Flat-Band Electronic Bipolarity in a Janus and Kagome van der Waals Semiconductor Nb₃TeI₇

Janus materials, a novel class of materials with two faces of different chemical compositions and electronic polarities, offer significant potential for various applications with catalytic reactions, chemical sensing, and optical or electronic responses. A key aspect for such functionalities is face-dependent electronic bipolarity, which is usually limited by the chemical distinction of terminated surfaces and has not been exploited in the semiconducting regime. Here, it is showed that a Janus and Kagome van der Waals (vdW) material Nb₃TeI₇ has ferroelectric-like coherent stacking of the Janus layers and hosts strong electronic bipolar states in the semiconducting regime. A large potential difference of ∼ 0.7 eV between the I₄ and TeI₃ terminated surfaces is observed, despite only one fourth of the I atoms being replaced by Te atoms on one side of the layers. Additionally, robust semiconducting properties with the face-dependent n-type and p-type field-effect transistor behaviors are demonstrated. These unique properties are attributed to Nb 4d orbital flat bands of the breathing-Kagome lattice, of which significantly large electron mass makes the semiconducting properties immune to impurity doping, and inherent strong electron correlation enhances asymmetric electron distribution, thereby amplifying a built-in electric field. These findings highlight that naturally-grown Janus and Kagome vdW semiconductors provide a promising material platform for utilizing strong electronic bipolarity in 2D-material-based applications.