Novel electronic and photonic properties of low-symmetry two-dimensional materials

In the past twelve years, the field of two-dimensional (2D) materials has been growing very rapidly with continuous innovations in both new materials and devices. Most of the 2D materials have relatively symmetrical 2D crystal lattices, which result in similar electrical and optical properties along different in-plane crystal directions. Since 2014, we have seen the emergence of a new class of 2D layered materials, which possess low symmetry crystal lattices. Such low symmetry 2D layered materials include black phosphorus (BP) and its arsenic alloys, as well as rhenium disulfide (ReS₂) and rhenium diselenide (ReSe₂) from the transition metal dichalcogenide (TMDC) family. Benefiting from their reduced crystal symmetry, they have distinct electrical and optical characteristics along different in-plane crystal directions, providing previously unexplored tunability on the performance of electrical and optical devices. In this paper, we will review the research from our group on studying the basic properties (crystal structure, electrical and optical properties) of low-symmetry 2D materials before discussing the novel electrical and photonic device applications they can enable.