Anisotropic thermal expansion of group-IV monochalcogenide monolayers

By employing first-principles calculations, we investigate the thermal expansion behavior of group-IV monochalcogenide monolayers. We find that these two-dimensional (2D) materials exhibit unusually large anisotropic thermal expansion, i.e., the two lattice parameters in the armchair and zigzag directions contract and expand, respectively, with increasing temperature, which is indicative of a second-order phase transition to a square lattice. Such anisotropic thermal expansion gives rise to a secondary pyroelectric response that is effectively 1–2 orders of magnitude larger than that of typical bulk pyroelectrics, suggesting that thermal expansion is essential to properly describe their electronic properties.