LAUSR

The exciton dynamics in WS2 from monolayer to four-layer was investigated by using fluorescence lifetime imaging measurement (FLIM). Negatively charged trions' transition process is measured and detected for the first time by using fluorescence detection method. Compared with neutral excitons, negatively charged trions have a longer fluorescence lifetime. Further exploration illustrated that the fluorescence lifetime of both neutral excitons and trions get longer with thickness increased. When WS2 added up from monolayer to four-layer, lifetimes of direct transition excitons and trions tend to increase over 10 times and 2.5 times, separately, while the lifetime of indirect transition excitons tends to be reduced by nearly 2.5 times. This layer-dependent signature is ascribed to the reduced binding energy in thicker WS2 at room temperature, which is verified by density theory functional (DFT) calculation. Although the direct transition exciton dominates the whole fluorescence decay process, it is influenced by trions and dark excitons. Based on the FLIM results, we proposed four main excitons transition channels for the first time during fluorescence luminescence process. Such layer-dependent transition channel conception helps to control the fluorescence lifetime which determines the efficiency of carriers' separation.