Intrinsic ferromagnetism and valley polarization in hydrogenated group V transition-metal dinitride (MN2H2, M = V/Nb/Ta) nanosheets: insights from first-principles.

Abstract

Due to the extraordinary electronic and magnetic properties, transition-metal dinitrides (TMDNs) and their derivatives are gaining importance in low-dimensional layered materials. In this work, through first-principles calculations, we have comprehensively investigated the structural and electronic properties of hydrogenated group V TMDN nanosheets. We find that surface hydrogenation can well stabilize the H-, T- and M-phase structures of group V TMDNs, for which the formed MN2H2 nanosheets have robust energetic, dynamical and thermal stabilities. Different from pristine MN2 systems, the H-phase system has become the most favorable structure of MN2H2 nanosheets. Intrinsic ferromagnetism is present in these H-MN2H2 nanosheets, which even exhibit bipolar magnetic semiconducting behaviors. More interestingly, large spontaneous valley polarization occurs in the H-MN2H2 nanosheets, and is attributed to the coexistence of remarkable spin-orbit coupling and magnetic exchange interactions according to the k·p model analysis. Among them, the H-NbN2H2 nanosheets are found to be a promising ferrovalley material, whose valley polarization value reaches as large as 0.11 eV and the Curie temperature is up to 225 K. Besides that, versatile electronic properties are obtained in the T- and M-phase structures of the MN2H2 nanosheets, which will be magnetic/nonmagnetic metals/semiconductors depending on the metal species and phase structures. Our study demonstrates that the hydrogenation can bring robust structural stabilities and unconventional electronic properties into the group V TMDN nanosheets, which enable many potential applications in spintronics and valleytronics.