Thickness and temperature dependent photoluminescence from few-layer MoS2

Semiconducting nanolayers of transition metal dichalcogenide such as MoS2 have been proposed as promising channel materials for future nanoelectronic and optoelectronic applications. One of the advantages of 2D materials comes from quantum confinement, enabling the indirect-to-direct bandgap transition as a function of the film thickness and strong photoluminescence in singlelayer form. In this stud y, we show the variation of photoluminescence and structural properties of few-layers MoS2. The flakes are grown on SiO2/Si by chemical vapor deposition using S and Mo powders. 2D material is characterized by electron and atomic force microscopies and temperature-dependent photoluminescence and Raman micro-spectroscopies. Atomic force microscopy reveals the presence of MoS2 monolayers, which are investigated by spectroscopy to compare the properties of different flakes. The PL spectrum of the flakes shows two broad bands associated to the A0, derived from an exciton, and A-, derived from a trion consists of an exciton combined with another electron, and B exciton. The light emission at room temperature is found to depend on the thickness and shape of MoS2 layers. For instance, photoluminescence close to the periphery of the flake is decreased compared to that in the center, which can be connected to a different concentration of defects. The nonradiative recombination can be related with the defect trapping and electron relaxation through the defect levels within bandgap. At the same time, the trion band is mostly decreased. In temperaturedependent measurements, the major trend is that the photoluminescence redshifts and intensity dramatically decreases from 82 to 200K. This can be attributed to the thermally activated nonradiative recombination with the increase of electron-phonon interaction. Heating from 200K to room temperature causes an increase of the photoluminescence intensity, which may be due to defect activation at higher temperatures. We can assume the presence of different mechanisms that concurrently influence the photoluminescence intensity.