Layer-Dependent Asymmetric Dipoles Induced Formation of Atomically Thin <i>p–n</i> Junctions

Conventional doping strategies for bulk materials become ineffective when applied to two-dimensional (2D) materials, posing a fundamental challenge to the realization of atomically thin <i>p-n</i> junction devices. Herein, we establish a general framework for engineering self-doped <i>p-n</i> junctions in 2D polarized semiconductors through layer-dependent asymmetric dipoles. Within a 2D metal/2D polarized semiconductor/2D metal sandwich model, a criterion for identifying <i>p-n</i> junction behavior is that the metal work function lies between work functions determined by the two surfaces of the polarized semiconductor so that electrons and holes can be selectively injected into the top and bottom metal layers. The surface work function disparity can be effectively tuned by increasing the number of polarized layers, thereby expanding compatibility with more metal electrodes. This work provides a universal strategy for realizing atomically thin <i>p-n</i> junction devices, laying the foundation for advancing 2D electronics in the post-Moore era.