ARTICLE https://doi.org/10.1038/s41467-020-19181-2 OPEN Blue emission at atomically sharp 1D heterojunctions between graphene and h-BN Gwangwoo Kim 1,11, Kyung Yeol Ma2,11, Minsu Park3, Minsu Kim1, Jonghyuk Jeon 4, Jinouk Song5, José Eduardo Barrios-Vargas 6, Yuta Sato 7, Yung-Chang Lin7, Kazu Suenaga 7, Stephan Roche8,9, Seunghyup Yoo5, Byeong-Hyeok Sohn4, Seokwoo Jeon3 & Hyeon Suk Shin 1,2,10 ✉ 1234567890():,; Atomically sharp heterojunctions in lateral two-dimensional heterostructures can provide the narrowest one-dimensional functionalities driven by unusual interfacial electronic states. For instance, the highly controlled growth of patchworks of graphene and hexagonal boron nitride (h-BN) would be a potential platform to explore unknown electronic, thermal, spin or optoelectronic property. However, to date, the possible emergence of physical properties and functionalities monitored by the interfaces between metallic graphene and insulating h-BN remains largely unexplored. Here, we demonstrate a blue emitting atomic-resolved hetero- junction between graphene and h-BN. Such emission is tentatively attributed to localized energy states formed at the disordered boundaries of h-BN and graphene. The weak blue emission at the heterojunctions in simple in-plane heterostructures of h-BN and graphene can be enhanced by increasing the density of the interface in graphene quantum dots array embedded in the h-BN monolayer. This work suggests that the narrowest, atomically resolved heterojunctions of in-plane two-dimensional heterostructures provides a future playground for optoelectronics. 1 Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea. 2 Department of Energy Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea. 3 Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea. 4 Department of Chemistry, Seoul National University, Seoul 08826, Republic of Korea. 5 School of Electrical Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea. 6 Departamento de Física y Química Teórica, Facultad de Química, UNAM, 04510 México City, México. 7 Nanomaterials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba 305-8565, Japan. 8 Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona Institute of Science and Technology, Campus UAB, 08193 Barcelona, Spain. 9 ICREA— Institució Catalana de Recerca i Estudis Avançats, 08010 Barcelona, Spain. 10 Low Dimensional Carbon Material Center, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea. 11These authors contributed equally: Gwangwoo Kim, Kyung Yeol Ma. ✉email: shin@unist.ac.kr NATURE COMMUNICATIONS | (2020)11:5359 | https://doi.org/10.1038/s41467-020-19181-2 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-19181-2 I n-plane two-dimensional (2D) heterostructures serve as a image mapping (Fig. 1f) at the 410 nm wavelength demon- perfect platform to explore one-dimensional (1D) function- strates the homogenous PL emission only at the circular shaped alities because of their atomically sharp (resolved) hetero- interface between the graphene and h-BN regions. The mag- junctions. Because the edge contacts offer easier band offset nified mapping images (Supplementary Fig. 1) clearly show the tuning, lateral heterostructures-based heterojunctions have led to luminescence occurring exclusively at the boundary. This PL improved performance for P-N rectifying diode1,2 and interlayer result is different from the previous results for the graphene-h- excitons3, as well as low contact resistance for metal/semi- BN zigzag interface that showed localized energy states near the conductor junctions4,5. These examples illustrate the emergence Fermi level10–12. Since the grown in-plane heterostructures of quantum phenomena arising from differences in the properties were fabricated in sequential growth, they resulted in epitaxial of two types of 2D materials divided by a heterojunction. How- hetero-junctions between graphene and h-BN with well-defined ever, it has been rarely reported that 1D heterojunctions them- zigzag edges. However, in our work, we fabricate hetero- selves show unknown properties and functionality. structures by using conversion reactions of h-BN to graphene Interestingly, a recent theoretical paper reported the formation and hence expect that a different atomic arrangement of the of localized density of states (LDOS) at the random, disordered heterojunctions emerges at boundaries. interface of hexagonal boron nitride (h-BN) and graphene6. Thus far, most of the theoretical calculations predicted that the zigzag and armchair interfaces between graphene and h-BN have half‐ Graphene quantum dots embedded in a h-BN monolayer. In metallic and semiconducting properties7–9. In addition, some order to increase the amount of interface per unit area for experimental attempts using scanning tunneling microscopy have improved PL intensity, we prepare in-plane heterostructures of identified a clear sharp peak close to zero bias, which was ascribed graphene quantum dots in a h-BN monolayer by a spatially to the localized interfacial states at the zigzag edges connecting controlled conversion on Pt nanoparticles (Pt NPs) prepared with graphene with h-BN10,11. Park et al. observed an enhanced local the aid of self-patterning diblock copolymer micelles16 (GQD/h- density of states at zigzag interfaces composed of several segments BN, Fig. 2a, b). The structure of GQD/h-BN is a better sample to forming 120° angles and found distinct states with energy levels of investigate in-depth the PL emission at the G/h-BN junctions −0.6 eV (or +0.6 eV) at C–B (or −N) terminated boundaries12. owing to the higher density of interfaces. Figure 2c presents the Finally, disordered interfaces between h-BN and graphene, which PL spectrum (red line) of the 7 nm-sized GQD/h-BN hetero- would be common in lateral heterostructures fabricated by pat- structures under excitation with a 266 nm laser. The blue PL terned regrowth13,14, have been observed experimentally, but the emission was observed with a peak wavelength of 410 nm, presence and role of localized energy states occurring at these equivalent to the PL result (blue line) of the G/h-BN interfaces. interfaces are unknown. Interestingly, a ~6 times enhancement in PL intensity is shown in Here, we demonstrate luminescent, atomic-resolved hetero- GQD/h-BN films due to the higher interfacial density compared junctions connecting graphene and h-BN domains in 2D in-plane to the G/h-BN samples. The interface length per unit area for heterostructures. Blue-emitting photoluminescence (PL) at GQD/h-BN (0.0082 nm−1) is ~6.3 times longer than that for the 410 nm wavelength from 1D heterojunctions of h-BN and gra- G/h-BN sample (0.0013 nm−1), which is comparable to the ratio phene was observed, which supposedly originates from the of PL intensities of the two samples. localized energy states at the disordered heterojunction between To understand the mechanism of blue PL emission at h-BN and graphene (G/h-BN junction). The weak blue emission 410 nm, we performed PL measurements on several control at the 1D heterojunctions in a simple in-plane heterostructure of samples. It is noted that no PL peaks (Fig. 2c) were observed h-BN and graphene can be enhanced by increasing the amount of from bare Pt NPs (green) and bare h-BN (black) on a SiO2/Si interface through embedding a graphene quantum dot array in a substrate. However, carbon doping in the h-BN monolayer h-BN monolayer (GQD/h-BN in-plane heterostructures). Finally, during the conversion reaction might be the origin. To check, we stacked four GQD/h-BN films by placing h-BN intercalation we achieved the conversion of h-BN on a SiO2/Si substrate layer between each film to improve the emission. without the Pt NP array under the same conditions (950 °C, CH4/Ar flow = 5/50 sccm). The PL emission was not observed in the sample (as shown in the blue spectrum of Supplementary Results Fig. 3c), indicating that the PL results originated from the in- Simple in-plane graphene/h-BN heterostructures. First, we plane heterostructures of GQD and h-BN. Furthermore, to examine simple in-plane graphene/h-BN heterostructures to check the PL emission of bare GQD without h-BN, a sample confirm that the desired heterostructures has formed15. Fig- was prepared on a CVD-grown graphene monolayer via the O2 ure 1a shows an SEM image of the circular graphene region plasma etching process by using the Pt NP array as a pattern embedded in the h-BN monolayer, confirming a clear boundary mask (Supplementary Fig. 4a)17. We can easily distinguish the between the graphene and h-BN regions. The boundary can be GQD arrays and SiO2 regions by SEM contrast (Supplementary observed in a magnified SEM image (Fig. 1b). To identify the Fig. 4b). The PL peak at 410 nm wavelength was not observed graphene and h-BN materials, we measured the Raman spectra from the bare GQD (as shown in the red spectrum of of the heterostructures after they were transferred onto a SiO2/ Supplementary Fig. 4c). Lastly, nano-sized holes of h-BN in Si substrate. The red and blue lines in Fig. 1c show the corre- the GQD/h-BN heterostructures may be one of the origins of sponding Raman spectra of the graphene (G, 2D peaks) and h- the PL (Supplementary Fig. 5). To check this possibility, a h-BN BN (E2g peak), respectively. Notably, the Raman mapping sheet with nano-sized holes was prepared by using hydrogen- image (Fig. 1e) in the 2D band (2630–2730 cm−1) clearly dif- etching of h-BN through the annealing process on Pt NPs in H2 ferentiates the graphene from the h-BN regions of the hetero- atmosphere15,18. The CVD-grown single-layer h-BN was structures. Figure 1d shows the PL spectra of the graphene transferred onto the Pt NPs/SiO2 substrate and etched away (red), h-BN (blue), and the interface between the graphene and on the Pt NPs by annealing at 700 °C in H2 flow (10 sccm). h-BN regions (green) by excitation with a 266 nm laser. Inter- Supplementary Fig. 5b shows an AFM image of a hydrogen- estingly, we observed a small PL peak at 410 nm wavelength etched h-BN film with holes transferred onto a SiO2 substrate. corresponding to the interface, while no PL emission was No PL emission was observed for this sample (as shown in observed in the graphene or h-BN regions. Furthermore, PL Supplementary Fig. 5c). 2 NATURE COMMUNICATIONS | (2020)11:5359 | https://doi.org/10.1038/s41467-020-19181-2 | www.nature.com/naturecommunications NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-19181-2 ARTICLE a c 1800 e 2D band 550 h-BN 1500 Intensity (a.u) 1200 Graphene 900 Graphene 600 h-BN 10 µm 300 10 µm 1200 1600 2000 2400 2800 100 Raman shift (cm–1) d 900 f 410 nm 340 b 870 G/h-BN junction Intensity (a.u) h-BN 840 Graphene 810 780 Graphene, h-BN 200 nm 10 µm 360 390 420 450 480 230 Wavelength (nm) Fig. 1 Characterization and PL analysis of graphene/h-BN in-plane heterostructures. a, b SEM images of in-plane graphene/h-BN heterostructures fabricated by conversion reaction of h-BN on a Pt (111) single crystal. The location of the magnified SEM image, (b), is marked by the blue rectangle in a. c Raman spectra of graphene (red) and h-BN (blue) regions on a SiO2/Si substrate using a 532-nm laser. d PL spectra of graphene (red), h-BN (blue) regions, and interface (green) using a 266-nm laser. e, f Raman and PL mapping images of the 2D band (2630–2730 cm−1) and 410 nm wavelength, respectively, for the area inside the red square in a. c a b 1100 h-BN 1050 GQD/h -BN Intensity (a.u) GQD GQD 1000 950 900 G/h-BN interface 850 GQD GQD GQD 800 750 Pt NP, h-BN 200 nm 360 390 420 450 480 Wavelength (nm) Fig. 2 PL analysis of GQD/h-BN in-plane heterostructures. a Schematic of the GQD/h-BN in-plane heterostructures fabricated by conversion of h-BN on Pt nanoparticles (Pt NPs). b SEM image of the GQD/h-BN heterostructures on a SiO2/Si substrate. c PL spectra of GQD/h-BN (red), Pt NPs (green), and bare h-BN (back). The PL spectrum of G/h-BN interface (blue) is the same spectrum as green one in Fig. 1d. All of samples were measured on a SiO2/Si substrate using 266-nm laser. Localized energy states at disordered G/h-BN boundary: origin photoexcitation (photons excite electrons to a higher energy of PL. We confirmed that the PL emission is not due to the levels). During the photoexcitation and photoemission, there is a intrinsic properties in the GQDs and nano-sized holes of h-BN by local charge redistribution along the interface which is only the absence of PL in control experiments. Actually, it would be possible at the interface. The redistribution is allowed by the difficult to attribute the PL definitely to the 1D graphene/h-BN graphene charge carrier lifetime. Additionally, we also theoreti- heterojunction because of the large area probed with a confocal cally checked the size dependence using graphene/h-BN poly- Raman system. PL from surrounding areas is also measured. crystalline samples with three different average grain sizes: 10, 20, Focusing the laser only on the atomic-resolved interface is very and 40 nm; as shown in Fig. 3a, the three samples show the same challenging in the current PL technique. Based on this, it is dif- two peaks located at −1.2 eV and 2.0 eV, which indicates that ficult to conclusively attribute the observed PL emission to the 1D there is no energy change of the localized state according to the interface. Alternatively, to elucidate the origin of PL, we theore- graphene or h-BN domain size. Thus, we believe that localized tically characterized the local energy states due to the disordered energy states are formed at the disordered G/h-BN heterojunc- G/h-BN junction. First, we set-up an in-plane graphene/h-BN tion, which result in PL emissions with a 400 nm wavelength. Our heterostructures lattice using molecular dynamics, in which there finding for the PL phenomenon at disordered G/h-BN hetero- is an orientation mismatch between the graphene and h-BN like junction and its origin are different from the previously reported in a polycrystalline sample. Due to the mismatch, the G/h-BN light emission in 2D materials caused by (i) doped Pt atoms20–22, junction (typically named grain boundary) is disordered; i.e., it is (ii) hydrogenated edges23,24 in large nanoholes, (iii) atomic formed by a variety of non-hexagonal rings. Second, we set-up a defects like NBVN (nitrogen substitution at boron site + nitrogen tight-binding Hamiltonian to describe the electronic properties vacancy)25,26 and carbon substitution at nitrogen sites27 in h-BN, (see details in the Supplementary Information). We calculate the and (iv) point defects in WSe228–32. Also, note that a slight dif- local density of states projected over all the sites at the interface ference of emission wavelength between theoretical and experi- by using the Kernel Polynomial Method19 to evaluate the Green mental results is due to limitations of a tight-binding function; this shows two characteristic peaks at −1.2 eV and Hamiltonian (TBH) calculation that cannot cover all experi- 2.0 eV (Fig. 3a). The energy separation of these peaks is, theo- mental parameters such as electron-electron interaction effects or retically, characterized by a 390 nm wavelength, which is close to Coulomb screening caused by the generated excitons. To support the experimental PL. The energy states corresponding to −1.2 eV above localized energy states at G/h-BN junctions, we need to and 2.0 eV are mainly located at the disordered G/h-BN hetero- show atomic structures at the disordered G/h-BN junctions. junctions, shown in Fig. 3b, c. PL is light emission after a Thus, we measured annular dark-field scanning tunneling NATURE COMMUNICATIONS | (2020)11:5359 | https://doi.org/10.1038/s41467-020-19181-2 | www.nature.com/naturecommunications 3 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-19181-2 a 10 b 10 nm Density of states of the GB sites [a.u.] E = 2.0 eV Graphene 20 nm 8 40 nm 6 h-BN ~ 390 nm 4 c E - -1.2 eV Graphene 2 0 h-BN -4 -2 0 2 4 E [eV] d e f B C Graphene N 1 nm h-BN 1 nm Fig. 3 Disordered G/h-BN junctions and their electronic properties. a Local density of state projected over all the sites at the graphene/h-BN interface for three different polycrystalline samples with 10 nm, 20 nm and 40 nm average grain size. As reference, dotted line corresponds to the density of states of a polycrystalline h-BN sample to show the energy location of the band gap. b, c Local density of states of each site in a disordered interface at 2.0 eV and −1.2 eV, respectively. It is clear that local density of states is higher at the structural defects of the interface. The units used are arbitrary. d, e Annular dark- field (ADF) STEM image showing a boundary between graphene and h-BN domains (top and bottom, respectively). Blue and orange circles denote 5- and 7-membered rings, respectively. The bright impurity atom found in the red box of e is identified as silicon by EELS, probably coming from the sample transfer process. f STEM-EELS elemental maps of boron, carbon, nitrogen and a possible structure model for the area indicated by a red square in e. electron microcopy (ADF-STEM) images and electron energy loss noting that we realized G/h-BN junctions with the enhanced spectroscopy (EELS) maps at the boundary of h-BN and graphene emission, isolated by h-BN intercalation layers in vertical direc- as shown in Fig. 3d–f and Supplementary Fig. S6. The high- tions. It indicates that the disordered G/h-BN junctions can be a resolution TEM images (Fig. 3d) clearly show the atomic struc- potential component in optoelectronics. As an example, we fab- ture of the interfaces between graphene (top region) and h-BN ricated a blue light emitting device using the GQD/h-BN het- (bottom region), where we observed the disordered boundaries erostructures with the disordered G/h-BN junctions. With only with 5-(blue spheres), and 7-membered rings (yellow spheres) GQD/h-BN heterostructures, it was confirmed that blue electro- consisting of C, B, and N atoms. Therefore, based on the TEM luminescence could be realized in LEDs without any energy results to support our simulated model, we suppose that the blue donor in the emitting layer (see details in the Supplementary emission comes from localized energy states at the disordered Figs. S8–S11). interface region between graphene and h-BN. Discussion Vertically stacked GQD/h-BN layers. To improve the weak PL In summary, we demonstrated blue light-emission at hetero- intensity from G/h-BN junctions, we fabricated the van der Waals junctions between graphene and h-BN domains. The emission heterostructures by vertically stacking GQD/h-BN layers using a likely originates from the localized energy states at the interface layer-by-layer assembly with a wet-transfer method to increase between graphene and h-BN on the in-plane heterostructures. We the amount of G/h-BN junctions (Fig. 4a). Two types of stacked fabricated GQD/h-BN heterostructures and stacked them with h- samples were prepared: one consists of four layers stack of GQD/ BN intercalation layers to increase the total interface length, h-BN films (4L GQD/h-BN) and the other inserts an additional thereby improving PL emission. The stacked 3D heterostructures h-BN intercalation layer between each GQD/h-BN film (4L had the GQD structures isolated in the vertical and planar GQD/h-BN with h-BN). As shown in Fig. 4b, the PL intensity of directions by the h-BN matrix, which improved their emission the 4L GQD/h-BN (red) was reduced compared to that of single- efficiency. This work suggests that the narrow 1D emission at layer GQD/h-BN film (black). This indicates that the PL heterojunctions of in-plane graphene/h-BN heterostructures quenching still occurs in a structure that simply stacks GQD/h- could be used for future optoelectronic devices. BN films due to photon reabsorption and nonradiative energy transfer between GQDs in the vertical direction33,34. However, Methods utilizing h-BN intercalation layers substantially improved the PL Growth of simple in-plane graphene/h-BN heterostructures. The in-plane intensity since the charges generated on the GQD/h-BN film were heterostructure of graphene and h-BN was prepared by a catalytic conversion not transported to GQDs in adjacent layers, blocked by the h-BN reaction of h-BN to graphene on a Pt metal substrate17. A single layer of h-BN was charge barrier (schematically shown in Fig. 4c). Although the synthesized on a Pt (111) single crystal using ammonia borane as a precursor by CVD method35. The resulting complex, h-BN/Pt(111), was then loaded into the intensity of 4L GQD/h-BN with h-BN does not improve to 4 center of a vacuum quartz tube placed in a furnace. The tube was pumped down to times that of the single-layer GQD/h-BN due to the reabsorption 0.21 torr with pure argon gas (50 sccm), and then heated to 1000 °C at a steady rate problem (see details in the Supplementary Fig. S7), it is worth for over 40 min. The reaction was initiated with a flow of methane (5 sccm) and 4 NATURE COMMUNICATIONS | (2020)11:5359 | https://doi.org/10.1038/s41467-020-19181-2 | www.nature.com/naturecommunications NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-19181-2 ARTICLE a GQDs embedded in h-BN b (GQD/h -BN) 4L GQD/h-BN with 3L h-BN sheets - 4L GQD/h-BN with h-BN - 1L GQD/h-BN - 4L GQD/h-BN Normalized intensity (a.u) 3L h-BN Improving 4L GQD/h -BN Quenching 360 390 420 450 480 Wavelength (nm) c GQD/GQD structure GQD/h-BN/GQD structure Inserting h+ h+ e- e- h+ h+ e- e- h-BN h+ e- Energy transfer Fig. 4 Vertically stacked GQD/h-BN layers with h-BN barriers. a Schematic of the preparation process for two kinds of stacked GQD/h-BN films. b PL spectra of stacked GQD/h-BN on a SiO2/Si substrate: a four layer stack of GQD/h-BN films (4L GQD/h-BN, red spectrum), 4L GQD/h-BN with an additional h-BN intercalation layer between each GQD/h-BN film (4L GQD/h-BN with h-BN, blue spectrum), and a single layer of GQD/h-BN film (black spectrum, same as the red spectrum in Fig. 2d). c Schematic of the nonradiative charge transfer between GQDs in adjacent layers. argon (50 sccm) gases to produce the in-plane heterostructures with a shorter tin oxide (ITO) (150 nm), poly(3,4-ethylenedioxythiopehe):poly(styrene sulfo- 10 min reaction time. nate) (PEDOT:PSS) (40 nm), poly(N-vinylcarbazole) (PVK) (20 nm), an emit- ting layer, 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBI) (60 nm), lithium fluoride (LiF) (1 nm), and Al (100 nm). A glass substrate pre- Growth of graphene quantum dots embedded in a h-BN sheet. A Pt nano- deposited with a 150 nm thick ITO electrode (half-etched) was cleaned in an particle (NP) array on a SiO2 substrate was prepared using self-patterning diblock ultrasonic bath using de-ionized (DI) water, acetone, and isopropyl alcohol, and copolymer micelles36. A single-layer of polystyrene-block-poly(4-vinylpyridine) then it was treated with air plasma using a plasma cleaner (PDC-32G, Harrick (PS-P4VP) micelles with H2PtCl6, a precursor of Pt NPs, in their cores was spin- Plasma). The PEDOT:PSS (CLEVIOS P VP AI 4083) was deposited by spin- coated on the SiO2 substrate. To fabricate the Pt NP array, the micellar film on the coating (3000 rpm for 40 s) and annealed at 150 °C for 30 min. Then, the PVK SiO2 was annealed at 400 °C for 30 min in air. The h-BN grown on Pt substrate (Mn: 25,000–50,000) was deposited onto the PEDOT:PSS layer by spin-coating were transferred onto the Pt NPs/SiO2 substrate using a wet-transfer method (1500 rpm for 40 s) using a PVK solution (15 mg/mL in chlorobenzene) and (electrochemical delamination). Then, the sample was loaded into the center of a annealed at 120 °C for 20 min. The fabricated GQD/h-BN was transferred onto vacuum quartz tube in a furnace for the conversion reaction. The tube was pumped the top of the PVK by using the wet-transfer method. The samples with PVK or down to 0.21 Torr with pure Argon gas (50 sccm). Then the furnace was heated to GQD/h-BN were loaded into a thermal evaporator (HS-1100, Digital Optics & 950 °C in 40 min. When the reaction started, methane gas (5 sccm) with Argon Vacuum) for deposition of the TPBI and LiF/Al under high vacuum (50 sccm) was flown as the source for graphene growth. During the reaction, the h- (~10−6 Torr). The current density-luminance-voltage characteristics of the BN region on the Pt NPs was converted to graphene, and after 10 min of growth a fabricated LED devices were evaluated using a Keithley 2400 source meter and a uniform GQD array embedded in the h-BN film was obtained15. calibrated photodiode (FDS100, Thorlab). The EL spectra were measured by a fiber-optic spectrometer (EPP2000, StellarNet). All measurement processes were Transfer of GQD/h-BN films to other substrates. The GQD/h-BN film on Pt performed in an inert environment. NPs/SiO2 was transferred onto other substrates via a wet-transfer method using HF and an aqua regia solution. First, polystyrene (PS) was spin-coated on the sample, and it was immersed in a HF solution (5% in DI water) to remove the SiO2 layer. Data availability Then, the floating PS film was transferred to the aqua regia solution (3:1 mixture of All data that support the findings of this study are present in the paper and the hydrochloric acid and nitric acid) to remove the Pt NPs. Finally, the film was supplementary materials, and additional data are available from the corresponding transferred onto the desired substrate and the PS film was removed with toluene to author upon reasonable request. obtain a GQD/h-BN film on the substrate. Code availability Characterization. Scanning electron microscopy (Verios 460, FEI) and atomic The codes that are used to generate results in this paper are available from the force microscopy (Dimension Icon, Bruker) were used to determine the surface corresponding author upon reasonable request. morphology of the samples. PL and Raman spectra were measured using a micro Raman spectroscope (alpha 300, WITec GmbH) with 266-nm and 532-nm lasers, respectively. The h-BN/graphene heterostructure sample was transferred to a Received: 19 March 2020; Accepted: 1 October 2020; microporous silicon nitride membrane TEM grid (Alliance Biosystems, Inc.) for scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy (EELS). JEOL JEM-2100F electron microscope equipped with JEOL Delta spherical aberration correctors and Gatan Quantum electron spectrometer was operated at an electron accelerating voltage of 60 kV. EELS chemical maps of boron, carbon and nitrogen were obtained by measuring their K References edge signals at each point of a scanned area. 1. Li, M.-Y. et al. Epitaxial growth of a monolayer WSe2-MoS2 lateral p-n junction with an atomically sharp interface. Science 349, 524–528 (2015). Fabrication of GQD/h-BN-based LED device and evaluation. A GQD-based 2. Duan, X. et al. 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