Unraveling the Defect Emission and Exciton-Lattice Interaction in Bilayer WS2

Defect states and exciton of two-dimensional semiconductors play an important role in fundamental research and device applications. Here, we reported the defect emissions and exciton-lattice interaction of bilayer WS2. These defect emissions show a very narrow linewidth, doublet peaks, spatial localization, saturation with pumping power and can survive up to 180 K. The behavior of these defect emissions means it should be a good candidate as a single photon source. Besides defect emissions, direct exciton and two indirect excitons due to band-to-band transition are identified. By analyzing the temperature-dependent photoluminescence (PL) spectra of excitons, we obtained the Debye temperature, exciton-phonon coupling constant, and pressure coefficient terms of all excitons. Combining the PL experiments and density functional theory calculations, we attributed two indirect excitons to the Lambda-K and Lambda-Gamma transitions, respectively. Our study not only gives a better understanding of the defect emissions and energy band structure in multilayer materials, but also provides an opportunity for defect and band engineering in two-dimensional layered systems.