2D janus niobium oxydihalide NbOXY: Multifunctional piezoelectric semiconductor for electronics, photonics, sensing and sustainable energy applications

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 carrier mobility of NbOXY exhibits an exceptionally strong anisotoropy ratio of ∼400, which is larger than most of the anisotropic 2D semiconductors. The exceptionally large deformation potential of NbOXYmonolayers leads to strong piezoresistive effect useful for strain sensing application. 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 out-of-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.