Ferroelectricity in 2D metal phosphorus trichalcogenides and van der Waals heterostructures for photocatalytic water splitting

Abstract

Integration of ferroelectricity into van der Waals heterostructures offers additional opportunities to control over the properties and functionalities of heterostructures by switching the direction of polarization via an external electric field. To discover potential ferroelectric monolayers that exhibit out-of-plane electric polarizations, we screen a family of metal phosphorus trichalcogenides M1M2P₂X₆ with M1 = Cu/Ag, M2 = In/Bi, X = S/Se using density functional theory calculations. We predict room-temperature ferroelectricity in CuInP₂S₆ and CuBiP₂S₆ monolayers with out-of-plane polarizations (P_s) of 0.59 μC cm⁻² and 0.35 μC cm⁻², respectively. The strong metal-chalcogenide M1–X bond in the two Cu and S-based systems is responsible for their high phase transition temperatures. The polarizations in ferroelectric monolayers can persist in van der Waals heterostructures, and band gaps as well as band alignment of the heterostructures can be tuned by switching the polarization direction. Finally, we demonstrate that both visible light absorption and type-II band alignment facilitating fast charge separation can be realized in CuInP₂S₆/Mn₂P₂S₆ and CuInP₂S₆/Zn₂P₂Se₆ ferroelectric heterostructures, which are promising for applications in photocatalytic water splitting.