Terahertz Scanning Tunneling Microscopy for Visualizing Ultrafast Electron Motion in Nanoscale Potential Variations

Studying the microscopic behavior of free carriers in materials at an ultrashort time scale is critical to developing semiconductor, optoelectronic, and other technologies satisfying the ever-increasing requirements for smaller sizes and higher speeds. Understanding the effect of local potential modulations and localized states due to nanoscale microstructures on carrier dynamics is essential to realize these requirements. Here, we used time-resolved scanning tunneling microscopy/spectroscopy (STM/STS) combined with a carrier-envelope phase (CEP)-controlled subcycle THz electric field, THz-STM, to probe the ultrafast motion of electrons photoinjected into C-60 multilayer structures grown on Au substrate. We have succeeded in demonstrating the time-resolved measurement of ultrafast electron dynamics with sub-nanoscale spatial resolution and subcycle time resolution for the first time and successfully visualized the electron motion triggered by the spatial variation in the lowest unoccupied molecular orbital (LUMO). The difference in the effects of molecular defects, such as a molecular vacancy and orientational disorder, was also clearly distinguished with single-molecular-level spatial resolution. This method is expected to play an important role in the precise evaluation of local electronic structures and dynamics for the future development of new functional materials and device elements.