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Monolayer 2D transition metal dichalcogenides (TMDs) show high sensitivity to the local dielectric environment, leading to modulation of their optoelectronic properties. Here, we report on the formation of localized trions in a MoS 2 /few-layer graphene van der Waals heterostructure. We performed temperature-dependent photoluminescence and Raman studies down to 80 K, to understand the mechanism for localized charge excitation, which shows contrasting behaviour with MoS 2 /SiO 2 . We attribute trion formation to optically induced charge transfer from few-layer graphene to MoS 2 . Our theoretical analysis and simulations comparing the dielectric screening between MoS 2 /SiO 2 and MoS 2 /few-layer graphene strongly suggest the dominance of excess charge carrier concentration over dielectric screening as the cause of trion formation. The concentration of charge carriers could be tuned actively with excitation power. Our findings provide an efficient approach for trion formation in MoS 2 and explain the mechanism behind charge transfer in the MoS 2 /few-layer graphene heterostructure.