Fig. 3: Mechanical properties of 2D auxetic oxides.

a GeO2 (101), b MO2 (110) (M = Os, Pb, Pd, Rh) monolayers expand/contract in the z-direction, when a stretch/compression in the x(y) direction is applied, exhibiting linear negative Poisson’s ratio effect. In contrast, c WO3 (110), d AgO (101) and CuO (101) monolayers expand in the z-direction, regardless if a stretch or compression in the x(y) direction is applied. The insets show the optimized crystal structures, where the red and gray sticks denote oxygen and the corresponding metal atoms, respectively. e, f In-plane positive Poisson’s ratios (PPR) of GeO2 (101), RhO2 (110), WO3 (110), and AgO (101) as a function of the in-plane angle. 0 degree corresponds to the x-axis.