BiOSe: 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 BiOSe semiconductors with ultrahigh predicted room-temperature carrier mobility (1790 cm V s for the monolayer). Thanks to its unique non-neutral layered structure, few-layer BiOSe contributes to a substantial interlayer charge transfer (>0.5 e) and almost the highest electrostatic potential difference (Δ) of ∼4 eV among the experimentally attainable 2D layered materials. More importantly, positioning graphene on different charged layers ([BiO] or [BiSe]) switches the charge transfer direction, inducing selective n-doping or p-doping. Furthermore, we can use polar BiOSe 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 (∼10 cm) 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.