  Two-dimensional (2D) niobium oxydihalide NbOI$_2$ has been recently
demonstrated as an excellent in-plane piezoelectric and nonlinear optical
materials. Here we show that Janus niobium oxydihalide, NbO$XY$ (X, Y = Cl, Br,
I and X$\neq$Y), is a multifunctional anisotropic semiconductor family with
exceptional piezoelectric, electronic, photocatalytic and optical properties.
NbO$XY$ are stable and mechancially flexible monolayers with band gap around
the visible light regime of $\sim 1.9$ eV. The anisotropic carrier mobility of
NbO$XY$ lies in the range of $10^3 \sim 10^4$ cm$^2$V$^{-1}$s$^{-1}$, which
represents 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-plane
piezoelectricity. Remarkably, NbO$XY$ exhibits an additional out-of-plane
piezoelectric response, $d_{32}$ as large as 0.55 pm/V. G$_0$W$_0$-BSE
calculation further reveals the strong linear optical dichroism of NbO$XY$ in
the visible-to-ultraviolet regime. The optical absorption peaks with $14\sim18$
\% in the deep UV regime ($5\sim6$ eV), outperforming the vast majority of
other 2D materials. The high carrier mobility, strong optical absorption,
sizable built-in electric field and band alignment compatible with overall
water splitting further suggest the strengths of NbO$XY$ in energy conversion
application. We further propose a directional stress sensing device to
demonstrate how the out-of-plane piezoelectricity can be harnessed for
functional device applications. Our findings unveil NbO$XY$ as an exceptional
multifunctional 2D semiconductor for flexible electronics, optoelectronics, UV
photonics, piezoelectric and sustainable energy applications.
