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

Abstract 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 Nb 3 TeI 7 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 I 4 and TeI 3 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 4 d 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.