Evolution of high-frequency Raman modes and their doping dependence in twisted bilayer MoS2

Abstract

Twisted van der Waals heterostructures unravel a new platform to study strongly correlated quantum phases. The interlayer coupling in these heterostructures is sensitive to twist angles ($\theta$) and key to controllably tune several exotic properties. Here, we demonstrate a systematic evolution of the interlayer coupling strength with twist angle in bilayer $\mathrm{Mo}{\mathrm{S}}_2$ using a combination of Raman spectroscopy and classical simulations. At zero doping, we show a \textit{monotonic} increment of the separation between the ${\mathrm{A}}_{1g}$ and ${\mathrm{E}}_{2g}^1$ mode frequencies as $\theta$ decreases from $10^{\circ }\to 1^{\circ }$, which saturates to that for a bilayer at small twist angles. Furthermore, using doping-dependent Raman spectroscopy we reveal $\theta$ dependent softening and broadening of the ${\mathrm{A}}_{1g}$ mode, whereas the ${\mathrm{E}}_{2g}^1$ mode remains unaffected. Using first principles-based simulations we demonstrate large (weak) electron-phonon coupling for the ${\mathrm{A}}_{1g}$ (${\mathrm{E}}_{2g}^1$) mode explaining the experimentally observed trends. Our study provides a non-destructive way to characterize the twist angle, the interlayer coupling and establishes the manipulation of phonons in twisted bilayer $\mathrm{Mo}{\mathrm{S}}_2$ (twistnonics).