α_<i>n</i>h_<i>m</i>-GeSe: a multifunctional semiconductor combining auxeticity and piezoelectricity

Multifunctional materials with outstanding performance have enormous potential applications in the next generation of nanodevices. Using first principles calculations, we design a series of multifunctional two-dimensional materials in monolayer α_<i>n</i>h_<i>m</i>-GeSe (<i>n</i>, <i>m</i> = 1, 2) that combine auxeticity and piezoelectricity. Due to the similar local structures of α-GeSe and h-GeSe, monolayer α_<i>n</i>h_<i>m</i>-GeSe can be designed through the combination of these two materials. Elastic constants and phonon dispersion curves confirm that all the structures are mechanically and dynamically stable. Monolayer α_<i>n</i>h_<i>m</i>-GeSe exhibits auxetic properties with an in-plane negative Poisson's ratio along the diagonal direction. An out-of-plane negative Poisson's ratio effect can be observed by applying tensile strain in the <i>x</i>-direction, which is beneficial for mechanical devices. Only a few materials are both in-plane and out-of-plane auxetic. In addition, monolayer α_<i>n</i>h_<i>m</i>-GeSe can exhibit electric polarization because of the breaking of the central symmetry, demonstrating in-plane piezoelectric properties with strong anisotropy. The above results make monolayer α_<i>n</i>h_<i>m</i>-GeSe an interesting multifunctional material and provide a candidate material for a nanoscale device.