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 $\mathrm{MoS_{2}}$ 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 $\mathrm{E^{1}_{2g}}$ 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 $\mathrm{E^{1}_{2g}}$ mode remains unaffected. Using first principles-based simulations we demonstrate large (weak) electron-phonon coupling for the $\mathrm{A_{1g}}$ ($\mathrm{E^{1}_{2g}}$) 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 $\mathrm{MoS_{2}}$ (twistnonics).