Unveiling nanoscale THz-STM imaging techniques on graphene nanoribbons with zigzag edge topology

The development of near-field THz microscopes has transcended the diffraction limitation traditionally constraining THz super-resolution imaging, heralding a new era of precision. Notably, Terahertz combined scanning tunneling microscopy (THz-STM) has distinguished itself by achieving unparalleled spatial resolution alongside remarkable temporal precision. Despite the significant advancements in THz-STM imaging research, a thorough exploration of its unique imaging features remains elusive, particularly in resolving local electronic spectroscopy. This study methodically explores THz-STM imaging over atomically precise 6-zigzag-edged graphene nanoribbons (6-ZGNR) on Au(111), employing a constant-current mode. The investigation reveals that intense THz-driven electric fields can induce irreversible alterations to the occupied and unoccupied state densities of the 6-ZGNR. Utilizing these THz-modified nanoribbons, analyses of both THz-driven STM imaging and THz current imaging with an external lock-in amplifier are carried out, and experimental factors affecting their imaging qualities have been investigated. It is demonstrated that the imaging with an external lock-in amplified THz current signal accurately captures the local electronic spectroscopy variations at the nanoscale. What we believe is a novel imaging technique proficiently delineates the features on the Au(111) surface and the 6-ZGNR, showcasing superior performance over direct terahertz-driven STM imaging of the samples.