Laser-induced anharmonicity vs thermally induced biaxial compressive strain in mono- and bilayer MoS2 grown via CVD

Kishore K Madapu,Sandip Dhara

doi:10.1063/5.0001863

Abstract

We studied the role of biaxial compressive strain in laser-induced anharmonicity effects of mono- and bilayer MoS2 grown by chemical vapor deposition. With the increased laser powers, the A1g phonon mode was strongly affected by the anharmonicity, whereas the E12g mode was negligibly affected by the anharmonic effects, which is attributed to the dominant nature of biaxial compressive strain over the effects of anharmonicity. In the case of the A1g phonon mode, anharmonicity effects dominated over the biaxial compressive strain owing to its large out-of-plane thermal expansion coefficient. The origin of biaxial compressive strain was explained by invoking the difference in linear thermal expansion coefficients of MoS2 and the substrate. Moreover, thermally induced biaxial compressive strain also influenced the exciton emission energy. The role of the substrate thermal conductivity was discussed in the light of anharmonicity of the MoS2 monolayer. In addition, the polarizability of Raman modes was found to be dependent on the second layer arrangement over the monolayer.

Highlights

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted a great deal of attention over the last decade.[1]
The frequency difference of these Raman modes depends on the number of layers; Raman spectroscopy indirectly evaluates the number of layers in the MoS2 layered structure.[4,13,14,15]
The MoS2 mono- and bilayer samples were grown on the SiO2(300 nm)/Si and c-Al2O3 substrate using the atmospheric pressure chemical vapor deposition (APCVD) technique

Summary

INTRODUCTION

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted a great deal of attention over the last decade.[1]. The effect of anharmonicity increases when the temperature of the sample increases In this context, the thermal properties of mono- and few-layer MoS2 were studied by several authors, and the phonon shifts were evaluated with temperature and laser heating.[24–32]. Several authors experimentally derived the first-order temperature coefficients of monolayer MoS2 using the temperature-dependent Raman studies. Laser-induced anharmonic effects in mono- and bilayer (BL) MoS2 are studied. The role of thermally induced biaxial compressive strain in the laser-induced anharmonicity of atomically thin MoS2 is discussed. With the increasing laser power, the anharmonicity effect dominates the A1g phonon mode. The thermal conductivity of the substrate plays a dominant role in laser-induced anharmonicity. The change in the Raman polarizability induced by anharmonicity is found to be different for the in-plane and out-ofplane vibrations of the monolayer. The energies of exciton emissions were found to depend on the excitation energy

EXPERIMENTAL DETAILS

RESULTS AND DISCUSSION

CONCLUSION