I. INTRODUCTION

Section:

ChooseTop of pageABSTRACTI. INTRODUCTION <<II. SYNTHESIS OF 2D III-N...III. EMERGING APPLICATION...IV. CONCLUSIONTwo-dimensional (2D) semiconductors, such as transitional metal dichalcogenides (TMDs), exhibit unique electronic, optical, excitonic, and quantum properties and correlations1–101. A. K. Geim and I. V. Grigorieva, Nature 499, 419–425 (2013). https://doi.org/10.1038/nature123852. J. S. Ross, P. Klement, A. M. Jones, N. J. Ghimire, J. Yan, D. G. Mandrus, T. Taniguchi, K. Watanabe, K. Kitamura, W. Yao, D. H. Cobden, and X. Xu, Nat. Nanotechnol. 9, 268–272 (2014). https://doi.org/10.1038/nnano.2014.263. S. Wu, S. Buckley, J. R. Schaibley, L. Feng, J. Yan, D. G. Mandrus, F. Hatami, W. Yao, J. Vuckovic, A. Majumdar, and X. Xu, Nature 520, 69–72 (2015). https://doi.org/10.1038/nature142904. M. Bernardi, M. Palummo, and J. C. Grossman, Nano Lett. 13, 3664–3670 (2013). https://doi.org/10.1021/nl401544y5. O. Lopez-Sanchez, D. Lembke, M. Kayci, A. Radenovic, and A. Kis, Nat. 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Kolobov, P. Fons, J. Tominaga, B. Hyot, and B. Andre, Nano Lett. 16, 4849–4856 (2016). https://doi.org/10.1021/acs.nanolett.6b0122549. D. Kecik, A. Onen, M. Konuk, E. Gurbuz, F. Ersan, S. Cahangirov, E. Akturk, E. Durgun, and S. Ciraci, Appl. Phys. Rev. 5, 011105 (2018). https://doi.org/10.1063/1.499037750. M. C. Lucking, W. Xie, D. H. Choe, D. West, T. M. Lu, and S. B. Zhang, Phys. Rev. Lett. 120, 086101 (2018). https://doi.org/10.1103/PhysRevLett.120.08610151. A. Onen, D. Kecik, E. Durgun, and S. Ciraci, Nanoscale 10, 21842–21850 (2018). https://doi.org/10.1039/C8NR05626A52. A. V. Kolobov, P. Fons, Y. Saito, J. Tominaga, B. Hyot, and B. André, Phys. Rev. Mater. 1, 024003 (2017). https://doi.org/10.1103/PhysRevMaterials.1.024003 have predicted that group III-nitride materials can adopt a stable graphite-like hexagonal crystal structure in their monolayer limit, exhibiting tunable energy bandgap from 2 to 9 eV. The recent theory also predicted that by exploiting the strong Coulomb interaction and excitons in monolayer III-nitrides, conventional low-efficiency AlGaN and indirect-bandgap hBN can be turned into high-brightness deep UV emitters,53,5453. A. Aiello, Y. Wu, A. Pandey, P. Wang, W. Lee, D. Bayerl, N. Sanders, Z. Deng, J. Gim, and K. Sun, Nano Lett. 19, 7852 (2019). https://doi.org/10.1021/acs.nanolett.9b0284754. D. Bayerl and E. Kioupakis, Appl. Phys. Lett. 115, 131101 (2019). https://doi.org/10.1063/1.5111546 which are the only alternative technology to replace mercury lamps for water and air purification. To date, however, it has remained difficult to achieve 2D III-nitrides beyond small-scale domains. Consequently, many fundamental optical, electrical, excitonic, and quantum properties of 2D III-nitrides have remained unknown. Therefore, the design, discovery, and development of 2D III-nitride nanostructures with innovative electronic, optical, excitonic, and quantum properties will enable a broad spectrum of applications, from high-efficiency light absorbers5555. Q. Wen, Y. Wu, P. Wang, D. Laleyan, D. Bayerl, E. Kioupakis, Z. Mi, and M. Kira, Appl. Phys. Lett. 116, 181103 (2020). https://doi.org/10.1063/5.0004119 or light harvesting, deep UV light emitters for water/air purification, and room temperature single photon sources, to architectures for excitonic optoelectronics.

In this Perspective, we discuss the recent developments of next generation 2D III-nitride semiconductors, including monolayer InN, GaN, AlN, and hBN. The quantum theory and design, epitaxy, and characterization are presented. Their applications in achieving efficient light generation in the deep UV and room-temperature single photon emitters are also described. This Perspective provides a broad overview on the synthesis of 2D III-nitrides with desired properties for applications in next-generation optoelectronic, excitonic, and quantum devices.

II. SYNTHESIS OF 2D III-NITRIDES

Section:

ChooseTop of pageABSTRACTI. INTRODUCTIONII. SYNTHESIS OF 2D III-N... <<III. EMERGING APPLICATION...IV. CONCLUSIONTo date, there is a significant gap between the theoretical prediction of 2D III-nitrides and the experimental realization of such structures. Although GaN-based 3D crystal structures have been extensively studied for LED lighting and power electronic applications, the epitaxy/synthesis of 2D III-nitrides is facing tremendous challenges. Depending on the preferred orbital hybridization configuration, III-nitrides can be divided into two categories. The sp2 hybridized materials (such as hBN)56,5756. A. Maity, S. J. Grenadier, J. Li, J. Y. Lin, and H. X. Jiang, Prog. Quantum Electron. 76, 100302 (2021). https://doi.org/10.1016/j.pquantelec.2020.10030257. H. Prevost, A. Andrieux-Ledier, N. Dorval, F. Fossard, J. S. Mérot, L. Schué, A. Plaud, E. Héripré, J. Barjon, and A. Loiseau, 2D Mater. 7, 045018 (2020). https://doi.org/10.1088/2053-1583/aba8ad have mixed in-plane ionic/covalent bonding and primary van der Waals (vdW) interaction between layers along the [0001] direction. The weak vdW interlayer interaction enables the mechanical exfoliation of 2D hBN.5858. C. R. Dean, A. F. Young, I. Meric, C. Lee, L. Wang, S. Sorgenfrei, K. Watanabe, T. Taniguchi, P. Kim, K. L. Shepard, and J. Hone, Nat. Nanotechnol. 5, 722–726 (2010). https://doi.org/10.1038/nnano.2010.172 The sp3 hybridized materials (such as InN, GaN, AlN, and their ternary and quaternary alloys) feature strong covalent/ionic bonding along all crystallographic directions,5959. J. Wu, J. Appl. Phys. 106, 011101 (2009). https://doi.org/10.1063/1.3155798 making it almost impossible to mechanically exfoliate 2D nitrides from their bulk format. Moreover, it remains challenging to directly thin down GaN bulk to a few atomic layers by cleaving the tetrahedrally bonded surface along the (0001) plane.6060. N. Sanders, D. Bayerl, G. Shi, K. A. Mengle, and E. Kioupakis, Nano Lett. 17, 7345–7349 (2017). https://doi.org/10.1021/acs.nanolett.7b03003The hybridization configuration also affects the nucleation and crystal growth of III-nitrides during epitaxial growth. For example, the surface adatoms tend to be adsorbed and nucleate at the propagating edges of hBN, which leads to the formation of a layered 2D structure.6161. J. Dong, L. Zhang, X. Dai, and F. Ding, Nat. Commun. 11, 5862 (2020). https://doi.org/10.1038/s41467-020-19752-3 Wurtzite III-nitrides growth is essentially a 3D process with growth rate along different crystallographic directions determined by the surface energy anisotropies,6262. V. Consonni, M. Knelangen, L. Geelhaar, A. Trampert, and H. Riechert, Phys. Rev. B 81, 085310 (2010). https://doi.org/10.1103/PhysRevB.81.085310 which makes it very difficult to realize only 2D growth in wurtzite III-nitrides. In addition, it is desired that the 2D structure can be separated from the epitaxial substrate and, therefore, can be transferred to arbitrary or functionalized substrates to facilitate subsequent device processing, which puts another demand on the material synthesis. Some of the major developments in the growth/synthesis 2D III-nitrides are briefly described as follows.

A. Monolayer III-nitrides

Monolayer III-nitrides are essentially quantum wells consisting of one atomic layer thin materials. By exploiting Coulomb engineering and excitons, Kioupakis et al. predicted that the radiative efficiency could be enhanced by up to three orders of magnitude due to the extreme quantum confinement, which reduces the polarization-induced separation of electrons and holes and enhances the Coulomb interaction.54,5554. D. Bayerl and E. Kioupakis, Appl. Phys. Lett. 115, 131101 (2019). https://doi.org/10.1063/1.511154655. Q. Wen, Y. Wu, P. Wang, D. Laleyan, D. Bayerl, E. Kioupakis, Z. Mi, and M. Kira, Appl. Phys. Lett. 116, 181103 (2020). https://doi.org/10.1063/5.0004119 Monolayer III-nitrides present an unexplored opportunity for realizing high-efficiency deep UV emission, which is critical for water/air purification and disinfection.6363. S. M. Sadaf, S. Zhao, Y. Wu, Y. H. Ra, X. Liu, S. Vanka, and Z. Mi, Nano Lett. 17, 1212–1218 (2017). https://doi.org/10.1021/acs.nanolett.6b05002 Experimentally, significantly enhanced Coulomb interaction can be achieved through quantum confinement between materials with large band offsets, wherein ultrawide bandgap Al(Ga)N materials are typically utilized as the barrier. The large band offsets ensure the confinement of charge carriers within the quantum wells.6464. Y. Wu, X. Liu, P. Wang, D. A. Laleyan, K. Sun, Y. Sun, C. Ahn, M. Kira, E. Kioupakis, and Z. Mi, Appl. Phys. Lett. 116, 013101 (2020). https://doi.org/10.1063/1.5124828 Extensive efforts have been devoted to achieving embedded ultrathin (In)GaN in the epilayer structure.65–6965. V. Davydov, E. M. Roginskii, Y. Kitaev, A. Smirnov, I. Eliseyev, E. Zavarin, W. Lundin, D. Nechaev, V. Jmerik, M. Smirnov, M. Pristovsek, and T. Shubina, Nanomaterials 11, 2396 (2021). https://doi.org/10.3390/nano1109239666. C. Liu, Y. K. Ooi, S. M. Islam, H. Xing, D. Jena, and J. Zhang, Appl. Phys. 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Mater. 28, 7978–7983 (2016). https://doi.org/10.1002/adma.201600990 However, interdiffusion of cations between the quantum well and barrier is often observed,70,7170. V. Jmerik, D. Nechaev, K. Orekhova, N. Prasolov, V. Kozlovsky, D. Sviridov, M. Zverev, N. Gamov, L. Grieger, Y. Wang, T. Wang, X. Wang, and S. Ivanov, Nanomaterials 11, 2553 (2021). https://doi.org/10.3390/nano1110255371. S. M. Islam, K. Lee, J. Verma, V. Protasenko, S. Rouvimov, S. Bharadwaj, H. Xing, and D. Jena, Appl. Phys. Lett. 110, 041108 (2017). https://doi.org/10.1063/1.4975068 which makes the quantum confinement and the Coulomb enhancement effect elusive.We have demonstrated the epitaxy of monolayer GaN embedded in AlN nanowires grown directly on Si substrate.64,7264. Y. Wu, X. Liu, P. Wang, D. A. Laleyan, K. Sun, Y. Sun, C. Ahn, M. Kira, E. Kioupakis, and Z. Mi, Appl. Phys. Lett. 116, 013101 (2020). https://doi.org/10.1063/1.512482872. J. Stachurski, S. Tamariz, G. Callsen, R. Butté, and N. Grandjean, Light Sci Appl. 11(14), 114 (2022). https://doi.org/10.1038/s41377-022-00799-4 The epitaxy was performed under nitrogen-rich conditions. Figure 2(a) shows the scanning transmission electron microscopy (STEM) image of the as-grown heterostructure, wherein a monolayer of GaN in AlN matrix with relatively sharp interfaces can be identified. The significantly improved interface quality compared with ultrathin GaN/AlN grown in epilayer structure can be attributed to the nitrogen-rich growth condition utilized for N-polar nanostructures. During molecular beam epitaxy (MBE), a metal-rich growth regime is generally used to ensure a smooth surface; however, atomic steps tend to be formed in this growth regime, which causes discontinuity in monolayer GaN.7070. V. Jmerik, D. Nechaev, K. Orekhova, N. Prasolov, V. Kozlovsky, D. Sviridov, M. Zverev, N. Gamov, L. Grieger, Y. Wang, T. Wang, X. Wang, and S. Ivanov, Nanomaterials 11, 2553 (2021). https://doi.org/10.3390/nano11102553 Under nitrogen-rich growth conditions, the growth front of the nanowires is free from excess metal adatoms, which can effectively prevent interfacial mixing due to variations in incorporation preferences of surface metal adatoms.With interfacial quality significantly improved, the optical properties of ultrathin GaN can be readily tuned by varying the thickness/growth duration. As shown in Fig. 2(b), monolayer GaN embedded in AlN matrix exhibits strong emission at 240 nm, whereas two monolayer corresponds to 270 nm emission. Theoretical and experimental results also showed that the electronic and optical gap both increase with increasing the AlN barrier thickness [Fig. 2(c)]. The measured photon energies match well with the optical gap. The difference between the electronic and optical gap results in an exciton binding energy of 230 meV, which is well above the thermal energy at room temperature and is about one order of magnitude larger than that of bulk GaN. The enhanced exciton binding energy essentially results from enhanced Coulomb interactions of the electron–hole pairs confined in the ultrathin quantum well. Beyond an excitation power of 0.3 W/cm2, the measured lifetime is relatively invariant with increasing excitation power [Fig. 2(d)], which agrees well with the features of exciton-related emission.7272. J. Stachurski, S. Tamariz, G. Callsen, R. Butté, and N. Grandjean, Light Sci Appl. 11(14), 114 (2022). https://doi.org/10.1038/s41377-022-00799-4

B. Graphene encapsulation

In 2016, Balushi et al. reported the growth of 2D GaN using graphene encapsulation.7373. Z. Y. Al Balushi, K. Wang, R. K. Ghosh, R. A. Vila, S. M. Eichfeld, J. D. Caldwell, X. Qin, Y. C. Lin, P. A. DeSario, G. Stone, S. Subramanian, D. F. Paul, R. M. Wallace, S. Datta, J. M. Redwing, and J. A. Robinson, Nat. Mater. 15, 1166–1171 (2016). https://doi.org/10.1038/nmat4742 In this process, a graphene capping layer is used to stabilize the 2D GaN layer and promote Frank–van der Merwe growth. The 2D GaN layer is formed between graphene and the underlying SiC substrate, as shown in Fig. 3(a1). A direct energy bandgap of 5 eV was measured, in good agreement with theory. The obtained 2D GaN has an R3m space group and covalently bonds with the epitaxial substrate as shown in Fig. 3(a2), making it almost impossible to separate the 2D GaN from the substrate. Such a method has also been used to realize 2D InN and 2D AlN recently.74,7574. W. Wang, Y. Zheng, X. Li, Y. Li, H. Zhao, L. Huang, Z. Yang, X. Zhang, and G. Li, Adv. Mater. 31, e1803448 (2019). https://doi.org/10.1002/adma.20180344875. B. Pecz, G. Nicotra, F. Giannazzo, R. Yakimova, A. Koos, and A. Kakanakova-Georgieva, Adv. Mater. 33, e2006660 (2021). https://doi.org/10.1002/adma.202006660 In these studies, the formation of 2D III-nitride layers was made possible by Ga (or Al) adatom intercalation through defects in the graphene layer. Consequently, the growth of 2D III-nitride layers is extremely sensitive to the defect distribution in graphene. In addition, this growth process results in the extensive formation of 3D III-nitrides on graphene, leading to a mixture of 3D and 2D structures. It is difficult to achieve uniform 2D III-nitrides beyond small-scale domains. To date, there have been no experimental studies of photoluminescence emission, charge carrier transport, and excitonic properties.

C. Surface-confined, metal-assisted growth

In 2018, Chen et al. showed that free-standing 2D wurtzite GaN can be synthesized on liquid gallium via a surface-confined nitridation reaction in a chemical vapor deposition chamber.7676. Y. Chen, K. Liu, J. Liu, T. Lv, B. Wei, T. Zhang, M. Zeng, Z. Wang, and L. Fu, J. Am. Chem. Soc. 140, 16392–16395 (2018). https://doi.org/10.1021/jacs.8b08351 In this process, the liquid gallium droplet is spread on a tungsten (W) foil, and urea is supplied as a precursor for the nitrogen source. The ultrathin GaN is also reflected by the low-magnification transmission electron microscopy (TEM) image, as shown in Fig. 3(b1). The wurtzite crystal structure is confirmed by the high-magnification TEM images, as shown in Figs. 3(b2) and 3(b3). The successful synthesis of 2D GaN was attributed to the stratified Ga/Ga-W structure, wherein an ultrathin liquid Ga layer exists at the growth front and participates in the growth of 2D GaN with micrometer size. At the growth temperature of 1080 °C, the Ga-W under the surficial Ga layer remains in a solid state and prevents thickening of the as-grown 2D GaN as excess N adatoms at the surface are deprived by the W atom due to its stronger nitridation ability. The obtained 2D GaN shows uniform lattice parameters and superior optical quality, including blue-shifted photoluminescence and enhanced internal quantum efficiency. More importantly, the 2D GaN can be transferred onto an SOI substrate, and a field effect transistor based on the 2D GaN was demonstrated. It is worth noting that 2D III-nitrides have much larger bandgap compared with 2D TMDs, and the large bandgap can potentially enable high on/off ratio in switching devices and is ideally suited for deep UV optoelectronics. Based on this synthesis method, ammonolysis of liquid Ga was also reported to obtain 2D GaN with a thickness of 1.3 nm.7777. N. Syed, A. Zavabeti, K. A. Messalea, E. Della Gaspera, A. Elbourne, A. Jannat, M. Mohiuddin, B. Y. Zhang, G. Zheng, L. Wang, S. P. Russo, E. Dorna, C. F. McConville, K. Kalantar-Zadeh, and T. Daeneke, J. Am. Chem. Soc. 141, 104–108 (2019). https://doi.org/10.1021/jacs.8b11483

D. Selective thermal evaporation

In 2019, Wang et al. reported the fabrication of atomically thin AlN by combining epitaxial casting with selective thermal evaporation.7878. P. Wang, T. Wang, H. Wang, X. Sun, P. Huang, B. Sheng, X. Rong, X. Zheng, Z. Chen, Y. Wang, D. Wang, H. Liu, F. Liu, L. Yang, D. Li, L. Chen, X. Yang, F. Xu, Z. Qin, J. Shi, T. Yu, W. Ge, B. Shen, and X. Wang, Adv. Funct. Mater. 29, 1902608 (2019). https://doi.org/10.1002/adfm.201902608 The first step of this synthesis process is epitaxial casting. GaN/AlN core/shell nanowires were vertically grown on a Si substrate using a plasma-assisted molecular beam epitaxy (MBE) system. The thickness of AlN shell was controlled by the growth duration. The second step is selective thermal evaporation. The as-grown GaN/AlN core/shell nanowires ensemble was dispersed on a hydrophilic Si substrate (or other high-temperature resistant hydrophilic substrate, such as sapphire or SiC), then loaded into an ultrahigh vacuum MBE chamber, and performed a thermal evaporation at a temperature higher than the GaN decomposition temperature but lower than the AlN decomposition temperature, as schematically shown in Fig. 3(c1). Due to the large difference in the decomposition activation energy for GaN and AlN, the GaN core nanowire was decomposed into Ga and N and further evaporated from the bottom open end. Therefore, only the robust AlN shell survived after the high-temperature thermal treatment, forming atomically thin AlN nanotubes as confirmed by the high-magnification TEM images [Fig. 3(c2)]. The bandgap of such quasi-2D AlN was characterized by using x-ray photoelectron spectroscopy and spatially resolved electron energy loss spectroscopy measurements, showing an extremely large bandgap of 9.2 ± 0.1 eV for the AlN nanotubes with a wall thickness of two monolayers.7878. P. Wang, T. Wang, H. Wang, X. Sun, P. Huang, B. Sheng, X. Rong, X. Zheng, Z. Chen, Y. Wang, D. Wang, H. Liu, F. Liu, L. Yang, D. Li, L. Chen, X. Yang, F. Xu, Z. Qin, J. Shi, T. Yu, W. Ge, B. Shen, and X. Wang, Adv. Funct. Mater. 29, 1902608 (2019). https://doi.org/10.1002/adfm.201902608 This bandgap value is consistent with the bandgap reported for the 2D AlN grown using the graphene encapsulation method.7474. W. Wang, Y. Zheng, X. Li, Y. Li, H. Zhao, L. Huang, Z. Yang, X. Zhang, and G. Li, Adv. Mater. 31, e1803448 (2019). https://doi.org/10.1002/adma.201803448

E. Ultrahigh temperature molecular beam epitaxy of monolayer hBN

Interestingly, hBN is both a structurally compatible material with Al(Ga)N and an ideal substrate for the emerging 2D TMD materials.58,7958. C. R. Dean, A. F. Young, I. Meric, C. Lee, L. Wang, S. Sorgenfrei, K. Watanabe, T. Taniguchi, P. Kim, K. L. Shepard, and J. Hone, Nat. Nanotechnol. 5, 722–726 (2010). https://doi.org/10.1038/nnano.2010.17279. E. Zdanowicz, A. P. Herman, K. Opołczyńska, S. Gorantla, W. Olszewski, J. Serafińczuk, D. Hommel, and R. Kudrawiec, ACS Appl. Mater. Interfaces 14(4), 6131–6137 (2022). https://doi.org/10.1021/acsami.1c20352 Previous studies of hBN have been largely focused on the use of mechanical exfoliation8080. A. Pakdel, Y. Bando, and D. Golberg, Chem. Soc. Rev. 43, 934–959 (2014). https://doi.org/10.1039/C3CS60260E or chemical vapor deposition,81,8281.  H. X. Jiang and J. Y. Lin, Patent No. 0268-1242 (2014).82. Y. Gao, W. Ren, T. Ma, Z. Liu, Y. Zhang, W.-B. Liu, L.-P. Ma, X. Ma, and H.-M. Cheng, ACS Nano 7, 5199–5206 (2013). https://doi.org/10.1021/nn4009356 which suffers from poor interface properties, high concentrations of unintentional impurity incorporation, and large densities of defects. As an opportunity for improving hBN quality, MBE offers ultimate control down to the atomic scale. However, the conventional MBE growth of BN has been limited by the lack of suitable techniques to evaporate boron source material (melting temperature 2076 °C), and the relatively low substrate heater temperature capabilities (often limited to below 1000 °C) to achieve enhanced surface migration of B atoms. We have recently implemented an unconventional MBE system, which is equipped with an ultrahigh temperature heater (up to 1800 °C) and a retrofitted multi-pocket e-beam source to provide stable thermal evaporation of ultrahigh purity elemental boron. We have demonstrated an hBN/AlN based light-emitting diode (LED), which can operate efficiently at 210 nm.8383. D. A. Laleyan, S. R. Zhao, S. Y. Woo, H. N. Tran, H. B. Le, T. Szkopek, H. Guo, G. A. Botton, and Z. T. Mi, Nano Lett. 17, 3738–3743 (2017). https://doi.org/10.1021/acs.nanolett.7b01068While 2D wurtzite III-nitrides and hBN have been demonstrated using various synthesis methods, there has been a growing interest to control over the nucleation and crystallinity of the 2D nitrides for functional device application. The lack of controllability originates from the complex nucleation environment, wherein the epitaxial substrate surface consists of atomic steps with different heights, crystalline edges with different interfacial energies, and polycrystalline domains with various orientations, which prevent a precise control of the synthesis of 2D nitrides. Recently, we have developed a scalable and controllable synthesis of monolayer 2D hBN on graphene by controlling the initial nucleation process.8484. P. Wang, W. Lee, J. P. Corbett, W. H. Koll, N. M. Vu, D. A. Laleyan, Q. Wen, Y. Wu, A. Pandey, J. Gim, D. Wang, D. Y. Qiu, R. Hovden, M. Kira, J. T. Heron, J. A. Gupta, E. Kioupakis, and Z. Mi, Adv. Mater. 34, e2201387 (2022). https://doi.org/10.1002/adma.202201387 As schematically shown in Fig. 4(a), the surface of graphene substrate has two kinds of interfaces, namely, zigzag (ZZ) atomic edges and armchair (AC) edges. In an uncontrolled epitaxy, hBN nucleates on both interfaces, causes random interfaces, and prevents obtaining large-scale unidirectional hBN with single crystallinity. For controlled synthesis, the nucleation at the ZZ atomic edges of graphene is significantly suppressed. This is achieved by performing epitaxy at an ultrahigh temperature (1600 °C), wherein only nucleation at energetically stable interfaces exists. Subsequently, hBN exclusively grows at AC atomic edges, which results in uniform, faultless, and wafer scale epitaxy of 2D hBN, as shown in Fig. 4(b). The controlled synthesis of monolayer 2D hBN offers a promising opportunity to explore the indirect-to-direct bandgap crossover in hBN.8585. K. A. Mengle and E. Kioupakis, APL Mater. 7, 021106 (2019). https://doi.org/10.1063/1.5087836Figure 4(c) shows the spectra collected from the monolayer hBN/HOPG sample measured at 12 K. The 5.86 eV peak is assigned to the multilayer hBN. The 6.12 eV peak matches well with the dip in the reflection spectrum measured at 300 K and can be assigned to the monolayer emission from hBN. Our first-principles calculations based on the density functional theory and the many-body perturbation theory results showed that free-standing monolayer hBN has a predicted bandgap of 8 eV. We found that the graphene layer adjacent to the hBN layer can cause giant bandgap renormalization of up to 1 eV, attributed to the metallic nature of graphene. This metallic nature can cause extreme screening of Coulomb interaction of charge carriers within hBN. For free-standing monolayer hBN, the calculated exciton binding energy can reach 2.3 eV, as shown in