Unconventional inner-TL electric polarization in TL-LaOBiS with ultrahigh carrier mobility.

Based on first principles calculations, we propose a new 2D ferroelectric material, triple-layer (TL) LaOBiS2, with an ultrahigh carrier mobility over 40 000 cm2 V-1 s-1 and large sunlight absorption. TL-LaOBiS2 is composed of a middle LaO layer and top-and-bottom BiS2 layers that can be possibly exfoliated from its bulk counterpart. We reveal that each BiS2 layer can hold spontaneous in-plane ferroelectric polarization that can be further enhanced by imposing extensive strain. Furthermore, we discover that TL-LaOBiS2 possesses unconventional inner-TL ferroelectric (FE), antiferroelectric (AFE) and orthogonal polarizations. The ground inner-TL AFE state can be flexibly driven into a nearly degenerate FE state. Moreover, the direct band gaps, optical absorption and the carrier mobilities of TL-LaOBiS2 can be effectively regulated by different ferroelectric polarization configurations. This finding of various ferroelectric states with ultrahigh mobility and excellent optical absorption in TL-LaOBiS2 provides a promising platform for future realization of two-dimensional ferroelectric photovoltaic devices.