Spin-Polarized Me–BNX (X = Al and P) with Negative Poisson’s Ratios: Implications for Two-Dimensional Transition-Metal-Free Spintronic Devices

In this theoretical work, we designed a series of two-dimensional (2D) Me–BNX systems by replacing the sp2-hybridized carbon atoms in Me–graphene with BN pairs. The Mg, Al, Si, P, Ti, and Ge elements were used as the X species to replace the sp3 carbon at the central position in the original Me–graphene template. The related 2D materials were investigated using density functional theory calculations. From the perspectives of energy, lattice dynamics, and mechanics, these Me–BNX systems are structurally stable. Interestingly, most Me–BNX possess very small Poisson's ratios, while four of them are even negative caused by the synergism of the mechanical and electronic effects. Some of the Me–BNX systems are semiconductors. Especially, the Me–BNMg and Me–BNAl systems have optimal band gaps of around 1.9 eV at the B3LYP level with a 30% hybridization. Importantly, the Me–BNAl and Me–BNP systems are spin-polarized due to the trapping of the unpaired electrons in Al and P atoms by the localized electric fields, although there is no transition metal element. Furthermore, the energy difference of the band gaps between the two spin states of Me–BNAl reaches approximately 3.4 eV. Therefore, the optimal band gap and significant gap difference of Me–BNAl imply its promising application potential for solar utilization and spintronics.