Doping Graphene Transistors Using Vertical Stacked Monolayer WS₂ Heterostructures Grown by Chemical Vapor Deposition

We study the interactions in graphene/WS₂ two-dimensional (2D) layered vertical heterostructures with variations in the areal coverage of graphene by the WS₂. All 2D materials were grown by chemical vapor deposition and transferred layer by layer. Photoluminescence (PL) spectroscopy of WS₂ on graphene showed PL quenching along with an increase in the ratio of exciton/trion emission, relative to WS₂ on SiO₂ surface, indicating a reduction in the n-type doping levels of WS₂ as well as reduced radiative recombination quantum yield. Electrical measurements of a total of 220 graphene field effect transistors with different WS₂ coverage showed double-Dirac points in the field effect measurements, where one is shifted closer toward the 0 V gate neutrality position due to the WS₂ coverage. Photoirradiation of the WS₂ on graphene region caused further Dirac point shifts, indicative of a reduction in the p-type doping levels of graphene, revealing that the photogenerated excitons in WS₂ are split across the heterostructure by electron transfer from WS₂ to graphene. Kelvin probe microscopy showed that regions of graphene covered with WS₂ had a smaller work function and supports the model of electron transfer from WS₂ to graphene. Our results demonstrate the formation of junctions within a graphene transistor through the spatial tuning of the work function of graphene using these 2D vertical heterostructures