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

Abstract

Two-dimensional (2D) niobium oxydihalide NbOI2 has been recently demonstrated as an excellent in-plane piezoelectric and nonlinear optical material. Here we show that Janus niobium oxydihalide, NbOXY (X, Y = Cl, Br, I and X =Y), is a multifunctional anisotropic semiconductor family with exceptional piezoelectric, electronic, photocatalytic and optical properties. NbOXY are stable and flexible monolayers with band gap around the visible light regime of ∼ 1.9 eV. The anisotropic carrier mobility of NbOXY lies in the range of 10 3 ∼ 10 4 cm 2 V −1 s −1 , which represents some of the highest among 2D semiconductors of bandgap 2 eV. Inversion symmetry breaking in Janus NbOXY generates sizable out-of-plane d31 piezoelectric response while still retaining a strong in-plane piezoelectricity. Remarkably, NbOXY exhibits an additional out-of-plane piezoelectric response, d32 as large as 0.55 pm/V. G0W0-BSE calculation further reveals the strong linear optical dichroism of NbOXY in the visible-to-ultraviolet regime. The optical absorption peaks of 14 ∼ 18 % in the deep UV regime (5 ∼ 6 eV) outperform 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 NbOXY in solar-to-hydrogen conversion. We further propose a directional stress sensing device to demonstrate how the outof-plane piezoelectricity can be harnessed for functional device applications. Our findings unveil NbOXY as an exceptional multifunctional 2D semiconductor for flexible electronics, optoelectronics, UV photonics, piezoelectronics and sustainable energy applications.