Collective excitations in 2D atomic layers: Recent perspectives

The strong advantage of two-dimensional (2D) materials is that they exhibit interesting physical properties down to the single unit layer (typically less than 1 nm). Such emergent properties are highly dependent on thickness and can be tunable via external parameters such as electric field, pressure, and/or chemical treatment. In addition, most 2D materials have hexagonal crystal structures; thus, it is possible to stack with other 2D materials to enrich the properties. This tunability and compatibility makes 2D materials and their heterostructures promising candidates for optoelectronic devices.