Screening two-dimensional pyroelectric materials based on pentagonal chains with large shift current

The bulk photovoltaic effect (BPVE) of two-dimensional (2D) polar crystals has received considerable attention in recent years due to its potential applications in high-efficiency energy harvesting. Here, we propose a class of pentagonal chain-based 2D pyroelectric $\mathrm{Cu}X{X}^{\prime }Y$ $(X, X^{\prime }=\mathrm{S},\mathrm{Se},\mathrm{Te};Y=\mathrm{Cl},\mathrm{Br},\mathrm{I})$ monolayers with in-plane polarization via a high-throughput screening method. By using first-principles calculations, we show that compared to the 2D nonpolar $\mathrm{Cu}{X}_{2}Y$ compounds, the alignment between the averaged transition intensity and shift vector of the polar $\mathrm{Cu}X{X}^{\prime }Y$ materials gives rise to the enhanced shift current responses reaching up to $500\mu \mathrm{A}{\mathrm{V}}^{–2}$ in the visible range, and the magnitude of the shift current is positively related to the polarity of the $X-X^{\prime }$ bond. More importantly, we find that all the studied materials have their exfoliation energies comparable to those of existing 2D materials, implying the feasibility for preparing $\mathrm{Cu}X{X}^{\prime }Y$ monolayers by mechanical exfoliation. Our study demonstrates the potential of utilizing pentagonal chains in the design of polar materials with strong BPVE responses.

Figure 1

(a) Cairo pentagonal tiling. The dashed lines represent a unit cell. The orange shaded region shows a polar chain structure with two different atoms on the three-coordinated vertices. The red arrow denotes the twofold screw axis as well as the polarization direction. (b) The pentagonal chain structure with polarization along the chain direction induced by nonequivalent three-coordinated atoms.

Figure 2

(a) Schematic of the pentagonal chain configuration in 2D materials. (b) Geometric structure of the nonpolar $\mathrm{Cu}{X}_{2}Y$ sheet composed of $\mathrm{Cu}{X}_4$ pentagonal chains and halogen atoms ($Y$) as the linker. (c) Geometric structure of the polar $\mathrm{Cu}X{X}^{\prime }Y$ sheet composed of $\mathrm{Cu}{X}_2{X^{\prime }}_2$ pentagonal chains and halogen atoms.

Figure 3

(a) Shift current tensor ${\sigma }^{yxx}$ of CuTeSBr, CuTeSeBr, and $\mathrm{Cu}{\mathrm{Te}}_2\mathrm{Br}$. The inset image illustrates the direction of ${\sigma }^{yxx}$ with respect to the orientation of the electric field and polarization axis. (b) The relationship between the integrated shift current and the Born effective charge $Z_{yy}^{*}\left(X^{\prime }\right).$

Figure 4

(a) Band structures of the CuTeSBr and $\mathrm{Cu}{\mathrm{Te}}_2\mathrm{Br}$ sheets. The red bands contribute most to the peak value of the shift current. (b) The shift current response between different band pairs. The red line for $\mathrm{V}{\mathrm{B}}_3\to \mathrm{C}{\mathrm{B}}_1$ represents the largest contribution. The (c) shift current, (d) transition intensity, and (e) aggregate shift vector contributed from the ($\mathrm{V}{\mathrm{B}}_3\to \mathrm{C}{\mathrm{B}}_1$) pair.

Figure 5

The $k$-resolved (a) shift current, (b) transition intensity, and (c) shift vector between $\mathrm{V}{\mathrm{B}}_3$ and $\mathrm{C}{\mathrm{B}}_1$ of CuTeSBr at $\hslash \omega =1.85$ eV. (d)–(f) The same physical quantities of $\mathrm{Cu}{\mathrm{Te}}_2\mathrm{Br}$ at $\hslash \omega =1.66$ eV.