Exciton Linewidth and Exciton-Phonon Coupling in 2H and 3R Bilayer WS2 Studied by Magnetic Circular Dichroism Spectrum

Layered transition metal dichalcogenides (TMDs) are a promising platform for new photonic and optoelectronic devices. Exciton-phonon interaction is critical in determining their characteristics such as the exciton coherence lifetime and linewidth. However, the exciton linewidth obtained by conventional reflection spectrum is greatly affected by the background signals, and the research into exciton-phonon coupling difference induced by stacking-order in multi-layer structures is still lacking. In this work, the temperature-dependent exciton linewidths of CVD-grown large-area monolayer, 2H and 3R-stacking bilayer WS2 based on a self-designed reflective magnetic circular dichroism (MCD) spectrum are systematically investigated. It is found that 2H-bilayer WS2 exhibits significantly larger exciton linewidth compared with monolayer and 3R bilayer, which can be attributed to the appearance of new phonon-assisted relaxation channels caused by interlayer coupling. Meanwhile, 3R bilayer with a redshifted exciton peak has a narrower linewidth than 2H phase because the interlayer hopping is suppressed, resulting in the absence of interlayer scattering channel. These results provide intuitive evidence for the exciton linewidth-broadening and exciton-phonon coupling in different stacked layers and open up new vistas for the development of TMD-based narrow-linewidth nano-sensors devices.