Prediction of novel 2D auxetic semiconductors with high carrier mobilities and unusually low lattice thermal conductivities

A perpetual search for new two-dimensional (2D) materials with novel properties has led to many important discoveries in recent times. Using evolutionary structure search methods combined with ab initio theory, we have found two low energy structural types of 2D A₂B (A=Cu, Ag, Au, and B = S, Se) auxetic semiconductors having direct bandgaps in the range 1.09−2.60 eV and high electron mobilities. Cu₂B and Ag₂B have the possibility of achieving highly anisotropic hole transport with the application of a uniaxial strain. Due to the ionic bonding nature, s-A₂B structures have unusually low lattice thermal conductivities down to 1.5 W m⁻¹ K⁻¹ at 300 K, showing great promise for new generation thermoelectric devices. Moreover, they are found to have ultralow Young’s moduli (down to 20 N/m), which are lower than most previously reported 2D materials. Finally, some of these structures are found to have in-plane negative Poisson’s ratios under the application of strain along the diagonal direction.