Bulk photovoltaic effect in MoSe$_2$ and Janus MoSSe sliding ferroelectrics

TL;DR

This work addresses the bulk photovoltaic effect (BPVE) in two-dimensional sliding ferroelectrics by combining interlayer sliding in MoSe$_2$ bilayers with intralayer polarization in Janus MoSSe. It uses first-principles density functional theory with spin-orbit coupling and van der Waals corrections, augmented by Wannier-interpolation to compute the second-order photoconductivity tensors $\sigma^{abc}$ (shift) and $\eta^{abc}$ (injection). By comparing MoSe$_2$, Janus I, Janus II, and Janus II' configurations under opposite polarization states, it finds a dramatic BPVE enhancement in Janus II, especially for the circular injection current, and shows that the response is predominantly governed by vertical chemical asymmetry rather than interlayer sliding. The results establish design rules for maximizing and tuning nonlinear optical responses in 2D ferroelectrics through a combination of interlayer sliding and Janus intralayer polarization, with potential implications for bias-free, ultrafast photovoltaics and optoelectronic devices.

Abstract

We present a first-principles study of the nonlinear optical properties of sliding ferroelectric bilayers based on MoSe$_2$ and Janus MoSSe. Two Janus configurations are considered: i) one bilayer where the two intralayer polarizations caused by Janus chemical asymmetry cancel each other out, yielding photocurrent spectra comparable to pristine MoSe$_2$ bilayers; ii) another bilayer where the intralayer polarizations add up, for which the photoresponses are strongly enhanced. Our results show that photocurrent generation in the polar Janus structures is predominantly governed by vertical chemical asymmetry, with limited dependence on the sliding direction. These findings highlight complementary design strategies: interlayer sliding enables sensitivity to external tuning, while the Janus intralayer polarization enhances photoresponses in the visible range. The interplay between composition and stacking therefore provides a versatile platform for tailoring light-matter interactions in 2D ferroelectric materials.

Figures (10)

• Figure 1: The side and top views of the bilayer crystal structures of (a) MoSe$_2$, (b) Janus I, and (c) Janus II, respectively. The Mo, Se and S atoms are represented by violet, green, and yellow spheres respectively. The blue, maroon, and green arrows represent the direction of the interlayer polarization (P$_{inter}$/P$_{inter'}$/P$_{inter"}$) for MoSe$_2$, Janus I, and Janus II respectively. The red arrows represent the direction of intralayer polarization (P$_{intra}$) exisisting within the Janus I and Janus II monolayers.
• Figure 2: The non-zero, independent components of the photoconductivity tensor for MoSe$_2$. (a) Shift current components (b) Circular injection current components.
• Figure 3: Comparision of BPVE response for MoSe$_2$, Janus I and Janus II bilayers for a) in-plane shift current component $\sigma_{xxy}$ (b) out-of-plane shift current component $\sigma_{zxx}$ and (c) circular injection current component $\eta_{xzx}$.
• Figure 4: Evolution of the (a) out-of-plane shift current $\sigma_{zxx}$ and (b) circular injection current $\eta_{xzx}$ components for Janus II, Janus II (slide) and Janus II'.
• Figure 5: Band structure along the high symmetry $k$-points for the bilayers of (a) MoSe$_2$ (b) Janus I (c) Janus II. The Fermi energy is set to zero in the energy scale.