Ultralow Frequency Interlayer Mode from Suppressed van der Waals Coupling in Polar Janus SMoSe/SWSe Heterostructure

Abstract

The broken mirror symmetry in Janus SMoSe and SWSe monolayers induces novel properties for photocatalytic, thermoelectric and photocatalytic devices. Interlayer coupling is critical in van der Waals (vdW) heterostructure for quantum transport and polaritonics. We investigate Janus SMoSe/SWSe vdW heterostructures with three stacking interfaces: S-S, S-Se, and Se-Se. The S-Se SMoSe/SWSe vdW heterostructure with lowest symmetry exhibits ultralow frequencies of in-plane shear (1.94 cm⁻¹) and out-of-plane breathing (4.47 cm⁻¹) modes due to weaker interlayer vdW restoring forces and a significant intrinsic vertical dipole moment. The reduced restoring forces are caused by the critical charge transfer across the vdW interface. Thus, the larger interlayer spacing in the S-Se SMoSe/SWSe heterostructure results in the suppressed vdW interlayer coupling for ultralow phonon frequencies. These findings advance understanding of tuning vdW coupling in polar Janus SMoSe/SWSe heterostructures by stacking engineering, providing theoretical insights for designing tunable nanoelectronic devices.