Tunable auxetic properties in group-IV monochalcogenide monolayers

Negative Poisson's ratio (NPR) is a counterintuitive material elastic constant that can lead to many unusual auxetic properties. Here, using first-principles calculations, we report tunable negative Poisson's ratio in the out-of-plane direction in group-IV monochalcogenide monolayers MX (M=Sn, Ge and X=S, Se). SnSe, GeS, and SnS monolayers have intrinsic NPR νzx ranging from -0.004 to -0.210 in armchair (x) tension, whereas GeSe monolayer possesses a much larger NPR νzy of -0.433 in zigzag (y) tension. Our analysis attributes the NPR effects to the relative position of M and X in the puckered structure and the smaller bending stiffness of M-X-M bond angle. We further established the correlation between electronic structures of materials and their crystal structures. It allows us to fine tune GeSe structure via electron doping, leading to a reversible and continuous change of νzy from -0.821 up to 0.895. We also demonstrate the concept of strain engineering GeSe monolayer to switch its Poisson's ratio νzx between two different values: 0.583 and -0.433. Our in-depth study provides not only fundamental knowledge but also practical routes for designing 2D smart materials with tunable negative Poisson's ratio, which are desirable for smart devices at small scale.