Curvature-induced phase transitions in two-dimensional polymorphic materials

Many two-dimensional (2D) materials are polymorphic and can exhibit multiple lattice structures with similar stability although the chemical compositions are fixed. However, controlled synthesis of these 2D materials with a desirable lattice structure remains a daunting task. Here, we employ the outstanding structural bendability of 2D materials enabled by their atomic thickness and propose a strategy to achieve a phase control by introducing curvature in the materials. This idea is built on the lattice-dependent bending stiffness of 2D materials which can lift the energy degeneracy of polymorphic phases upon bending deformation. We apply our idea to the switching of stability between black and blue phosphorus monolayers as well as between semiconducting and metallic MoTe₂ monolayers, which are supported by first-principles calculations including vibrational energy corrections. The black phosphorus can be transformed into blue phosphorus at a curvature of over 0.062 Å ⁻¹ and room temperature. Moreover, the curvature-induced phase engineering can even lead to new lattice structures, as exemplified by the emergence of hexagonal buckled SnO monolayer at small curvatures.