Tuneable Poisson's ratio of monolayer GeS and Ge₂SSe

A tuneable Poisson's ratio is desirable to extend the applications of 2D materials. We demonstrate that Poisson's ratio of monolayer GeS depends approximately linear on the applied strain and electric field strength. In contrast, monolayer Ge₂SSe is subject to abrupt switching between positive and negative values (sign reversal) at particular critical values of the strain and electric field strength. The tuneability of Poisson's ratio is attributed to the interplay of different bending stiffnesses. A structural transition is identified as origin of the sign reversal in the case of monolayer Ge₂SSe.