Extraordinary piezoelectric effect induced in two-dimensional rare earth monochalcogenides via reducing system dimensionality

Piezoelectricity is pivotal for applications in micro/nanoelectromechanical systems (MEMS/NEMS). Inducing such a property in 2D systems via the reduction of the dimensionality of their corresponding 3D bulk is here explored. Based on DFT theory and Gaussian-type-localized basis sets, the structural, electronic, mechanical, and piezoelectric properties of both 3D and 2D rare earth monochalcogenides RmX (Rm = Tm, Yb, Lu, and X = S, Se, Te) are investigated using the CRYSTAL code. Most intriguingly, the 2D LuX compounds display a buckled structure, where the Lu and X atoms protrude from the monolayer surface leading to an additional out-of-plane piezoelectric effect; (e31 = 2 104.84, 1 770.28, 1 689.79 pC/m, and d31 = 56.37, 49.76, and 147.90 pm/V for LuS, LuSe, and LuTe, respectively). Such piezoelectric response is two orders of magnitude larger than the one of recently reported 2D ferroelectric MXenes, and is nearly thirty times larger than the commonly used AlN and GaN bulk structures. Furthermore, the reduced elastic constants obtained, when compared to other 2D materials, confirm the flexibility and softness of the considered 2D systems.