Two-dimensional transition-metal dichalcogenides (2DTMDs) monolayers have attracted massive research attention over the past decade due to their unique electronic, optical, and mechanical properties, which remarkably differ from the properties of their respective bulk materials. As a prototypical 2DTMD, monolayer molybdenum disulfide (1L-MoS2) transforms from an indirect bandgap (1.2 eV) semiconductor in its bulk form to a direct bandgap (1.8 eV) semiconductor, owing to which photoluminescence (PL) emerges in 1L-MoS2 with sensitive photo response. Optically generated electron-hole pairs in 1L-MoS2 form stable excitonic states due to large Coulomb interactions in atomically thin layers. Many-body bound states, such as charged exciton (trion), can be formed through the interplay between exciton and a charge carrier by tuning the electron densities in 1L-MoS2, thereby enabling variable PL properties to meet different optoelectronic applications, such as photodetectors, photovoltaics, sensors, and light emitters. Various approaches, including electrical gating, chemical doping, defect engineering, covalent functionalization, and plasma treatment have been employed to realize the PL enhancement or quenching of 1L-MoS2. Nevertheless, these approaches either require complicated methods, are sensitive to chemical contamination, or cause structural damage to the structure. Stacking of 1L-MoS2 with other 2D materials, such as graphene, hBN, or other TMDs by van der Waals forces, also makes the excitonic states of MoS2 adjustable through quantum coupling. Among which, graphene is a semimetal composed of sp 2 -hybridized carbon atoms in a honeycomb lattice array, exhibiting outstanding properties such as high carrier mobility up to 10 5 cm 2 V -1 s -1 at room temperature, quantum electronic transport, high optical transparency (~97.7%), and large tunability of the Fermi level. These superiorities make it an ideal heterogeneous layer to change the excitonic states in MoS2 through interlayer charge transfer. It is reported that the PL of 1L-MoS2 becomes quenched when stacked with pristine graphene. Due to the Schottky barrier at the interface, the photogenerated holes are injected from the valence band of MoS2 to graphene, while the photogenerated electrons in the conductance band of MoS2 are not allowed to diffuse to graphene. Derivatives of graphene, such as graphene oxides (GO), oxo-functionalized graphene (oxo-G), reduced oxo-functionalized graphene (r-oxo-G), and porous graphene, have been used to enhance the PL intensity of 1L-MoS2. Owing to the rich electron-withdrawing groups in GO and oxo-G, such as hydroxyl, epoxy, and organosulfates, or the defective sites in r-oxo-G and porous graphene, a p-doping effect with PL enhancement of the neutral exciton is observed in 1L-MoS2. In addition to the non-covalent heterostructures, Chen et al. from the Hirsch group fabricate a covalently linked MoS2/graphene heterostructure generating PL changes in 1L-MoS2. Firstly, a 4-bromophenyl functionalized MoS2 layer is prepared on a Si/SiO2 substrate by reacting generated negatively charged MoS2 with 4-bromobenzenediazonium tetrafluoroborate. Subsequently, a graphene layer is transferred on top. Through the lasertriggered free radical reaction of 4-bromophenyl groups, the overlaying graphene is covalently attached from underneath to the functionalized MoS2. However, the covalent heterostructure shows remarkable PL quenching, which is likely due to the efficient charge transfer at the covalently bonded interface. Structures like those require complex preparation processes, and the PL changes are considered as a result of charge transfer. However, 2D materials are not only sensitive to charge-transfer doping but also susceptible to interlayer interactions as they are atomically thin. Interlayer van der Waals interactions and interlayer distance are very important factors but are rarely considered in studying the PL of graphene/MoS2 heterostructures (G/MoS2).