Polytypism in few-layer gallium selenide

Gallium selenide (GaSe) is one of the layered group-III metal monochalcogenides, which has an indirect bandgap in the monolayer and a direct bandgap in bulk unlike other conventional transition metal dichalcogenides (TMDs) such as MoX2 and WX2 (X = S and Se). Four polytypes of bulk GaSe, designated as β-, e-, γ-, and δ-GaSe, have been reported. Since different polytypes result in different optical and electrical properties even with the same...