Structural and quantum-state phase transitions in van der Waals layered materials

Van der Waals layered transition metal dichalcogenides can exist in many different atomic and electronic phases. Such diverse polymorphisms not only provide a route for investigating novel topological states, such as quantum spin Hall insulators, superconductors and Weyl semimetals, but may also have applications in fields ranging from electronic and optical/quantum devices to electrochemical catalysis. And the methods for triggering robust phase transitions between polymorphs are evolving and diversifying—several growth processes, high-pressure/strain methods, and optical, electronic and chemical treatments have been developed. Here, we discuss recent progress on phase transitions and the related physics in layered materials, and demonstrate unique features compared with conventional solid-state materials. With diverse polymorphisms and phase transitions that can be triggered using many methods, layered transition metal dichalcogenides are attractive materials for realizing novel topological states, as well as for a range of other applications.