Multifunctional Binary Monolayers Ge x P y : Tunable Band Gap, Ferromagnetism, and Photocatalyst for Water Splitting.

The most stable structures of two-dimensional Ge x P y and Ge x As y monolayers with different stoichiometries (e.g., GeP, GeP2 , and GeP3 ) are explored systematically through the combination of the particle-swarm optimization technique and density functional theory optimization. For GeP3 , we show that the newly predicted most stable C2/ m structure is 0.16 eV/atom lower in energy than the state-of-the-art P3̅m1 structure reported previously ( Nano Lett. 2017, 17, 1833). The computed electronic band structures suggest that all the stable and metastable monolayers of Ge x P y are semiconductors with highly tunable band gaps under the biaxial strain, allowing strain engineering of their band gaps within nearly the whole visible-light range. More interestingly, the hole doping can convert the C2/ m GeP3 monolayer from nonmagnetic to ferromagnetic because of its unique valence band structure. For the GeP2 monolayer, the predicted most stable Pmc21 structure is a (quasi) direct-gap semiconductor that possesses a high electron mobility of ∼800 cm2 V-1 s-1 along the k a direction, which is much higher than that of MoS2 (∼200 cm2 V-1 s-1 ). More importantly, the Pmc21 GeP2 monolayer not only can serve as an n-type channel material in field-effect transistors but also can be an effective catalyst for splitting water.