Direct Observation of Molecular Orbitals in an Individual Single-Molecule Magnet Mn12 on Bi(111)

Abstract

Single-molecule nanomagnets have unique quantum properties, and their potential applications require characterization and accessibility of individual single-molecule magnets on various substrates. We develop a gentle tip-deposition method to bring individual prototype single-molecule magnets, manganese-12-acetate (Mn12) molecules, onto the semimetallic Bi(111) surface without linker molecules, using low-temperature scanning tunneling microscopy. We are able to identify both the almost flat-lying and side-lying orientations of Mn12 molecules at 4.5 K. Energy-resolved spectroscopic mapping enables the first observation of several molecular orbitals of individual Mn12 molecules in real space, which is consistent with density functional theory calculations. Both experimental and theoretical results suggest that an energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the almost flat-lying Mn12 is only 40% of such a gap for an isolated (free) Mn12 molecule, which is caused by charge transfer from the metallic surface states of Bi to the Mn12. Despite the reduction of this gap, STM images show that the local lattices of Bi(111) covered with Mn12 remain essentially intact, indicating that Mn12-Bi interactions are not strong. Our findings open an avenue to address directly the local structural and electronic properties of individual single-molecule magnets on solid substrates.