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

Two-disness.onal (2D) Mobium oxydihalide NbOI2 has been tecently demonstrated as an eaLellent in-plane piemeleetric epsilon nd nonlinear optical nuterial. Her. we show that Janus niobium oxydihalide, NbOXY (X. Y = CI, ar, I and X p Y). is a multifunctional anisorropic semiconduccor tamily with exceptional niezoelecttic, electronic, photocatalytic and optical properties. NbOXY are stable and .lexible monolayers with band gap around the visible light regime of - 1.9 eV. The canter mobility of NbOXY exhibits an exceptionally strong anisotoropy ratio of ' 4tt,, which is larger than most of the anisotropic 2D semiconductors. The exceptionally large deformation potential of NbOXYmonolayeis leads to strong piezoresistive effect useful for strain sensing application. Inversion symmemy 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 outof-plane piezoelectric response, 632 as large as 0.55 pm/V. GoWo-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) outpmfoini the vast majority of other 2D materials. The high canier 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, optoelecnonics, UV photonics, piezoelectronics and sustainable energy applications.