Strong vertical piezoelectricity in flexible Janus HfFX (X = Cl, Br, I) monolayers: A first-principles study

The advancement of piezoelectric materials has been constrained by limited understanding of their intrinsic mechanisms and relatively low vertical piezoelectric performance. This study introduces Janus HfFX (X = Cl, Br, I) monolayers and investigates their piezoelectric properties, revealing vertical piezoelectric strain coefficients that exceed those of other Janus materials by 1–2 orders of magnitude. The enhanced piezoelectric performance is attributed to Bader charge differences and electronegativity difference ratios, consistent with the proposed P-R mechanisms. Furthermore, piezoelectric performance tends to improve with increasing electronegativity difference ratios. The enhanced vertical piezoelectric response is also attributed to strong built-in electric fields. Additionally, the results reveal an inherent coupling between piezoelectricity and carrier transport, where a lower polarization electric fields is associated with higher hole mobility. This study not only highlights HfFX monolayers as promising candidate applications in energy conversion and tactile sensing but also deepens our understanding of piezoelectricity in two-dimensional materials.