Direct visualization of carrier density modulation effect around individual charged impurities

Charged impurities are one of the dominant sources for carrier scattering in modern electronic devices, and their impact significantly increases as the size of devices approaches atomic limitation, making it essential to understand how carriers scatter off impurities at the atomic level. Despite the importance, the electronic transport behaviors, especially the local electrochemical potential distribution induced by electrical current, around charged impurities have remained experimentally elusive. Here, by using multiprobe scanning tunneling potentiometry, we directly visualize the dipolar potential distribution over a region of several nanometers around individual charged impurities in epitaxial bilayer graphene. The orientations of those dipoles, aligning or antialigning with local current, are dictated by charge polarities of impurities, providing direct evidence of the carrier density modulation effect proposed by Landauer in 1976. By in situ tuning local current directions, we demonstrate the active control of the orientations of these dipoles. Our studies advance the understanding of charged-impurity scattering from ensemble average to the single-impurity level and paves the way to explore more quantum transport phenomena at atomic scale.