Nevertheless, the F-G/MoS2 shows comparable PL enhancement for MoS2 to that of GO/MoS2 as we reported in previous works, demonstrating that the electronic effect of functional groups is not the only factor of PL enhancement of MoS2. To elucidate further effects, the reversely stacked MoS2/F-G heterostructure was fabricated. As exhibited in the schematic structure in Figure , the F-G placed on top of MoS2 can keep the electronwithdrawing effect induced by phenyl groups, but have better interlayer contact with the graphene lattice, compared to F-G/MoS2, as shown in Figure . Firstly, a mechanically exfoliated 1L-MoS2 was transferred onto a Si/SiO2 (300 nm) wafer. Then a monolayer graphene was transferred on the 1L-MoS2 to form a MoS2/G heterostructure. (Figure ). The MoS2/G heterostructure was annealed in vacuum at 200 °C for 2h, and then dip-coated with a thin layer of BPO (5 mM, in acetone) of the graphene surface of the heterostructure (MoS2/G/BPO). The laser functionalization process is identical to the previously described for the functionalization of hBN/G/BPO. However, here, the in situ functionalization is directly carried out on the surface of the MoS2/G heterostructure without further transfer process, compared to F-G/MoS2. The laser irradiation area on MoS2/G is marked by the red box (region III) as shown in Figure . After 30 s laser exposure, the functionalized area (region III) on MoS2/G shows a high ID/IG intensity of around 2.2 with a slight increase in the PL intensity of MoS2 (Figure and). In contrast, the non-functionalized area of MoS2/G (region II) shows PL quenching of MoS2. In Figure , the average Raman spectra of MoS2/F-G (region III), MoS2/non-F-G (region II), and isolated MoS2 (region I) from the three different areas on the MoS2/G heterostructure are shown. The E 1 2g (385.4 cm -1 ) and the A1g (404.1 cm -1 ) peaks in the Raman spectrum of the isolated 1L-MoS2 layer confirm the monolayer characteristic of the MoS2 flake. Vibrational modes of both 1L-MoS2 and 1Lgraphene appear in the Raman spectra of MoS2/F-G and MoS2/non-F-G regions, indicating the formation of heterostructures. The D peak appears in MoS2/F-G after functionalization with the attenuation of the 2D peak, and the degree of functionalization is around 1%, which is consistent with hBN/F-G/MoS2 (Figure ). However, as the PL spectra in Figure reveal, the PL intensity of MoS2 only increases marginally by 1. The differentiated PL characteristics of F-G/MoS2 and MoS2/F-G prove that the structural disparity notably affects the PL of MoS2. PL spectra of 1L-MoS2 on four different structures of F-G/MoS2, MoS2/F-G, G/MoS2, and pristine 1L-MoS2 on Si/SiO2 were compared (Figure ). To gain a better understanding of spectral changes, peaks were fitted using Lorentzian functions. Corresponding to three typical excitonic states of neutral exciton (A 0 , 1.89 eV), negatively charged trion (A -, 1.85 eV), and B exciton (2.02 eV), the PL spectra of F-G/MoS2, MoS2/F-G, G/MoS2, and MoS2 were fitted by three Lorentzian peaks (Figure ). Detailed parameters about fitted PL peaks by the Lorentz function are exhibited in Table . In pristine 1L-MoS2, the PL spectrum is dominated by the trion state, which arises from reduced dielectric screening in gapped 2D crystals and the relatively heavy particle band masses associated with the Mo d-manifolds. In contrast to pristine MoS2, the A -peak increases in G/MoS2 and the A 0 peak was obviously weakened. Conversely, the A 0 excitons are the dominant contributors in the PL emission with a 1.8-fold enhancement in MoS2/F-G and a 5fold enhancement in F-G/MoS2 compared to isolated MoS2, which indicates that MoS2 becomes p-doped by F-G. The A 0 exciton peak intensity in F-G/MoS2 is 11 times higher than that in G/MoS2. As illustrated in Figure , electrons transfer from MoS2 to F-G for F-G/MoS2 and MoS2/F-G. For G/MoS2, MoS2 donates electrons to pristine graphene. Even though F-G shows electron-withdrawing ability. Whether it was placed on top or under MoS2, there is a significant difference in the effect on enhancing the PL of MoS2. This result indicates that the p-doping is not the only reason for the PL increase of MoS2. In F-G/MoS2, the MoS2 is supported directly by the phenyl groups due to the high functionalization degree on the graphene surface, which increases the interlayer distance and results in a weakened van der Waals interaction between G and MoS2. Compared to G/MoS2, the interlayer contact between MoS2 and the graphene basal plane of F-G/MoS2 is largely reduced, which hinders the electron transfer from the carbon plane to MoS2, and consequently enhances the PL intensity. Moreover, the reversibility of the functionalization effect on the PL enhancement of MoS2 is investigated. The hBN/(non-)F-G/MoS2 heterostructure was annealed at 400 °C for 2 h in vacuum to break the C-Aryl bond for restoring graphene (Figure ). As shown in the Raman spectra in Figure , the Raman signal of de-F-G recovers the narrow and most intense 2D peak. The sharp D peak vanishes, although a broad peak remains, most likely resulting from trapped dissociated phenyl groups between graphene and MoS2. Accordingly, the PL enhancement from F-G also disappears after de-functionalization (Figure and). The PL intensity of de-F-G/MoS2 is slightly lower than that of the isolated MoS2 area because of the interlayer Schottky barrier. However, the maximum of the PL peak of de-F-G/MoS2 is redshifted on account of the decoupled phenyl groups (Figure ). The PL recovers mainly because of the annealing process, which breaks the covalent bond of the phenyl functional group perpendicular to the graphene plane, leading to the free orientation of the dissociated phenyl groups and restoration of the interlayer spacing.