Figure To keep the explanation of heterostructures concise, herein, we define the vertical stacking order of the heterojunction layers as (substrate/)bottom layer/upper layer. As shown schematically in Figure , few-layer hBN with a thickness of 6 nm (Figure ) were exfoliated and transferred on a Si/SiO2 (300 nm) wafer substrate. Then, a monolayer of graphene exfoliated from HOPG was transferred on the hBN flake (hBN/G). The hBN/G structure was subsequently annealed in vacuum at 150 °C for 2 h to remove water and organic residues originating from the transfer process. The microscope and AFM image of the hBN/G heterostructure is shown in Figure and Figure , respectively. We find that the hBN layer stabilizes the flat structure of the graphene flakes during repeated solution treatments because hBN is flat compared to Si/SiO2. Then, a thin layer of BPO film was dip-coated on the surface of hBN/G (hBN/G/BPO) using a 5 mM BPO solution in acetone followed by drying for 5 min under ambient conditions (Figure ). The photochemical functionalization of graphene was carried out by 532 nm Raman laser treatment at a power of 1 mW. Under the laser irradiation, the BPO decomposes to phenyl radicals and CO2 by accepting a hot electron from photoexcited graphene. The phenyl radicals subsequently attack on the basal plane of graphene. To exclude any damaging effect from the laser energy input in combination with solvent residues, we conducted reference experiments for pristine graphene and graphene treated with acetone. Therefore, the reference structures were treated for accumulated laser irradiation times from 1 s to 300 s. According to the Raman spectra displayed in Figure , the non-defective graphene of hBN/G is stable under those conditions. For hBN/G/BPO, we find that the degree of functionalization of graphene is radiation time-dependent. Accordingly, with accumulated laser acquisition time from 1 s to 50 s, the maximum functionalization density was indicated by Raman spectra (Figure ). Thus, for further experiments, we used 30 s irradiation time for the monotopic functionalization of hBN/G. The laser illuminates only the bottom half part of the graphene flake by using the mapping function of the scanning Raman instrument as marked in the green area in Figure and the red box in Figure . After laser-induced functionalization, the residual BPO film was washed off carefully with acetone. The readout process of Raman spectra of the asfunctionalized hBN/G was accomplished with a lower laser power of 0.5 mW and 1 s irradiation time. As can be seen in the ID/IG Raman intensity mapping in Figure and the schematic structure in Figure , the illuminated region can be clearly distinguished as a result of functionalization (F-G), unlike the rest of the graphene flake (non-F-G). The average Raman spectra of pristine hBN/G before functionalization, the hBN/F-G, and hBN/non-F-G are depicted in Figure , respectively. There are two dominant peaks in the Raman spectra of pristine hBN/G and hBN/non-F-G: G-band (at 1580 cm -1 ) and 2D-band (at 2680 cm -1 ).