Single-Layer BI: A Multifunctional Semiconductor with Ferroelectricity, Ultrahigh Carrier Mobility, and Negative Poisson’s Ratio

Developing two-dimensional multifunctional materials is highly desirable for nanoscale device applications. Herein, by means of first-principles calculations, we demonstrate a promising two-dimensional multifunctional semiconductor with ferroelectricity, ultrahigh carrier mobility, and negative Poisson's ratio in single-layer $\mathrm{BI}$. We show that single-layer $\mathrm{BI}$ exhibits intrinsic ferroelectricity, derived from its asymmetric structure, and the ferroelectricity features an in-plane electric polarization as large as $3.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}{\mathrm{C}}_{{\mathrm{m}}^{\ensuremath{-}1}}$, which is beneficial for nonvolatile memories. Owing to its large band dispersion, single-layer $\mathrm{BI}$ is also found to harbor an extremely high carrier mobility ($1.17\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{0.1em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}{\mathrm{V}}^{\ensuremath{-}1}{\mathrm{s}}^{\ensuremath{-}1}$), even comparable to that of graphene, suggesting its potential for high-performance electronics and bulk photovoltaic effect. This, combined with the moderate band gap, renders single-layer $\mathrm{BI}$ with a high absorption coefficient (${10}^{5}\mathrm{cm}{\mathrm{m}}_{\ensuremath{-}1}$) from near-infrared to ultraviolet light. In addition, we unveil that single-layer $\mathrm{BI}$ is a long-sought-after auxetic material with a negative Poisson's ratio of \ensuremath{-}0.31. All of these findings make single-layer $\mathrm{BI}$ a compelling multifunctional material, offering a versatile platform for diverse nanoscale devices applications.