Fig. 3

Nearly-free-electron like LUMO band of C₆₀ monolayer on BP. a DFT calculated DOS of an unsupported C₆₀ monolayer lattice with the same geometry as on a BP support, shown in Fig. 1f. The calculated DOS reproduces the two major peaks and one shoulder for the LUMO complex in the STS data. b The calculated band structure of the C₆₀ lattice in Fig. 1f. The three pairs of bonding–antibonding bands of LUMO-a, -b, and -c are represented by purple, bright green and dark green colors, respectively. The band formed by the superatom s-SAMO orbitals, is indicated in orange. Most importantly, the LUMO-a band is as dispersive as the NFE s-SAMO band. c The spatial distribution of wave function square of the LUMO-a bonding band at the Γ point. The distribution shows a 2D delocalized character as observed in the STS image (Fig. 2e). The black circles highlight the four arms between adjacent molecules along the arm-chair direction, where the bonding interaction dominates. The blue circles highlight the two arms along zig-zag direction. The iso-surface value of the image is 0.0005 and the largest value is 0.02. d 3D image of Fig. 3c shows that the 2D delocalized probability density distribution has two layers in real space. e The top and side views of spatial distributions of LUMO-a, -b, -c orbitals of an isolated C₆₀ molecule with the same geometry as on BP surface. Z represents the direction vertical to the plane determined by the arm-chair and zig-zag directions. It is evident that the in-plane C atoms in the LUMO-a state have high density in the molecular plane and thus contribute to intermolecular interactions. By contrast The LUMO-b and -c have density that projects above and below the plane, and hence, their intermolecular hybridization is weak