Two-dimensional Janus 1T - CrXY ( X=S , Se, Te, Y=F

Ferrovalley, spin, and piezoelectric polarizations are novel characteristics for electronic materials, and so far, there are few reports that these properties coexist in a single system. By first-principles calculations, we predict a series of highly stable intrinsic ferrovalley materials, i.e., single layer $\mathrm{Cr}\mathit{XY}$ ($X=\mathrm{S}$, Se, Te, $Y=\mathrm{F}$, Cl, Br, I) with large valley polarization up 76.1 meV and appropriate Curie temperature higher than room temperature. For CrSF, the large bandgap of spin-up (spin-down) is as high as 1.96 eV (3.11 eV), which is beneficial for generating 100% spin polarized carriers by optical excitation or electrical gating. The quasianomalous valley Hall effect with valley contrast properties can be induced by compressive strain in CrSI and CrTeX ($X=\mathrm{Cl}$, Br, I). Meanwhile, the large in-plane $(\ensuremath{-}5.78\ensuremath{-}3.54\phantom{\rule{0.16em}{0ex}}\mathrm{pm}/\mathrm{V})$ and outside plane piezoelectric polarization ($\ensuremath{-}5.03\ensuremath{-}2.53\phantom{\rule{0.16em}{0ex}}\mathrm{pm}/\mathrm{V}$) of $\mathrm{Cr}\mathit{XY}$ are higher than most reported two-dimensional materials. Our work provides a pathway for a wide variety of applications in nanoelectronics, spintronics, valleytronics, piezoelectrics, and other demanding areas.