Origin of Janus-induced polarization enhancement in sliding ferroelectrics

Sliding ferroelectricity, characterized by an out-of-plane electric polarization originating from asymmetric interlayer charge distribution in noncentrosymmetric van der Waals bilayers or multilayers, suffers from generally low polarization. Here, we propose that interface charge density enhancement is a key mechanism for increasing out-of-plane polarization in sliding ferroelectrics, which is demonstrated through first-principles calculations on two-dimensional Janus material MoSeS. The tail-to-tail stacked MoSeS bilayer (t-MoSeS) exhibits significantly larger polarization than other bilayers, which mainly origins from reduced interlayer distances and enhanced interlayer charge accumulation due to Janus structures. A systematic analysis reveals that the polarization of t-MoSeS displays heightened (reduced) sensitivity to interlayer distance (lattice constant), resulting in the Janus-induced polarization enhancement $(\mathrm{\ensuremath{\Delta}}P)$ amplified at smaller interlayer distances and lattice constants. Furthermore, we theoretically verify the scaling law $\mathrm{\ensuremath{\Delta}}P\ensuremath{\propto}{P}_{0}^{2}$, directly linking the polarization enhancement to the Janus monolayer polarity $({P}_{0})$. Our work establishes interface charge engineering as a fundamental design principle for high-polarization sliding ferroelectrics, with Janus materials providing a promising implementation pathway, resolving the structure-charge-polarization interplay and advancing the frontier of two-dimensional ferroelectric materials.