Sub-molecular magnetic imaging of individual organic radicals using electron spin resonance in a scanning tunneling microscope

Abstract

Electron spin resonance (ESR) of individual atoms and small molecules has extended the capabilities of scanning tunneling microscopes to give very high energy resolution and quantum control, transforming individual adsorbed atoms into sensitive detectors of the local magnetic field. Here we present spin resonance of individual organic radical molecules adsorbed on an ultra-thin MgO film on silver. We find that several planar fluorenone derivatives become charged to anions upon adsorption on the insulating film. This spontaneous charging quenches the spin of the radicals, and transforms stable molecules into radical anions. These radicals are driven spin-resonantly by the radio-frequency electric field from the tip and sensed locally by magnetoresistance. ESR allowed magnetic resonance imaging of the delocalized unpaired electron in these molecules. We used Fe-terminated tips and halogen-functionalized Fe tips in a 1-Kelvin microscope. Potential applications include the investigation of coupled molecular spins and graphene nanoribbon edge states, and the transfer of a spin-resonant molecule to the microscope tip in order to provide a versatile scanning ESR sensor.