Strong Vertical Piezoelectricity of CuGeTeBr and CuGeTeI Monolayers: Promising Candidates for Flexible Nanoscale Piezoelectric Devices

The pursuit of two-dimensional materials that simultaneously exhibit exceptional piezoelectric performance and robust structural stability remains a significant challenge for electromechanical applications. In this study, we construct novel CuGeTeX (X = Br and I) monolayer structures and employ first-principles calculations to systematically investigate the structural, electronic, and piezoelectric properties. Phonon spectrum analysis, ab initio molecular dynamics simulations, and elastic constants evaluations confirm the dynamical, thermal, and mechanical stability of these systems. Both monolayers exhibit semiconducting behavior, satisfying an essential prerequisite for piezoelectric response. The asymmetric atomic configurations induce pronounced Bader charge redistribution arising from differences in atomic electronegativity, resulting in substantial intrinsic polarization and a remarkable enhancement of the piezoelectric coefficients. Notably, CuGeTeI monolayer demonstrates exceptionally robust vertical piezoelectric coefficients with d 31 = 2.04 pm/V. Moreover, these two monolayers have superior elastic flexibility, which is identified as the critical factor underlying this enhanced piezoelectric performance. Furthermore, the systematic variation in vertical piezoelectric coefficients across the Br and I atoms is directly attributed to the change in electronegativity difference between constituent atoms agree with the P (piezoelectricity)–R (electronegativity difference ratio) mechanism. This research provides a theoretical foundation for CuGeTeI monolayers as promising candidates in flexible nanoscale electronics and wearable sensors while offering fundamental insights into the design principles of high-performance piezoelectric materials.