ⅠⅠⅠ. Results and discussion 15 3.1 Synthesis and characterization of WS2 monolayer 15 3.2 Fabrication of the Graphene/1L-WS2/Graphene heterostructures 18 3.3 Observation of charge carrier transfer of Graphene/1L-WS2/Graphene heterostructures 20 3.4 Investigation of photoluminescence of Graphene/1L-WS2/Graphene heterostructures 22