2D Janus铌氧二卤化物NbO $XY$：用于电子、光子学和可持续能源应用的多功能高迁移率压电半导体
2D Janus Niobium Oxydihalide NbO$XY$: Multifunctional High-Mobility Piezoelectric Semiconductor for Electronics, Photonics and Sustainable Energy Applications
摘要：
二维（2D）二卤氧化铌NbOI $_2$最近被证明是一种优异的面内压电和非线性光学材料。在这里，我们表明，Janus铌氧二卤化物，NbO $XY$（X，Y = Cl，Br，I和X $\neq$ Y），是一个多功能的各向异性的半导体家庭具有特殊的压电，电子，光催化和光学性能。NbO $XY$是稳定的和机械柔性的单分子膜，其带隙在$\sim 1.9$ eV的可见光区域附近。NbO $XY$的各向异性载流子迁移率在$10^3 \sim 10^4$ cm $^2$ V $^{-1}$ s $^{-1}$的范围内，其代表带隙$\gtrsim 2$ eV的2D半导体中的一些最高的。在Janus NbO $XY$中的反转对称性破缺产生相当大的面外$d_{31}$压电响应，同时仍然保持强的面内压电性。值得注意的是，NbO $XY$表现出额外的面外压电响应，$d_{32}$高达0.55 pm/V。G $_0$ W $_0$-BSE计算进一步揭示了NbO $XY$在可见光至紫外光范围内的强线性光学二向色性。$14\sim18$的光学吸收峰在深紫外区（$5\sim6$ eV），优于绝大多数其他2D材料。高载流子迁移率、强光吸收、相当大的内建电场和与整体水分裂相容的能带排列进一步表明了NbO $XY$在能量转换应用中的优势。我们进一步提出了一个方向性的应力传感装置，以证明如何利用平面外的压电功能器件的应用。我们的研究结果揭示了NbO $XY$作为一种特殊的多功能2D半导体，用于柔性电子，光电子，紫外光子学，压电和可持续能源应用。
英文摘要：
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.
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