Highly Dispersive Nearly Free Electron Bands at a 2D-Assembled C₆₀Monolayer

Superatomic molecular orbitals (SAMOs), which are atom-like diffuse orbitals formed at a molecule, play an important role in controlling the electronic functionality at one-to three-dimensional (1D-3D) assemblies of organic nanomaterials because they form highly dispersive and nearly free electron (NFE) bands, such as a bulk metal, by mixing their wavefunctions with neighbors. Herein, we identify a series of delocalized SAMOs at a two-dimensional assembled C60 fullerene monolayer utilizing resonant angle-resolved two-photon photoemission spectroscopy and theoretical calculations. SAMOs exhibit distinct NFE band dispersion characteristics to be hybridized between the diffuse orbitals at a 2D-assembly, unlike the well-known frontier unoccupied π∗ orbitals with localized wave functions at the carbon framework of C60. Density functional theory calculations for the NFE bands quantitatively reproduce the experimental observations including their energies and effective electron masses. These NFE bands comprising SAMOs above the Fermi level dominate charge transfer or photofunctional properties at the 1D-3D molecular assemblies, particularly when the band energies are lowered through a doping strategy.