Strain engineering and hydrogen effect for two-dimensional ferroelectricity in monolayer group-IV monochalcogenides MX (M = Sn, Ge; X = Se, Te, S)

Two-dimensional (2D) ferroelectric compounds are a special class of materials that meet the need for devices miniaturization, which can lead to a wide range of applications. Here, we investigate ferroelectric properties of monolayer group-IV monochalcogenides MX ( M = Sn, Ge; X = Se, Te, S) via strain engineering, and their effects with contaminated hydrogen are also discussed. GeSe, GeTe, and GeS do not go through transition up to the compressive strain of –5%, and consequently have good ferroelectric parameters for device applications that can be further improved by applying strain. According to the calculated ferroelectric properties and the band gaps of these materials, we find that their band gap can be adjusted by strain for excellent photovoltaic applications. In addition, we have determined the most stable hydrogen occupancy location in the monolayer SnS and SnTe. It reveals that H prefers to absorb on SnS and SnTe monolayers as molecules rather than atomic H. As a result, hydrogen molecules have little effect on the polarization and electronic structure of monolayer SnTe and SnS.