Vibrational identification of Janus PtXO(X=S,Se) monolayers with strong out-of-plane piezoelectricity

The breaking of structural symmetries make two-dimensional Janus materials have additional degrees of freedom that can lead to novel electronic and piezoelectric properties. Although Janus PtSSe and other analogous materials have recently been synthesized or predicted, the discovery of Janus piezoelectrics with large vertical piezoelectricity and a systematic understanding of the piezoelectricity of Pt-based Janus materials are still lacking. Here, we identify two new Janus piezoelectric $\mathrm{Pt}X\text{O}$ ($X=\mathrm{S},\mathrm{Se}$) monolayers with strong out-of-plane piezoelectricity by first-principles calculations. Unlike the symmetric binary systems, $\mathrm{Pt}X\text{O}$ exhibit additional Raman-active modes and their activities can be regularly tuned by the biaxial compressive (tensile) strain, which simultaneously ensures the mechanic, dynamic, and thermal stabilities of $\mathrm{Pt}X\text{O}$. Notably, the out-of-plane ${e}_{31}\mathrm{s}$ of $\mathrm{Pt}X\text{O}$ are one order larger than those of Janus PtSSe, PtSTe, and PtSeTe. The electronic parts are found to play the dominant role in determining the piezoelectricity of all Pt-based Janus materials, based on which we reveal that the piezoelectric performances of Pt-based Janus materials are generally in proportion to the Bader charge difference and the electronegativity difference ratio of the two asymmetric layers. Moreover, the vertical piezoelectricity can be further enhanced by the compressive strain. Our results indicate that Janus $\mathrm{Pt}X\text{O}$ are candidates for the potential applications on nanoscale piezoelectrics; the discussions about piezoelectricity would help to predict or design other transition metal dichalcogenide based Janus materials with strong out-of-plane piezoelectricity in advance.