between C₆₀ molecules through a substrate control of intermolecular interactions. We show by STM and theory that the black phosphorus (BP) substrate organizes C₆₀ molecules into a compressed monolayer and imposes a favorable orientation that optimizes the intermolecular π-π couplings, resulting in a nearly-free-electron (NFE) like lowest unoccupied molecular orbital (LUMO) band in C₆₀ monolayers, acquiring an effective mass of 0.53-0.70 and has carrier mobility of ~200 to 440 cm²V^-1s^-1. Our results show that the desirable NFE band does happen for a molecule like C₆₀if the π-π interactions are favored by an appropriate substrate template. The fact that the π-π vdW interactions could trigger the NFE like electronic band formation implies that templating of intermolecular interactions by vdW forces on otherwise weakly interacting substrates provides a promising strategy for tailoring remarkable electronic properties in organic materials for electronics and optoelectronics.