Multifunctional Binary Monolayers Ge_xP_y: Tunable Band Gap, Ferromagnetism, and Photocatalyst for Water Splitting

The most stable structures of two-dimensional Ge_xP_y and Ge_xAs_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 P3 m1 structure reported previously (Nano Lett. 2017, 17, 1833). The computed electronic band structures suggest that all the stable and metastable monolayers of Ge_xP_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.