Blue emission at atomically sharp 1D heterojunctions between graphene and h-BN

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 heterojunction 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.


Fabrication of bare GQDs array without h-BN
In order to check PL emission on bare GQD without h-BN, the sample was prepared using a single-layer graphene grown on Pt foil and O 2 plasma etching process, shown in Figure S3a.
First, the graphene monolayer grown on the Pt foil is transferred onto a SiO 2 /Si substrate by electrochemical delamination method. After aligning the Pt NPs array on the top of graphene by self-assembly process 1 , O 2 plasma etching is performed on the sample at 50 W for 1 min, and in the graphene etching process, Pt NPs array is used as a pattern mask. After the reaction, Pt NPs are removed with aqua regia solution.

Supplementary Figure 4 | Characterizations of GQDs array without h-BN matrix. a,
Scheme of bare GQDs prepared by the O 2 plasma treatment of CVD grown graphene. b, SEM image of a bare GQDs (~7 nm) array prepared in (a). c, PL spectra of bare GQD (red spectrum). 6

Fabrication of h-BN sheet with nano-sized holes
The h-BN sheet with nano-sized holes was prepared by using hydrogen-etching of h-BN 2 though the annealing process on Pt NPs in H 2 atmosphere. First, after aligning the Pt NPs array on the SiO 2 /Si substrate by self-assembly process 1

Theoretical model
In this section, we detail the models of the disordered interfaces between graphene and hBN in-plane heterostructures. Disordered interfaces are found when there is an orientation mismatch between graphene and hBN crystals. Thus, we set-up three different polycrystalline lattices with graphene and hBN crystals of 10 nm, 20 nm and 40 nm average grain size. The polycrystalline samples with average grains size were created using a Voronoi diagram. Each Voronoi cell was filled with randomly oriented honeycomb crystals. In order to achieve a thermodynamically stable structure we annealed the lattice using molecular dynamics as implemented in LAMMPS, with the parameters found in reference. 4 We calculated the electronic properties using a nearest-neighbor tight-binding Hamiltonian fitted to a Wannierization of DFT calculations. 4 The local density of states projected over all the sites at the graphene-hBN interface (grain boundary sites, GB) was calculated by evaluating the imaginary part of the Green function using the Kernel Polynomial Method with 3000 moments and the Lorentz kernel. 5

Vertically stacking of GQD/h-BN layers with h-BN intercalation layers
It is known that the stacks of GQDs in the solid state induced photon reabsorption and nonradiative energy transfer which indicates partial PL quenching. 6,7 That is, at the stacked sample, the excited electrons of GQDs may nonradiatively relax to ground states through couplings with neighboring ones (by reabsorption and energy transfer). We solved the issue of nonradiative energy transfer by inserting 3L h-BN as an intercalation layer between GQD/h-BN layers, but the generated photons from GQD/h-BN can be still re-absorbed by adjacent layers. However, note that 3L h-BN as an intercalation layer may not fully prevent the nonradiative energy transfer because charge tunneling through 3L h-BN may occur still.

Comparing the PL emission in 1 layer-, 2 layers-, 3 layers-, and 4 layers-stacked GQD/h-BN
films with h-BN intercalation layers, the extent of enhanced PL intensity (blue solid line) did not increase by 2, 3, or 4 times (red dotted line) proportionally as shown in Supplementary Fig.   S7.

GQD/h-BN layers in an OLED device
The device architecture is shown in Supplementary Fig. S8a, with a simple structure 8,9 consisting of ITO/PEDOT:PSS/PVK/GQD/TPBI/LiF/Al (Supplementary Fig. S9). The PEDOT:PSS, PVK, and TPBI were used as hole injection, hole transport (HTL), and electron transport layers (ETL), respectively. Supplementary Fig. S8b shows the electronic band structure of the device. Note that the reference device is fabricated without GQD/h-BN while having the same structure as above. The current density-voltage curves for two devices of GQD/h-BN and reference ( Supplementary Fig. S8c) show that the turn-on voltage decreases from 6 V to 4 V when the GQD/h-BN with G/BN junctions is used as the emitting layer in the device. Moreover, a higher current density at a lower voltage (~5 V) was observed in the device with GQD/h-BN. This indicates that the G/BN junctions in the GQD/h-BN heterostructures may provide additional carrier transport, resulting in enhancement of the overall current density.
The normalized EL spectrum (red) of the GQD/h-BN device in Supplementary Fig. S8d shows blue emission with a peak wavelength at 410 nm, which is consistent with the PL spectrum of GQD/h-BN with G/BN junctions (Fig. 2c). However, the reference device without the GQD/h-BN emitting layer showed the broader emission peak at 430 nm ( Supplementary Fig. S8d, black spectrum). The external quantum efficiency (EQE) of our best GQD/h-BN device was 0.5% ( Supplementary Fig. 10).

External Quantum Efficiency (EQE) of the GQD/h-BN devices
The EQE value was calculated by using the following formula.