Stable puckered C2N2 nanosheet with giant anisotropic hole carrier mobility: insights from first-principles

Abstract

Very recently, a black-phosphorus-like nitrogen (bp-N) bulk has been synthesized in high-pressure experiments [D. Laniel, et al., Phys. Rev. Let., 2020, 124, 216001 and C. Ji, et al., Sci. Adv., 2020, 6, eaba9206]. However, the corresponding bp-N monolayer, i.e. a puckered N nanosheet, is unstable, which will decompose to loose N zigzag lines. Based on first-principles calculations, we find that the puckered geometry can be stabilized by inserting C₂ dimers between the upper and lower N zigzag lines. The formed C₂N₂ nanosheet possesses good energetic, dynamical, mechanical, thermal and chemical stabilities, which can maintain the free-standing state even in the presence of O₂ gas. Owing to the phosphorene-like geometrical character, this C₂N₂ nanosheet exhibits an auxetic mechanical behaviour with a negative Poisson's ratio in the out-of-plane direction. The C₂N₂ nanosheet is an indirect-gap semiconductor with a noticeable linear dichroism. More interestingly, a giant anisotropy appears in the hole mobility of the C₂N₂ nanosheet, which has a high (low) hole mobility of about 1700 (20) cm² V⁻¹ s⁻¹ along the armchair (zigzag) direction. The corresponding anisotropic ratio is as large as 85 in the C₂N₂ nanosheet, which is the maximum mobility anisotropy reported in two-dimensional materials. The high hole mobility and large anisotropic ratio are also preserved in the multilayer C₂N₂ nanosheets. Our study demonstrates that the puckered N-based nanosheets possess robust structural stabilities and unconventional physical properties, which endow the system with many potential applications in flexible electronics, optoelectronics, and direction-sensitive sensors.