Article

https://doi.org/10.1038/s41467-023-43928-2

Resonant exciton transfer in mixeddimensional heterostructures for overcoming dimensional restrictions in opticalprocesses
Received: 21 August 2023

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Accepted: 24 November 2023

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

5,

Nanomaterials exhibit unique optical phenomena, in particular excitonic
quantum processes occurring at room temperature. The low dimensionality,
however, imposes strict requirements for conventional optical excitation, andan approach for bypassing such restrictions is desirable. Here we report on
exciton transfer in carbon-nanotube/tungsten-diselenide heterostructures,
where band alignment can be systematically varied. The mixed-dimensional
heterostructures display a pronounced exciton reservoir effect where thelonger-lifetime excitons within the two-dimensional semiconductor are funneled into carbon nanotubes through diffusion. This new excitation pathway
presents several advantages, including larger absorption areas, broadband
spectral response, and polarization-independent efﬁciency. When bandalignment is resonant, we observe substantially more efﬁcient excitation via
tungsten diselenide compared to direct excitation of the nanotube. We further
demonstrate simultaneous bright emission from an array of carbon nanotubes
with varied chiralities and orientations. Our ﬁndings show the potential of
mixed-dimensional heterostructures and band alignment engineering forenergy harvesting and quantum applications through exciton manipulation.

The emergence of low-dimensional materials, encompassing onedimensional (1D) carbon nanotubes (CNTs) and two-dimensional (2D)
transition metal dichalcogenides (TMDs), has resulted in the discovery
of unique physical phenomena not seen in bulk materials1–4. The
reduced screening of Coulomb interactions in these materials leads totightly bound excitons consisting of electron-hole pairs that remain
stable at room temperature, playing a central role in opticalprocesses5–8. In CNTs, the 1D electronic structure gives rise to van Hove

singularities within the density of states, consequently forming discrete excitonic transitions. The resultant excitons are robustly conﬁned within the 1D channel, exhibiting small Bohr radii9, short
excitonic radiative lifetimes10, extended diffusion lengths11, efﬁcient
exciton-exciton annihilation12, and single-photon emission at room
temperature13.
While CNTs offer many intriguing luminescence properties due totheir 1D nature, this dimensionality also imposes strict limitations on

1

Nanoscale Quantum Photonics Laboratory, RIKEN Cluster for Pioneering Research, Saitama 351-0198, Japan. 2Quantum Optoelectronics Research Team,RIKEN Center for Advanced Photonics, Saitama 351-0198, Japan. 3Platform Photonics Research Center, National Institute of Advanced Industrial Science andTechnology (AIST), Ibaraki 305-8568, Japan. 4Department of Physics, Keio University, Yokohama 223-8522, Japan. 5Department of Physics, University ofTsukuba, Ibaraki 305-8571, Japan. 6Department of Mechanical Engineering, The University of Tokyo, Tokyo 113-8656, Japan. 7Department of Materials
e-mail: nan.fang@riken.jp; yuichiro.kato@riken.jp
Engineering, The University of Tokyo, Tokyo 113-8656, Japan.

Nature Communications | (2023)14:8152

1

Articlehttps://doi.org/10.1038/s41467-023-43928-2

Results
Exciton transfer in mixed-dimensional heterostructures
We begin by studying mixed-dimensional heterostructures consistingof a 2D material ﬂake on top of an individual air-suspended CNT. The
CNTs are grown over trenches by chemical vapor deposition. For the
2D material, WSe2 is selected owing to its high stability at room temperature and a large bandgap that could offer sufﬁciently high energy
excitons for transferring into CNTs. A WSe2 ﬂake is placed on top of thetubes by utilizing the anthracene-assisted transfer technique26,27. The
CNT/WSe2 heterostructures are therefore entirely free-standing as
depicted in Fig. 1a, b, which eliminates the substrate-induced inhomogeneity and dielectric screening effects28,29. The morphology of theheterostructure is examined with an atomic force microscope (Supplementary Fig. 1), and an intimate contact between the CNT and theWSe2 is conﬁrmed.
Photoluminescence excitation (PLE) spectroscopy can provide
evidence for the exciton transfer process. We ﬁrst focus on a CNT withchirality (n, m) = (9, 8) as an example. Figure 1c shows a PLE map from apristine suspended (9, 8) CNT before the transfer, with single main
peaks for each of the excitation and emission energies corresponding
to E22 and E11 transitions, respectively12. After transferring a monolayer
WSe2 ﬂake, PL is still bright with 77% of the intensity before the transfer(Fig. 1d). Both E11 and E22 peaks show redshifts of 33 and 54 meV,c
1.7

Prisne(9,8) CNT

1.6

3820
PL (counts/sec)

Excitaon energy (eV)aexciton transfer efﬁciency can be signiﬁcantly modulated due to bandalignment. The transfer process shows a resonant behavior, leading toa pronounced enhancement in excitation with fast transfer from theWSe2 exciton states. Employing this unique excitation process, wedemonstrate simultaneous bright emission from an array of CNTs withvaried chiralities and orientations. These ﬁndings highlight exciton
harvesting using mixed-dimensional heterostructures as a novel
approach for overcoming the dimensional limitations in opticalprocesses.

1.5
1.4
1.3
0.8

0.9

1.0

1.1

0

e
Normalized PL (arb. units)

the optical excitation process. Spatially, the diameter of a single-walled
CNT is ~1 nm, which is considerably smaller than the diffraction-limited
diameter of ~1 μm for incident laser light. This pronounced size difference restricts the interaction area between CNTs and light, inherently leading to a limited number of photons absorbed14. Spectrally,
the isolated E11 and E22 transitions in the absorption spectrum necessitate wavelength-tunable lasers to excite CNTs of the desiredchirality15. Furthermore, the 1D conﬁnement of excitons in CNTs
results in nearly perfect linear excitation polarization dependence,
requiring the polarization angle to be aligned with the nanotube axis16.
These limitations can be largely mitigated in higher-dimensional
materials, such as 2D materials17. A much more efﬁcient process,therefore, would involve connecting excitation in 2D materials withemission from 1D CNTs via exciton transfer, a phenomenon frequently
observed in molecular complexes, biological systems, and semiconductor heterostructures18–21. High-quality interface needed for
efﬁcient coupling can be constructed by taking advantage of the vander Waals nature, as the 1D and 2D materials can be stacked together toform mixed-dimensional heterostructures without any lattice matching constraints22,23. With the ideal van der Waals interface, the bandalignment in the heterostructure should be mainly governed by theband structure of each individual material rather than the interface, as
is the case in the Anderson model24,25. The chirality-dependent bandgap of CNTs thus introduces an important degree of freedom into thissystem to engineer the band alignment, potentially enabling effective
modulation of the exciton transfer process.
Here we investigate the exciton transfer process in CNT/tungsten
diselenide (WSe2) mixed-dimensional heterostructures. Comparedwith conventional E22 excitation in CNTs, the WSe2-based excitation
offers an immensely larger absorption area, broader spectral response,and polarization-independent efﬁciency, all of which stem from the 2D
nature of the material. Following an extensive exploration of CNT
chirality and WSe2 layer number combinations, we observe that the

E22

CNT/WSe2
PLE

0.5 CNT
PLE

0.0
1.3

CNT

1.7

Aer transfer
of 1L WSe2

1.6

2000
PL (counts/sec)

WSe2

Excitaon energy (eV)dWSe2 PL

1.4

1.5

1.5
1.4

0.8

0.9

1.0

1.1

1.7

f
90
135

0

EA

45

E22

180

0

225

1.3

1.6

Energy (eV)

Emission energy (eV)
b

EA

1.0

315
270

Emission energy (eV)Fig. 1 | Signature of exciton transfer in a 1D-2D heterostructure. a A schematic ofa suspended CNT/WSe2 heterostructure. b An optical microscope image of the(9,8) CNT/1L WSe2 heterostructure. The substrate is SiO2/Si. The scale bar is 5 μm.
c, d PLE maps of the (9,8) CNT before (c) and after (d) the transfer of the 1L WSe2.
The excitation power is 10 μW and the excitation polarization is aligned to CNT axis.
e Normalized PLE spectra of integrated E11 emission for the pristine (9,8) CNT(orange) and after the transfer of 1L WSe2 (green), and a PL spectrum taken from the

Nature Communications | (2023)14:8152

suspended 1L WSe2 region away from the nanotube in the same sample (gray). The
PL spectrum is normalized to the EA peak. f Excitation polarization dependence for
EA excitation (1.676 eV, red circles) and E22 excitation (1.521 eV, blue circles). PL
emission is plotted as a function of angle with respect to the trench, where 0
degrees correspond to the direction along the trench. The excitation power is 10
μW. The blue line is a ﬁt to a cosine squared function.

2

Articlehttps://doi.org/10.1038/s41467-023-43928-2

CNT

E22 excitaon

E22
E11

0

0

1

Time (ns)EA excitaon

EA

f

WSe2
CNT

1

Time (ns)

2

1

0.1

−2

GS
0

−1 0
1
Posion (μm)

2

EA excitaonh11
PL (arb. units)

0

EA excitaon

E22
E11

0

E22 excitaon

g

0
d

1

1

GS

2

b
PL intensity (arb. units)ePL (arb. units)

1

WSe2

PL (arb. units)

PL intensity (arb. units)

EA

E22 excitaon

PL (arb. units)

c

a

0.1

−2

−1 0
1
Posion (μm)

2Fig. 2 | Exciton reservoir effect in 1D-2D heterostructures. a, b PL decay curves
taken from the (9,8) CNT/4L WSe2 heterostructure at E22 (1.494 eV, a) and EA
excitation (1.653 eV, b). Experimental results and IRF are indicated by gray lines andgreen lines, respectively. Blue and red lines are ﬁts as explained in the text.
c,d, Energy level diagrams showing the exciton dynamics for E22 (c) and EA excitation (d). GS indicates the ground state. e, f Normalized PL intensity maps from the(9,8) CNT/1L WSe2 heterostructure at E22 (1.494 eV, e) and EA excitation (1.653 eV, f).

The PL images are constructed by integrating PL emission over a 30-meV-wide
spectral window centered at the E11 energy. The trench is indicated by the green
broken lines and the CNTs are indicated by the broken gray lines. The scale bar is 1
μm. g, h Line proﬁles from the normalized PL intensity maps indicated by whitebroken lines in (e), (f), respectively. Black dots are experimental results and lines are
ﬁts. The excitation power is 20 nW for (a, b) and 10 μW for (e, f).

respectively, arising from substantial dielectric screening effect ofWSe223. Remarkably, a prominent high-energy peak appears at ~1.673
eV in the excitation dependence.
The origin of the peak can be identiﬁed by comparing the PLE
spectrum of the heterostructure with the PL spectrum of the suspended WSe2 (Fig. 1e). The PL spectrum (gray curve) shows a single
peak from the A excitons, which overlaps with the newly appeared
peak in the PLE spectrum of the heterostructure (green curve). The
new peak is therefore denoted as EA, indicating a process whereabsorption by WSe2 A excitons results in emission from CNT E11
state. We hypothesize that the exciton transfer process is responsible for coupling the excited and the ground states within different
materials. The EA peak and the E22 peak in the PLE spectra (green
curve) are comparable in their height, showing that the exciton
transfer process is highly efﬁcient despite the different dimensionalities. We also note that the EA peak in the PLE spectrum showsa considerable tail on the high energy side, which can be explainedby the continuum of absorption in WSe2 arising from interband
transitions30.
Excitation polarization measurements offer insights into thedimensionality characteristics of the different excitation pathways. A
strong linear polarization dependence emerges from the E22 excitation
peak as shown in Fig. 1f, consistent with the 1D nature of E22 excitons.
Conversely, the EA excitation peak does not display a discernible linear
polarization, thereby reﬂecting the 2D nature of A excitons. This
notable polarization distinction enables selective excitation of thesamples, either by the A exciton or the E22 exciton (Supplementary
Fig. 2), thus introducing an additional degree of freedom for excitation
manipulation.
Similar exciton transfer processes are observed for heterostructures with thin WSe2 layers, varying from monolayer (1L) to quadlayer (4L). However, this process is substantially less pronounced in
WSe2 ﬂakes with a thickness of approximately 10 nm (Supplementary
Fig. 3). For subsequent analysis, we therefore concentrate primarily on
few-layer WSe2 samples from 1L to 4L.

Exciton reservoir effect in mixed-dimensional heterostructures

Nature Communications | (2023)14:8152

Having identiﬁed the exciton transfer process in CNT/WSe2 heterostructures, we carry out time-resolved PL measurements to explore thedynamics of the exciton transfer. A (9,8) CNT/4L WSe2 heterostructure
is investigated by comparing the time-resolved PL signals from E11
peaks under two distinct excitation conditions.
For E22 excitation, a rapid decay curve is observed (Fig. 2a). We
extract the decay lifetime as 52 ps using exponential ﬁtting convoluted
with the instrument response function (IRF). This small value is consistent with the bright exciton lifetime for suspended (9,8) CNTs11.
Interestingly, when under EA excitation, a much slower decay curve isobserved (Fig. 2b).
The marked change in the decay time cannot be ascribed to theincrease of E11 lifetime since no obvious excitation dependence in E11
excitons is observed in terms of spectral peak energy and linewidth.
Instead, it can be understood by considering the exciton transfer
dynamics, which include WSe2 A exciton generation, recombination,and transfer processes (Fig. 2c, d).
In the case of E22 excitation, given that the relaxation time from E22
to E11 excitons is on the order of femtoseconds31, the rate equation forE11 exciton population n11 is expressed as
dn11n= G  11 ,
dt
τ 11

ð1Þ

where G represents generation rate by the laser, and τ11 = 52 ps is the
lifetime of E11 excitons. When excited at the EA peak, the rate equation
for n11 is expressed as
dn11 QnA n11
=

,
dt
τT
τ 11

ð2Þ

where Q is the fraction of A excitons interacting with the CNT relative
to the total A exciton population nA, and τT represents the transfer
time. It is worth noting that the rise time in Fig. 2b is as short as that in3

Articlehttps://doi.org/10.1038/s41467-023-43928-2

Fig. 2a and below the resolution limited by IRF, indicating that thetransfer is a fast process.
The rate equation for nA in the WSe2 ﬂake can be represented asdnA
ð1  QÞnA QnA
=G 

,
dt
τA
τT

ð3Þ

where τA is the A exciton lifetime. In Eq. (3), we consider Q to be
negligibly small as only a small portion of A excitons are transferred
due to the higher dimensionality of WSe2. The solution to Eq. (3) istherefore proportional to expðt=τ A Þ, and we can accurately reproduce the time-resolved PL in Fig. 2b by setting τA = 500 ps and solving
Eq. (2). The A exciton lifetime extracted is one order of magnitudelarger than that in CNTs (52 ps) as expected from the smaller binding
energy, and consistent with previously reported values from mechanically exfoliated WSe232. Essentially, the WSe2 ﬂake serves as an exciton
reservoir, continuously supplying excitons to CNTs which leads to theslow decay curve. We note that a smaller τA of ~200 ps is observed in
the case of a thinner WSe2 bilayer (Supplementary Fig. 4).
The exciton reservoir effect is also apparent in imaging measurements. We use a three-dimensional motorized stage to scan the(9,8) CNT/1L WSe2 sample shown in Fig. 1 for obtaining PL images.
Since the collection spot is much larger than the excitation spot,
emission outside the excitation laser is also collected while the image
resolution is determined by the laser spot size. The images therefore
represent excitation efﬁciency proﬁles for CNT PL, which can be used
to identify non-local excitation processes. Figure 2e displays a PL
excitation image with the excitation at the E22 peak, presenting atypical suspended CNT image with the length of the bright PL area
constrained by the trench and the width determined by the Gaussianlaser proﬁle. Notably, the PL excitation image for EA excitation appears
spatially expanded, as shown in Fig. 2f. This enlarged PL image indicates that A excitons excited at a distance also funnel into the CNT
after diffusion. The WSe2 ﬂake therefore also acts as a spatial reservoir
for exciting CNTs. We also note that the image enlargement occurs not
only in the suspended region but also on the substrate, indicating that
A excitons are more resilient to substrate effects than E11 excitons.
Line proﬁles of the PL images along the trench for E22 and EA
excitation are illustrated in Fig. 2g and h, respectively. The E22-excited
proﬁle is ﬁtted using a Gaussian function, yielding a 1/e2 radius of thelaser spot r = 0.58 μm. For the EA excitation, the PL intensity is proportional to the density of A exciton transferred to the CNT position,
assuming that the exciton transfer is a linear process. The line proﬁle at
EA excitation is hence ﬁtted to a solution of a steady-state onedimensional diffusion equation
2

D

d nA ðxÞ nA ðxÞG
2 2

+ pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ e2x =r = 0,
dx 2
τA
2πr 2

ð4Þ

where x is the position along the trench direction and D is the diffusion coefﬁcient. Givenpﬃﬃﬃﬃﬃﬃﬃﬃﬃ r = 0.58 μm, the simulated proﬁle with adiffusion length L = Dτ A = 0:60 μm matches the experimental data
as shown in Fig. 2h. The diffusion lengths are also estimated forheterostructures with thicker WSe2 ﬂakes, reaching up to 1.10 μm(Supplementary Fig. 5). It is worth mentioning that the diffusion
length here is larger than previously reported values33, possibly due
to the suspended structure of the WSe2 ﬂake which has reduced
scattering sites.

Band alignment tuning and resonant exciton transfer
The efﬁcient exciton reservoir effect observed in both spatial andtemporal domains suggests the possibility of a greatly enhanced
exciton transfer under suitable band alignment. To explore the optimized structure, we take advantage of the variable bandgap of CNT
with chirality. We have fabricated 34 CNT/WSe2 heterostructures using
Nature Communications | (2023)14:8152

Fig. 3 | Band alignment tuning and resonant exciton transfer. a–e Normalized
PLE spectra of integrated E11 emission for the (10,5) CNT/1L WSe2 sample (a), (9,7)
CNT/2L and 3L WSe2 samples (b), (10,5) CNT/2L and 3L WSe2 samples (c), (11,3)
CNT/2L and 3L WSe2 samples (d), (12,1) CNT/2L and 3L WSe2 samples (e). The PLE
spectra are normalized to the E22 peak intensity. In (b–e), baselines of PLE spectra
for the samples with 2L and 3L WSe2 are 0 and 1, respectively. The excitation power
is 10 μW. f Factor α as a function of E11 energy from all samples. CNT families of 22,
25, 28, and others are indicated by green, blue, red, and gray circles, respectively.
Broken lines are guide to the eye. (Inset) Schematics of band alignment of CNT/
WSe2 heterostructures. With varying CNT chirality, the band alignment transition
from type-I to type-II occurs. Black arrows indicate the exciton tunneling process,while a red arrow indicates the resonant tunneling of holes when the valencebands match.

17 different chiralities (see Supplementary Table 1 for the list of samples) and have quantitatively characterized the exciton transfer
process.
Figure 3a displays the PLE spectrum from a (10,5) CNT/1L WSe2
heterostructure, which has demonstrated the largest enhancement of
the exciton transfer. The EA peak is dominating the spectrum, and theE22 peak is almost overwhelmed. Gaussian peak ﬁts are carried out todistinguish the two excitation peaks. The efﬁciency factor α = I(EA)/
I(E22) is deﬁned to quantify the EA excitation process with respect tothe E22 excitation, where I(EA) and I(E22) are the peak intensities of EA
and E22, respectively. In this heterostructure, the exciton population ismultiplied by α = 3.5 compared with E22 excitation. Under such aneffective excitation through A excitons, we note that the excitonexciton annihilation (EEA) process12 is strong and the α factor isunderestimated. A PLE measurement at a low laser power to avoid the
EEA effect leads to an increased α factor of 6.2 (see Supplementary
Fig. 6). From the α factor, we estimate that approximately 3.8% of the A
excitons transfer into the nanotube, taking into account the different
absorption areas and absorption coefﬁcients (see Supplementary
Note 8). While this value may not seem high, it is actually substantial
because the exciton population in WSe2 is orders of magnitude larger
than in CNTs.
4

Articlehttps://doi.org/10.1038/s41467-023-43928-2

To elucidate the origin of the enhanced transfer, we plot the PLE
spectra for the heterostructures with bilayer (2L) and trilayer (3L) WSe2
transferred onto CNTs with varying chiralities of (9,7), (10,5), (11,3), and(12,1) in Fig. 3b–e, respectively. The selected chiralities here belong to
the same family (2n + m = 25). The PLE spectra from (9,7) and (11,3)
samples exhibit clear EA peaks, although weaker than that in (10,5)
CNTs. In contrast, the EA peak is unresolved in (12,1) samples, indicating a strong suppression of the exciton transfer process.
The observed chirality-dependent exciton transfer processes
cannot be explained by the commonly observed Förster-type energy
transfer in 2D heterostructures34,35. For the energy transfer process
where WSe2 A exciton is the donor and CNT E22 exciton is the acceptor,
we do not expect changes in the transfer efﬁciency unless the spectral
overlap is modulated substantially. The chiralities shown in Fig. 3b–e
have similar E22 energies but exhibit large changes in the efﬁciency
factors, inconsistent with Förster-type energy transfer.
Instead, we interpret our data with a direct exciton tunnelingprocess, which should be sensitive to the band alignment between thetwo materials36,37. For CNT/2L WSe2 samples, the E11 energies for chiralities of (9,7), (10,5), (11,3), and (12,1) are 0.914, 0.973, 1.008, and 1.046
eV, respectively. Since the E11 energy reﬂects the bandgap of CNTs, theband alignment should be varied, as illustrated in Fig. 3f. For chiralities
with smaller bandgap, type-I heterojunction is expected where excitontunneling is allowed. As the bandgap gets larger, the valence band
offset between CNT and WSe2 becomes smaller. When the bands
match, we expect resonant tunneling for the holes, and this maybe thecase for (10,5) CNTs with signiﬁcantly enhanced EA excitation peak.
With further increase in the bandgap, transition to type-II heterojunction should occur. The exciton transfer would then become
inhibited due to the barrier for the hole tunneling, which is consistent
with the PLE spectra for (12,1).
We perform density functional theory studies to examine theband alignment within the heterostructure (Supplementary Fig. 7).
Results show that offsets in the valence band is less signiﬁcant thanthat in the conduction band. The variation of chirality from lower E11
energy to a higher one ultimately instigates the reversal of the valenceband offset, thus leading to the transition from type-I to type-II. The

theoretical ﬁndings corroborate with the empirical results describedabove, supporting the picture that band alignment transition is taking
place and conﬁrming the exciton transfer process.
The transition is more clearly observed by plotting the α factor
against the E11 energy, as shown in Fig. 3f. For family number 25, theresonant peak can be seen at E11 energy around 0.97 eV. The bandalignment transitions are consistently observed for families 22 and 28
with the threshold E11 energy of the transition increasing with thefamily number. The CNT families, recognized for impacting the band
structure of CNTs via trigonal warping and curvature effects38, mayalso be inﬂuencing the band alignment in the heterostructures here. It
should be noted that there is no observable resonance in families 22
and 28, which could be attributed to the absence of a chirality with anexactly matched band energy.
The highly efﬁcient exciton transfer at the resonance suggests it isa rapid process, and we perform Monte Carlo simulations to gain moreinsight. Exciton generation, diffusion, transfer, and recombination
processes are included, and we ﬁnd the transfer time τT = 1.1 ps
reproduce the experimental value of α = 6.2 (see Supplementary
Note 8). In contrast, mixed-dimensional heterostructures consisting ofzero-dimensional quantum dots and 2D TMDs show a long exciton
transfer time in the range of 1–10 ns39,40. Such a long transfer time is
primarily caused by complex interfaces consisting of shells andligands. The transfer process observed in this study is remarkably fastand comparable with homogeneous dimensional structures, such as
2D-2D TMD and 1D-1D CNT heterostructures34,41. The rapid transfer
process can be ascribed to the intimate contact between the CNT and
WSe2, facilitated by the uniform van der Waals interface and the presence of a resonant tunneling condition.

Overcoming the dimensional restrictions on excitation in a CNT
array/WSe2 heterostructureWith relaxed optical selection rules and pronounced exciton reservoir
effects, the A exciton based excitation has a more distinct effect on anarray of CNTs. The array under investigation comprises of four tubes
located from left to right with chiralities of (8,7), (9,8), (10,5), and(10,5), and is covered by a monolayer ﬂake. Figure 4a shows a PL

EA excitaon
EA

E22

E22 excitaon

Fig. 4 | Simultaneous excitation of a CNT array in a mixed-dimensional
heterostructure. a, b PL intensity maps for EA (1.664 eV, a) and E22 (1.494 eV, b)
excitation. The PL images are constructed by integrating the full PL spectrum. The
CNT chiralities are (8,7), (9,8), (10,5), and (10,5) from left to right, and the positions
and orientations are indicated by the broken gray lines. The scale bar is 1 μm. c Line

Nature Communications | (2023)14:8152

proﬁles indicated by white broken lines in (a, red) and (b, blue). d PL spectra taken
at Positions 1, 2, and 3 as indicated in (c). The upper three PL spectra are taken with
EA excitation and the bottom three PL spectra are taken with E22 excitation. The
excitation power is 3 μW.

5Article
excitation image with EA excitation, whereas Fig. 4b is an excitation
image taken with E22 excitation for the (9,8) tube. The excitation at the
EA energy results in high intensity PL from all tubes and a quasi-2D
excitation image spanning the trench. We note that the two (10,5)
CNTs exhibit different PL intensities, which is primarily due to theirdifferent lengths12. The line proﬁle (Fig. 4c, red) shows spatially continuous emission over 10 μm, resulting from overlapping tube images
enlarged by exciton diffusion. In comparison, tube images are completely isolated in Fig. 4b and its line proﬁle (Fig. 4c, blue). Furthermore, the (8,7) tube is unnoticeable since the excitation energy isdetuned from its E22 energy and the polarization angle is misaligned. In
general, achieving simultaneous excitation across all tubes remains achallenge due to different E22 energies and varying angles amongst
the CNTs.
The diffusion process can be over one micron (Supplementary
Fig. 5), allowing for enhanced versatility in exciting multiple tubes. In
Fig. 4d, we plot PL spectra at three representative positions for EA
excitation (red) and E22 excitation (blue). Simultaneous PL emission
from spatially distant (9,8) and (10,5) tubes is demonstrated by EA
excitation at the region in between (Position 2, red). In contrast, no PL
is observed under E22 excitation at this position (blue).

Discussion
In conclusion, an efﬁcient exciton transfer process within CNT/WSe2
heterostructures has been observed. Excitation via WSe2 A excitonspresents several distinctive features compared to traditional E22 excitation: larger absorption areas, broader spectra, and tolerance topolarization misalignment. Through the examination of 17 different
CNT chiralities, it has been conﬁrmed that the exciton transfer is signiﬁcantly modulated by the band alignment. The (10,5) CNT/WSe2
heterostructure demonstrates the resonant exciton transfer, leading
to a considerable enhancement of EA excitation efﬁciency and a rapidtransfer time of 1.1 ps. The ability to simultaneously illuminate CNT
arrays of varying chiralities and orientations, resulting in a quasi-2D
image under EA excitation, is also demonstrated. These observations
propose that the exciton transfer process in mixed-dimensional heterostructures could be harnessed for generation of excitons in CNTs,
thus providing a promising avenue to surpass the excitation constraints in low-dimensional materials. The high tunability of the bandalignment indicates the potential for more intriguing excitons toemerge in this system, for example, indirect excitonic states at themixed-dimensional interface under type-II band alignment.

https://doi.org/10.1038/s41467-023-43928-2

suspended chirality-identiﬁed CNTs with lengths in the range of 0.5 to
2.0 μm.

Anthracene crystal growth
For the purpose of stamping WSe2 ﬂakes onto CNTs, anthracene
crystals are grown through an in-air sublimation process26,27. The
procedure commences with the placement of anthracene powder ontoa glass slide, which is subsequently heated to 80 °C. A secondary glass
slide is positioned above the anthracene source, generally maintaininga 1 mm gap. Following this setup, thin, large-area single crystals begin
to grow out of the glass surface. To enhance the growth of thin, largearea crystals, the glass slides are patterned using ink from commercially available markers, effectively inhibiting the nucleation of threedimensional crystals. The typical growth time for anthracene crystals is 10 h.

WSe2 transfer using anthracene crystals
WSe2 crystals are purchased from HQ graphene. Flakes of WSe2 are
prepared on conventional 90-nm-thick SiO2/Si substrates by employing a mechanical exfoliation technique. Layer number is determined by
optical contrast. A polydimethylsiloxane (PDMS) sheet supported byglass is used to collect a single anthracene crystal, creating ananthracene/PDMS stamp. The targeted WSe2 ﬂakes on the substrate
are subsequently picked up by pressing the anthracene/PDMS stamp.
Rapid separation of the stamp (>10 mm/s) ensures the adherence of
the anthracene crystal to the PDMS sheet. The stamp is then applied to
the receiving 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. Gradual retraction of the PDMS (<0.2 μm/s) allows for theanthracene crystal carrying the WSe2 ﬂake to settle on the receiving
substrate. The anthracene crystal is then removed via sublimation in
air at 110 °C over a 10-min period, leaving a clean suspended CNT/WSe2
heterostructure. This completely dry process eliminates the risk ofcontamination from solvent. Moreover, the solid single-crystal
anthracene serves to shield the 2D ﬂakes and the CNT throughout
the transfer, ensuring that the CNT/WSe2 heterostructure experiences
minimal strain26,27. We note that some of the CNT/WSe2 samples
exhibit substantially small E11 and E22 energy shifts of less than 5 meV,suggesting a lack of contact between the CNT and the WSe2. Thesesamples are excluded in the following study.

Photoluminescence measurements

Methods
Air-suspended carbon nanotube growth
In preparing air-suspended CNTs, we utilize silicon dioxide (SiO2)/
silicon (Si) substrates with trenches12. The process begins with patterning of alignment markers and trenches onto the Si substrates,
utilizing electron-beam lithography. The trenches, with lengths ofapproximately 900 μm and widths varying between 0.5 and 3.0 μm, aresubsequently formed by dry etching. The substrate is then subjectedto thermal oxidation, resulting in the formation of a SiO2 ﬁlm inside thetrenches, with a thickness typically ranging from 60 to 70 nm. The next
step involves a further instance of electron-beam lithography to designate catalyst areas along the trench edges. An iron (Fe) ﬁlm with athickness of approximately 1.5 Å is then deposited by an electron beam
evaporator, serving as a catalyst in the CNT growth. The ﬁnal step of
the process is the synthesis of CNTs, achieved through alcohol chemical vapor deposition at a temperature of 800 °C for 1 min. Optimization of the iron ﬁlm thickness is crucial in controlling the yield for
predominantly producing isolated synthesized CNTs. PLE spectroscopy is conducted to identify the chiralities of suspended CNTs bycomparing E11 and E22 energies to tabulated data12. In order to form theheterostructures with WSe2, we speciﬁcally select isolated, fully

Nature Communications | (2023)14:8152

A custom-built confocal microscopy apparatus is utilized to perform
PL measurements for CNT E11 excitons at ambient temperature within
an atmosphere of dry nitrogen gas12,23. A variable wavelength Ti:sapphire laser serves as a continuous-wave excitation source, with its
power controlled via neutral density ﬁlters. The laser beam is focused
onto the samples with an objective lens featuring a numerical apertureof 0.65 and a working distance of 4.5 mm. The 1/e2 diameter of the laser
spot is 1.16 μm. The confocal pinhole deﬁnes the collection spot size,
which is approximately 5.4 μm in diameter. A longpass ﬁlter with a cuton wavelength of 1050 nm is used to collect the PL emission from
CNTs. PL is gathered through the same objective lens and detected bya liquid-nitrogen-cooled 1024-pixel indium gallium arsenide diodearray attached to a spectrometer. A 150-lines/mm grating is used toobtain a dispersion of 0.52 nm/pixel at a wavelength of 1340 nm. PL
excitation images are taken by using a three-dimensional motorizedstage to scan the CNT/WSe2 samples in our confocal microscopy system. A PL spectrum is collected at each position, and PL excitation
images are constructed by integrating the CNT emission. For photoluminescence measurements of WSe2 A excitons, a 532-nm laser and acharge-coupled device camera are employed. Polarization is aligned tothe tube unless otherwise noted.

6Article
Time-resolved measurements
Approximately 100 femtosecond pulses at a repetition rate of 76 MHz
from a Ti:sapphire laser is utilized for excitation. The laser beam is
directed onto the sample using an objective lens with a numericalaperture of 0.85 and a working distance of 1.48 mm. The PL from thecenter of the nanotube within the heterostructure is coupled to asuperconducting single-photon detector with an optical ﬁber, and atime-correlated single-photon counting module is used to collect thedata. IRFs dependent on the detection wavelength are acquired bydispersing supercontinuum white light pulses with a spectrometer.
The experiments are conducted at room temperature in air.

Data availability
All the data generated in this study have been deposited in the R2DMSGakuNinRDM database at https://dmsgrdm.riken.jp/sdgab/. Source
data are provided with this paper.

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7

Article

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.,
JP21H05233 to S.O., JP22F22350 to Y.R.C., JP23H00262, 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. are supported by the
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 all
the samples. Y.R.C. performed atomic force microscope measurements
and assisted in other measurements. D.Y. and S.F. contributed to thetime-resolved PL measurements. M. M., Y.G., and S.O. performed density functional theory calculations. Y.R.C., C.F.F., and K.N. assisted insample preparation. K.O. aided in the development of the anthraceneassisted transfer method. Y.K.K. supervised the project. N.F. and Y.K.K.
co-wrote the manuscript, with all authors providing input and commentson the manuscript.

Competing interests
The authors declare no competing interests.

Additional information
Supplementary information The online version contains
supplementary material available athttps://doi.org/10.1038/s41467-023-43928-2.

Nature Communications | (2023)14:8152

https://doi.org/10.1038/s41467-023-43928-2
Correspondence and requests for materials should be addressed to N.
Fang or Y. K. Kato.
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