Phonon and exciton temperature-dependent properties of twisted MoS${}_2$

Abstract

In the present work, Raman and photoluminescence spectroscopies were used to study the dynamics of phonons and different excitons of MoS${}_2$ bilayer under a rotation of 29${}^{\circ }$ dependent of the temperature. The twisted bilayer (T-2L) of MoS${}_2$ was obtained through mechanical exfoliation, and subsequently rotated using a dimethyl polysiloxane (PDMS) substrate and deterministic transferred to a SiO${}_2$ substrate. The Raman spectrum of the twisted bilayer presents three peaks E${}^{\prime }$ (386 cm${}^{-1}$), A${}_1^{\prime }$ (405 cm${}^{-1}$) and an FA${}^{\prime }$ peak at approximately 409 cm${}^{-1}$ linked to the A${}_1^{\prime }$ mode, which is attributed to a Moiré pattern phonon. Both modes (A${}_1^{\prime }$ and FA${}^{\prime }$) are dependent of light polarization in a way that demonstrates an effective coupling between the layers. It was also verified through the Grüneisen parameter, an increase in the anharmonicity of the mode in the E${}^{\prime }$ plane and a decrease the same for A${}_1^{\prime }$. In the PL measurements, the appearance of an exciton in T-2L was verified, which generated a second shoulder measured at $\approx$1.58 eV attributed to an indirect transition of an I trion. The interaction between the rotated monolayers of MoS${}_2$ proved to be an important parameter for possible fine-tuning of the properties of bilayer samples.

9 pages, 4 figures, article