Electron Spin Resonance Scanning Tunneling Microscope of Non-Magnetic Molecules

TL;DR

This paper demonstrates the first ESR-STM measurements on non-magnetic molecules, specifically C60 radical ions on graphene, revealing charge state dependence, hyperfine interactions, and potential relaxation time insights.

Contribution

It introduces ESR-STM for non-magnetic molecules, expanding the technique's applicability beyond paramagnetic species.

Findings

ESR-STM signal at g=2.00 matches macroscopic ESR of fullerene radical ion

Signal depends on bias voltage, reflecting molecular charge states

Observation of hyperfine splitting and phase changes related to relaxation times

Abstract

Electron Spin Resonance-Scanning Tunneling Microscopy (ESR-STM) of C60 radical ion on graphene is a first demonstration on radical ions. ESR-STM signal at g=2.00 was measured in accordance with macroscopic ESR of fullerene 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. The linewidth provides information on the lifetime of the free radical. In several experiments, a 13C hyperfine splitting was observed, and a change in the phase of the ESR-STM peak as a function of the speed of the magnetic field sweep was changed. This might be used to provide information on the relaxation times of the molecules. In parallel, ESR-STM signal at g=1.6 was ascribed to Tungsten oxide (W5+) at the tip apex, which is not bias voltage dependent. The ability of ESR-STM to explore non paramagnetic molecules, significantly broadens the scope of this technique.