Coexistence of In-Plane and Out-of-Plane Piezoelectricity in Janus WXSiN₂ (X = S, Se, Te) Monolayers: Potential Implications for Nanoscale Piezoelectric Devices

Two-dimensional (2D) Janus materials are characterized by vertical polarization due to their asymmetric structures, and they possess significant potential for electronic and piezoelectric applications; yet they face challenges in achieving superior piezoelectric performance. This study explores the piezoelectric properties of a type of Janus WXSiN 2 (X = S, Se, Te) monolayers using first-principles calculations. The structural stability of these monolayers is verified by dynamic, thermodynamic, and mechanical stability analyses. The calculations demonstrate that the broken mirror symmetry results in relatively larger in-plane and out-of-plane piezoelectric coefficients in these WXSiN 2 monolayers. The piezoelectric coefficients adhere to the electronegativity difference ratio principle, where an increase in electronegativity differences leads to enhanced piezoelectric performance. The larger piezoelectric coefficients are closely related to the presence of relatively strong built-in electric fields and larger Born effective charges, providing insights into the physical mechanisms underlying their unique properties. Notably, WTeSiN 2 exhibits the most robust piezoelectric performance among these three monolayers, suggesting considerable potential for applications in nanoscale piezoelectric devices. This study highlights the substantial out-of-plane piezoelectric capabilities of Janus WXSiN 2 monolayers and provides reliable evidence for the further development of 2D Janus materials with strong piezoelectricity.