Revealing the interlayer van der Waals coupling of bi-layer and tri-layer MoS using terahertz coherent phonon spectroscopy.

In this research, we applied THz coherent phonon spectroscopy to optically probe the vibrational modes of the epitaxially-grown bi-layer and tri-layer MoS on sapphire substrate. The layers' THz vibration is displacively stimulated and temporally retrieved by near-UV femtosecond laser pulses, revealing Raman-active and Raman-inactive modes in one measurement. With the complete breathing modes revealed, here we extend the linear chain model by considering the elastic contact with the substrate and vdWs coupling of the next nearest MoS layer to analyze the effective spring constants. We further considered the intralayer stiffness as a correction term to acquire the actual interlayer vdWs coupling. Our THz phonon spectroscopy results indicate the interlayer spring constants of 9.03 × 10 N/m and 9.86 × 10 N/m for bi-layer and tri-layer respectively. The extended model further suggests that a non-negligible substrate mechanical coupling and next nearest neighbor vdWs coupling of 1.48 × 10 N/m and 1.04 × 10 N/m have to be considered.