Coexistence of intrinsic piezoelectricity and nontrivial band topology in monolayer InXO (X = Se and Te)

Abstract

The combination of piezoelectricity with other unique properties (like the topological insulating phase and intrinsic ferromagnetism) in two-dimensional (2D) materials is worthy of much intensive study. In this work, the piezoelectric properties of 2D topological insulators InXO (X = Se and Te), derived from monolayer InX (X = Se and Te) by double-side oxygen functionalization, are studied by density functional theory (DFT). Large piezoelectric strain coefficients (e.g. d₁₁ = −13.02 pm V⁻¹ for InSeO and d₁₁ = −9.64 pm V⁻¹ for InTeO) are predicted, which are comparable and even higher than those of many other familiar 2D materials. Moreover, we propose two strategies to enhance the piezoelectric response of monolayer InXO (X = Se and Te). Firstly, biaxial strain (0.94–1.06) is applied, and the d₁₁ (absolute value) is increased by 53%/56% for monolayer InSeO/InTeO at 1.06 strain, which is due to the increased e₁₁ (absolute value) and reduced C₁₁–C₁₂. In the considered strain range, the InXO (X = Se and Te) monolayers are always 2D topological insulators, which confirms the coexistence of piezoelectricity and nontrivial band topology. Secondly, a Janus monolayer In₂SeTeO₂ is designed by replacing the top Se/Te atomic layer in monolayer InSeO/InTeO with Te/Se atoms, and is dynamically and mechanically stable. More excitingly, Janus monolayer In₂SeTeO₂ is also a 2D topological insulator with sizeable bulk gap of up to 0.158 eV, confirming the coexistence of intrinsic piezoelectricity and its topological nature. The calculated d₁₁ is −9.9 pm V⁻¹, which is in the middle of those of InSeO and InTeO monolayers. Finally, the carrier mobilities of monolayer InXO (X = Se and Te) are obtained, which show a rather pronounced anisotropy between electrons and holes, and are almost isotropic between the armchair and zigzag directions. Our work implies that it is possible to use the piezotronic effect to control the quantum transport process, ultimately leading to the novel device applications of monolayer InXO (X = Se and Te), and can stimulate further experimental work.