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 $3.3\times {10}^{-10}{\mathrm{C}}_{{\mathrm{m}}^{-1}}$, 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 ($1.17\times {10}^5\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$), 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 (${10}^5\mathrm{cm}{\mathrm{m}}_{-1}$) 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 −0.31. All of these findings make single-layer $\mathrm{BI}$ a compelling multifunctional material, offering a versatile platform for diverse nanoscale devices applications.

Figure 1

(a) Top and side views of the crystal structure of SL $\mathrm{BI}$ with red lines highlighting the unit cell. (b) Electron localization function (ELF) of SL $\mathrm{BI}$, wherein red and blue areas indicate accumulated and vanishing electron density, respectively. (c) 2D Brillouin zone of SL $\mathrm{BI}$. (d) Phonon band dispersions for SL $\mathrm{BI}$. (e) Variations of the total energy of SL $\mathrm{BI}$ during AIMD simulations at 300 K. Insets in (e) show snapshots of the structure taken from the end of the AIMD simulation at 300 K.

Figure 2

(a) Minimum-energy pathway, along with corresponding structures, of FE-PE-FE transition in SL $\mathrm{BI}$ obtained from the cNEB calculation; corresponding shortest $\mathrm{B}$—$\mathrm{I}$ bonds in the 2D plane are highlighted in pink. (b) Double-well potential versus polarization of SL $\mathrm{BI}$. (c) Energy barrier and polarizations as a function of in-plane strain for SL $\mathrm{BI}$. (d) FE domain walls of SL $\mathrm{BI}$ (d_b are displacements along the +b axis).

Figure 3

(a) Band structure and projected density of states for SL $\mathrm{BI}$. Fermi level is set to 0 eV. (b) Optical absorption coefficients of SL $\mathrm{BI}$ under strain from −3% to 3%.

Figure 4

Polar diagrams of (a) Young’s modulus Y(θ) and (b) Poisson’s ratio ν(θ) of SL $\mathrm{BI}$. Evolution of local structures of SL in the x-z (c) and y-z (d) planes under uniaxial tension strain.