Vibrational identification of oxygen adsorbed PtX2 (X=S,Se) with large vertical piezoelectricity

Among the monolayer transition metal dichalcogenide (TMDC) MX${}_2$ family, pristine MX${}_2$ with $T$-phase structures such as PtX${}_2$ generally have no piezoelectricity. Though the good surface air-stability of PtX${}_2$ (X=S,Se) were reported by experiments, the effect of oxygen adsorption on piezoelectricity is unknown. Here, taking PtX${}_2$ (X=S,Se) as prototype and using the first-principles calculations, the PtX${}_2$@O (X=S,Se), where one side of PtX${}_2$ layer are fully adsorbed by O, are identified as piezoelectrics with strong out-of-plane piezoelectricity and large carrier mobility. PtX${}_2$@O are checked to have mechanic, dynamic and thermal stabilities. Unlike PtX${}_2$, PtX${}_2$@O possess more Raman-active modes whose Raman-activities and peak positions can be regularly tuned by strain. The predicted electron mobility of PtX${}_2$@O reaches up to an order of magnitude of 10${}^3$ cm${}^2$ V${}^{\mathrm{-1}}$ s${}^{\mathrm{-1}}$. Notably, the robust piezoelectricity is realized by the O adsorption. The in-plane(out-of-plane) $d_{11}$ ($d_{31}$) reaches up to 7.84(1.01) pm/V. The $d_{31}$ is an order of magnitude larger than that of other typical MX${}_2$-based 2D piezoelectrics, including PtSSe, MoSTe, etc. The large vertical piezoelectricity is attributed to the asymmetric charge transfers between the up and down layers which induce large potential-step ($\Delta$V). The trend of $\Delta$V generally coincides with that of $e_{31}$ ($d_{31}$), the larger $\Delta$V is, the larger $e_{31}$ ($d_{31}$) could be obtained. Our results not only show the potential use of PtX${}_2$@O on 2D piezoelectric devices, but also indicate the crucial role of $\Delta$V-engineering for pre-design of piezoelectrics. The findings can be extended to realization of piezoelectricity of other MX${}_2$ with $T$-phase structures such as HfX${}_2$, SnX${}_2$.