Two-Dimensional Square-A₂B (A = Cu, Ag, Au, and B = S, Se): Auxetic Semiconductors with High Carrier Mobilities and Unusually Low Lattice Thermal Conductivities

Using evolutionary structure search combined with ab initio theory, we investigate the electronic, thermal, and mechanical properties of two-dimensional (2D) A₂B (A = Cu, Ag, Au, and B = S, Se) auxetic semiconductors. Two types of structures are found to have low energy, namely, s(I/II)-A₂B, which have direct bandgaps in the range 1.09-2.60 eV and high electron mobilities. Among these semiconductors, Cu₂B and Ag₂B have light holes with 2 orders of magnitude larger mobility than the heavy holes, up to 9.51 × 10⁴ cm² V^-1 s^-1, giving 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, which are quite promising for new generation thermoelectric devices. Besides, s-A₂B structures show extraordinary flexibility with ultralow Young's moduli (down to 20 N/m), which are lower than most previously reported 2D materials. Moreover, under strain along the diagonal direction, five of the structures have in-plane negative Poisson's ratios.