Electron spin resonance and sub-molecular resolution magnetic resonance imaging of single organic radicals with the scanning tunneling microscope.*

Single atoms and molecules constitute the ultimate spatial limit for magnetic data storage, quantum sensors and quantum information units. Recently, increased attention has been given to organic spin radicals owing to advances in synthetic strategies used to generate magnetic edge states in nanographenes and spin arrays in covalent organic frameworks. Concurrently, electron spin resonance with the scanning tunneling microscope (ESR-STM) has recently been developed to allow atomic-scale spin resonance experiments on individual atoms and molecules with coherent control. Here we describe our efforts to measure the spin resonance of single organic radicals adsorbed on ultra-thin MgO grown on a metal support. We find that multiple molecular species become charged to anions upon adsorption via electron transfer from the underlying substrate. This spontaneous charging quenches stable radicals and generates radical anions depending on the molecular species. For radical anions, we successfully measure spin resonance using both conventional Fe-terminated tips as well as halogen-functionalized tips. Further, we demonstrate magnetic resonance imaging (MRI) of delocalized molecular spins with sub-molecular spatial resolution. The MRI images reveal unexpectedly rich detail arising from the highly localized exchange interaction between the magnetic tip and each atomic spin center of the delocalized electron. Our results suggest the universality of ESR-STM to probe arbitrary magnetic adsorbates and serves as an important milestone for our goal to employ single molecules as scanning spin-resonant quantum magnetometers.