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ArticleHexagonal Boron Nitride As an Ideal Substrate for Carbon Nanotube
Photonics
N. Fang, K. Otsuka, A. Ishii, T. Taniguchi, K. Watanabe, K. Nagashio, and Y. K. Kato*
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sı Supporting Information*

ABSTRACT: Hexagonal boron nitride is widely used as a
substrate for two-dimensional materials in both electronic andphotonic devices. Here, we demonstrate that two-dimensional
hexagonal boron nitride is also an ideal substrate for onedimensional single-walled carbon nanotubes. Nanotubes directly
attached to hexagonal boron nitride show bright photoluminescence with narrow line width at room temperature, comparable toair-suspended nanotubes. Using photoluminescence excitation
spectroscopy, we unambiguously assign the chiralities of nanotubes
on boron nitride by tracking individual tubes before and after
contact with boron nitride. Although hexagonal boron nitride has alow dielectric constant and is attached to only one side of thenanotubes, we observe that optical transition energies are redshifted as much as ∼50 meV from the air-suspended nanotubes. We also perform statistical measurements on more than 400 tubes,
and the redshifts are found to be dependent on tube diameter. This work opens up new possibilities for all-solid-state carbon
nanotube photonic devices by utilizing hexagonal boron nitride substrates.
KEYWORDS: carbon nanotubes, hexagonal boron nitride, photoluminescence, hybrid-dimensional system, heterostructureH

optical properties on h-BN because of the low defect density,
chemical inertness, hydrophobic nature, and the absence of
dangling bonds on the surface. In fact, suppression of spectral
ﬂuctuations at low temperatures has been observed,17suggesting the potential beneﬁts of using h-BN as a substrate
for CNTs. Making use of h-BN eﬀects on excitons in
nanotubes and eliminating the strong substrate quenching
would open up a pathway to all-solid-state CNT photonics.
Here we investigate two diﬀerent types of heterostructuresconsisting of h-BN and CNT by using PL excitation (PLE)
spectroscopy. In samples where h-BN ﬂakes are transferred on
individual air-suspended CNTs, the chiralities are unambiguously assigned, and the same tubes are tracked for h-BN eﬀects.
Bright luminescence with a narrow line width is observed fromthe CNTs directly attached to h-BN at room temperature,indicating weak quenching and small broadening eﬀect from hBN. Excitonic energies are found to exhibit considerable
redshifts as well. We also study CNTs transferred on h-BN
ﬂakes to obtain statistical information on the energy shifts fromexagonal boron nitride (h-BN), a two-dimensional (2D)
material, has played a key role in the development of vander Waals devices. Because of the atomically thin nature, bothelectrical and optical properties of 2D materials are extremely
sensitive to the substrate they contact.1−4 Compared withconventional substrates such as SiO2/Si, h-BN is atomically ﬂatwith low defect density, thereby enabling substantial improvements of device performance and discovery of novel physicalphenomena. In electronic devices, the use of h-BN has led tothe mobility enhancement of single-layer graphene,2,5 as well as
the appearance of superconductivity for double-layer graphene
at the magic angle.6 Encapsulation by h-BN reduces the line
widths of photoluminescence (PL) from transition metal
dichalcogenides,7,8 and interlayer excitons9,10 in h-BN/WSe2/
MoS2/h-BN heterostructures are utilized in novel excitonic
devices.11
It is natural to envision a continued success of h-BN
substrates in hybrid-dimensional systems, where heterostructures consisting of materials with mixed dimensions are
employed.12 In particular, we expect that the advantages ofh-BN will be most evident in structures involving onedimensional (1D) carbon nanotubes (CNTs), which would
comprise 1D/2D heterostructures. The van der Waals
interaction should be similar to graphene/h-BN heterostructures, leading to a high-quality atomically ﬂat interface. In
comparison to conventional solid-state substrates that cause
strong PL quenching of CNTs,13−16 we anticipate improved© 2020 American Chemical Society

Received: March 12, 2020
Published: June 2, 2020

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ArticleThe h-BN/CNT structures are investigated with a homebuilt confocal PL microscopy system at room temperature.18,23−27 A wavelength-tunable Ti:sapphire laser is used
for excitation after controlling its power P by neutral density
ﬁlters, and the polarization is adjusted by a half-wave plate tobe parallel with the CNT being measured. The laser beam is
focused on the sample using an objective lens with a numericalaperture of 0.8 and a working distance of 3.4 mm. The
reﬂected beam from the sample is detected by a photodiode tomap out an image, which can be used to identify the h-BN
ﬂakes. PL is collected through the same objective lens anddetected using a liquid-nitrogen-cooled 1024 pixel InGaAs
diode array attached to a spectrometer. All measurements are
performed in dry nitrogen to avoid the formation of waterinduced defects.
Figure 1c shows a reﬂectivity image of the h-BN ﬂake withthe target CNT below, reproducing the shape of the h-BN
ﬂake observed in the optical microscope image (Figure 1b).
Using the reﬂectivity image, we conﬁrm that the nanotube isunderneath the h-BN ﬂake and PL spectroscopy is performed.
A PL image of a tube before and after the transfer of h-BN isshown in Figure S1 with no distinct change, implying ahomogeneous interface between CNT and h-BN.
We characterize the h-BN eﬀects on the CNTs bycomparing PLE maps of individual CNTs before and after
the transfer of h-BN (Figure 1d,e). A clean PLE map can beobtained even after the transfer, since the PL intensity remains
suﬃciently strong. PL intensity is stable over 1 h withextremely small spectral ﬂuctuations, as shown in Figure S2,
which is consistent with CNTs grown on h-BN.17 The
reduction ratio of PL intensity after transfer compared tobefore transfer averages to 0.48 over 8 tubes, while the PL
reduction ratio is typically less than 0.01 in the current solidstate substrates such as SiO2/Si, quartz, and metals.13−16,28 It is
known that ﬂuorescence from an emitter close to an interface
is directed into the medium with a higher refractive index.29,30
We calculate this eﬀect, as shown in Figure S3, and ﬁnd thatthe radiated power of the h-BN/CNT sample has anenhancement factor of 1.7 over air-suspended CNT. The
modest reduction of PL after the transfer of h-BN therefore
cannot be simply explained by the directed emission, but
indicates the quenching eﬀect from h-BN is weak, making hBN a promising substrate for light-emitting CNT devices. We
point out that the full width at half-maximum for E11 emission
shows only a slight increase by 2.2 meV. The line width of hBN/CNT is much smaller than CNTs on other solid-state
substrates,13−15,28 while the inhomogeneous line width broadening of the ground exciton of WS2(WSe2) in 2D WS2(WSe2)/h-BN heterostructures is reported to be ∼2 meV at low
temperatures.1 The small change in the line width at room
temperature suggest that our 1D/2D heterostructures show
comparable interfacial properties to 2D/2D heterostructures.
The PLE maps also show substantial modiﬁcations of theexcitonic energies. We identify the nanotube chirality from the
PLE map before transfer (Figure 1d) to be (10,8) by extracting
the E11 and the E22 energies from the emission and theexcitation resonances, respectively.18 After the transfer of hBN, we observe ΔE11 = 21 meV and ΔE22 = 30 meV, where
ΔEii is the excitonic energy reduction and i is the index of thetransition. Dielectric screening from h-BN is expected to causesome redshift, but the observed shifts are large considering thefact that h-BN is only attached to one side of the CNT. The
excitonic energy shifts are further investigated in detail byover 400 tubes. The redshifts are found to be dependent on the
CNT diameter, and smaller-diameter tubes show unexpectedly
large E11 and E22 redshifts of ∼50 meV. The observed
magnitude of the shifts are too large to explain by dielectric
constants alone, considering that h-BN is only attached to one
side of the CNT and the other side remains unscreened. The
results demonstrate the ideal properties of h-BN as a substrate
for CNT photonic devices.
We begin by studying h-BN/CNT heterostructures consisting of an h-BN ﬂake on top of an individual air-suspended
CNT, as shown in Figure 1a. First, the air-suspended CNTs

Figure 1. (a) Device schematic of an air-suspended CNT with an hBN ﬂake transferred on top. (b) Optical microscope image of a typicalh-BN/CNT sample. The broken-line box indicates the area measuredin (c). (c) Reﬂectivity image of the h-BN/CNT sample shown in (b).
Scale bars in (b) and (c) are 20 and 5 μm, respectively. (d) and (e)PLE maps of an air-suspended (10,8) CNT before and after thetransfer of a ∼20 nm thick h-BN ﬂake, respectively. The PLE maps aretaken with P = 10 μW. The line widths for E11 emission before andafter the transfer are 9.9 and 12.1 meV, respectively.

are prepared on silicon substrates.18 Electron-beam lithography
and dry etching processes are used to fabricate trenches andalignment marks, followed by thermal oxidation to form 70 nm
thick SiO2. Catalyst regions near the trenches are patterned bya second electron beam lithography step, and a 1.5 Å Fe ﬁlm isdeposited as a catalyst for the CNT growth by an electronbeam evaporator. The CNTs are synthesized by alcohol
chemical vapor deposition at 800 °C for 1 min.19 The Fe ﬁlm
thickness is optimized to control the yield such that most of
the synthesized CNTs are isolated. Next, an h-BN ﬂake istransferred on top of a CNT with the help of the marks on thesubstrate. The h-BN ﬂakes are prepared on polydimethylsiloxane (PDMS) by mechanical exfoliation from crystals supplied
by NIMS and then transferred on the target CNTs at 120 °C
by using a micromanipulator system.4 Transferred h-BN
thickness ranges within 20−90 nm, as determined by atomic
force microscopy (AFM) and optical contrast.20−22 A typical hBN/CNT sample is shown in Figure 1b.
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Articletaking advantage of h-BN/CNT heterostructures where theredshifts can be tracked for individual nanotubes with known
chirality.
We ﬁrst consider the sensitivity of the redshifts on h-BN
thickness. Two h-BN/CNT samples are prepared from (13,5)
nanotubes using h-BN ﬂakes with thicknesses of 20 and 71 nm,
which we will refer to as thin and thick h-BN samples,
respectively (insets of Figure 2a,b). We integrate the PLE maps

Figure 3. PLE peak energies of 19 h-BN/CNT samples before h-BNtransfer (blue dots) and after h-BN transfer (red dots) obtained fromLorentzian ﬁts. Lines connect the energies before and after the h-BNtransfer for each sample.

the bottom side of the CNT should be unscreened. We also
observe some tube-to-tube variations of the shift values within
the same chirality. The (10,8) CNTs show an anticorrelated
dispersion of the E11 and E22 energies after the transfer, which
may be caused by strain.32−35
Up to this point, we have been focusing on h-BN/CNT
samples with individual nanotubes that allow for chirality
identiﬁcation before h-BN transfer. Now that we understandthe general behavior of the CNTs upon contact with h-BN, we
are in a good position to study an ensemble of CNTs directlytransferred onto h-BN without prior characterization. CNTs
are ﬁrst grown on a SiO2/Si substrate, and we prepare h-BN
ﬂakes on another SiO2/Si substrate by mechanical exfoliation.4
The CNTs are picked up by using a PDMS/anthracene stamp
and transferred on the target h-BN ﬂake by using the
micromanipulator system. PDMS is peeled oﬀ and anthracene
is sublimated at 110 °C, leaving a clean surface for CNTs. We
will refer to this structure as CNT/h-BN, and thecorresponding schematic and optical microscope image areshown in Figure 4a and b, respectively. It is noted that CNTs
are transferred on the surrounding SiO2/Si substratesimultaneously due to the large area of the PDMS/anthracene
stamp. An AFM phase image of CNTs on both h-BN and the
SiO2/Si substrate is shown in Figure 4c, and it is observed thatthe h-BN ﬂake shows smaller surface roughness than that of
the SiO2/Si substrate. Many randomly oriented CNTs are
ﬁrmly attached to both h-BN and the SiO2/Si substrate
without showing any loose ends or segments. Zoomed-in AFM
images are shown in Figure S4, where the clean surface of
CNTs is observed.
Integrated PL images are taken in the same region as the
AFM image and shown in Figure 4d. Although CNTs aretransferred on both the h-BN ﬂake and the SiO2/Si substrate,
only CNTs on h-BN show bright PL. In previous reports oftransferred CNTs on quartz or glass substrates, the dominantfactor that aﬀects the PL intensity was pointed out not to be
the substrate itself but the elimination of interaction between
substrates and CNTs by the transfer process.14 In contrast, ourresults reveal that the PL quenching eﬀect is signiﬁcantly

Figure 2. (a, b) Integrated and normalized PL spectra for airsuspended (13,5) CNTs before (blue) and after (red) the transfer ofh-BN ﬂakes with thicknesses of (a) 20 nm and (b) 71 nm. The insetsin (a) and (b) are optical microscope images of the h-BN/CNT
samples, and scale bars are 20 μm. (c, d) Integrated and normalized
PLE spectra for the same CNTs before (blue) and after (red) thetransfer of h-BN ﬂakes with thicknesses of (c) 20 and (d) 71 nm. P is
ﬁxed at 10 μW.

for each nanotube along the excitation energy axis to obtain PL
spectra, and the normalized spectra are shown in Figure 2a,b.
The thin and thick h-BN samples show ΔE11 = 23.8 and 24.4
meV, respectively. Similarly, PLE spectra shown in Figure 2c,dare obtained by integrating the PLE maps along the emission
energy axis, where we observe ΔE22 = 18.6 and 18.7 meV for
the thin and thick h-BN samples, respectively. For both ΔE11
and ΔE22, there are no notable changes between the two
samples. We have checked the eﬀect of h-BN thickness in three
more sets of h-BN/CNT samples for other chiralities, and thediﬀerences in ΔE11 and ΔE22 between samples with thin (<25nm) and thick (>50 nm) h-BN are found to be less than 3
meV, with no clear thickness dependence. The insensitivity ofthe energy shifts on h-BN thickness indicates that 20 nm ﬂakes
are thick enough for the full screening of CNT excitons in theperpendicular direction. We note that E11 and E22 energy shifts
can be considered independent of the h-BN thickness
hereafter, as h-BN ﬂakes thinner than 20 nm are not used inthis study.
As the redshifts are not sensitive to the h-BN thickness, thegeneral behavior of the excitonic energy shifts caused by h-BN
can be identiﬁed by measuring more individual tubes with
diﬀerent chiralities. E11 and E22 energies before and after thetransfer of h-BN are extracted and plotted in Figure 3. All 19
CNTs show redshifts in E11 and E22, which is consistent withthe dielectric screening eﬀect. The average ΔE11 and ΔE22 are
24.3 and 22.0 meV, respectively, comparable to the energy
diﬀerence of micelle-wrapped tubes with respect to airsuspended tubes.31 Again, the values of the shifts are
considered large, as h-BN only sits on top of the CNT and1775

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ArticleFigure 4. (a) Schematic of CNTs transferred on an h-BN ﬂake. (b) An optical microscope image of the CNT/h-BN sample. White and black boxes
indicate the areas measured in (c) and (d), respectively. (c) AFM phase image at the edge of the h-BN ﬂake. (d) An integrated PL image at theedge of the h-BN ﬂake. In order to detect nanotubes of various chiralities, four PL images are taken using excitation energies at 1.442, 1.512, 1.570,
and 1.699 eV with P = 100 μW. The four images are constructed by integrating the emission spectra, and the sum of the PL images is shown here.
The boundary of the h-BN ﬂake, as conﬁrmed by a reﬂectivity measurement is indicated by a broken line. Scale bars in (b), (c), and (d) are 50, 2,
and 4 μm, respectively.

Figure 5. (a) PLE peaks of 430 measured CNT/h-BN samples (light red dots). Exciton energies for air-suspended CNTs (blue dots)18 and values
for CNT/h-BN (red open circle) calculated from eq 1 are also shown. (b) and (c) ΔE11 and ΔE22, respectively, as a function of 1/dt for (8,7),(9,7), (9,8), (10,5), (10,6), (10,8), (11,3), (11,6), (11,7), (12,7), and (13,5) CNTs. The chiralities have been limited to those with unambiguous
assignment. Green dots indicate type I tubes, while orange triangles indicate type II tubes. Error bars in (b) and (c) represent standard deviations.

transfer in CNT bundles.36 PLE maps with clear features are
selected and the E11 and E22 peak positions are plotted in
Figure 5a. Since the dielectric environment for CNTs in the
CNT/h-BN structure should, in principle, be the same as thosein the h-BN/CNT samples, redshifts in E11 and E22 are
expected to be similar. By examining the correlations with thepeak positions in Figure 3, we assign the chiralities for the
CNT/h-BN sample. In comparison with the h-BN/CNT
structures, however, the CNT/h-BN sample shows slightly
larger redshifts for CNTs with the same chirality. The
variations of the redshifts are also larger, with a standard
deviation of E11 in the range of 4−11 meV. The increased
redshifts could be attributed to better adhesion between thetransferred CNTs and h-BN, while the variations might beaﬀected by the substrate itself and indicate the superior
compatibility between 1D and 2D materials. Compared withconventional substrates, the h-BN substrate is atomically ﬂatwith no dangling bonds and low defects density, which might
contribute to the considerable suppression of the substratequenching eﬀect.
In order to obtain statistical information from the ensembleof CNTs, we have collected more than 400 PLE maps from the
CNT/h-BN samples. Typical PLE spectra are provided in
Figure S5. Unlike the CNTs grown over trenches in Figure 1,
the transferred CNTs can form bundles because of theirrandom orientation and high density. We observe some PLE
maps showing excitation spectra with smeared-out features,and we exclude these PLE maps as they indicate exciton energy
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ArticleTable 1. Average E11 and E22 Peak Energies for Carbon Nanotubes on h-BN Obtained from Lorentzian Fits of PLE Mapsa
E11

E22

(n, m)

nm

meV

nm

meV

(8,6)
(8,7)
(9,7)
(9,8)
(10,5)(10,6)(10,8)(11,3)(11,6)(11,7)(12,7)(13,5)

1201.8 ± 5.5
1288.7 ± 8.0
1343.7 ± 14.1
1424.3 ± 8.1
1266.4 ± 13.2
1397.1 ± 17.0
1478.0 ± 10.2
1223.7 ± 7.3
1415.5 ± 10.6
1529.7 ± 12.8
1550.1 ± 13.3
1502.3 ± 9.0

1031.6 ± 4.7
962.1 ± 6.0
922.7 ± 9.7
870.5 ± 4.9
979.0 ± 10.2
887.4 ± 10.8
838.8 ± 5.8
1013.2 ± 6.0
875.9 ± 6.5
810.5 ± 6.8
799.8 ± 6.9
825.3 ± 4.9

729.7 ± 5.2
740.1 ± 4.5
802.5 ± 4.7
818.3 ± 5.4
794.8 ± 3.0
766.3 ± 5.6
873.0 ± 5.4
804.9 ± 6.6
863.8 ± 5.5
852.8 ± 5.7
930.3 ± 6.0
926.7 ± 4.5

1699.0 ± 12.2
1675.2 ± 10.1
1544.9 ± 9.0
1515.1 ± 10.1
1559.8 ± 5.9
1618.0 ± 11.8
1420.2 ± 8.8
1540.3 ± 12.7
1435.3 ± 9.2
1453.8 ± 9.7
1332.7 ± 8.6
1337.9 ± 6.5a
The error values are standard deviations. Some chiralities are not listed, since the large energy variations prevent us from diﬀerentiating thechiralities with similar energies such as (13,2) and (14,0).

and out-of-plane dielectric constants for bulk h-BN are 6.9 and
3.8, respectively,42 whereas the value for acetone is 20.7.
Moreover, CNTs on h-BN should not be screened on the top
side, while CNTs in acetone are fully surrounded by thesolvent. These two factors should lead to reduced screening
eﬀects for CNT/h-BN in a simple dielectric screening picture.
Instead, it appears that the h-BN substrate eﬃciently screens
the excitons in the CNTs, and our observation shows that thedielectric constants alone cannot explain the large redshift
caused by h-BN. It is suggested that there are additional
mechanisms contributing to the spectral changes, possibly
perturbations to electronic structures. Further theoretical study
would be needed to clarify the origin and to explain eq 1. We
note that larger ΔEii is expected in 2D materials other than hBN because they usually have larger dielectric constants, whichis also promising for the dielectric engineering of CNTs.
In summary, we have investigated h-BN eﬀects on the
optical properties of CNTs by performing PL spectroscopy on
h-BN/CNT and CNT/h-BN heterostructures at room temperature. We have demonstrated that CNTs directly attached to hBN are highly luminescent with narrow line widths of ∼12
meV, which is comparable to air-suspended CNTs. The
substrate quenching and broadening eﬀects on the h-BN
substrates are found to be much weaker than those inconventional substrates such as quartz and SiO2/Si. In
addition, the anomalously large redshifts in E11 and E22 of∼50 meV are observed, despite the fact that h-BN has a low
dielectric constant and is only attached to one side of the
CNT. The statistical results of PLE maps from over 400 tubes
identify the relationship between the redshifts and the CNT
diameter. These ﬁndings highlight the superior properties of hBN for 1D/2D hybrid-dimensional photonics and open a newpathway for manipulating excitons in CNTs.

caused by the formation of bundles in the randomly oriented
CNTs, as seen in Figure 4c. The average E11 and E22 forassignable chiralities are summarized in Table 1.
Statistical results of the E11 and E22 peak positions in Figure
5a allow us to identify other features of the excitonic energy
shifts. We ﬁnd that the energy shifts ΔEii compared to airsuspended CNTs18 are largely aﬀected by CNT diameter dt.
Kataura plots of ΔE11 and ΔE22 are shown in Figure 5b,c for
CNTs with diﬀerent chiralities, and both ΔE11 and ΔE22
clearly increase with 1/dt. We point out that smaller-diameter
CNTs show anomalously large redshifts in E11 and E22 of ∼50
meV. A similar dt dependence of ΔE11 has been observed for
CNTs immersed in organic solvents, which is explained by amodel including both external screening from the dielectric
environment and internal screening from the CNT itself.37 The
screening from the nanotube is weaker compared to h-BN, andtherefore, CNTs with larger dt will have weaker screening eﬀect
and smaller ΔE11. In Figure 5b,c, we have observed that ΔEii isnearly linear with 1/dt, and therefore, we describe the data by
an empirical equation
ΔEii = A + B /dt(1)

where A and B are ﬁtting parameters. For ΔE11, the values of A
and B are −16.3 and 58.4 meV·nm, respectively. For ΔE22, we
ﬁnd A = −41.4 meV and B = 90.1 meV·nm. The slope B for
ΔE22 is larger than that for ΔE11, implying that excitons fromthe higher subband are more sensitive to the environment. The
electric ﬁelds of E22 excitons extend further out compared toE11 excitons, which might contribute to the higher sensitivity.38
By using eq 1, we can also deduce Eii peaks of other chiralities
that have not been used in the linear ﬁt. The calculated E11 andE22 peaks based on eq 1 are plotted in Figure 5a as open red
circles and are located almost at the center of the scatteredexperimental points for each chirality.
ΔEii is reported to be also dependent on the chiral angles of
CNTs. Ohno et al. have observed chiral angle dependence ofΔEii caused by changes in the eﬀective mass,39 while Jiang et al.
have calculated that ΔEii also has family dependence due totrigonal warping and curvature eﬀects.40 We have not observed
clear chiral angle or family dependence, which might be due tothe large energy variations.
Surprisingly, the redshifts of E11 in CNTs on the solid-stateh-BN substrate are comparable to CNTs in solvents with muchstronger dielectric screening such as acetone.41 The in-plane■

ASSOCIATED CONTENT

sı Supporting Information*

The Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acsphotonics.0c00406.
PL images of a CNT before and after the transfer of hBN, time traces of PL for a CNT before and after thetransfer of h-BN, a schematic for estimating theenhancement of radiated power by an h-BN layer,
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AFM images of CNTs on SiO2/Si and h-BN, PLE mapsof individual CNTs and bundled CNTs on h-BN (PDF)

AUTHOR INFORMATION

Corresponding Author

Y. K. Kato − Nanoscale Quantum Photonics Laboratory,
RIKEN Cluster for Pioneering Research, Saitama 351-0198,
Japan; Quantum Optoelectronics Research Team, RIKEN
Center for Advanced Photonics, Saitama 351-0198, Japan;
orcid.org/0000-0002-9942-1459; Email: yuichiro.kato@
riken.jp
Authors

N. Fang − Nanoscale Quantum Photonics Laboratory, RIKEN
Cluster for Pioneering Research, Saitama 351-0198, Japan;
Quantum Optoelectronics Research Team, RIKEN Center for
Advanced Photonics, Saitama 351-0198, Japan; orcid.org/
0000-0002-1053-1900
K. Otsuka − Nanoscale Quantum Photonics Laboratory, RIKEN
Cluster for Pioneering Research, Saitama 351-0198, Japan;
orcid.org/0000-0002-6694-0738
A. Ishii − Nanoscale Quantum Photonics Laboratory, RIKEN
Cluster for Pioneering Research, Saitama 351-0198, Japan;
Quantum Optoelectronics Research Team, RIKEN Center for
Advanced Photonics, Saitama 351-0198, Japan; orcid.org/
0000-0001-9011-4121
T. Taniguchi − National Institute for Materials Science, Ibaraki
305-0044, Japan; orcid.org/0000-0002-1467-3105
K. Watanabe − National Institute for Materials Science, Ibaraki
305-0044, Japan; orcid.org/0000-0003-3701-8119
K. Nagashio − Department of Materials Engineering, The
University of Tokyo, Tokyo 113-8656, Japan; orcid.org/
0000-0003-1181-8644
Complete contact information is available at:https://pubs.acs.org/10.1021/acsphotonics.0c00406
Notes

The authors declare no competing ﬁnancial interest.

■

ACKNOWLEDGMENTS
Work supported in part by JSPS (KAKENHI JP19K23593,
JP16H05962, JP19H00755), MIC (SCOPE 191503001), andMEXT (Nanotechnology Platform JPMXP09F19UT0075).
Growth of hexagonal boron nitride crystals supported by the
MEXT Element Strategy Initiative to Form Core ResearchCenter, Grant Number JPMXP0112101001 and JST (CREST
JPMJCR15F3). K.O. is supported by a JSPS Research
Fellowship. We acknowledge the Advanced Manufacturing
Support Team at RIKEN and T. Nishimura for technicalassistance.

■

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