Anisotropic two-dimensional screening at the surface of black phosphorus

Black phosphorus (BP), in contrast to most van der Waals semiconductors, exhibits a anisotropic crystal structure within each atomic layer, resulting in electronic in-plane anisotropy reflected in the effective masses1,2, carrier mobility3,4, excitonic wavefunctions5, and plasmonic dispersion6,7. Yet, despite the important role of screening underlying many physical properties, a detailed picture of the dielectric response is missing. Here, we use the adsorption of individual potassium atoms on the surface of BP to vary the near-surface doping over a wide range, while simultaneously probing the dielectric screening via the ordering of the adsorbed atoms. The strongly anisotropic screening leads to the formation of potassium chains with a well-defined orientation. The dimensionality and anisotropy of the screening is found to be consistent with the presence of a band-bending induced confinement potential near the surface. Exploiting doping-dependent anisotropic screening opens up numerous possibilities to tune transport, interactions, and excitation dynamics in black phosphorus.