Unexpected band gap evolution and high carrier mobility sparked by the orbital variation in two-dimensional GaGeX (X = S, Se, Te)

As elemental group material, germanium, enjoys a high status in the field of two-dimensional (2D) materials. However, 2D germanene is a gapless semiconductor, which greatly limits its application in optoelectronic devices. Herein, we found an interesting class of 2D materials, named GaGeX (X = S, Se, Te), consisting of two outer GaX layers and an inner germanene, a sandwich structure stacked together by interlayer Ga-Ge bonds. Compared with known 2D materials (such as MoX2), the band gaps gradually decrease in the order of sulfide, selenide, and telluride, the electronic calculations show that they have a unique band gaps reversely enlarging trend of 0.03, 0.46 and 0.72 eV from GaGeS to GaGeTe, respectively. Moreover, GaGeX is predicted to have superior mobility, especially with a hole mobility of up to 5510 cm(2) V-1 s(-1) in GaGeTe. These novel phenomena are found closely related to the orbital variation. Our findings render these 2D materials potential candidates for future optoelectronic devices.