# Fileset

[13214.full.pdf](https://mdr.nims.go.jp/filesets/85ad155e-08dc-4c61-be7f-639b42b8c66b/download)

## Creator

Maciej Koperski, Diana Vaclavkova, [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), Kostya S. Novoselov, Marek Potemski

## Rights

[Creative Commons BY Attribution 4.0 International](https://creativecommons.org/licenses/by/4.0/)

## Other metadata

[Midgap radiative centers in carbon-enriched hexagonal boron nitride](https://mdr.nims.go.jp/datasets/64a08a29-2862-4a0c-a031-dd83c987d249)

## Fulltext

Midgap radiative centers in carbon-enriched hexagonal boron nitrideMidgap radiative centers in carbon-enriched hexagonalboron nitrideMaciej Koperskia,1, Diana Vaclavkovab, Kenji Watanabe (渡邊賢司)c, Takashi Taniguchid, Kostya S. Novoselova,1,and Marek Potemskib,e,1aDepartment of Materials Science and Engineering, National University of Singapore, 117575, Singapore; bLaboratoire National des Champs MagnétiquesIntenses, CNRS–Université Grenoble Alpes–Université Paul Sabatier–Institut National des Sciences Appliquées Toulouse–European Magnetic FieldLaboratory, 38042 Grenoble, France; cResearch Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044,Japan; dInternational Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan; and eInstituteof Experimental Physics, Faculty of Physics, University of Warsaw, PL-02-093 Warsaw, PolandContributed by Kostya S. Novoselov, April 16, 2020 (sent for review March 6, 2020; reviewed by Pawel Hawrylak and Luis Viña)When serving as a protection tissue and/or inducing a periodiclateral modulation for/in atomically thin crystals, hexagonal boronnitride (hBN) has revolutionized the research on van der Waalsheterostructures. By itself, hBN appears as an emergent wide-bandgap material, which, importantly, can be optically bright inthe far-ultraviolet range and which frequently displays midgapdefect-related centers of yet-unclear origin, but, interestingly, act-ing as single-photon emitters. Controlling the hBN doping is ofparticular interest in view of the possible practical use of this ma-terial. Here, we demonstrate that enriching hBN with carbon (C)activates an optical response of this material in the form of a seriesof well-defined resonances in visible and near-infrared regions,which appear in the luminescence spectra measured underbelow-bandgap excitation. Two, qualitatively different, C-relatedradiative centers are identified: One follows the Franck–Condonprinciple that describes transitions between two defect states withemission/annihilation of optical phonons, and the other showsatomic-like resonances characteristic of intradefect transitions.With a detailed characterization of the energy structure and emis-sion dynamics of these radiative centers, we contribute to the de-velopment of controlled doping of hBN with midgap centers.hexagonal boron nitride | defects | midgap centers | single-photon emittersWell-defined defect centers play a crucial role in condensed-matter physics and material science. That is particularlyevident in case of semiconductors and wide-gap insulators, wheremidgap levels may arise due to various imperfections of thecrystal lattice. These additional states may manifest themselvesin many ways, e.g., giving rise to photoluminescence (1) or par-ticipating in charge-tunneling processes (2). They may also provideversatile functionalities, ranging from lasing (3) to pressure sensing(4, 5). When properly isolated and controlled, individual defectsmay become single-photon sources (6) or building blocks in devicesrelevant for metrology in nanoscale (7).One of the materials with interesting, yet incompletely un-derstood, defect centers is hexagonal boron nitride (hBN). Dueto a large value of the bandgap (8), equal to 5 eV, the opticalresponse of hBN crystals may span a very broad spectral range(9). Indeed, a deep-ultraviolet (deep-UV) interband photo-luminescence (10, 11) is known to be accompanied by 4.2-eVdefect-related emission resonances (12), as well as visible(reaching near-infrared wavelengths) spectrally narrow emissionlines (13–18). The two latter emitters have been demonstrated tobe capable of single-photon emission, but their microscopic or-igin remains unclear. For the sake of a comprehensive un-derstanding, the atomistic structure of these defects needs to beidentified. Also, from practical perspectives, the methods of theircontrolled creation should be developed. One of the mostplausible candidates to account for these defect centers, accordingto predictions of low-formation energy and large migration bar-riers (19), are carbon (C)-related impurities. In this work, we ex-plore this hypothesis by inspecting C-doped hBN (hBN:C) crystals,uncovering their robust and intricate optical response in the visibleand near-infrared spectral range.For the purpose of a comparative study of pristine andC-doped samples, hBN single crystals were grown by a high-pressure, temperature-gradient method. These specimens areknown to exhibit very low defect density, and their character-ization by photoluminescence and/or cathodoluminescence usu-ally does not show any substantial signatures of defect-relatedlight emission. Several hBN crystals originating from the samegrowth experiment were placed in a graphite furnace andannealed at high temperature. This procedure had an immedi-ately observable impact on the hBN crystals. They changed theirappearance from colorless and transparent to yellow, henceproviding a first indication that C is incorporated in the hBNmaterial. In order to characterize these crystals through opticalmethods, we isolated layers of various thicknesses via standardtechniques of mechanical exfoliation and deposited them ontoultraflat silicon substrates.C-doping is believed to introduce shallow and deep levels inhBN and has been recently reported to modify its photo-luminescence spectra in the far-UV region (20). Here, we ex-amine the optical response of C-enriched hBN in the visible andnear-infrared spectral range. To start with their characterization,SignificanceWell‐defined defect centers play a crucial role in condensed-matter physics. That is particularly evident in the case ofsemiconductors and wide‐gap insulators, where midgap levelsprovide a number of versatile functionalities: from lasing topressure sensing. Individual defects may become single-photonsources—crucial for applications in quantum technology. Re-cently, such defects have been observed in hexagonal boronnitride (hBN). Here, a method to fabricate stable and re-producible defects in hBN is introduced. Large bandgap ofboron nitride allows the use of such defects in many quantumapplications. Fabrication and characterization of such defects inhBN, which has become a relevant, emerging, wide-bandgap2D material, is an important step toward novel functionalitiesof van der Waals heterostructures.Author contributions: M.K. and M.P. designed research; M.K., D.V., and M.P. performedresearch; K.W. and T.T. contributed new reagents/analytic tools; M.K., D.V., K.S.N., andM.P. analyzed data; and M.K., K.S.N., and M.P. wrote the paper.Reviewers: P.H., University of Ottawa; and L.V., Universidad Autónoma de Madrid.The authors declare no competing interest.This open access article is distributed under Creative Commons Attribution License 4.0(CC BY).1To whom correspondence may be addressed. Email: msemaci@nus.edu.sg, kostya@nus.edu.sg, or marek.potemski@lncmi.cnrs.fr.This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2003895117/-/DCSupplemental.First published June 1, 2020.13214–13219 | PNAS | June 16, 2020 | vol. 117 | no. 24 www.pnas.org/cgi/doi/10.1073/pnas.2003895117Downloaded at National Institute for Materials Science on July 11, 2020 https://orcid.org/0000-0003-3701-8119http://orcid.org/0000-0003-4972-5371http://orcid.org/0000-0001-8881-6618http://crossmark.crossref.org/dialog/?doi=10.1073/pnas.2003895117&domain=pdfhttp://creativecommons.org/licenses/by/4.0/http://creativecommons.org/licenses/by/4.0/mailto:msemaci@nus.edu.sgmailto:kostya@nus.edu.sgmailto:kostya@nus.edu.sgmailto:marek.potemski@lncmi.cnrs.frhttps://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2003895117/-/DCSupplementalhttps://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2003895117/-/DCSupplementalhttps://www.pnas.org/cgi/doi/10.1073/pnas.2003895117we inspected the low-temperature (5 K) microoptical response ofmultiple species of both pristine and hBN:C by exciting themwith a 2.41-eV laser beam. Representative optical spectra arepresented in Fig. 1. The dominant resonance observed for thepristine hBN films was due to an optical-phonon, Raman scat-tering peak at 1,365 cm−1. The optical response of the hBN:Cspecies was, on the other hand, very rich. Let us note that thepresented spectra were rather homogenous across the in-vestigated samples. Multiple samples showed the same opticalresponse, and the micro-optical mapping demonstrated that theintensity of emission correlates well with the thickness of thehBN:C film, but no qualitative differences were discerned. Se-lected maps may be found in SI Appendix, Fig. S1. This findingencourages the interpretation of the observed light emission asoptical transitions in an ensemble of C-related defects in hBNmaterials. Such defect centers clearly introduce well-definedmidgap levels that enable the radiative, intracenter recombina-tion processes. We will proceed with further characterization ofthe resonances seen in the optical spectra of hBN:C films anddiscuss their possible origin, together with the assessment of thephysical character of the emitting states.Franck–Condon Spectrum of a Defect Strongly Coupled toOptical PhononsWe firstly focused on the interpretation of the optical responseof hBN:C in the energy range 1.6 to 2.05 eV. The resonancesappearing therein, depicted in Fig. 2A, formed a pattern char-acteristic of a Franck–Condon type of spectrum (21, 22). Whenlocalized electronic levels are strongly coupled to optical phonons,photo-excitation elevates electrons between two molecular-likestates while simultaneously annihilating (in absorption processes)or creating (in emission processes) phonons. Such a description hasbeen found to accurately account for the optical transitions betweendefect states in many types of solid-state structures, the renownedexample being a nitrogen-vacancy (NV) center in diamond (23–25).Notably, a Frank–Condon spectrum has been also identified inhBN, but in the near-UV spectral region (around 4.1 eV), andlikely not related to a defect induced by C-doping (20). In theFranck–Condon picture, the highest energy resonance constitutes atransition without involvement of phonons, the so-called zero-phonon line (ZPL), which we observed at 1.995 eV. The magni-fied view of the ZPL is presented in Fig. 2B to illustrate theasymmetric shape of this resonance. Such a feature is usually at-tributed to a homogenous broadening associated with acousticphonons (26, 27). That is, in a simple view, because processes withthe annihilation of phonons are restricted to low-energy vibrationalmodes according to their thermal distribution, while the processeswith the creation of phonons may involve modes of any energy. TheZPL resonance was accompanied by a low-energy phonon sideband formed by vibronic resonances due to local* and/or quasilocal†phonons coupled to the electronic states partaking in the opticaltransition. The absorption and emission processes that are relevantfor the optical response of a defect strongly coupled to opticalphonons and following the Franck–Condon model are schemati-cally presented in Fig. 2C.The fundamental figures of merit characterizing a Franck–Condon emitter are Debye–Waller (28, 29) and Huang–Rhys(30) factors, which are used to define and determine electron–phonon coupling strength. The Debye–Waller factor is calcu-lated as the spectral weight of the ZPL with respect to the totalemission intensity of the defect. For our emitters, we found theDebye–Waller factor (w) to be equal to 3.0%, which is compa-rable with the value obtained for other defect centers in wide-gapmaterials (25). The Huang–Rhys factor (S) is less straightforwardto establish; hence, we used a phenomenological formula(31–34) S = −ln(w) to obtain an estimation of an average valueFig. 1. Low-temperature (5 K) micro-optical response of a pristine hBN film(lower spectrum) as compared to that of hBN:C film (upper spectrum) under514.4-nm (2.41 eV) single-frequency DPSS laser excitation. The spectrum ofthe pristine hBN film is dominated by the conventional Raman-scatteringresponse of hBN (phonon peak at 1,365 cm−1) and of the underneath sili-con substrate (phonon excitation at 524 cm−1). The hBN:C films display muchmore complex optical spectrum with multiple photoluminescence reso-nances accompanying the two major Raman scattering lines seen for pristinematerial. a.u., arbitrary units.AB CDFig. 2. (A) The low-temperature spectrum of an hBN:C film under 2.41-eVlaser excitation is presented in greater detail to demonstrate the existenceof multiple resonances. (B) The highest-energy resonance, identified as aZPL, is located at 1.995 eV and displays an asymmetric line shape. (C) Thisspectrum is interpreted in terms of Franck–Condon model with transitionsbetween two electronic levels coupled to equidistant (in harmonic oscillatorapproximation) vibrational modes. The ZPL is emphasized by using thickerarrows to mark it in the scheme. (D) The evolution of the emission spectrumwith temperature shows that the distinctive resonances are conspicuousexclusively at low temperature. The spectra are shifted vertically for bettervisibility. A.u., arbitrary units.*Lattice oscillatory modes related to movement of atoms in the close neighborhood ofthe defect, hence creating vibration states highly localized in the real space.†Lattice vibrations composed of modes that are delocalized in real space, but whosecontribution to the phonon spectrum is altered by the presence of the defect and effec-tively leading to an appearance of a phonon-like resonance.Koperski et al. PNAS | June 16, 2020 | vol. 117 | no. 24 | 13215APPLIEDPHYSICALSCIENCESDownloaded at National Institute for Materials Science on July 11, 2020 https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2003895117/-/DCSupplementalof phonons emitted in a single recombination process thatyielded, in our case, 3.5. Such a number appears to be reasonablebased on the characteristics of the phonon side-band spectrum(Fig. 2A). It is formed by one well-pronounced resonance 50.7 meVbelow the ZPL, followed by several significantly weaker peaks.Let us note that the energy of the consecutive resonances doesnot correspond to multiple phonon replicas of an individual vi-brational mode. One needs to consider at least two differentphonons to account for four resonances marked in the figure.This observation may act as an indication that the defect levelspartaking in the recombination process are efficiently coupled tomore than one vibrational mode.Further information about the optical properties of this defectcenter comes from the temperature dependence of the emissionspectrum, which is presented in Fig. 2D. The distinctive resonancesdiscernible in the lowest temperature spectrum were less apparentat higher temperatures, so that their evolution can be traced onlyup to 150 K. The broad emission band persisted up to room tem-perature, but the total integrated intensity decreased when in-creasing the sample temperature. Such behavior is understandablewithin the Franck–Condon model. The initial state of the re-combination process, which is responsible for the emergence of theZPL and the associated phonon replicas, depopulates at highertemperature in favor of higher-energy vibronic states. Therefore,the distinctive resonances, which are strictly tied to the populationof initial state of the ZPL transition, become less pronounced athigher temperature. The decrease of the total emission intensitymay be attributed to an activation of nonradiative processes.The 1.54-eV Emission Doublet and Associated Higher-EnergyTransitionsThe Franck–Condon model cannot account for the entirety ofthe emission spectrum that we observed for the hBN:C flakes.Another well-pronounced feature appeared as a doublet of res-onances at 1.54 eV. As shown in Fig. 3A, these lines persisted upto room temperature. At low temperature, they took the form ofnarrow, distinctive resonances, which became accompanied byhigher- and lower-energy shoulders when the temperature was in-creased. Such behavior may be expected of a defect that is effec-tively decoupled from optical phonons of the host lattice (e.g., dueto a particular symmetry of the electronic states). Usually, the op-tical transitions may still involve, especially at higher temperature,creation and/or annihilation of acoustic phonons (35), as suchprocesses do not require the strict selection rules.An important example of a defect characterized by similarproperties is a Cr3+ substitution for Al ions in corundum (Al2O3)crystals. Crystals with a substantial concentration of Cr3+ im-purities (commonly known as ruby) display a robust photo-luminescence in the form of a lower-energy doublet (R lines)accompanied by weaker, higher-energy transitions (B lines)arising due to transitions between intrinsic levels of Cr3+ ions(36). A very similar pattern was discernible in the optical re-sponse of hBN:C material. However, quantitative differencesbetween the narrow-line emission resonances in hBN:C and rubycrystals were apparent. Most importantly, the splitting betweenthe two components of the doublet was significantly larger in theformer material and equal to about 10 meV. Such splitting mayarise due to spin-orbit coupling and/or lattice distortions (37),which lift the degeneracy of states involved in the recombinationprocess (38, 39). Higher-energy narrow lines were also visible,and their most pronounced representatives appeared around2.30 eV. They displayed qualitatively identical temperatureevolution (Fig. 3B) as the fundamental 1.54-eV doublet; hence, itis plausible that the higher-energy narrow resonances appeareddue to transitions within the same defect center.Coexistence of Various Types of Defect Spectra in hBN:CA C atom may create two simple impurity types in hBN: a sub-stitution for boron (forming donor states) or a substitution fornitrogen (forming acceptor states). Their more complicatedvariations, such as C substitution for boron combined with ad-jacent nitrogen vacancy, have also been proposed as a possibleorigin of narrow-lines luminescence in hBN (40). Such defectcenters give rise to a ground-state midgap level that may exist invarious charge states. Transitions between different charge statescould give rise to a Franck–Condon type of spectra, as was ob-served in our hBN:C samples.However, an alien atom in the lattice structure introduces, inprinciple, an entire structure of levels formed by various con-figurations of its electrons on atomic valence shells. The intra-defect transitions between these levels may give rise to narrowemission lines akin to those commonly observed in atomicspectroscopy. The resonances due to transitions between atomic-like levels may persist when atoms become impurities in crystals.The observation of such resonances gives insight into the alter-nation of the atomic-level structure caused by the crystal envi-ronment, as well as coupling of the electronic states with latticemotion. The emission doublet at 1.54 eV observed in hBN:C,accompanied by higher-energy transitions, exhibited character-istics of such intradefect transitions.The possibility to observe both types of transitions stronglydepends on two factors: the details of absorption spectra thatdetermine the efficiency of occupying the appropriate emittingsates and the time scales of recombination (both radiative and1.50 1.55 1.60 2.25 2.30Energy (eV)Intensity(a.u.)Energy (eV)5 K30 K60 K90 K120 K150 K180 K210 K240 K270 K300 KA B.)Intensity(a.uFig. 3. The impact of temperature on the 1.54-eV doublet of resonances (A)and a set of higher-energy narrow resonances (B) is presented. All of thelines in this energy range exhibit qualitatively identical evolution withtemperature, as they persist up to room temperature while displaying sig-nificant broadening in form of asymmetric exponential-like tails, which ac-company the main lines at higher temperature. Such observation inspires theattribution of these narrow lines to optical transitions within the same de-fect center between levels that couple to acoustic phonons. The spectrataken at various temperatures have been shifted vertically for enhancedclarity. a.u., arbitrary units.13216 | www.pnas.org/cgi/doi/10.1073/pnas.2003895117 Koperski et al.Downloaded at National Institute for Materials Science on July 11, 2020 https://www.pnas.org/cgi/doi/10.1073/pnas.2003895117nonradiative) and/or relaxation processes. We have investigatedthese aspects of our emitting centers by performing photo-luminescence excitation (PLE) spectroscopy and measurementsof emission-decay times. The PLE spectra for the ZPL of theFranck–Condon-type spectrum and both resonances forming the1.54-eV doublet are presented in Fig. 4A.The ZPL is efficiently excited by photons from a broad rangeof wavelengths. There is a general trend of increasing emissionintensity for the higher energy of excitation, combined with theappearance of additional absorption resonances. In the Franck–Condon model, the resonant absorption may appear due toalignment of the energy of excitation photon with a ZPL tran-sition energy enlarged by the energy of an appropriate phonon.Alternatively, the enhanced absorption may correspond to theexistence of higher-energy electronic levels (e.g., of a differentcharge state of the defect). The major absorption resonance at2.477 eV was energetically more displaced from the ZPL than aphonon-assisted transition would allow; hence, it is most likely ofthe latter type.The 1.54-eV emission doublet clearly exhibited a resonantcharacter of excitation mechanisms. When studying intradefecttransitions, the resonances in absorption-like spectra usuallycorrespond to higher-energy states of the defect, as is the case,e.g., for Cr3+ impurities of ruby (41). The PLE spectra of the1.54-eV doublet unveiled prominent resonances, for which theemission intensity was over an order of magnitude stronger thanin nonresonant regime. Both lines of the doublet shared a lower-energy resonance at 1.709 eV (about 0.17 eV above the emissionenergy). The higher-energy line of the doublet displayed also ahigher-energy resonance at 2.443 eV, which was absent in theexcitation spectrum for the lower-energy line. Such an observa-tion suggests that there exists a splitting in the initial state of therecombination processes, which gives rise to the 1.54-eV emis-sion doublet. The existence of a robust resonance at 2.443 eV isindicative of a presence of a higher-energy defect level, whoseoccupation may be transferred efficiently, through a selectiverelaxation process, only to the state that constitutes the origin ofthe higher-energy resonance of the 1.54-eV doublet. The ob-servations based on the PLE spectra of the emission doublet maybe summarized in the form of a Jablonski diagram (42) thatshows the most pronounced atomic-like states of the defect to-gether with corresponding transitions that are required for dis-cerning the main features of the emission and absorption spectra(SI Appendix, Fig. S5).Overall, the distinctive character of the excitation spectraclearly demonstrated that the efficiency of emission of a partic-ular type of defect is strongly dependent on excitation condi-tions. Existence of robust resonances provides an opportunity tocontrol which type of emission spectrum would be dominant inhBN:C samples.Further information about the emission processes may beinferred from PL decay transients. These were measured forhigh-energy excitation (2.824 eV) with a 100-ps pulsed laser (seeFig. 4B for the PL spectrum measured under pulsed excitation,which shows qualitatively identical features as the PL spectrumunder continuous-wave excitation). The decay profile for theZPL of the Franck–Condon-type (Fig. 4C) defect showed amultiexponential character. As the ZPL appears on top of thephonon side band, we associated the short time component (witha characteristic time τ = 1.3 ± 0.2 ns) to the emission dynamics ofthe ZPL and the weaker, long time component (with a charac-teristic time τ = 8.1 ± 2.2 ns) to the phonon-assisted re-combination that contributes to the side band. The dynamics ofthe 1.54-eV emission doublet (Fig. 4 D and E) was quite dif-ferent. The lower-energy line showed a monoexponential decaywith characteristic time τ = 1.0 ± 0.1 ns. The decay time of thehigher-energy resonance was faster and fell below the measur-able time in our set-up (≤730 ± 100 ps). The monoexponentialcharacter of the decay transient suggests that the emission pro-cess is unperturbed by the existence of intermediate states.The nanosecond lifetimes of the carriers participating in therecombination processes, which give rise to the 1.54-eV doubletA BCDEFig. 4. (A) PLE spectra measured at 4 K are presented for the Franck–Condon-type emitting center (Upper; green curve represents emission intensity of ZPL)and for the 1.54-eV emission doublet (Lower; purple and blue curves represent emission intensity of higher and lower energy resonances, respectively). The PLspectrum in A, Upper (black curve) is excited at 2.477 eV, and the PL spectrum in A, Lower (black curve) is excited at 2.095 eV. The spectra have been shiftedvertically for clarity. The time-resolved study of emission dynamics was performed with a 437-nm (2.824-eV) picosecond laser. (B) The time-integrated lowtemperature (4 K) PL spectrum under such excitation is demonstrated. (C–E) All three fundamental transitions are visible, and the decay profiles are shown forZPL (C), higher-energy resonance (D), and lower-energy resonance (E) of the 1.54-eV doublet. The orange transients in C–E represent the decay profile of thelaser and are indicative of the lower limit of the measurable decay time. a.u., arbitrary units.Koperski et al. PNAS | June 16, 2020 | vol. 117 | no. 24 | 13217APPLIEDPHYSICALSCIENCESDownloaded at National Institute for Materials Science on July 11, 2020 https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2003895117/-/DCSupplementaland the 1.995-eV ZPL, demonstrate that these qualitativelydifferent emitting centers are capable of light emission withsimilar efficiency. The longer lifetime component observable atthe energy of the ZPL provides an additional argument that thecarriers occupying Franck–Condon states may recombine viamany body processes, e.g., involving creation of phonons.SummaryWe have presented a comprehensive study of the optical re-sponse of C-enriched hBN crystals. We have found the emissionsignatures of two types of emission centers that display proper-ties with many parallelisms to other well-known defect centers insolids, such as NV centers in diamond or Cr3+ impurities in rubycrystals. Through the combination of excitation and time-resolved spectroscopy, we have unveiled an intricate energystructure of the emitting defect centers and proposed a plausibleinterpretation of the recombination processes. Our findingsdemonstrate that C-related defects in hBN offer the possibility tostudy fundamental defect physics in two-dimensional (2D) crys-tals, which are compatible with constantly evolving and improv-ing van der Waals technology. The successful demonstration ofcreating well-defined defect levels may inspire similar efforts tointroduce various dopants into other compounds of the family of2D materials. The properties of our defect centers stronglysupport and promote the notion that hBN is a material suitablefor a variety of opto-electronic applications, including devicesoperating in visible and near-infrared spectral range.MethodsCrystal Growth. Single crystals of hBN were grown by the temperature-gradient method under high-pressure and high-temperature conditions.Colorless and transparent single crystals of hBN were obtained (43). Toachieve C-doping, selected crystals were annealed for 1 h at 2,000 °C withnitrogen gas flow by using high-frequency furnace with graphite susceptor.Sample Preparation. Thin films of pristine hBN and hBN:C were isolated bymechanical exfoliation and deposited onto ultraflat silicon substrates cleanedby plasma ashing. A common technique of two-step exfoliation based onpolydimethylsiloxane films was used. The samples were inspected in anoptical microscope in order to locate individual flakes, whose thickness couldbe estimated by their optical contrast with respect to the substrate.Optical Spectroscopy. The optical spectra were measured in a confocalmicrospectroscopy set-up. A cold-finger cryostat was used to cool down thesamples to 4 K, and a resistor-based heater allowed control of the temper-ature. A long working-distance objective (50×) and a set of appropriatefilters/polarizers were employed to achieve focalization of a laser beamdown to a spot of about 1-μm diameter and collection of light from thesample. A 500-cm spectrometer with a charge-coupled device camera pro-vided spectral resolution and light-detection capabilities. The emissionspectra were measured by using 514.4-nm narrow-linewidth diode pumpedsolid-state (DPSS) laser. The excitation spectroscopy was realized with abroad-band supercontinuum light source coupled to a spectrometer toachieve tunable monochromatic (linewidth of about 2 nm) excitation. A100-ps, 437-nm laser was used to perform time-resolved measurements. Anavalanche photodiode (APD) counted the photons to obtain PL decay pro-files. By measuring the laser pulses directly with the APD (orange curves inFig. 4 C–E), we established the lowest measurable decay time to be 730 ± 100 ps.The decay profiles were fitted with exponential profiles:I(t) = I0 * exp(−tτ),where I0 is a multiplicative constant, and τ is the characteristic decay time.The measurements of PL spectra in a magnetic field (presented in SIAppendix, Fig. S6) were done in a superconducting magnet coupled to ahelium-bath cryostat. A fiber-based probe hosting a miniaturized opticaltable with a set of lenses, mirrors, and filters was used to focalize the laserand collect emitter light. Piezo-stages with x–y–z motion capabilities wereused to position the sample. The sample was cooled down via helium-exchange gas.Data Availability. All of the supplementary data on the optical character-ization of the hBN:C films are provided in SI Appendix, Figs. S1–S6.ACKNOWLEDGMENTS. The work has been supported by the EuropeanUnion (EU) Graphene Flagship project and the Atomically Thin Semiconduc-tors for Future Optoelectronics project (TEAM programme of the Founda-tion for Polish Science, cofinanced by the EU within the European RegionalDevelopment Fund). K.W. and T.T. were supported by the Elemental Strat-egy Initiative conducted by the Japan Ministry of Education, Culture, Sports,Science, and Technology, and the Japan Science and Technology AgencyCore Research for Evolutional Science and Technology Grant JPMJCR15F3.1. M. A. Reshchikov, H. Morkoç, Luminescence properties of defects in GaN. J. Appl.Phys. 97, 95 (2005).2. U. Chandni, K. Watanabe, T. Taniguchi, J. P. Eisenstein, Evidence for defect-mediatedtunneling in hexagonal boron nitride-based junctions. Nano Lett. 15, 7329–7333(2015).3. T. H. Maiman, Stimulated optical radiation in ruby. Nature 187, 493–494 (1960).4. J. D. Barnett, S. Block, G. J. Piermarini, An optical fluorescence system for quan-titative pressure measurement in the diamond-anvil cell. Rev. Sci. Instrum. 44, 1–9(1973).5. J. Yen, M. Nicol, Temperature dependence of the ruby luminescence method formeasuring high pressures. J. Appl. Phys. 72, 5535–5538 (1992).6. R. Brouri, A. Beveratos, J. P. Poizat, P. Grangier, Photon antibunching in the fluo-rescence of individual color centers in diamond. Opt. Lett. 25, 1294–1296 (2000).7. R. Schirhagl, K. Chang, M. Loretz, C. L. Degen, “Nitrogen-vacancy centers in diamond:Nanoscale sensors for physics and biology” in Annual Review of Physical Chemistry, M.A. Johnson, T. J. Martinez, Eds. (Annual Reviews, Palo Alto, CA, 2014), vol. 65, pp. 83–105.8. G. Cassabois, P. Valvin, B. Gil, Hexagonal boron nitride is an indirect bandgap semi-conductor. Nat. Photonics 10, 262 (2016).9. J. D. Caldwell et al., Photonics with hexagonal boron nitride. Nat. Rev. Mater. 4, 552–567 (2019).10. K. Watanabe, T. Taniguchi, H. Kanda, Direct-bandgap properties and evidence forultraviolet lasing of hexagonal boron nitride single crystal. Nat. Mater. 3, 404–409(2004).11. Y. Kubota, K. Watanabe, O. Tsuda, T. Taniguchi, Deep ultraviolet light-emittinghexagonal boron nitride synthesized at atmospheric pressure. Science 317, 932–934(2007).12. M. R. Uddin, J. Li, J. Y. Lin, H. X. Jiang, Probing carbon impurities in hexagonal boronnitride epilayers. Appl. Phys. Lett. 110, 182107 (2017).13. T. T. Tran, K. Bray, M. J. Ford, M. Toth, I. Aharonovich, Quantum emission fromhexagonal boron nitride monolayers. Nat. Nanotechnol. 11, 37–41 (2016).14. L. J. Martinez et al., Efficient single photon emission from a high-purity hexagonalboron nitride crystal. Phys. Rev. B 94, 121405 (2016).15. M. Koperski, K. Nogajewski, M. Potemski, Single photon emitters in boron nitride:More than a supplementary material. Opt. Commun. 411, 158–165 (2018).16. M. Toth, I. Aharonovich, “Single photon sources in atomically thin materials” in An-nual Review of Physical Chemistry, M. A. Johnson, T. J. Martinez, Eds. (Annual Re-views, Palo Alto, CA, 2019), vol. 70, pp. 123–142.17. N. Chejanovsky et al., Single spin resonance in a van der Waals embedded para-magnetic defect. arXiv:1906.05903 (13 June 2019).18. A. Gottscholl et al., Room temperature initialisation and readout of intrinsic spindefects in a van der Waals crystal. arXiv:1906.03774 (10 June 2019).19. L. Weston, D. Wickramaratne, M. Mackoit, A. Alkauskas, C. G. Van de Walle, Nativepoint defects and impurities in hexagonal boron nitride. Phys. Rev. B 97, 214104(2018).20. T. Pelini et al., Shallow and deep levels in carbon-doped hexagonal boron nitridecrystals. Phys. Rev. Mater. 3, 094001 (2019).21. J. Franck, Elementary processes of photochemical reactions. Trans. Faraday Soc. 21,0536–0542 (1926).22. E. Condon, A theory of intensity distribution in band systems. Phys. Rev. 28, 1182–1201 (1926).23. F. Jelezko et al., Spectroscopy of single N-V centers in diamond. Single Mol. 2, 255–260(2001).24. F. Jelezko, J. Wrachtrup, Single defect centres in diamond: A review. Phys. StatusSolidi A Appl. Mat. 203, 3207–3225 (2006).25. A. Alkauskas, B. B. Buckley, D. D. Awschalom, C. G. Van de Walle, First-principlestheory of the luminescence lineshape for the triplet transition in diamond NV cen-tres. New J. Phys. 16, 073026 (2014).26. A. Kiel, Temperature-dependent linewidth of excited states in crystals. I. Linebroadening due to adiabatic variation of the local fields. Phys. Rev. 126, 1292–1297(1962).27. R. H. Silsbee, Thermal broadening of the Mössbauer line and of narrow-line electronicspectra in solids. Phys. Rev. 128, 1726–2835 (1962).28. P. Debye, Interferenz von Röntgenstrahlen und Wärmebewegung. Ann. Phys. 348,49–92 (1913).29. I. Waller, Zur Frage der Einwirkung der Wärmebewegung auf die Interferenz vonRöntgenstrahlen. Z. Phys. 17, 398–408 (1923).13218 | www.pnas.org/cgi/doi/10.1073/pnas.2003895117 Koperski et al.Downloaded at National Institute for Materials Science on July 11, 2020 https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2003895117/-/DCSupplementalhttps://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2003895117/-/DCSupplementalhttps://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2003895117/-/DCSupplementalhttps://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2003895117/-/DCSupplementalhttps://www.pnas.org/cgi/doi/10.1073/pnas.200389511730. K. Huang, A. Rhys, Theory of light absorption and non-radiative transitions inF-centres. Proc. R. Soc. London Ser. A Math. Phys Sci. 204, 406–423 (1950).31. M. Lax, The Franck-Condon principle and its application to crystals. J. Chem. Phys. 20,1752–1760 (1952).32. J. J. Markham, Interaction of normal modes with electron traps. Rev. Mod. Phys. 31,956–989 (1959).33. G. Davies, Vibronic spectra in diamond. J. Phys. C Solid State Phys. 7, 3797–3809(1974).34. G. Davies, The Jahn-Teller effect and vibronic coupling at deep levels in diamond.Rep. Prog. Phys. 44, 787–830 (1981).35. U. Rothamel, J. Heber, W. Grill, Vibronic sidebands in ruby. Z. Phys. B Condens. Matter50, 297–304 (1983).36. S. Sugano, Y. Tanabe, The line spectra of Cr-3+ ion in crystals. Discuss. Faraday Soc. 26,43–48 (1958).37. S. M. Sharma, Y. M. Gupta, Theoretical analysis of R-line shifts of ruby subjected todifferent deformation conditions. Phys. Rev. B Condens. Matter 43, 879–893 (1991).38. H. Klein, U. Scherz, M. Schulz, H. Setyono, K. Wisznewski, Temperature dependenceof the EPR spectrum of ruby. Z. Phys. B Condens. Matter 28, 149–157 (1977).39. W. C. Zheng, S. Y. Wu, Theoretical studies of the temperature dependence of zero-fieldsplitting of Cr3+ centers in ruby. Phys. Rev. B Condens. Matter 54, 1117–1122 (1996).40. S. A. Tawfik et al., First-principles investigation of quantum emission from hBN de-fects. Nanoscale 9, 13575–13582 (2017).41. J. A. Calviello, E. W. Fisher, Z. H. Heller, Direct 2T1-2E phonon relaxation in ruby and itseffect upon R-line breadth. J. Appl. Phys. 37, 3156 (1966).42. A. Jablonski, Efficiency of anti-Stokes fluorescence in dyes. Nature 131, 839–840(1933).43. T. Taniguchi, K. Watanabe, Synthesis of high-purity boron nitride single crystals underhigh pressure by using Ba-BN solvent. J. Cryst. Growth 303, 525–529 (2007).Koperski et al. PNAS | June 16, 2020 | vol. 117 | no. 24 | 13219APPLIEDPHYSICALSCIENCESDownloaded at National Institute for Materials Science on July 11, 2020