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ACS Nano
ACS Nano
nn
ancac3
ACS Nano
1936-0851
1936-086X
American Chemical Society

39163482
10.1021/acsnano.4c03640
Article
Atomic and Electronic Structure of Defects in hBN: Enhancing Single-Defect Functionalities
Qiu Zhizhan †‡
Vaklinova Kristina ‡
Huang Pengru †‡
Grzeszczyk Magdalena †‡
Watanabe Kenji ∥
https://orcid.org/0000-0002-1467-3105
Taniguchi Takashi ⊥
Novoselov Kostya S. †‡
https://orcid.org/0000-0002-3690-8235
Lu Jiong *‡#
https://orcid.org/0000-0002-8301-914X
Koperski Maciej *†‡
† Department of Materials Science and Engineering, National University of Singapore, 117575 Singapore
‡ Institute for Functional Intelligent Materials, National University of Singapore, 117544 Singapore
∥ Research Center for Electronic and Optical Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
⊥ Research Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
# Department of Chemistry, National University of Singapore, 117543 Singapore
* Email: chmluj@nus.edu.sg.
* Email: msemaci@nus.edu.sg.
20 08 2024
03 09 2024
18 35 2403524043
17 03 2024
08 08 2024
07 08 2024
© 2024 The Authors. Published by American Chemical Society
2024
The Authors
https://creativecommons.org/licenses/by-nc-nd/4.0/ Permits non-commercial access and re-use, provided that author attribution and integrity are maintained; but does not permit creation of adaptations or other derivative works (https://creativecommons.org/licenses/by-nc-nd/4.0/).

Defect centers in insulators play a critical role in creating important functionalities in materials: prototype qubits, single-photon sources, magnetic field probes, and pressure sensors. These functionalities are highly dependent on their midgap electronic structure and orbital/spin wave function contributions. However, in most cases, these fundamental properties remain unknown or speculative due to the defects being deeply embedded beneath the surface of highly resistive host crystals, thus impeding access through surface probes. Here, we directly inspected the atomic and electronic structures of defects in thin carbon-doped hexagonal boron nitride (hBN:C) by using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Such investigation adds direct information about the electronic midgap states to the well-established photoluminescence response (including single-photon emission) of intentionally created carbon defects in the most commonly investigated van der Waals insulator. Our joint atomic-scale experimental and theoretical investigations reveal two main categories of defects: (1) single-site defects manifesting as donor-like states with atomically resolved structures observable via STM and (2) multisite defect complexes exhibiting a ladder of empty and occupied midgap states characterized by distinct spatial geometries. Combining direct probing of midgap states through tunneling spectroscopy with the inspection of the optical response of insulators hosting specific defect structures holds promise for creating and enhancing functionalities realized with individual defects in the quantum limit. These findings underscore not only the versatility of hBN:C as a platform for quantum defect engineering but also its potential to drive advancements in atomic-scale optoelectronics.

2D insulators
hexagonal boron nitride
single defects
discrete midgap states
wave function imaging
Air Force Office of Scientific Research 10.13039/100000181 FA8655-21-1-7026 Ministry of Education, Culture, Sports, Science and Technology 10.13039/501100001700 NA Japan Society for the Promotion of Science 10.13039/501100001691 23H02052 Japan Society for the Promotion of Science 10.13039/501100001691 20H00354 Ministry of Education - Singapore 10.13039/501100001459 MOE-T2EP50122-0012 Ministry of Education - Singapore 10.13039/501100001459 MOE-T2EP10221-0005 Ministry of Education - Singapore 10.13039/501100001459 MOE-T2EP10123-0004 Ministry of Education - Singapore 10.13039/501100001459 MOE2018-T3-1-005 Ministry of Education - Singapore 10.13039/501100001459 EDUN C-33-18-279-V12, I-FIM Agency for Science, Technology and Research 10.13039/501100001348 M21K2c0113 document-id-old-9nn4c03640
document-id-new-14nn4c03640
ccc-price
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pmcDefect centers found or created in insulating crystals, such as diamond, metal oxides, silicon carbide, or gallium nitride, can modify the fundamental electronic properties of the host material and lead to diverse phenomena. This has resulted in a variety of functionalities, such as testbed quantum systems in the form of qubits or entangled centers,1 photoluminescence (PL) markers,2 sensors of pressure or local magnetic fields in nanoscale,3 lasing media,4 or single-photon sources.5 However, the development of these functionalities has primarily been restricted to bulk crystals, with defect centers buried deep beneath the crystal surface. This has made it challenging to study the defect-related characteristics in the band structure of the host material via local surface probes, thereby preventing access to comprehensive information about the atomic and electronic structures of specific defects. These properties of defects were rather inferred speculatively through other methods, such as optical spectroscopy, which was limited by spatial resolution and sensitivity to transitions only between electronic states, fulfilling electric dipole, spin, and momentum selection rules. However, in this study, we demonstrate that advances in two-dimensional (2D) materials have made it possible to embed defects in atomically thin films and investigate them directly through scanning probes employing electrons tunneling through the insulating barrier. The combination of 2D insulators and scanning tunneling probes allows for the direct visualization of the midgap ladders of discrete energy states and their corresponding spatial features.

Directly applying scanning tunneling microscopy (STM) to investigate the bulk insulator surface is challenging, primarily stemming from its large band gap and high contact resistance. To overcome this limitation, alternative methods were developed by incorporating top-layer graphene,6−8 including tunneling into a semimetallic graphene monolayer deposited on bulk hBN crystal.9 In such cases, electrically active defects in the insulator become discernible in STM by imposing the local Coulomb potential on the electrons in the semimetallic graphene. However, the microscopic information about the atomic structure and the energy landscape of the midgap defect states in hBN remained inaccessible since the tunneling current was predominantly contributed by the electronic states of semimetallic graphene. An alternative approach involves tunneling through the atomically thin film toward the conducting substrates, which has been widely employed to investigate the intrinsic properties of 2D semiconductors or insulators.10−16 Inspired by previous studies, we investigated a thin hexagonal boron nitride (hBN) film deposited on a bulk graphite substrate. The thickness of the hBN film was limited to no more than three layers, ensuring a notable tunneling conductance indicated by prior research.17 The thin hBN film was isolated through mechanical exfoliation from a bulk crystal modified by annealing in a graphite furnace.18 This procedure was found to induce radiative centers with well-established spectral characteristics that were revealed by their PL response.19 This enabled the systematic study of the optical properties of defects, leading to the development of single-photon emitters,20 the sensitivity of the dielectric environment in the single-defect limit,20 and vertical tunneling light emitting diodes utilizing intradefect optical transitions.21 The characteristics of the optical resonances were compared with the theoretically predicted optical excitations in quantum-embedding schemes,20 leading to early proposals to attribute PL resonances to defects of specific atomic structures based on carbon substitutions and vacancies.22 To achieve a fundamental understanding of the link between atomic structures, electronic band structures, and optical response of defects, we investigated the carbon-doped hBN (hBN:C) that has already been used to develop functionalities at a single-defect level.

The present study provides information on the defect properties in hBN and its potential as a platform for defect engineering. The development of tools for characterizing the atomic and electronic structures of individual defects,23 as demonstrated here through a combination of STM, scanning tunneling spectroscopy (STS), and optical spectroscopy, is pivotal for further advancements in various domains, including single-photon emission,24−26 sensing capabilities,20 integration of quantum light with optoelectronic devices,21 and photonic structures.

Results and Discussion

Enabling STM and STS characterization of atomically thin hBN required a sample of a specifically designed structure. To achieve this, we constructed Si/SiO2/graphite/hBN:C heterostructures, where the (semi)metallic graphite substrate was contacted by a gold electrode, facilitating electrical grounding and a convenient STM tip approach. The fabricated sample underwent extended annealing at around 310 °C in a high vacuum environment, typically overnight, to achieve sufficient surface cleanliness for atomically resolved STM imaging. The optical and atomic force microscopy (AFM) images of our sample with three layers of hBN:C are presented in Figure 1(a,b), while detailed information on the fabrication process can be found in the Methods section. The inspection of the optical response of this heterodevice was consistent with previous observations of radiative centers in hBN:C. Spatial maps of PL intensity, monitored at a specific energy of 2.075 eV corresponding to single-photon emitters in thin layers of hBN:C,20 revealed localized emission originating only from regions covered by hBN:C film as seen in Figure 1(c). Generally, the introduction of midgap defect levels can activate different types of radiative processes. Transitions between the electronic bands of the host crystals and the defect states are typically characterized by broad emission bands indicative of a continuous dispersion of multiple conduction and/or valence sub-bands. The transitions between two defect states, in the limit of weak electron–phonon coupling, give rise to narrow PL resonances due to the discrete character of midgap levels. These two cases, i.e., defect to the band and intradefect radiative transitions, are schematically depicted in Figure 1(d). Their respective signatures (broad-band emission and narrow resonances) coexist in the PL spectrum of the Si/SiO2/graphite/hBN:C sample shown in Figure 1(e). The narrow resonance at 2.075 eV exhibited an 85 meV blue shift relative to the resonances observable in bulk hBN:C films (see Figure 1(f) for a comparison between different hBN and hBN:C samples). Such modification of the emission energy was attributed previously to the effect of enhanced dielectric screening.20 Hence, the blue shift is expected in Si/SiO2/graphite/hBN:C sample due to the proximity of thin hBN:C film to strongly screening metallic graphite substrate.

Figure 1 Photoluminescence (PL) spectroscopy of the Si/SiO2/graphite/hBN:C device. (a) Optical image of the sample presents the region with graphite/hBN:C heterostructure. (b) Atomic force microscopy image enabled the identification of three layers hBN:C film. (c) Map of integrated PL intensity at the energy of 2.075 eV revealed localized emission from hBN:C. Maps (b, c) correspond to the same area of the sample. (d) A diagram demonstrating optical transitions involving midgap defect states and the electronic sub-bands of the host material. (e) Representative low-temperature optical spectrum from the location marked with a circle in the PL map in panel (e). Optical response comprises Raman scattering resonances from graphite (2D and G bands), silicon (Si), and PL signal from hBN:C in the form of narrow resonances and broad bands. The broad-band component was deconvoluted into three bands, as highlighted by the Gaussian peaks. (f) Comparison between PL spectra measured for several hBN and hBN:C films characterized by varied thickness.

To directly investigate the defects in the thin hBN:C film, we conducted the STM characterization of the sample in the experimental configuration presented schematically in Figure 2(a). For a large band gap system like hBN, we found it advantageous to initially perform STM imaging at a large bias VS of approximately 4.7 V. The large sample bias applied was comparable to the average work function of the tip and sample, positioning the STM operation within the bias range akin to the field emission regime. This strategy notably mitigated the risk of tip crashing while concurrently facilitating the mapping of the local electrostatic field of electrically active defects. Figure 2(b,c) presents representative bias-dependent STM images with electrically active defects manifesting as protrusions/indentations for the positive/negative sample bias. A close examination revealed that the individual defect exhibited protrusions/indentations with a symmetric Gaussian-shaped extension characterized by a diameter of 1–2 nm in Figure 2(d,e). Such an observation signified that these commonly observed defects are positively charged, creating a local electrostatic field that facilitates electron tunneling into empty states and suppresses hole tunneling into occupied states.27−30 Additional large-scale STM images were presented in the SI Appendix in Figure S1, which indicated that positively charged defects were prevalent among electrically active defects.

Figure 2 Low-temperature (T = 4.7 K) scanning tunneling microscopy (STM) of the Si/SiO2/graphite/hBN:C device. (a) Diagram demonstrates the experimental configuration, where the graphite substrate was grounded via a golden electrode and bias was applied to the STM tip. The large-scale STM images revealed the presence of multiple defect centers at (b) with the positive sample bias VS = 4.7 V and (c) with the negative sample bias VS = −5.0 V. The tunneling current was It = 15 pA and It = 10 pA for (b, c), respectively. The individual defects emerged as bright protrusions at (d) positive sample bias VS = 4.5 V and as a dark halo at (e) negative sample bias VS = −5.0 V. The tunneling current was It = 100 pA and It = 150 pA for (d, e), respectively.

Out of the population of positively charged defects, a subset of individual centers were selected at random for a detailed inspection of their properties. Atom-resolved STM images of positively charged defects, summarized in Figure S2, served as the basis for this examination. Based on the symmetry observed in atom-resolved STM images, the examined defects could be categorized into two main groups: single-site defects characterized by a symmetric lattice, and multisite defects characterized by an asymmetric lattice and a complex interplay between atomic and electronic structures. Figure 3(a–d) illustrates a representative type of positively charged single-site defects, referred to as type I defects. The STM image acquired at VS = 4.3 V (Figure 3(a)) revealed an extended Coulomb potential. At a lower bias of VS = 2 V, the defect center exhibited a significantly brightened dot feature (Figure 3(b)). The surrounding lattice remained close to the crystallographic structure of pristine hBN, suggesting that the type I defect is related to a single heteroatom substitution. This interpretation is further supported by the STM image at VS = −3.4 V, which unambiguously revealed a single-atom site defect center, as seen in Figure 3(c). Given the significant concentration of carbon in the hBN:C crystals identified previously via secondary ion mass spectroscopy,31 the heteroatom substitution can be attributed to a carbon atom. Such an impurity appeared systematically in this sample, and further examples of defects with analogous characteristics are presented in the SI Appendix in Figure S2. Determining whether the substitution occurred at a nitrogen or boron site could not be realized exclusively relying on STM imaging, which was analyzed based on data presented in Figure S3. Identifying the heteroatom substitutional position in the lattice was instead supported by dI/dV spectroscopic measurement, as shown in Figure 3(d). The tunneling spectroscopy for the pristine three-layer hBN region revealed a band gap of 6.20 ± 0.29 eV via the inspection of the onsets indicative of the valence and the conduction band edges.32 The detailed procedure for establishing these onsets is presented in Figure S4. In contrast, the tunneling dI/dV spectrum corresponding to a defect site of the first type exhibited a lower onset of the conduction band minimum (CBM). The reduction in the CBM energy can be attributed to a significant downward band bending caused by the positively charged defect. That requires a donor-like defect level mixing with the CBM state, which can only occur in the considered scenario through carbon substitution for boron (CB+). The attribution of the observed defect to carbon substitution fornitrogen CN structure can be excluded since CN gives rise to an acceptor-type midgap level residing closely to the valence band maximum, hence incapable of carrying a positive charge, as established in previous reports based on predictions by density functional theory (DFT) calculations.22

Figure 3 Imaging and spectroscopy of single-site defects in hBN:C. (a–c) STM images of the defect of the first type. The images were obtained at different sample biases: (a) VS = 4.3 V, (b) VS = 2.0 V, and (c) VS = −3.4 V. The tunneling current was It = 30 pA, and the sample temperature was T = 77 K in all three cases. Notably, the observed defect structure shares the same trifold symmetry inherited from the resolved lattice, providing evidence that the resolved lattice is indeed the hBN lattice. (d) Scanning tunneling spectroscopy (STS) transients are demonstrated for the pristine hBN lattice (red curve) and the defect of the first type (blue curve). The tip position is marked as red and blue crosses in panel (a). (e, f) STM images of the defect of the second type. The images were obtained at different sample biases: (e) VS = 4.3 V and (f) VS = −4.9 V. The tunneling current was It = 500 pA, and the sample temperature was T = 4.7 K in both cases. The single vacancy character of this defect was evident in (g) an image with a subtracted nonperiodic contribution achieved via fast Fourier transform (FFT) filtering. The FFT image is demonstrated in the inset of panel (g). (h) STS transient for the defect of the second type was measured at the location marked by a blue cross in panel (e).

We also identified a sparsely occurring type II defect, despite its identical signature in the STM image acquired at VS = 4.3 V, presented in Figure 3(e). The atomic structure of the second defect type was starkly different from the previous case, as shown in the STM image at VS = −4.9 V in Figure 3(f). The distinct structural characteristics of this defect were discernible after employing fast Fourier transform (FFT) filtering to remove the circular negative-contrast background, which is associated with the extended Coulomb potential (refer to Figure S5 for more details about the FFT methods). As demonstrated in Figure 3(g), a single-atom vacancy became apparent, giving rise to a defect center of D3h symmetry. The emergence of this defect was accompanied by local stretching of the surrounding hBN lattice, which constitutes a differentiating factor between a vacancy and an impurity. Further examples of STM images of type II defects are shown in Figure S6. The dI/dV spectrum of the vacancy, presented in Figure 3(h), revealed the presence of a ladder of narrow resonances below the CBM. Therefore, the tunneling spectroscopy indicated that the vacancy acts as a donor-like defect coupled with phonon excitations, which will be discussed in detail later. These characteristics are consistent with the vacancy at a nitrogen site (VN+) and in contrast with acceptor-like features of a vacancy at a boron site (VB).22

The fundamental properties of CB and VN defects can be comprehended in the framework of DFT. As discussed in previous reports,22 CB defect gives rise to a single-defect level below the CBM due to an excess electron provided by carbon at the boron lattice site. The optical response of such a defect can be related to transitions between the defect level and the conduction band, which constitutes a plausible origin of the broad emission bands observed in the PL spectra of the hBN:C device. For the positive charge state of the defect, the excess electron can relax to the metallic state in the graphite substrate; thereby, the ionized CB+ defect exhibits the same number of electrons as boron in the neutral state. This is reflected in the DFT band structure of an hBN unit cell containing a CB+ defect, where an empty level emerges below the conduction band, as demonstrated in Figure 4(a). Such electronic configuration can favor the band-bending effect revealed by the STS data. The band structure characterizing the VN+ defect is more intricate due to the presence of three broken bonds that give rise to three defect levels. All three levels reside below the conduction band, with the two levels located closer to the conduction band edge being nearly degenerate, as seen in Figure 4(b). Although the optical response of the VN defect could in principle combine the defect to the band and intradefect transitions, the predicted emission energy based on many-body models applied to intra-VN transitions does not correspond to the single-photon emitters observed so far in thin layers of hBN:C.20

Figure 4 Predictions of the midgap energy levels due to defects in the positive charge state in hBN. Density functional theory (DFT) calculations revealed the hBN band structure in the presence of positively charged defects: (a) carbon substitution for boron (CB+) and (b) nitrogen vacancy (VN+). The red and blue colors represent the opposite spin directions of the wave function, respectively.

The tunneling spectra exhibited resonances that were not restricted to midgap defect states exclusively. These resonances can also arise from tunneling events that are facilitated by the emission of phonons into the hBN lattice. This gives rise to replicated resonances at higher VS values, as illustrated in Figure 5(a). The phonon excitations coupled to electrons occupying a defect state can often be modeled as harmonic oscillators. This model predicts equidistant resonances separated by the energy of a phonon. The midgap resonances observed for the VN+ defect were found to be consistent with this scenario. Specifically, they were reproduced by seven Gaussian peaks split by ℏω1 = 220 ± 7 meV, as demonstrated in Figure 5(b). However, shifts in the position of resonances induced by the electric field originating from the STM tip often result in an overestimation of the phonon energy in STS experiments, as demonstrated in Figure S7. Therefore, it is plausible that the dominant electron–phonon coupling mechanism is related to the Raman-active optical phonon mode in hBN at 195 meV, which corresponds to a bond-stretching vibrational mode.33 Furthermore, the lowest energy resonance at VS = 1.75 V exhibited a fine structure that could be reproduced by considering replicas of two phonon modes of the energy: ℏω2 = 20 ± 1 meV and ℏω3 = 30 ± 3 meV, as shown in Figure S8. Such low-energy vibrational modes coupled to an individual defect are likely to originate from local phonons that are spatially localized within the stretched lattice around the defect, as visualized by the STM images for defect VN+. It is noteworthy that prior studies have underscored the important role of impurity states efficiently coupled to phonon modes in the optical response of single-photon emitters in hBN:C,19,34 which is evidenced by the common observation of phonon side bands accompanying the zero-phonon line, as can be seen in the PL spectra from hBN:C in Figure 1(f). This suggests that replicated resonances due to a strong vibronic response in the tunneling spectrum may aid in the identification of potential defect candidates for the single-photon emission among the myriad of defects present within hBN:C.

Figure 5 Electron–phonon coupling revealed by STS characterization. (a) Diagram presents an inelastic tunneling process via a defect state assisted by multiple phonons. (b) Small-scale dI/dV spectrum of the defect of the second type exhibited a series of resonances, replicated by fitting seven equidistant Gaussian peaks with a splitting of ℏω1 = 220 ± 7 meV.

After elucidating the nature and electronic properties of positively charged single-site defects, we proceeded to explore multisite defects characterized by multiple midgap states coupled with vibrational modes (Figure 6(a)), which we denoted as type III defects. The observed characteristics suggest a more intricate atomic structure in comparison to a single-atom lattice modification. We hypothesize that the type III defect is associated with a complex of carbon impurities and/or vacancies, given the mechanism of reduced formation energies driven by the creation of molecular-like bonds in multisite defects in hBN.22,35,36 Above the Fermi level, the dI/dV spectrum revealed the presence of two unoccupied defect states located at VS = 0.13 V and VS = 0.24 V strongly coupled to the phonon mode of the energy ℏω1 = 220 meV (Figure 6(b)). Below the Fermi level, a prominent resonant peak appeared at VS = −0.63 V. The spatially resolved STS data, presented in Figure 6(c), demonstrated that the resonance shifted to higher energy when the STM tip approached the defect center. Such an observation excluded the possibility of a tip-induced charging peak, as a charging peak would shift in the opposite direction.28 Therefore, the physical origin of the resonance at VS = −0.63 V can be attributed to the occupied defect state. The occupied and empty defect states displayed wave functions of distinct geometry, as illustrated by bias-dependent dI/dV mapping, presented in Figure 6(d). The occupied state at VS = −0.63 V exhibited an asymmetric wave function centered at the position of the defect, while the empty defect state at VS = 0.13 V revealed a wave function consisting of a dim center enclosed by a ring-like protrusion. While directly identifying the nature of the observed multisite defects is challenging based on the limited information, the characteristics of the observed defect may suggest some possibilities. For instance, the presence of multiple in-gap states near the Fermi level with two unoccupied states and one occupied state coincided with the predicted electronic structure of CBVN defect,20 which was previously proposed as the origin of single-photon emission in hBN films. This implication aligns with our direct observation of both constituent defects CB and VN. However, at present, we cannot exclude more complicated defect centers such as CBCNVN or CBVNVB.

Figure 6 Defects with a ladder of midgap states and strong electron–phonon coupling. (a) dI/dV spectra for a pristine hBN lattice (green) and the defect of a third type (blue). The inset demonstrates the STM image of the defect of the third type under sample bias VS = −0.8 V and tunneling current It = 30 pA. The sample temperature was T = 77 K. (b) A small-scale dI/dV spectrum for the defect of the third type, replicated by two subsets of equidistant Gaussian peaks with equal splitting of ℏω1 = 220 ± 7 meV. (c) Spatially resolved dI/dV spectra across the defect of the third type. (d) dI/dV maps of the defect of the third type measured at different sample biases: VS = −0.63 V and VS = 0.13 V. The corresponding position of the tip is marked by “+” symbols.

Conclusions

In conclusion, we investigated the properties of individual defects in atomically thin hBN layers through the defect-mediated tunneling of electrons toward a metallic substrate. A combination of STM, STS, and optical spectroscopy has proven to be a comprehensive toolset, enabling the exploration of the energy landscape, wave functions, and optically active transitions arising due to fundamental defects in a van der Waals insulator. We identified CB+ and VN+ centers among the large population of defects in hBN:C. Given a relatively high mobility of single-site carbon substitution at evaluated temperature, intentionally introduced carbon impurities may combine further with other types of atomic defects to create thermodynamically stable complex defect structures, hosting a ladder of midgap states with sizable electron–phonon coupling. The characterized electronic structure of the defect complex enables a many-body intradefect transition that potentially constitutes the origin of single-photon emission observed in thin layers of hBN. The direct access to midgap states in hBN presents the possibility of creating functionalities at a single-defect level. We envisage that the next challenge for hBN-based research lies in developing scanning tunneling luminescence,37 which would enable local electrical excitation of single-photon sources at the atomic limit.

Methods

Sample Fabrication

Graphite and hBN:C crystals were mechanically cleaved onto silicon wafers with 300 and 90 nm layers of SiO2, respectively. Large-area graphite films were selected to act as conductive substrates for atomically thin hBN:C films. The hBN:C flakes were transferred onto the graphite substrate via a pickup technique. The hBN:C flakes of the desired thickness were identified by their optical contrast and atomic force microscopy. They were lifted from the Si/SiO2 wafer with a poly(dimethylsiloxane)/polycarbonate stamp at 100 °C. Subsequently, the hBN:C flakes were released onto the graphite film together with the polycarbonate film at 180 °C, which renders a high-quality interface between the two materials. The sample was then annealed at 180 °C on a hot plate and washed in dichloromethane, acetone, and isopropanol to remove the polymer residue. Before the STM measurements, the sample was annealed in situ in an ultrahigh vacuum at 310 °C for 12 h to remove surface residues and adsorbates.

Optical Characterization

The PL spectra were measured in microscopic (1 μm spot size) backscattering geometry with 514 nm laser excitation. The sample was cooled down to 1.6 K through helium exchange gas. The sample was mounted on x-y-z piezo positioners that allowed for a PL mapping experiment. The PL signal from the sample was collected by a multimode fiber with 50 mm core diameter, dispersed by a 0.75 m spectrometer with 300 g/mm grating, and detected a by liquid-nitrogen-cooled charge-coupled device camera.

STM and STS Measurements

Our STM and STS measurements were conducted in the Createc LT-STM system with a base pressure lower than 10–10 mbar at 4.7 or 77 K. Performing STM/STS measurements at 4.7 K improved the energy resolution in tunneling spectroscopy at the cost of elevated risk of crashing the tip due to the reduced sample conductivity at low temperatures. However, the outcomes do not yield a qualitatively distinct result regarding the atomic and electronic structure of defects in hBN:C when compared to the STM/STS results obtained at 77 K. The tungsten tip was calibrated spectroscopically against the surface state of Au(111) substrate. All of the dI/dV spectra were measured through a standard lock-in technique with a modulated voltage of 3–10 mV at the frequency of 700–900 Hz.

DFT Calculations

The DFT calculations were performed using a generalized gradient approximation (GGA) to the exchange–correlation functional proposed by Pedew, Burke, and Ernzerhof (PBE).38 All calculations were spin-polarized and used the projector augmented wave (PAW) pseudopotential supplied with the VASP code.39,40 A plane wave cutoff of 500 eV was used. Pristine single-layer hBN was first optimized for the lattice geometry using the conventional cell and a 21 × 21 × 1 Monkhurst-Pack reciprocal space grid with an energy tolerance of 0.01 eV. A vacuum spacing of 20 Å was used to separate periodic images of the single layer and the lattice vectors in a conjugate gradient approach. A 5 × 5 × 1 supercell was used for the calculations of the defect properties in hBN. The structures were fully optimized using the conjugate gradient algorithm until the residual atomic forces were smaller than 10 meV/Å. A G-centered 12 × 12 k-point sampling was used for the Brillouin-zone integration. The method for the calculation of the formation energy of different defects existing in various charge states at the N-rich condition was the same as in ref (36).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsnano.4c03640.Additional experimental data on imaging defect centers in hBN through scanning tunneling microscopy; establishing hBN band gap through scanning tunneling spectroscopy; demonstration of band-bending effects, and analysis of the low-energy phonon modes emergent in replicated tunneling resonances (PDF)

Supplementary Material

nn4c03640_si_001.pdf

The authors declare no competing financial interest.

Acknowledgments

This project was supported by the Ministry of Education (Singapore) through the Research Centre of Excellence program (grant EDUN C-33-18-279-V12, I-FIM), AcRF Tier 3 (MOE2018-T3-1-005) and MOE Tier 2 grants (MOE-T2EP10223-0004, MOE-T2EP10123-0004, and MOE-T2EP50122-0012), and Agency for Science, Technology and Research (A*STAR) under MTC Individual Research Grants (M21K2c0113). This material is based upon work supported by the Air Force Office of Scientific Research and the Office of Naval Research Global under award number FA8655-21-1-7026. K.W. and T.T. acknowledge support from the JSPS KAKENHI (Grant Numbers 20H00354 and 23H02052) and World Premier International Research Center Initiative (WPI), MEXT, Japan. P.H. acknowledges the support from the National Key Research and Development Program (2021YFB3802400) and the National Natural Science Foundation of China (52161037). The computational work for this article was performed on computational resources at the National Supercomputing Center of Singapore (NSCC).
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