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Two-dimensional (2D) niobium oxydihalide NbOI$_2$ has been recentlydemonstrated as an excellent in-plane piezoelectric and nonlinear opticalmaterials. Here we show that Janus niobium oxydihalide, NbO$XY$ (X, Y = Cl, Br,I and X$\neq$Y), is a multifunctional anisotropic semiconductor family withexceptional piezoelectric, electronic, photocatalytic and optical properties.NbO$XY$ are stable and mechancially flexible monolayers with band gap aroundthe visible light regime of $\sim 1.9$ eV. The anisotropic carrier mobility ofNbO$XY$ lies in the range of $10^3 \sim 10^4$ cm$^2$V$^{-1}$s$^{-1}$, whichrepresents some of the highest among 2D semiconductors of bandgap $\gtrsim 2$eV. Inversion symmetry breaking in Janus NbO$XY$ generates sizable out-of-plane$d_{31}$ piezoelectric response while still retaining a strong in-planepiezoelectricity. Remarkably, NbO$XY$ exhibits an additional out-of-planepiezoelectric response, $d_{32}$ as large as 0.55 pm/V. G$_0$W$_0$-BSEcalculation further reveals the strong linear optical dichroism of NbO$XY$ inthe visible-to-ultraviolet regime. The optical absorption peaks with $14\sim18$\% in the deep UV regime ($5\sim6$ eV), outperforming the vast majority ofother 2D materials. The high carrier mobility, strong optical absorption,sizable built-in electric field and band alignment compatible with overallwater splitting further suggest the strengths of NbO$XY$ in energy conversionapplication. We further propose a directional stress sensing device todemonstrate how the out-of-plane piezoelectricity can be harnessed forfunctional device applications. Our findings unveil NbO$XY$ as an exceptionalmultifunctional 2D semiconductor for flexible electronics, optoelectronics, UVphotonics, piezoelectric and sustainable energy applications.

2D Janus Niobium Oxydihalide NbO$XY$: Multifunctional High-Mobility  Piezoelectric Semiconductor for Electronics, Photonics and Sustainable Energy  Applications