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Ferroelectrics have many significant applications in electric devices, such as capacitor or random-access memory, tuning the efficiency of solar cell. Although atomic-thick ferroelectrics are the necessary components for high-density electric devices or nanoscale devices, the development of such materials still faces a big challenge because of the limitation of intrinsic mechanism. Here, we reported that in-plane atomic-thick ferroelectricity can be induced by vertical electric field in phosphorene nanoribbons (PNRs). Through symmetry arguments, we predicted that ferroelectric direction is perpendicular to the direction of external electric field and lies in the plane. Further confirmed by the comprehensive first-principles calculations, we showed that such ferroelectricity is induced by the electron-polarization, which is different from the structural distortion in traditional ferroelectrics and the recent experimental discovery of in-plane atomic-thick ferroelectrics (Science 2016, 353, 274). Moreover, we found that the value of electronic polarization in bilayer is much larger than that in monolayer. Our results show that electron-polarization ferroelectricity maybe the most promising candidate for atomic-thick ferroelectrics. [ABSTRACT FROM AUTHOR]

High-temperature ferromagnetic two-dimensional (2D) materials with flat surfaces have been a long-sought goal due to their potential in spintronics applications. Through comprehensive first-principles calculations, we show that the recently synthesized MoN2 monolayer is such a material; it is ferromagnetic with a Curie temperature of nearly 420 K, which is higher than that of any flat 2D magnetic materials studied to date. This novel property, made possible by the electron-deficient nitrogen ions, render transition-metal dinitrides monolayers with unique electronic properties which can be switched from the ferromagnetic metals in MoN2, ZrN2, and TcN2 to half-metallic ones in YN2. Transition-metal dinitrides monolayers may, therefore, serve as good candidates for spintronics devices. [ABSTRACT FROM AUTHOR]