Article

https://doi.org/10.1038/s41467-024-47099-6

Room-temperature quantum emission frominterface excitons in mixed-dimensional
heterostructures
Received: 5 September 2023

Check for updates

1234567890():,;

1234567890():,;

Accepted: 19 March 2024

N. Fang 1 , Y. R. Chang 1, S. Fujii 2,3, D. Yamashita2,4, M. Maruyama
Y. Gao5, C. F. Fong 1, D. Kozawa1,2,6, K. Otsuka 1,7, K. Nagashio 8,
S. Okada 5 & Y. K. Kato 1,2

5,

The development of van der Waals heterostructures has introduced unconventional phenomena that emerge at atomically precise interfaces. For
example, interlayer excitons in two-dimensional transition metal dichalcogenides show intriguing optical properties at low temperatures. Here we
report on room-temperature observation of interface excitons in mixeddimensional heterostructures consisting of two-dimensional tungsten diselenide and one-dimensional carbon nanotubes. Bright emission peaks originating from the interface are identiﬁed, spanning a broad energy range within
the telecommunication wavelengths. The effect of band alignment is investigated by systematically varying the nanotube bandgap, and we assign the new
peaks to interface excitons as they only appear in type-II heterostructures.
Room-temperature localization of low-energy interface excitons is indicatedby extended lifetimes as well as small excitation saturation powers, and photon correlation measurements conﬁrm antibunching. With mixed-dimensionalvan der Waals heterostructures where band alignment can be engineered, newopportunities for quantum photonics are envisioned.

The discovery of van der Waals (vdW) materials, including twodimensional (2D) transition metal dichalcogenides (TMDs) and graphene, has brought about a revolution in the assembly of artiﬁcial
heterostructures by allowing for the combination of two different
materials without the constraints of lattice matching. Such an unprecedented level of ﬂexibility in heterostructure design has led to theemergence of novel properties not seen in individual materials. A
prime example is twisted bilayer graphene at magic angles, which
exhibits exotic phases such as correlated insulating states1 andsuperconductivity2. Another notable development is the stacking of
two TMDs, resulting in the observation of unique excitons known asinterlayer excitons, characterized by electrons and holes located inseparate layers3–6. The spatially indirect nature of interlayer excitons
imparts them with distinct properties, including long exciton
lifetimes3, extended diffusion lengths7, large valley polarization8, andsigniﬁcant modulation by moiré potentials9,10.
The existing vdW heterostructures comprise 2D materials withsimilar lattice structure, excitonic characteristics, and inherently
identical dimensions. Development of vdW heterostructures thatencompass lower dimensional materials may give rise to uniqueinterface exciton states resulting from the mixed dimensionality.
Carbon nanotubes (CNTs), a typical one-dimensional (1D) material, areideal for such heterostructures as they have all bonds conﬁned to thetube itself11,12. CNTs interact with 2D materials through weak vdW

1

Nanoscale Quantum Photonics Laboratory, RIKEN Cluster for Pioneering Research, Saitama, Japan. 2Quantum Optoelectronics Research Team, RIKEN Center
for Advanced Photonics, Saitama, Japan. 3Department of Physics, Keio University, Yokohama, Japan. 4Platform Photonics Research Center, National Instituteof Advanced Industrial Science and Technology (AIST), Ibaraki, Japan. 5Department of Physics, University of Tsukuba, Ibaraki, Japan. 6Research Center for
Materials Nanoarchitectonics, National Institute for Materials Science, Ibaraki, Japan. 7Department of Mechanical Engineering, The University of Tokyo,
e-mail: nan.fang@riken.jp; yuichiro.kato@riken.jp
Tokyo, Japan. 8Department of Materials Engineering, The University of Tokyo, Tokyo, Japan.

Nature Communications | (2024)15:2871

1

Article
forces, resulting in well-deﬁned, atomically sharp interfaces13,14. The
chirality-dependent bandgap of CNTs can be utilized to tune the bandalignment as demonstrated in exciton transfer process15, allowing forunambiguous identiﬁcation of excitonic states at the 1D-2D interface.
Here we report on the observation of multiple emergent excitonic
peaks in the 1D-2D CNT/tungsten diselenide (WSe2) heterostructuresat room temperature. These peaks appear exclusively at the interface
region with a broad energy range lower than CNT E11 states, and theirdependence on the chirality of CNTs and the layer number of WSe2 isinvestigated. The emergence of the peaks is found to be highly correlated with the band alignment, and they are interpreted as interface
excitons. Prominent linear polarization, low excitation saturation
power, and a long lifetime are characteristic of low-energy interface
excitons, suggesting strong conﬁnement. Through photon correlation
measurements, room-temperature antibunching has been conﬁrmed.
These ﬁndings expand the existing concept of spatially indirect excitons based on 2D heterostructures to 1D systems, demonstrating signiﬁcant potential of the interface excitons for nanophotonics andquantum information processing.

Results and discussion
Emerging peaks in 1D-2D mixed-dimensional heterostructures
The CNT/WSe2 heterostructures under investigation are entirely freestanding to preclude substrate effects5,16, as depicted in Fig. 1a, b. Highquality CNTs are initially grown over trenches (see Methods and Supplementary Fig. 1), followed by placement of a WSe2 ﬂake upon the
tubes using the anthracene-assisted transfer technique17,18. We perform
photoluminescence excitation (PLE) measurements to determine theFig. 1 | Emergence of interface excitons in 1D-2D heterostructures. a A schematicof a suspended carbon nanotube (CNT)/WSe2 heterostructure. b An optical
microscope image of the (9,4) CNT/1L WSe2 heterostructure. The substrate is SiO2/
Si. The red and white dashed lines indicate CNT and WSe2, respectively. The scale
bar represents 5 μm. c The band diagram of the (9,4) CNT/1L WSe2 heterostructure,illustrating the excitons in CNT and WSe2 as well as the interface exciton. d A
photoluminescence (PL) spectrum of the pristine, suspended (9,4) CNT prior to
WSe2 transfer. e A PL spectrum immediately following the transfer of 1L WSe2 toform the heterostructure. Emergent peaks are denoted by IX. Arrows indicate
unstable IX peaks. The inset depicts the time-trace of the integrated PL intensity for

Nature Communications | (2024)15:2871

https://doi.org/10.1038/s41467-024-47099-6

chirality before transfer for all samples. A (9,4) CNT is selected as arepresentative case, forming type-II band alignment with WSe215. Suchalignment should establish emergent excitonic states between the
CNT conduction band minimum and the WSe2 valence band maximum(Fig. 1c). The formation of the indicated indirect excitons generally
requires charge transfer, which is plausible as exciton transfer has been
observed in similar heterostructures with type-I band alignment15.
Room-temperature photoluminescence (PL) spectroscopy is
employed to investigate the excitonic states present within theheterostructure19–21. The PL spectrum of the pristine, suspended (9,4)
CNT displays a singular peak at 1.143 eV, corresponding to the E11transition (Fig. 1d)11. After the transfer of the monolayer WSe2 ﬂake, theCNT E11 peak is redshifted to 1.102 eV (Fig. 1e) as a result of thedielectric screening effect14, indicating intimate contact between thetwo materials. Notably, two excitonic peaks arise with energies lower
than E11. These peaks cannot be attributed to the suspended monolayer (1L) WSe2 emission as only the A exciton peak at 1.658 eV is
expected (see Supplementary Fig. 2). The transfer process is notexpected to introduce defects, since CNT/hexagonal boron nitride
heterostructures prepared in a similar manner does not exhibit such
low-energy peaks14, suggesting the existence of emergent excitonic
states in the CNT/WSe2 heterostructure. The newly emerged peaks are
initially unstable and exhibit temporal blinking (inset in Fig. 1e). Such
PL evolution is not observed in sp3 defects, which exhibit more stableemission once formed22,23. Following a spectral development involving
ﬂuctuations of the peaks, the unstable excitonic states vanish andstable states remain for which we perform the subsequent measurements (see Supplementary Fig. 3). Figure 1f presents the stable PL

the IX peak indicated by the orange arrow. The peaks at 1.011 and 0.962 eV ind, e are K-momentum exciton peaks. f, A PL spectrum from the same sample after
approximately eight hours of optical measurements. E11 indicates the CNT E11 state,
and IX1 and IX2 indicate the emergent excitonic states. g, h PL intensity maps of E11
g and IX1 h. The edges of the trench are indicated by the green broken lines. The
scale bar represents 1 μm. i A photoluminescence excitation (PLE) map of the (9,4)
CNT/1L WSe2 heterostructure. The excitation power is 4.5 and 5 μW for f andi, and 10 μW otherwise. The excitation energy is 1.730 eV for d, e, 1.680 eV forf, and 1.653 eV for g, h.

2Article
spectrum, featuring two prominent peaks at 0.924 eV and 0.821 eV,
denoted as IX1 and IX2, respectively. With an energy difference ofapproximately 0.278 eV from E11, IX2 is at a lower energy than any
reported dark or defect states in (9,4) CNTs24–26. Such low-energy
peaks are also observed in other monolayer WSe2 heterostructureswith type-II band alignment (see Supplementary Fig. 4). We hypothesize these states to be interface excitons, which could possess substantially lower excitonic energies as determined by theheterostructure band alignment.
The spatial and spectral correlation between E11 excitons and IXs isinvestigated by conducting PL imaging measurement and PLE spectroscopy. Figure 1g is an integrated PL image from E11, exhibiting
strong signal above the trench due to the quenching from the silicon
dioxide (SiO2)/silicon (Si) substrate. Both IX1 and IX2 peaks areobserved precisely at the position of the E11 peak, as demonstrated by
the IX1 and IX2 images displayed in Fig. 1h and Supplementary Fig. 5,
respectively. Such spatial overlap with the CNT emission indicates that
IX peaks cannot be explained by emission from randomly distributed
defect states within WSe2. In the PLE map (Fig. 1i), E11 shows a strong
response to excitation energy, which is identiﬁed as CNT E22 transition(Supplementary Fig. 6). Similar response as E11 is observed for IX1 andIX2 peaks, implying that the carriers forming the IXs are supplied fromthe CNT. In comparison, we do not observe a clear signature of the
WSe2 A exciton peak in the PLE map. Considering IXs only emerge
upon transfer of the WSe2 ﬂake, the spatial and spectral overlap with
CNT supports the hypothesis that they originate from the interface.

Band alignment effect on IX peaks
Since interface excitons form between the two materials, manipulating
the heterostructure band alignment should affect the IXs6,27. It is possible to vary the CNT bandgap (Fig. 2a) by studying different CNT
chiralities (Fig. 2b), whereas WSe2 bandgap can be altered (Fig. 2c)through the layer number (Fig. 2d, Supplementary Fig. 2).
We ﬁrst investigate the chirality dependence, which signiﬁcantly
modulates the CNT bandgap. The band alignment is systematicallytuned by utilizing CNT/WSe2 heterostructures with different CNT
chiralities as illustrated in Fig. 2b. WSe2 layer numbers ≤ 4 are used forheterostructures since IX peaks can be observed as shown in the casefor (9,4) CNTs with bilayer (2L) and quadlayer (4L) WSe2. It is notedthat the valance band maximum of WSe2 changes from the K point tothe Γ point with increasing the layer number, but correlation with the
behavior of IX peaks cannot be observed. Multiple IX peaks appear in(9,4), (12,1), (8,6), (8,7), and (14,0) CNTs, which have large E11 energies
and therefore large bandgaps. This is consistent with the expectationthat a large bandgap is favorable for type-II band alignment as depicted
in Fig. 2a. It is noted that sp3 defects in CNTs generally introduce
doublet peaks23, different from the numerous IX peaks observed here
(see Supplementary Fig. 3). The observed IX peaks span a broad energy
range within the telecommunication wavelengths. Note that the lowenergy peaks approach the edge of the spectral window, suggesting
the possibility of lower-energy IX peaks existing beyond our current
detection capability. Meanwhile, the highest energy peak in eachchirality is located close to E11, with a difference of ~ 0.05 eV.
Remarkably, a further decrease in the bandgap leads to the disappearance of IXs (Supplementary Fig. 7). The presence of the IX peaks
is determined by chirality, consistent with the transition in bandalignment from type-II to type-I that is observed in a density functional
theory simulation15. We note that for sp3 defects, there exists no suchtransition and any chirality CNT can form sp3 defects25. We therefore
identify IXs as interface excitons.
The IX peaks in the PL spectra from various heterostructures are
summarized in Fig. 2e by plotting the number of the peaks(N) observed during time-trace measurements as a function of E11(see Supplementary Table 1 for the list of samples). Two distinct
regions can be seen below and above 0.94 eV, corresponding to type-I

Nature Communications | (2024)15:2871

https://doi.org/10.1038/s41467-024-47099-6

and type-II alignment, respectively (see Supplementary Fig. 8). IX
peaks are absent for type-I band alignment, whereas numerous peaks
appear for type-II alignment. Exciton transfer observed in similar heterostructures show anticorrelation with the appearance of the IX
peaks, consistent with the band alignment transition (Supplementary
Fig. 9)15.
The dependence of IXs on the number of WSe2 layers (Fig. 2c) ismore subtle, since the number of layers does not signiﬁcantly modulate the bandgap in comparison to CNT chirality (Supplementary
Fig. 10). For example, (9,4) CNT/WSe2 heterostructures are consistent
with complete type-II band alignment irrespective of the WSe2 layer
number (Figs. 1f and 2b). We therefore study (10,5) CNT/WSe2 heterostructures located at the band alignment transition15, as they should
be sensitive to the small changes in WSe2 bandgap. The PL spectrum of
the 1L WSe2 heterostructure is presented in Fig. 2d, which does notshow any observable IX peaks. In contrast, the trilayer (3L) WSe2 heterostructure reveals a discernible IX peak in between the E11 excitonand the trion (T) peaks. Two IX peaks appear for the 4L WSe2 heterostructure, with increased PL intensity for the higher energy IX peak andan additional lower energy IX peak besides the trion peak. This layernumber dependent behavior of the IX peaks can be explained by theband alignment transition, as shown in Fig. 2c.

Optical properties of the interface excitons
The interface excitons display several distinct features different fromthe E11 excitons. In Fig. 3a, we ﬁrst examine the emission polarization
dependence of IX1 and IX2 for the (9,4) CNT/1L WSe2 sample used inFig. 1. PL from IXs exhibits near ideal linear polarization of > 95%,
consistent with conﬁnement in the 1D channel11,28. We also note thatthe polarization angle of IX1 and IX2 deviates from that of E11 polarization by 13.7°, potentially suggesting some distortion of the optical
dipole moment in the IXs (see Supplementary Fig. 11). Owing to the useof a conventional normal-incidence photoluminescence setup which
predominantly detects in-plane dipoles, the vertical component
remains unresolved in our experiments.
Time-resolved PL is then performed for E11 and IX2 excitons, as the
lifetime of IXs is expected to be long due to the spatially indirect
nature3,7. PL decay curves corresponding to E11 and IX2 are measured asshown in Fig. 3b, and the lifetime is extracted by reconvolution ﬁtting
using the instrument response function (IRF). Two decay components
are obtained for the E11 PL decay curve, as is the case for suspended
CNTs: A main fast component with a lifetime of 59 ps associated withthe bright states, and a small slow component with a lifetime of 646 ps
associated with the dark states12. In contrast, only one decay component is observed for IX2 with a long lifetime of 673 ps, consistent withthe reduced optical dipole moment.
The interface excitons exhibit considerably bright emission at low
excitation powers as shown in Fig. 3c. At a low power of 0.04 μW, both
IX1 and IX2 display bright emission with the IX1 PL even exceeding theE11 PL. Such high intensity emission from interface excitons is unexpected at room temperature. Generally, indirect excitons exhibit weakPL emission due to diminished dipole coupling, which is the case for
2D-2D heterostructures where interlayer excitons can hardly beobserved at room temperature3,4,6. The evident PL from interface
excitons in our system can be ascribed to two primary factors. Firstly,
we employ a fully suspended structure, which reduces the substrateinduced screening effect and helps sustain the dipole strength. Secondly, the wavefunction of π-orbitals in CNTs, which extend signiﬁcantly out of the tube, could reduce the spatially indirect nature of
the interface excitons.
Emission intensity of the interface excitons exhibit intriguing
power dependence as shown in Fig. 3d. Both IX1 and IX2 PL nearlysaturate with a low threshold power of approximately 0.6 μW, while
the E11 PL increases substantially. The saturation observed is much
more pronounced compared to interlayer excitons in 2D-2D
3

Articlehttps://doi.org/10.1038/s41467-024-47099-6

Fig. 2 | Band alignment effect on IXs. a Band diagram of heterostructures constructed from CNTs with various chiralities. b Chirality-dependent PL spectra of 1D2D heterostructures. We deﬁne a heterostructure nomenclature where (9,4) CNT/
2L WSe2 is represented by (9,4)/2L, with other samples following the same convention. Excitation energy is adjusted to CNT E22 state for each heterostructure.
Excitation power values are 4, 6, 5, and 5 μW for (9,4)/2L, (12,1)/2L, (8,7)/2L, and(14,3)/3L heterostructures, respectively, and 10 μW for other samples. Arrows
indicate IX peaks. Samples exhibiting IX peaks may also host other unstable IX
peaks on a longer timescale, which is not indicated here. The peaks from
K-momentum states are carefully checked and excluded in the analysis of IXs.

c Band diagram of heterostructures constructed from WSe2 with different layer
numbers. d PL spectra from (10,5)/1L, (10,5)/3L, and (10,5)/4L samples. The excitation energy is adjusted to 1.653 eV and the excitation power is 5 μW for the (10,5)/
4L sample and 10 μW for others. Red arrows indicate IX peaks while the gray arrowdenotes trion peaks T. e The plot of the number of the IX peaks N as a function of E11
energy for all samples. It should be noted that N encompasses all IX peaks observed
during measurements, and some of them are not illustrated in b. More spectra of IX
peaks are shown in Supplementary Figs. 3, 7, and 12. The energies of all IX peaks canbe found in Supplementary Fig. 8.

heterostructures3. It is suspected that the IXs are further conﬁned to alower dimension, that is, 0D. Interface excitons in CNTs may be more
readily localized than interlayer excitons in 2D-2D heterostructures
because of the lower dimensionality. In general, localized states showmuch stronger saturation behavior than free states because of thestate-ﬁlling effects29,30. In other samples where interface excitons also
emerge, we ﬁnd that the saturation behavior depends on their energies(see Supplementary Fig. 12). The IX peaks with energies substantiallylower than the E11 energy display stronger saturation behaviors. This
could be explained by the deeper trap potential in the conﬁnement of

Nature Communications | (2024)15:2871

4

Articlehttps://doi.org/10.1038/s41467-024-47099-6

Fig. 3 | Optical properties of low-energy interface excitons. a Emission polarization dependence of PL emission from E11 (blue circles), IX1 (green dots), and IX2(red circles). PL emission is plotted as a function of angle with respect to the trench.
The lines are ﬁts to a cosine squared function. The degree of polarization is estimated by ðI max  I min Þ=ðI max + I min Þ, where Iax (I min ) is the maximum (minimum) PL
emission intensity. The excitation energy is adjusted to E22 of 1.653 eV with a powerof 5 μW. b PL decay curves taken from the CNT/WSe2 heterostructures for E11 (bluedots) and IX2 (red dots). Shortpass (1.033 eV) and longpass ﬁlters (0.919 eV) areused for the measurements of E11 and IX2, respectively. The pulsed laser is used herewith the power adjusted to 2 nW, and the excitation energy is 1.653 eV. The graycurves are the instrument response function (IRF), and the black curves are theexponential ﬁtting curves convoluted with the IRF. c PL spectra at powers of 0.04,
0.4, and 4 μW from bottom to top. The excitation energy is 1.680 eV. d Integrated
PL intensity as a function of the laser power for E11 (blue), IX1 (green), and IX2 (red),respectively.

Fig. 4 | Room-temperature quantum emission from low-energy interface
excitons. a–c Second-order correlation statistics of the IX peak during a short andb long time scales, and of c the E11 peak for the (9,4) CNT/2L WSe2 sample. The
excitation energy is adjusted to E22 of 1.653 eV with a power of 2 μW. Shortpass(1.033 eV) and longpass ﬁlters (0.886 eV) are used for the measurements of E11 andIX, respectively. The gray lines are experimental results, and the red lines are theﬁttings. See Supplementary Note 14 for the background correction procedure used
in a, b. For the second-correlation statistics data in b, c, four data points are binned
together to reduce the noise. See Supplementary Note 15 and 16 for additional data.

the interface excitons. The localization is also supported by theobserved blinking noise from the unstable IXs that host only “on” and“off” states (see Supplementary Figs. 3 and 13). The pure two-level
noise is exclusively observed in quasi-0D systems, such as quantum
dots and single molecules31,32, suggesting that the interface exciton
state could function as a single-photon emitter.

respectively. The value of 1 − α is 0.280, revealing high single-photon
purity by considering the effect of bunching behavior. It is noteworthy
that most of the low-energy interface excitons display similar peak
linewidths and power saturation behavior (see Supplementary Fig. 12),
implying that each of them acts as a quantum emitter. This is further
supported by the high reproducibility of the antibunching behavior inother samples (see Supplementary Figs. 15 and 16). It is noted that anelectron in CNT would sit at the Γ point in the CNT Brillouin zone, and ahole will be at the K point and the Γ point in monolayer and 2-4L WSe2,
respectively15. The corresponding interface excitons therefore could
be momentum indirect. However, the strong localization observed
would relax the momentum selection rule, potentially explaining thebright PL emission from the interface excitons.
For comparison, g(2)(τ) from E11 excitons does not exhibit any
antibunching or bunching behavior, as shown in Fig. 4c. Under pulsed
laser excitation, the E11 excitons are known to go through an efﬁcient
exciton-exciton annihilation (EEA) process that could result in SPE36.
The exciton density is generally lower with continuous-wave excitation, hindering the SPE through EEA. The conﬁnement effect is crucial
for SPE, and the absence of any antibunching behavior therefore
indicates the 1D free feature of E11 excitons.
We now discuss the possible origins of the localized interface
exciton states, which would require a potential depth exceeding a few
multiples of the thermal energy. The conﬁnement can be provided by

Room-temperature quantum emission from interface excitons
To elucidate the quantum nature of interface excitons, we conduct
photon correlation measurements on a (9,4) CNT/2L WSe2 sample
under continuous-wave laser excitation. Two primary peaks areobserved in this sample, originating from the stable low-energy IX andE11 (see Supplementary Figs. 3 and 14). The second-order correlationg(2)(τ) from the IX is shown in Fig. 4a, after background correction. A
distinct antibunching dip is observed at zero delay with g(2)(0) = 0.467.
Over a longer timescale, we also observe a bunching peak (Fig. 4b).
The bunching behavior is often observed in single-photon sources andis associated with the dynamics between excited states and other dark,charged, or meta-stable states33,34. We employ the equationg ð2Þ ðτÞ = ½1  α expðjτj=τ A Þ*½1 + β expðjτj=τ B Þ to ﬁt the observedg(2)(τ) statistics, where factors α and β quantify the degree of antibunching and bunching with values of 0.720 and 0.666, respectively35.
τA and τB indicate the timescales of the antibunching dip and thebunching peak, with values of 0.148 ns and 3.752 ns from the ﬁtting,Nature Communications | (2024)15:2871

5

Article
defect states, for example in materials such as diamond37, siliconcarbide38, hexagonal boron nitride34, and CNTs39. It is unlikely that
defects in CNTs play a role, since we use pristine suspended CNTs
containing negligible exciton quenching sites11,36. We note that they
have been characterized under low-power conditions within dry
nitrogen gas environment, precluding the formation of defects in the
CNTs11. In comparison, WSe2 ﬂakes inherently encompass a range of
defect states, spanning from single vacancies to complex vacancy
clusters40,41. Among them, it is known that single tungsten vacancies
induce defects states close to the valance band41, which could be
responsible for the conﬁnement. The defect-bound interface excitons
may be formed, similar to the case of interlayer excitons in 2D-2D
heterostructures42. The variability of the emission energy is consistent
with the picture in which defects play a role, as the location and thegeometry of the defect with respect to the CNT can inﬂuence theexcitonic states.
As another possible explanation, inhomogeneous strain could
also contribute to the localization of interface excitons as in the case ofmonolayer WSe243. The morphology of the heterostructure is examined with an atomic force microscope (Supplementary Fig. 17), and aclean CNT/WSe2 interface is conﬁrmed which would facilitate theformation of interface excitons. In one heterostructure, we observe ashallow local dip with a depth of ~ 3 nm and a width of ~350 nm, which
may conﬁne interface excitons nearby. Such nanoscale strain mightalso impact the sample through various mechanisms from van der
Waals gap ﬂuctuation44 to lattice reconstruction45, in a manner similar
to 2D-2D heterostructures. While the exact impact of strain remains
unclear, the spatially indirect nature renders interface excitons more
susceptible to the aforementioned effects than intralayer excitons.
In conclusion, we observe numerous IX peaks at the CNT/WSe2
interface below the CNT E11 energy, spanning the telecommunication
wavelengths. By systematically varying the chirality of CNTs and thelayer number of WSe2, we are able to assign the peaks to interface
excitons as they only appear for type-II band alignment. The low
saturation power and the long lifetime indicate that low-energy interface
excitons are strongly conﬁned, and photon antibunching is conﬁrmed.
The observation of interface excitons as room-temperature quantum
emitters at telecommunication bands opens up new opportunities forapplications in quantum technologies and optoelectronics, underscoring the emerging potential of mixed-dimensional heterostructures.

Methods
Air-suspended carbon nanotubes
We prepare air-suspended CNTs using trenched SiO2/Si substrates11.
First, we pattern alignment markers and trenches with lengths of
900 μm and widths ranging from 0.5 to 3.0 μm onto the Si substrates
using electron-beam lithography, followed by dry etching. We thenthermally oxidize the substrate to form a SiO2 ﬁlm, with a thickness
ranging from 60 to 70 nm. Another electron-beam lithography process
is used to deﬁne catalyst regions along the edges of the trenches. A
1.5 Å thick iron (Fe) ﬁlm is deposited as a catalyst for CNT growth using
an electron beam evaporator. CNTs are synthesized by alcohol chemical vapor deposition at 800 °C for 1 min. The Fe ﬁlm thickness is
optimized to control the yield for preparing isolated CNTs. The PL
images and PL polarization measurements are performed to exclude
the existence of any quenching sites in the CNTs, and only the tubes
showing a smooth proﬁle along the length and a high polarization
degree > 90% are selected and used for the preparation of theheterostructures11,12,36. We select isolated, fully suspended chiralityidentiﬁed CNTs with lengths ranging from 0.5 to 2.0 μm to form theheterostructures with WSe2.

Anthracene crystal growth
For transferring WSe2 ﬂakes onto CNTs, we grow anthracene crystals
through an in-air sublimation process17,18. Anthracene powder is heated

Nature Communications | (2024)15:2871

https://doi.org/10.1038/s41467-024-47099-6

to 80 °C on a glass slide, while another glass slide is placed 1 mm above
the anthracene source. Thin and large-area single crystals are thengrown on the glass surface. To promote the growth of large-area single
crystals, we pattern the glass slides using ink from commercial markers. The typical growth time for anthracene crystals is 10 h.

Transfer of WSe2 by anthracene crystals
First, WSe2 (HQ graphene) ﬂakes are prepared on 90-nm-thick SiO2/Si
substrates using mechanical exfoliation, and the layer number isdetermined by optical contrast. An anthracene single crystal is picked
up with a glass-supported PDMS sheet to form an anthracene/PDMS
stamp. Next, the WSe2 ﬂakes are picked up by pressing the anthracene/
PDMS stamp against a substrate with the target WSe2 ﬂakes. The stamp
is quickly separated ( > 10 mm/s) to ensure that the anthracene crystal
remains attached to the PDMS sheet. The stamp is then applied to thereceiving substrate with the desired chirality-identiﬁed CNT, whose
position is determined by a prior measurement. Precise positionalignment is accomplished with the aid of markers prepared on thesubstrate. By slowly peeling off the PDMS ( < 0.2 μm/s), the anthracene
crystal with the WSe2 ﬂake is released on the receiving substrate.
Sublimation of anthracene in air at 110 °C for 10 min removes theanthracene crystal, leaving behind a clean suspended CNT/WSe2 heterostructure. This all-dry process eliminates contamination from solvents, and the solid single-crystal anthracene protects the 2D ﬂakes
and the CNT during the transfer, ensuring that the CNT/WSe2 heterostructure experiences minimal strain17,18.

PL measurements
A homebuilt confocal microscopy system is employed to perform PL
measurements for interface excitons and E11 excitons at room temperature in dry nitrogen gas11,14. We utilize a wavelength-tunable continuous-wave Ti:sapphire laser for excitation, with its power controlled
by neutral density ﬁlters. The excitation polarization angle is adjusted
to be parallel to the CNT axis and the emission polarization angle
dependence is measured using a half-wave plate followed by a polarizer placed in front of a spectrometer. The laser beam is focused on thesamples with an objective lens that has a numerical aperture of 0.65
and a working distance of 4.5 mm. The 1/e2 spot diameter and thecollection spot size deﬁned by a confocal pinhole are approximately
1.2 and 5.4 μm, respectively. PL is collected through the same objective
lens and detected using a liquid-nitrogen-cooled 1024-pixel InGaAs
diode array connected to the spectrometer. A 150-lines/mm grating isused to obtain a dispersion of 0.52 nm/pixel at a wavelength of1340 nm. For photoluminescence measurements of WSe2 A excitons, a
532-nm laser and a charge-coupled device camera are employed.

Time-resolved and photon correlation measurements
Approximately 100 femtosecond pulses at a repetition rate of 76 MHz
from a Ti:sapphire laser is utilized for time-resolved measurements.
The excitation laser beam is focused onto the sample using an objective lens with a numerical aperture of 0.85 and a working distance of
1.48 mm. The PL from the center of the nanotube within the heterostructure is coupled to a superconducting single-photon detector with
an optical ﬁber, and a time-correlated single-photon counting module
is used to collect the data. IRFs dependent on the detection wavelength are acquired by dispersing supercontinuum white light pulses
with a spectrometer. Photon correlation measurements are carried out
using a Hanbury-Brown-Twiss setup with a 50:50 ﬁber coupler under
excitation with a continuous-wave laser. The experiments are conducted at room temperature.

Data availability
All the data generated in this study have been deposited into the
R2DMS-GakuNinRDM database, and are accessible at https://
dmsgrdm.riken.jp/42tsf/.
6

Article

References
1.

Cao, Y. et al. Correlated insulator behaviour at half-ﬁlling in magicangle graphene superlattices. Nature 556, 80 (2018).
2. Cao, Y. et al. Unconventional superconductivity in magic-angle
graphene superlattices. Nature 556, 43 (2018).
3. Rivera, P. et al. Observation of long-lived interlayer excitons inmonolayer MoSe2-WSe2 heterostructures. Nat. Commun. 6,6242 (2015).
4. Xiong, R. et al. Correlated insulator of excitons in MoSe2-WSe2
moiré superlattices. Science 380, 860 (2023).
5. Sun, X. et al. Enhanced interactions of interlayer excitons in freestanding heterobilayers. Nature 610, 478 (2022).
6. Ubrig, N. et al. Design of van der Waals interfaces for broadspectrum optoelectronics. Nat. Mater. 19, 299 (2020).
7. Unuchek, D. et al. Room-temperature electrical control of exciton
ﬂux in a van der Waals heterostructure. Nature 560, 340 (2018).
8. Rivera, P. et al. Valley-polarized exciton dynamics in a 2D semiconductor heterostructure. Science 351, 688 (2016).
9. Jin, C. et al. Observation of moiré excitons in WSe2/WS2 heterostructure superlattices. Nature 567, 76 (2019).
10. Seyler, K. L. et al. Signatures of moiré-trapped valley excitons in
MoSe2/WSe2 heterobilayers. Nature 567, 66 (2019).
11. Ishii, A., Yoshida, M. & Kato, Y. K. Exciton diffusion, end quenching,and exciton-exciton annihilation in individual air-suspended carbon
nanotubes. Phys. Rev. B 91, 125427 (2015).
12. Ishii, A., Yoshida, M. & Kato, Y. K. High efﬁciency dark-to-bright exciton conversion in carbon nanotubes. Phys. Rev. X 9, 041048 (2019).
13. Jariwala, D., Marks, T. J. & Hersam, M. C. Mixed-dimensional van der
Waals heterostructures. Nat. Mater. 16, 170 (2016).
14. Fang, N. et al. Hexagonal boron nitride as an ideal substrate forcarbon nanotube photonics. ACS Photonics 7, 1773 (2020).
15. Fang, N. et al. Resonant exciton transfer in mixed-dimensional
heterostructures for overcoming dimensional restrictions in opticalprocesses. Nat. Commun. 14, 8152 (2023).
16. Lefebvre, J., Homma, Y. & Finnie, P. Bright band gap photoluminescence from unprocessed single-walled carbon nanotubes.
Phys. Rev. Lett. 90, 217401 (2003).
17. Otsuka, K. et al. Deterministic transfer of optical-quality carbon
nanotubes for atomically deﬁned technology. Nat. Commun. 12,3138 (2021).
18. Fang, N. et al. Quantization of mode shifts in nanocavities integrated
with atomically thin sheets. Adv. Opt. Mater. 10, 2200538 (2022).
19. Jiang, M. et al. Gate-controlled generation of optical pulse trainsusing individual carbon nanotubes. Nat. Commun. 6, 6335 (2015).
20. Uda, T., Yoshida, M., Ishii, A. & Kato, Y. K. Electric-ﬁeld induced
activation of dark excitonic states in carbon nanotubes. Nano Lett.
16, 2278 (2016).
21. Otsuka, K., Ishii, A. & Kato, Y. K. Super-resolution ﬂuorescenceimaging of carbon nanotubes using a nonlinear excitonic process.
Opt. Express 27, 17463 (2019).
22. Piao, Y. et al. Brightening of carbon nanotube photoluminescencethrough the incorporation of sp3 defects. Nat. Chem. 5, 840 (2013).
23. He, X. et al. Tunable room-temperature single-photon emission attelecom wavelengths from sp3 defects in carbon nanotubes. Nat.
Photon. 11, 577 (2017).
24. Matsunaga, R., Matsuda, K. & Kanemitsu, Y. Origin of low-energy
photoluminescence peaks in single carbon nanotubes: K-momentum dark excitons and triplet dark excitons. Phys. Rev. B 81,
033401 (2010).
25. Kozawa, D. et al. Formation of organic color centers in airsuspended carbon nanotubes using vapor-phase reaction. Nat.
Commun. 13, 2814 (2022).
26. Yu, B. et al. ortho-substituted aryldiazonium design for the defectconﬁguration-controlled photoluminescent functionalization ofchiral single-walled carbon nanotubes. ACS Nano 16, 21452 (2022).

Nature Communications | (2024)15:2871

https://doi.org/10.1038/s41467-024-47099-6
27. Tan, Q. et al. Layer-engineered interlayer excitons. Sci. Adv. 7,
1 (2021).
28. Bai, Y. et al. Excitons in strain-induced one-dimensional moiré
potentials at transition metal dichalcogenide heterojunctions. Nat.
Mater. 19, 1068 (2020).
29. Settele, S. et al. Synthetic control over the binding conﬁguration of
luminescent sp3-defects in single-walled carbon nanotubes. Nat.
Commun. 12, 2119 (2021).
30. Shinokita, K., Miyauchi, Y., Watanabe, K., Taniguchi, T. & Matsuda, K.
Resonant coupling of a moiré exciton to a phonon in a WSe2/MoSe2
heterobilayer. Nano Lett. 21, 5938 (2021).
31. Nirmal, M. et al. Fluorescence intermittency in single cadmium
selenide nanocrystals. Nature 383, 802 (1996).
32. Moerner, W. E. & Orrit, M. Illuminating single molecules in condensed matter. Science 283, 1670 (1999).
33. Sallen, G. et al. Exciton dynamics of a single quantum dot
embedded in a nanowire. Phys. Rev. B 80, 085310 (2009).
34. Tran, T. T., Bray, K., Ford, M. J., Toth, M. & Aharonovich, I. Quantum
emission from hexagonal boron nitride monolayers. Nat. Nanotechnol. 11, 37 (2016).
35. Gao, K., Solovev, I., Holmes, M., Arita, M. & Arakawa, Y.
Nanosecond-scale spectral diffusion in the single photon emissionof a gan quantum dot. AIP Adv. 7, 125216 (2017).
36. Ishii, A., Yoshida, M. & Kato, Y. K. Room-temperature single-photon
emission from micrometer-long air-suspended carbon nanotubes.
Phys. Rev. Appl. 8, 054039 (2017).
37. Babinec, T. M. et al. A diamond nanowire single-photon source. Nat.
Nanotechnol. 5, 195 (2010).
38. Castelletto, S. et al. A silicon carbide room-temperature singlephoton source. Nat. Mater. 13, 151 (2014).
39. Ma, X., Hartmann, N. F., Baldwin, J. K. S., Doorn, S. K. & Htoon, H.
Room-temperature single-photon generation from solitary dopantsof carbon nanotubes. Nat. Nanotechnol. 10, 671 (2015).
40. Linhart, L. et al. Localized intervalley defect excitons as singlephoton emitters in WSe2. Phys. Rev. Lett. 123, 146401 (2019).
41. Zhang, S. et al. Defect structure of localized excitons in a WSe2
monolayer. Phys. Rev. Lett. 119, 046101 (2017).
42. Zhao, H. et al. Manipulating interlayer excitons for near-infrared
quantum light generation. Nano Lett. 23, 11006 (2023).
43. Parto, K., Azzam, S. I., Banerjee, K. & Moody, G. Defect and strainengineering of monolayer WSe2 enables site-controlled singlephoton emission up to 150 K. Nat. Commun. 12, 3585 (2021).
44. Shin, B. G. et al. Indirect bandgap puddles in monolayer MoS2 bysubstrate-induced local strain. Adv. Mater. 28, 9378 (2016).
45. Zhao, S. et al. Excitons in mesoscopically reconstructed moiré
heterostructures. Nat. Nanotechnol. 18, 572 (2023).

Acknowledgements
Parts of this study are supported by JSPS (KAKENHI JP22K14624 to D.Y.,
JP22K14625 to S.F., JP21K14484 to M.M., JP22K14623 to C.F.F.,
JP22H01893 to D.K., JP21H05233 to S.O., JP23H00262, JP22F22350,
JP20H02558 to Y.K.K.) and MEXT (ARIM JPMXP1222UT1135). Y.R.C. is
supported by JSPS (International Research Fellow). N.F. and C.F.F. aresupported by RIKEN Special Postdoctoral Researcher Program. We
thank the Advanced Manufacturing Support Team at RIKEN for technicalassistance.

Author contributions
N.F. carried out sample preparation and performed measurements on
the samples. Y.R.C. performed atomic force microscope measurements
and assisted in sample preparation. Y.R.C., C.F.F., and K.N. assisted in
optical measurements. D.Y., S.F., and D.K. contributed to the timeresolved PL and photon correlation measurements. M. M., Y.G., and S.O.
performed density functional theory calculations. K.O. aided in thedevelopment of the anthracene-assisted dry transfer method. Y.K.K.

7

Articlehttps://doi.org/10.1038/s41467-024-47099-6

supervised the project. N.F. and Y.K.K. co-wrote the manuscript, with all
authors providing input and comments on the manuscript.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional afﬁliations.

Competing interests

Open Access This article is licensed under a Creative Commons
Attribution 4.0 International License, which permits use, sharing,
adaptation, distribution and reproduction in any medium or format, aslong as you give appropriate credit to the original author(s) and thesource, provide a link to the Creative Commons licence, and indicate ifchanges were made. The images or other third party material in thisarticle are included in the article’s Creative Commons licence, unlessindicated otherwise in a credit line to the material. If material is notincluded in the article’s Creative Commons licence and your intended
use is not permitted by statutory regulation or exceeds the permitteduse, you will need to obtain permission directly from the copyrightholder. To view a copy of this licence, visit http://creativecommons.org/
licenses/by/4.0/.

The authors declare no competing interests.

Additional information
Supplementary information The online version contains
supplementary material available athttps://doi.org/10.1038/s41467-024-47099-6.
Correspondence and requests for materials should be addressed to
N. Fang or Y. K. Kato.
Peer review information Nature Communications thanks the anonymous reviewers for their contribution to the peer review of this work. A
peer review ﬁle is available.

© The Author(s) 2024
Reprints and permissions information is available at
http://www.nature.com/reprints

Nature Communications | (2024)15:2871

8