Conditions for the synthesis of high-quality 2D MoS2 as evidenced by narrow excitonic linewidths

Abstract

In 2D-MoS2, photoluminescence (PL) studies reveal two primary excitonic peaks, A-excitons and B excitons, each coming from a branch of the spin-orbit split valence bands at the K-points of its initial Brillouin zone. Additionally, electron injection caused by a substrate causes n-type doping of monolayer MoS2. This in turn causes the production of stable trions, A-, with a somewhat lower peak-energy position. The line width of these excitonic peaks also gives us an indication of the homogeneity of the material and the crystal structure. The line shape is predicted to be narrow and follows a Gaussian distribution in an ideal scenario where the material is homogeneous and all transitions are direct. As inhomogeneity and lattice vibrations, phonons increase, there are additional contributions from indirect transitions, and the line shape begins to become wider. Other major factors which influence the line shape and width of the PL peak are thermal effects including exciton-phonon coupling and density of states, as well as doping concentrations. Hence, through PL it is possible to analyze the effect of each factor individually towards the crystallinity and the quality of MoS2, keeping the other factors constant. As a result, the line width is a strong indicator of the quality of the grown 2D MoS2 crystal. The main goal of this research project is to investigate synthesis conditions for the narrowest A excitonic line width based on this theory and to optimize the conditions for high-quality monolayer MoS2. Each obtained PL spectrum was a Gaussian curve- fitted to obtain the relative contributions and positions of excitons and trions. The line width or the Full-Width Half Maximum of all the PL spectrums were obtained. According to the theory discussed above, the narrowest excitonic line widths were chosen out of all the PL spectrums of each parameter. Time, temperature, and the precursor concentrations were varied, and the productanalyzed to get a better insight into their effects on the quality of 2D MoS2.--Author's abstract