Bi₃O_2.5Se₂: a two-dimensional high-mobility polar semiconductor with large interlayer and interfacial charge transfer

Two-dimensional semiconductors with large intrinsic polarity are highly attractive for applications in high-speed electronics, ultrafast and highly sensitive photodetectors and photocatalysis. However, previous studies mainly focus on neutral layered polar 2D materials with limited vertical dipoles and electrostatic potential difference (typically <1.5 eV). Here, using the first-principles calculations, we systematically investigated the polarity of few-layer Bi₃O_2.5Se₂ semiconductors with ultrahigh predicted room-temperature carrier mobility (1790 cm² V^-1 s^-1 for the monolayer). Thanks to its unique non-neutral layered structure, few-layer Bi₃O_2.5Se₂ contributes to a substantial interlayer charge transfer (>0.5 e^-) and almost the highest electrostatic potential difference (ΔΦ) of ∼4 eV among the experimentally attainable 2D layered materials. More importantly, positioning graphene on different charged layers ([Bi₂O_2.5]⁺ or [BiSe₂]^-) switches the charge transfer direction, inducing selective n-doping or p-doping. Furthermore, we can use polar Bi₃O_2.5Se₂ as an exemplary assisted gate to gain additional holes or electrons except for the external electric field, thus breaking the traditional limitations of gate tunability (∼10¹⁴ cm^-2) observed in experimental settings. Our work not only expands the family of polar 2D semiconductors, but also makes a conceptual advance on using them as an assisted gate in transistors.