Recent advance in two-dimensional group-IV metal chalcogenides

Abstract

As complementary materials to zero-band-gap graphene, two-dimensional (2D) transition metal dichalcogenides (TMDCs) with sizable band gaps have attracted much attention. However, 2D group-IV metal chalcogenides such as SnSe and GeS have rarely been studied. Group-IV metal chalcogenides have been studied for a long time in the field of solar cell and photoelectric converter owing to their appropriate bandgaps. Recently, the highest ZT value of thermoelectric was discovered in the bulk SnSe crystal. It has been reported that layered group-IV metal chalcogenides are phase change materials, which endows them enriched properties. Theoretical calculations predict that 2D group-IV metal chalcogenides have excellent thermoelectricity, ferroelectricity and piezoelectricity and thus are promising building blocks for functional devices. However, these intriguing properties of 2D group-IV metal chalcogenides have scarcely been detected from experimental aspects. A number of reviews on 2D atomic crystals have included a brief introduction of group-IV metal chalcogenides, but a specific review on the recent advance in 2D group-IV metal chalcogenides is necessary to provide in-depth insights to this emerging family of 2D materials. In this review, we first summarized the crystallographic structures and band-gaps of 2D group-IV metal chalcogenides, revealed the dependence of band-gaps with number of layers and also correlated the properties with the structures of these 2D group-IV metal chalcogenides. We then discussed the progress in the synthesis of these materials and concluded that the lacking of controllable synthesis methods greatly hindered the studies on these materials. We also summarized the properties of these 2D materials from both theoretical calculations and experimental aspects. When the thickness of these materials is reduced to monolayer, they exhibit outstanding thermoelectricity, ferroelectricity and piezoelectricity compared with other 2D atomic crystal such as the most widely studied graphene and MoS2. The similar structures between 2D group-IV metal chalcogenides and black phosphorus provide them with anisotropic optical properties. However, their superior stability under ambient condition makes them promising candidates for practical devices. Finally, we summarized the current research status and challenges, along with the perspectives of applications in the 2D group-IV metal chalcogenides.