Superatom nearly-free-electron-like bands in C60 assemblies
The electronic and optical properties of molecules and molecular solids are generally considered from the perspective of the frontier orbitals and their intermolecular interactions.In this review,we introduce a new paradigm for understanding molecular orbitals,their intermolecular hybridizations,and how they lead to nearly-free-electron(NFE)-like bands in molecular assemblies.Achieving NFE-like bands in organic semiconductor materials will significantly influence their use in electronic and optoelectronic applications.C60,one of the most extensively investigated semiconducting molecules,is used as a prototype to demonstrate the emergence of two types of NFE bands in van der Waals(vdW)C60 assemblies.First,we show that NFE bands are hybridized with superatom molecular orbitals(SAMOs).SAMOs result from long-range polarization interactions,mainly shown in the screening of an external charge at a solid/vacuum interface.The polarization interaction is responsible for the universal image potential and the related unoccupied image potential(IP)states.In C60,the concave side of the C sheet reduces the energy of the IP states,converting them into diffusive atom-like orbitals called SAMOs.SAMOs,originating from the universal polarization interaction,are commonly found in molecules/assemblies with hollow structures,such as graphene layers,carbon nanotubes,fullerenes,and aromatic molecules.Due to the delocalization of the IP states,SAMOs diffuse like atom-like orbitals.Then,they hybridize similarly to atoms in a metal,forming NFE bands in C60 assemblies where the intermolecular distances are significant vdW distances.In C60 assemblies,the predicted electron mobilities of SAMOs are 21600-58100 cm2V-1s-1.Second,we highlight concatenated NFE bands with superatom covalent-like quasi-bonding.In C60 monolayers assembled on black phosphorus surfaces,π-π interactions are favored with suitable substrate vdW templating,initiating covalent-like quasi-bonding interactions between C60,similar to covalent bonding in atoms.This leads to a C60 NFE-like conduction band,with calculated electron mobilities of～200 to 440 cm2 V-1 s-1,exceeding those of most organic semiconductor materials.In contrast to the traditional perspective where physicists and chemists consider the intermolecular interactions between C60,SAMOs and covalent-like quasi-bonding behave differently.In the SAMO case,SAMOs have higher energy and are delocalized IP states that hybridize at larger intermolecular distances to form NFE bands.In covalent-like quasi-bonding interactions,lower energy frontier orbitals are hybridized at compressed intermolecular distances to form NFE bands.In the proposed framework,the energy of the molecular orbitals align with their delocalization character and is consistent with the intermolecular distances where the NFE bands can be formed.This introduces a new paradigm for understanding molecular interactions aligned with the method of atom manufacturing,where whole molecules/assemblies behave as superatoms,and intermolecular interactions can be tailored using unique superatomic interactions to generate an organic semiconductor with unique properties,such as vdW organic semiconductors with ultra-high carrier mobilities.
C60van der Waals distancesuperatom molecular orbitalscovalent-like quasi-bondingnearly-free-electron like bandscarrier mobility
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