ESR-STM on diamagnetic molecule: C₆₀ on graphene

Electron Spin Resonance-Scanning Tunneling Microscopy (ESR-STM) of C₆₀ radical ion on graphene is a first demonstration of ESR-STM on diamagnetic molecules. ESR-STM signal at g_average=2.0±0.1 was measured in accordance with macroscopic ESR of C₆₀ radical ion. The ESR-STM signal was bias voltage dependent, as it reflects the charge state of the molecule. The signal appears in the bias voltage which enables the ionization of the lowest unoccupied molecular orbital (LUMO) – creation of radical anion, and the highest occupied molecular orbital (HOMO) – creation of a radical cation of the C₆₀ molecule when it deposited on graphene. In parallel, ESR-STM signal at g_average=1.7±0.1 was ascribed to Tungsten oxide (WO₃) at the tip apex. In several experiments, triplet spectrum was observed, and we ascribed their origin to zero-field splitting of doubly ionized C₁₂₀O^-2 dimer, as argued in previous ESR experiments of C₆₀ samples. Second possibility is hyperfine coupling with two ¹³C nuclei. In addition, we further validate the interference mechanism previously suggested for ESR-STM noise spectroscopy. The ability of ESR-STM to observe ESR of diamagnetic molecules in parallel with observing their electronic structure, provides a general single molecule identification technique.