atomic structure of carbon centres in hbn: towards
TRANSCRIPT
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Atomic structure of carbon centres in hBN: towards engineering of single photon sources
Z. Qiu1, K. Vaklinova2, P. Huang1,3, M. Grzeszczyk1,3, H. Yang4, K. Watanabe5, T. Taniguchi6,
K. S. Novoselov1,3, J. Lu2,4,*, M. Koperski1,3,**
1 Department of Materials Science and Engineering, National University of Singapore, 117575, Singapore 2 Centre for Advanced 2D Materials, National University of Singapore, 117546, Singapore
3 Institute for Functional Intelligent Materials, National University of Singapore, 117544, Singapore 4 Department of Chemistry, National University of Singapore, 117543, Singapore
5 Research Center for Functional Materials, National Institute for Materials Science, Tsukuba 305‐0044, Japan 6 International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 305‐0044, Japan
E‐mail: * [email protected]; ** [email protected]
Revealing atomic and electronic structure of quantum defects in carbon doped
hexagonal boron nitride (hBN:C) is crucial for the development of future technology to
integrate them within solid state devices. Here, we investigate atomically thin hBN:C
films deposited on graphite substrates via scanning tunnelling microscopy and
spectroscopy. We observe positively charged defect centres which lower the onset of
differential conductance below the conduction band of the host material and induce
narrow resonances in the mid‐gap regime of the tunnelling spectra. We corroborate
atomically‐resolved lattice structure of defect sites and their mid‐gap states observed
in tunnelling dI/dV spectra with the predictions of the atomic and electronic structure
of carbon impurities in hexagonal boron nitride by density functional theory. Our
findings demonstrate that carbon substitution for boron constitutes a stable and
energetically favourable impurity. Its unique properties, when analysed comparatively
with other plausible defect centres, provide a good agreement with our experimental
observations.
Defect centres in crystals can transform the characteristics of host materials by modulating
their fundamental electronic properties or even raising novel phenomena. Recently, optical inspection
of hexagonal boron nitride (hBN) unveiled a variety of luminescent centres that act as single photon
sources in extraordinarily broad spectral range1‐4. The wide band gap character of hBN5,6 enables
formation of emitters that are optically active in ultraviolet, visible and infrared region. Moreover,
these luminescent centres appear to be particularly resilient by displaying stable photoluminescence
(PL) and single photon emission in ambient conditions7, despite being unprotected from environment
in atomically thin hBN films.
Currently, the pending identification of specific defects in hBN constitutes a fundamental
limitation in understanding the electronic structure of the emergent luminescent centres. Resolving
this issue can establish novel atomically precise systems based on doped hBN that provide
functionalities in the domain of spin manipulation, quantum sensors and light sources akin to modern
utilities of well‐known defect centres in 3D lattices. Moreover, creation and characterisation of specific
defects in atomically thin films, down to individual monolayers, is likely to enable a range of new
applications. These may include efficient coupling of the emissive centres to photonic structures,
fabrication of strongly planarized single‐spin devices or electrically driven on‐demand single photon
sources.
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In order to progress towards these exciting functionalities, we identify defect centres in
carbon‐doped few‐layers hBN films via scanning tunnelling probes8,9. By displaying homogenous and
reproducible optical response, these films became perfect candidates for the inspection of the nature
of the emergent defects in atomic scale. In this work, we utilize the two‐dimensional (2D) crystal
structure of hBN, to create heterostructures enabling characterisation of defect centres by scanning
tunnelling microscopy (STM) and scanning tunnelling spectroscopy (STS)10‐18. Particularly, we inspect
thin layers of carbon doped hBN (hBN:C), unveiling the presence of a specific defect centre: a carbon
substitution for boron (CB). We explore its atomic and electronic structure by the tunnelling probes
and corroborate our findings with density functional theory (DFT) predations of the properties of
single‐particle mid gap states arising due to such carbon impurity.
The hBN crystals used for this study were grown by the temperature gradient method in high‐
pressure and high‐temperature conditions19. The carbon doping of hBN was induced by annealing the
pristine hBN specimens in a graphite furnace for 1 h at 2000 °C. The inspection of low temperature
(1.6 K) PL spectra of exfoliated hBN:C films demonstrate a spatially homogenous optical response as
illustrated in Fig. 1. A specific pattern of optical resonances arises within a spectral region of 1 eVA
width in visible and near infrared range. The reproducible multi‐line pattern seen in the PL spectra is
enriched by additional resonances at specific spots of the hBN:C films which can be identified by PL
mapping experiments (Fig. 1(c,d)). Overall, the complex optical response of the hBN:C films is unlikely
to arise exclusively due to a single type of a defect centre hence further characterisation methods are
needed to unveil their atomic and electronic structure. In order to enable STM inspection of highly
A The energy region of observable PL resonances is limited by the detection range of a silicon charge coupled device camera (at low energy side) and by the laser excitation conditions (at high energy side).
Figure 1. Optical characterisation of exfoliated hBN:C films. (a) The PL spectra were measured for several
flakes under 514 nm (2.41 eV) excitation conditions at low temperature. Multiple resonances form a
reproducible spectral pattern with additional lines appearing at specific spots within the hBN:C films. (b) The
optical images of the flakes confronted with PL mapping experiments, when (c) the PL intensity is monitored
at the energy of particular lines, demonstrate homogenous optical response and (d) enables identification of
spots with localised emitters.
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resistive hBN crystals, we fabricated heterostructures that are composed of 3 atomic layers (3L) of
hBN:C deposited onto conductive graphite layers that allows probing of the hBN:C lattice via local
tunnelling of charge carries. The use of several layers of hBN:C allows for the top layer to be sufficiently
decoupled from graphite, whereas still allowing investigation of STM. A schematic representation of
our experiment is shown in Fig. 2a. The initial low temperature (T = 4.7 K) STM characterisation of our
samples reveals emergence of a moiré superlattice within the region of the heterostructure14,20‐26.
Based on the STM image presented in Fig. 2b, we establish the periodicity of the moiré pattern to be
4.16 ± 0.01 nm, which corresponds to a twist angle of θ=3.3° between the graphite and hBN:C
hexagonal lattices. Independently, STM image of hBN:C acquired over the surface region free of
defects/impurities resolves the lattice constant of 0.25 ± 0.01 nm, consistent with that of pristine
hBN27.
Multiple defect centres manifest themselves in large‐scale STM images via modulation of the
density of states available for the tunnelling processes (Fig. 3a). The best contrast for visualisation of
these defect sites is achieved at relatively high positive sample bias VS, at which individual defect site
exhibits a circular protrusion. A statistical analysis of multiple STM images reveals a defect density of
2.4×1010 cm‐2 per atomic layer in our hBN:C material (under the assumption of a homogeneous
distributions of the defect sites across the entire heterostructure). It is also noted that defect centres
show three representative apparent heights in the constant‐current STM images (Fig. S2), suggesting
that these defects are located in three different layers of hBN:C flakes. The majority of defect sites
display bright contrast at positive sample bias and dark halo at negative sample bias, as illustrated in
Fig. 3b and 3c respectively. This bias‐dependent STM contrast signifies that defect centres are
positively charged, creating a local electrostatic field that facilities (suppresses) electron (hole)
tunneling into empty states (occupied states), respectively28,29. The atomic‐resolution STM image
taken at 𝑉 3 V (Fig. 3d) reveals intact atomic lattice expected for hBN:C but with a slight
modification of lattice contrast, suggesting that these defects are likely created by the substitution of
N or B atoms with carbon atoms.
Figure 2. STM characterisation of a van der Waals hBN:C/graphite heterostructure. (a) A schematic
illustration shows our experimental configuration. The graphite substrate is grounded via a golden electrode
while the sample bias (VS) is applied to the STM tip. (b) Atomically‐resolved STM image of hBN:C layers
reveals emergence of a moiré superlattice in the region of interest (VS = 3.2 V, It = 10 pA, T = 4.7 K). We utilise
the appearance of the moiré pattern as an indicator that we probe the tunnelling processes within the area
of the hBN:C/graphite heterostructure.
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For further identification of the emergent defect types, we performed spatially resolved dI/dV
spectroscopic measurement over a pristine hBN lattice site and this dominant defect siteB. Within the
region of perfect hBN crystal structure, the onsets of the differential conductance correspond to the
edges of the valence and conduction bands for the negative and positive sample biases, respectively.
The band edges are better determined by further conducting the logarithm of dI/dV curve shown in
Figure 4a, whereby the band gap of 3L hBN film deposited on a graphite substrate is determined to be
6.20 0.29 eV (refer to Fig. S4 in S. I. appendix30). In contrast to perfect hBN region, dI/dV spectrum
taken over this dominant defect in hBN reveals prominent mid‐gap states spanning over a bias range
of 1.5 𝑉 𝑉 3.25 𝑉 . Multiple narrow resonances appear which form a band of about 1.5 eV width
below the conduction band. We interpret the appearance of the mid‐gap states as evidence of charge
tunnelling processes via vacant defect‐induced level(s). The broadband character of the sub‐
conduction band states indicates the existence of tunnelling processes with simultaneous emission of
phonons31 into the hBN lattice, forming repetitive replicas of the electronic states in dI/dV curves. The
comparison between the dI/dV resonances with the phonon density of states for the hBN lattice is
presented in Fig. S6.
There are two possible types of defects that can introduce donor states below the conduction
band without creating acceptor states above the valence band: (1) nitrogen vacancy (VN) and (2) carbon
substitution for boron (CB). As both types of defects could give rise to the tunnelling characteristics
observed in dI/dV spectra, we performed additional analysis of the defect stability and electronic
structure by DFT. Firstly, we compared the formation energy of single site and double site defects in
carbon doped hBN crystal as presented in Fig. 5a. We found that among single site defects, VN is
characterised by the largest and CB by that lowest formation energy. Moreover, our optical
characterisation of carbon doped specimen reveals activation of optical response that follows Franck‐
B The inspected defect sites were randomly selected from the group of defects identified in the large scale mapping experiment that show positive/negative contrast at positive/negative sample bias. We found that the presence of the donor level below the conduction band is a common observation for these types of defect. The lattice imperfections with the opposite contrast display unstable dI/dV characteristics.
Figure 3. Real‐space STM imaging of defect centres. The STM experiment allows for the visualisation of
atomic structures of defects within individual atomic layers of hBN:C. (a) Large‐scale STM image reveals the
presence of multiple defect centres (𝑉 6.8 V, 𝐼 10 pA). The majority of the defect centres exhibit (b)
bright protrusion acquired at positive sample biases 𝑉 4.5 V, 𝐼 100 pA) and (c) dark halo acquired at negative sample biases (𝑉 5 V, 𝐼 100 pA). (d) Lattice‐resolved STM image reveals the atomic
structure of individual defect centres (𝑉 3 V, 𝐼 50 pA). Scale bar is 2 nm in panel (b‐d).
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Condon principle with a significant electron‐phonon coupling strength8,9. The emergence of multiple
phonon replicas, creating phonon sidebands that dominate over zero‐phonon line in the optical
spectra, is consistent with the multiline band observable in dI/dV spectra taken at the defect sites.
Therefore, we conclude that CB is a common defect centre in carbon doped hBN samples that is likely
to give rise to mid‐gap photoluminescence.
From the point of view of electronic structure32‐35 of CB defect, carbon introduces one
additional valence electron at the lattice site of boron, which gives rise to a half‐filled mid‐gap level
below the conduction band. The orbital composition of the defect‐induced level originates mostly from
the pz orbital of the C atom. If the energy position of such level is higher than the Fermi level in graphite
– the extra electron will tunnel to graphite substrate, leaving the defect positively charged36, consistent
with our STM observation.
In conclusion, scanning probe techniques allowed us to identify CB as a defect centre
observable in carbon doped hBN crystals. We explored the atomic and electronic structure of this
defect via STM and STS characterisation in combination with an analysis of its properties based on DFT
theory. Our findings provide insights into the formation of a stable carbon‐substituted defect centre
in favour of other possible carbon‐based impurities. The 2D character of hBN allows isolation of
atomically thin films that host stable defect, which creates an opportunity to integrate them within
solid state and/or flexible membrane‐type devices. It is plausible, that with the increasing
understanding of the atomic structure of defects in hBN, they may become realistic counterparts for
other useful defect centres in wide gap materials, such as nitrogen‐vacancy centres in diamond.
Figure 4. Mid‐gap states introduced by the defect. (a) dI/dV curves taken over a pristine hBN lattice and (b)
over a defect site. The impact of the defect site is readily seen in the tunnelling spectra as a shift of the onset
of dI/dV signal on the positive bias side combined with an emergence of a narrow resonance. (c) A small‐
scale dI/dV STS curve over a defect site for 1.5 𝑉 𝑉 3.25 𝑉.
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Methods
Sample fabrication. Graphite and hBN:C crystals were mechanically cleaved onto silicon wafers with
300 nm and 90 nm layers of SiO2, respectively. Large‐area graphite films were selected to act as
conductive substrates for atomically thin hBN:C films (3 layers). The hBN:C flakes were transferred
onto the graphite substrate via a pickup technique. The hBN:C flakes of desired thickness were
identified by their optical contrast and atomic force microscopy. They were lifted from the
Si/SiO2 wafer with a polydimethylsiloxane/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
residues. Prior to the STM measurements, the sample was annealed in situ in ultra‐high vacuum at
310°C for 12 h to remove surface residues and adsorbates.
Optical characterisation. The photoluminescence (PL) spectra were measured in microscopic (1 m
spot size) back‐scattering 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 PL
mapping experiment. The PL signal from the sample was collected by a multimode fiber with 50 m
core diameter, dispersed by a 0.75 m spectrometer with 300 g/mm grating and detected by liquid‐
nitrogen‐cooled charge coupled device camera.
Figure 5. The density functional theory predicts the formation energy and electronic structure of various
types of single site and double site defect centres in carbon doped hBN lattices. (a) The values of the
formation energy are presented for boron vacancy (VB), nitrogen vacancy (VN), carbon substitution for boron
(CB), carbon substitution for nitrogen (CN) and their combinations CBCN, CNVB and CBVN. (b) The electronic
structure of CB opens tunnelling pathways that are consistent with the tunnelling dI/dV spectra observable
for defect sites in carbon doped hBN films. (c) Schematic illustration of the atomic structure of single site and
double site defect centres in hBN:C.
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STM and STS measurements. Our STM and STS measurements were conducted at 4.7 K in the Createc
LT‐STM system with a base pressure lower than 10‐10 mbar. Tungsten tip was calibrated
spectroscopically against the surface state of Au(111) substrate. All 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) 37. All
calculations were spin‐polarized and used the projector augmented wave (PAW) pseudo‐potential
supplied with the VASP code38,39. A plane wave cut‐off of 500 eV was used. Pristine single layer hBN
was first optimized for the lattice geometry using the conventional cell and a 21x21x1 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 5x5x1 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 ‐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 the Ref.32.
Acknowledgements
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 (MOE2019‐T2‐2‐044). This material is based upon work supported by the Air Force European
office of Aerospace Research and Development under award number FA8655‐21‐1‐7026. K.W. and T.T.
acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan (Grant
Number JPMXP0112101001) and JSPS KAKENHI (Grant Numbers JP19H05790 and JP20H00354).
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