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ArticleCite This: J. Phys. Chem. C 2019, 123, 5776−5781

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Organic Molecular Tuning of Many-Body Interaction Energies in AirSuspended Carbon Nanotubes
Shunsuke Tanaka,†,‡ Keigo Otsuka,†,‡ Kensuke Kimura,§,∥ Akihiro Ishii,†,‡ Hiroshi Imada,§
Yousoo Kim,§ and Yuichiro K. Kato*,†,‡†

Quantum Optoelectronics Research Team, RIKEN Center for Advanced Photonics, Saitama 351-0198, Japan
Nanoscale Quantum Photonics Laboratory and §Surface and Interface Science Laboratory, RIKEN Cluster for Pioneering Research,
Saitama 351-0198, Japan∥
Department of Advanced Materials Science, School of Frontier Sciences, The University of Tokyo, Chiba 277-8561, Japan
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‡

ABSTRACT: We investigate adsorption eﬀects of copper phthalocyanine
molecules on excitons and trions in air-suspended carbon nanotubes. Using
photoluminescence excitation spectroscopy, we observe that exciton energy
red-shifts gradually with the molecular deposition thickness. The trion
emission is also observed at large deposition amounts, which indicates chargetransfer between the phthalocyanine molecules and carbon nanotubes.
Analysis of the spectra for individual tubes reveals a correlation between the
exciton−trion energy separation and the exciton emission energy, showing thatthe many-body interaction energies scale similarly with the moleculardielectric screening.

■

INTRODUCTION
Single-walled carbon nanotubes (CNTs) have attracted
considerable attention due to their remarkable physical andelectronic properties, and much eﬀort has been devoted tofunctionalize CNTs for expanding their capabilities.1−4 Inparticular, noncovalent functionalization with organic molecules is a powerful strategy for developing CNT-based devices
such as photovoltaics and photodetectors.5−10 The interactions
between the organic molecules and CNTs are considered to beless perturbative compared to covalent modiﬁcation, andtherefore the outstanding properties of CNTs can bepreserved. Organic molecules such as porphyrin andphthalocyanine couple to nanotubes through π−π interactions
to modify the charge density of CNTs, while the emission
energy is reduced due to the molecular screening.11−16
Furthermore, there are unique exciton dynamics at the organic
molecule/CNT interface including energy and charge transfer.14,17−22
In the measurements performed for nanotubes in solutions,however, the existence of solvent molecules inevitablycomplicate the interpretation by inﬂuencing the interactions.
Additionally, the solvents themselves provide molecular
screening, reducing the dielectric eﬀects of the organic
molecules. Investigation of molecular adsorption on airsuspended nanotubes would provide invaluable information
toward fundamental understanding of the adsorption eﬀects, as
thermal evaporation techniques23,24 allow for moleculardeposition without introducing other molecules. The use ofpristine tubes should further enable drastic dielectric© 2019 American Chemical Society

modiﬁcation for developing noncovalently functionalized
CNT devices.
Here, we demonstrate control over many-body interactionenergies in air-suspended CNTs by copper phthalocyanine(CuPc) molecule adsorption. The molecules are deposited on
chirality-assigned CNTs by thermal evaporation, and we
perform photoluminescence (PL) spectroscopy of the CuPc/
CNT hybrid. From the averaged PL and PL excitation (PLE)
spectra for various chiralities, we ﬁnd that the E11 and E22
resonances red-shift with increasing deposition thickness.
Furthermore, a new emission peak is observed at an energy
below the E11 peak, which is attributed to trion emission. Data
from individual tubes reveal a good correlation between
exciton−trion energy separation and the E11 energy, suggesting
the existence of a universal relationship. We consider a model
assuming power law scaling of the interaction energies with
dielectric constant to explain the observed correlation.

■

EXPERIMENTAL SECTION

Air-Suspended CNTs. Our suspended CNTs are grownover trenches on bare Si substrates (Figure 1a). We perform
electron beam lithography and dry etching to form thetrenches, and another electron beam lithography step deﬁnes
catalyst areas near the trenches. Fe(III) acetylacetonate andfumed silica dispersed in ethanol are spin-coated as catalyst for

Received: December 28, 2018
Revised: February 12, 2019
Published: February 13, 2019
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The Journal of Physical Chemistry C

480−520 °C under a vacuum of less than 10−4 Pa. A glass slideis also placed in the chamber to quantify the deposition
thickness from absorbance of CuPc peak at 2.0 eV.27
Calibration is performed by measuring the actual thickness
by a surface proﬁler for two CuPc ﬁlms with 80 and 167 nm.
Four samples with diﬀerent deposition amounts are preparedby changing the evaporation time, with nominal thickness on
the substrate of 3, 7, 16, and 26 nm.

■

RESULTS AND DISCUSSION
CuPc Adsorption Eﬀects. In order to investigate themolecular adsorption eﬀects, we compare the nanotubes before
and after molecular deposition. A scanning electron microscopy image of a typical CNT after the evaporation is shown in
Figure 1b. Bright spots are observed sparsely on the suspended
CNTs, indicating inhomogeneous adsorption of CuPc. Figure
1c−h shows PL spectra, PL images, and polarization
dependence for a suspended CNT before and after CuPc
deposition for the 26 nm sample. We note that the data before
deposition have been taken prior to heating and therefore air
molecules are adsorbed. In the PLE map before the deposition(Figure 1c), the emission occurs at the E11 energy and the E22transition can be observed as a resonance in the excitationenergy. The values for the E11 and E22 resonances are
consistent with those for a (9,7) air-suspended nanotube,25
indicating that air-molecules such as water are adsorbed.28,29
Figure 1f shows a PLE map of the same tube after thedeposition, and we ﬁnd diﬀerences in the peak energy, the line
width, and the intensity. Both of the E11 and E22 resonances
show red-shift as well as broadening, and the PL intensity
decreases signiﬁcantly with the molecular adsorption. Additionally, a new emission band appears in an energy region
below the E11 emission. The PL image of the molecularadsorbed nanotube is slightly smaller compared to that before
the evaporation (Figure 1g,d), which is likely due to theinhomogeneous adsorption as observed in Figure 1b. In
comparison, the laser polarization dependence does not change
before and after the deposition (Figure 1e,h).
E11 and E22 Resonance Energy Shifts. The PLE maps ofthe nanotubes for the four samples are measured to examine
the deposition amount dependence of the adsorption eﬀects.
We integrate the PLE maps for each nanotube along theexcitation and emission energy axes to obtain PL and PLE
spectra, respectively, and the spectra for nanotubes with thesame chiralities are averaged within each sample. The
ensemble averaged PL and PLE spectra for (9,7) tubes areshown in Figure 2a,b, respectively. In the PL spectra (Figure2a), the intensity decreases and the E11 resonance energy redshifts with the increase of the deposition thickness. Two peaks
are observed for the 3 nm sample, corresponding to emission
from regions of nanotubes with and without the CuPc
molecule adsorption. In contrast, the E11 emission shows
only one peak in the PL spectrum for the 7 nm sample,
suggesting that the nanotube surface is fully covered with themolecules. For samples thicker than 7 nm, the new emission
band is observed as in Figure 1f, and we will refer to this peakas the T emission. While the relative intensity of the Temission becomes higher compared to the E11 emission as thedeposition amount increases, the absolute intensity does notseem to depend much on the amount of CuPc molecules.
In the PLE spectra (Figure 2b), the E22 resonance also redshifts with increasing deposition thickness, and the simultaneous red shifts for the E11 and E22 resonances can be

Figure 1. (a) Schematic image of our sample. (b) Scanning electron
microscope image of a CuPc/CNT hybrid. (c−e) PLE map, PL
image, and laser polarization angle dependence, respectively, for a(9,7) nanotube in the 26 nm sample before CuPc deposition. For(d,e), the PL intensity is obtained by integrating PL over a 10 meVwide spectral window centered at 0.96 eV. (f−h) PLE map, PL image,and laser polarization angle dependence, respectively, for the same
tube after the deposition. For (g,h), the PL intensity is obtained byintegrating PL over a 10 meV wide spectral window centered at 0.91
eV. (b,d,g) Scale bars are 1 μm. Panels (c−e) are taken with P = 5
μW, and (f−h) are taken with P = 300 μW.

CNT growth. CNTs are synthesized by alcohol chemical vapor
deposition at 800 °C for 1 min.25
PL Microscopy. The PL measurements are carried out witha home-built confocal microspectroscopy system.25,26 A
continuous-wave Ti:sapphire laser is used for excitation witha power P, and an objective lens with a numerical aperture of
0.8 and a working distance of 3.4 mm focuses the laser onto
the sample. The linear polarization of the excitation laser isrotated using a half-wave plate and PL from the sample isdetected by an InGaAs photodiode array detector attached to aspectrometer. The laser polarization is parallel to the tube axis
for measuring PLE maps and PL images. All measurements are
conducted at room temperature in dry nitrogen.
Sample Characterization before CuPc Deposition. We
characterize the suspended nanotubes before the deposition.
CNTs are located by line scans along the trenches, and PLE
measurements are performed. We check whether only a single
peak is observed in the emission spectrum and whether the E11
and E22 resonance energies match tabulated data for airsuspended nanotubes. 25 If a nanotube satisﬁes these
conditions, the position and the chirality of the nanotube arerecorded into a list for the measurements after the deposition.
CuPc Deposition on Nanotubes. After the characterization, the sample is placed in a vacuum chamber forevaporation of CuPc (Sigma-Aldrich) and is kept at about 80°C for 10 min to remove air molecules before the evaporation.
CuPc molecules are deposited on suspended CNTs in thechamber at room temperature using an evaporator heated to
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molecules to the CuPc molecules. The shifts show similar
behavior, and the energies decrease monotonically with thedeposition thickness. The E11 energy shifts by more than 60
meV, which is considerably larger than the shift caused byorganic molecule adsorption in solutions (∼20 meV).11,14 The
peak areas obtained from the ﬁtting are also shown in Figure
2d, drastically decreasing as the thickness increases. A possible
cause for the intensity reduction is carrier-induced PL
quenching28,33,34 as charge transfer between a metalphthalocyanine and a CNT has been predicted by a density
functional calculation.12 The energy shift and peak area for theT emission are also plotted in Figure 2c,d, respectively, andboth the energy and intensity roughly stay constant.
Using the same analysis procedure, the E11 and E22
resonance energies for various chiralities are obtained (Figure3a). Red shifts of both resonances are observed for all

Figure 2. (a,b) Ensemble averaged PL and PLE spectra, respectively,
for (9,7) tubes of the samples before deposition (black), 3 nm sample(red), 7 nm sample (green), 16 nm sample (blue), and 26 nm sample(purple). We measure 3−10 nanotubes for each sample and checkthat the same tube is measured before and after the deposition fromthe location and the laser polarization dependence. The nanotubes
before and after the deposition are measured with P = 100 μW and P= 300 μW, respectively. The peak shift and peak area obtained fromthe ﬁtting are shown in (c,d), respectively. The E11 and E22 peak shifts
are compared to the peak energies before the deposition, and the E11
and E22 peak areas are normalized by those before the deposition. The
peak shift for the T emission is compared to the peak energy for the 7
nm sample, and the peak area for the T emission is normalized by thatfor the E11 resonance before the deposition. The error bars are smaller
than the symbols.

Figure 3. (a) Deposition thickness dependence of the E11 and E22
resonance energies for various chiralities. The data are obtained by thesame procedure as in Figure 2. The excitation energy integration
windows are from 1.44 to 1.63 eV for (9,7), (10,5), (11,3), and (12,1)
or from 1.35 to 1.50 eV for the others. If the E11 resonance band issplit into two as for the 3 nm sample in Figure 2a, we have obtained apeak area weighted average of the emission energy by ﬁtting thespectrum with two Lorentz functions. The integration window for theemission energy is from 0.77 to 1.09 eV for all chiralities. The ﬁlled
squares, open squares, ﬁlled circles, open circles, and ﬁlled triangles
correspond to the samples before deposition, 3 nm sample, 7 nm
sample, 16 nm sample, and 26 nm sample, respectively. (b) Peakshifts of the E11 and E22 resonance energies for the 26 nm sample
relative to those before the deposition are shown as a function of thediameter. The error bars are smaller than the symbols.

explained by the molecular dielectric screening.29,30 For the 26
nm sample, the E22 resonance peak is on top of a broad
background slope, which may be attributed to the tail of the
CuPc absorption peak at 2.0 eV.27 Exciton transfer from CuPc
aggregates to CNTs would be consistent with the appearanceof the absorption tail in the PLE spectra.11,17 It has beensuggested that the exciton transfer time is less than 100 fs innoncovalently bound nanotube/porphyrin hybrids,14 andtherefore the exciton transfer from the CuPc molecules tothe nanotubes could occur before the intersystem crossing
process (∼500 fs).31
To evaluate the E11, E22, and T emission energy shifts foreach sample, we extract the peak positions by ﬁtting theensemble averaged PL and PLE spectra. A Lorentz function isused to ﬁt the E11 resonance in the PL spectra, except for the 3
nm sample where two Lorentz functions are used to obtain aweighted average of the E11 emission energy. The T emission
band is also ﬁtted by a Lorentz function for the PL spectra of
the samples with more than 7 nm thickness. For the 3 nm
sample, there is a diﬃculty for distinguishing the new emission
band from K-momentum exciton emission band32 due to the
low intensity. In the case of the PLE spectra, we extract the E22
resonance energy by ﬁtting with a Lorentzian on top of a linear
function. In Figure 2c, we plot the energy shifts of the E11 andE22 resonances for (9,7) nanotubes relative to the energies
before the evaporation. We note that the energy shifts are
induced by changing the surface molecules from the air

chiralities measured, consistent with the dielectric screening
eﬀects due to the adsorbed molecules. The molecule-induced
shifts of the E11 and E22 resonances for the 26 nm sample areplotted as a function of nanotube diameter in Figure 3b.
Although we expect that the shifts become gradually smaller as
the diameter increases,30,35,36 there is no clear dependence.
The scatter of the data points is likely caused by large tube-totube deviations of adsorption amounts. Averaging the shifts ofthe E11 and E22 resonances for all chiralities, we obtain valuesof −58 and −67 meV, respectively. Comparable emission
energy shifts have been observed for nanotubes under
dielectric screening by solvents,30,37 explaining why the shifts
due to organic molecule adsorption in solutions are suppressed.
We evaluate the change in the exciton resonance energy byconsidering the reduction of the electronic many-bodyinteractions due to molecular adsorption. The E11 exciton
energy is determined by E11 = Esp + Ese − Eeb, where Esp is the
single-particle band gap, Ese is the self-energy from repulsive
electron interactions, and Eeb is the exciton binding energy
from the attractive electron−hole interactions.38,39 Because the5778

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energy separation.28,34 As a trion is a bound state of a carrier
and an exciton,41 we expect trion formation if there is chargetransfer between CuPc molecules and CNTs. Such a picture
can also explain the E11 intensity reduction with the molecularadsorption. The weak dependence of the T emission intensity
on deposition thickness is also consistent, as the ﬁrst layer of
CuPc should have formed for thickness larger than 7 nm.
The exciton−trion energy separation for the CuPc adsorbed
state is smaller than that for the air molecule adsorbed state by
about 70 meV for tubes with diameter d = 1 nm, indicatingthat exciton−carrier interactions are considerably screened by
the CuPc molecules. It is known that the exciton−trion energy
separation ΔEX−T follows the relation ΔEX−T = Etb + Est, where
Etb = A/d is the trion binding energy and Est = B/d2 is the
singlet-triplet splitting energy while A and B are proportionality
constants.34,41 For the air molecule adsorbed state, A = 105
meV·nm and B = 70 meV·nm2.34 We ﬁt the data by ΔEX−T =(A/d + B/d2) × CT, under an assumption that both A and B
scale by the same factor CT upon the organic molecule
adsorption. The result is shown as a blue line in Figure 4d,
giving CT = 0.67, which is comparable to the value of CX.
Correlation between ΔEX−T and E11. So far we have been
discussing the ensemble spectra, but data for the individualtubes provide additional insights to molecular screening eﬀects
on the many-body interactions. By ﬁtting the PL spectra foreach nanotube with the same procedure as the ensemble
spectra, we obtain the energies for the T and E11 emission invarious molecular screening environments. In Figure 5a,
ΔEX−T values for (9,7) individual nanotubes are plotted as afunction of the E11 exciton emission energy. We ﬁnd that there
is a good correlation between ΔEX−T and E11, where ΔEX−T
becomes smaller as the E11 emission energy red shifts. We also
plot data points for (9,7) suspended nanotubes with andmagnitude of the self-energy is larger than that of the exciton
binding energy, the E11 exciton energy is higher than Esp by thenet many-body correlation energy Ese − Eeb. Assuming thatthese interactions scale by the same factor CX, the shift of theE11 exciton energy ΔE11 relative to the air-molecule adsorbed
state is given by ΔE11 = (Ese − Eeb) × (CX − 1). For airmolecule-adsorbed nanotubes with 1 nm diameter, Ese and Eeb
have been estimated to be 910 and 660 meV, respectively.29
From the average ΔE11 of −58 meV for the 26 nm sample, CX
is calculated to be 0.77, which corresponds to a reduction of
the many-body interactions with the molecular adsorption by
23%. Furthermore, CX values for the 3 nm sample, 7 nm
sample, and 16 nm sample are estimated to be 0.94, 0.83, and0.78, respectively, showing that the many-body interactionenergies reduce monotonically with increasing the molecularamounts. The absolute value of the average ΔE22 for the 26 nm
sample is slightly larger than that of the average ΔE11, which isreasonable because the net many-body correlation energy for
the E22 excitons is larger than that for the E11 excitons.30,40
T Emission. We now turn our attention to the low energy T
emission. Figure 4a is a PLE map for a (9,7) nanotube in the

Figure 4. (a) PLE map for a (9,7) tube in the 26 nm sample. (b,c) PL
images for T and E11 exciton emission, respectively, for the same tube.
Spectral integration windows centered at 0.81 and 0.92 eV with awidth of 10 meV are used for (b,c), respectively. Scale bars are 1
μm. (d) ΔEX−T for the 26 nm sample is shown as a function of thediameter. All of the data are taken at P = 300 μW. The error bars are
smaller than the symbols.

26 nm sample, and there are two peaks at 0.81 and 0.92 eVcorresponding to the T and E11 emission, respectively. The
absorption resonance energies for both emission are almost thesame, and the PL images at the T and E11 energies overlap
spatially (Figure 4b,c). The spectral and spatial coincidence
indicates that the T emission comes from the same tube. We
deﬁne the energy separation between the T and E11 peaks to be
ΔEX−T, for example ΔEX−T = 0.11 eV in Figure 4a. The T
emission energy for various chiralities is obtained by ﬁtting theensemble averaged PL spectra with a Lorentz function, andΔEX−T is plotted as a function of the diameter in Figure 4d.
Because the K-momentum exciton emission should appear at
0.13−0.14 eV below the E11 emission peak,32,34 it cannot
explain the T emission with ΔEX−T = 0.10−0.12 eV. In
addition, intensity of the K-momentum exciton emission istypically 1% of the bright E11 exciton emission, but the Temission intensity can be comparable to the E11 emission.34 A
more plausible interpretation would be emission from trions, as
the energy separation can be 0.10−0.14 eV under dielectric
screening within the diameter range shown in Figure 4d.41,42
The observed ΔEX−T tends to become smaller as the diameterincreases, consistent with the behavior for the exciton−trion

Figure 5. (a−c) Exciton−trion energy separation for (9,7), (13,2),and (11,3) tubes are shown as a function of the E11 emission energy.
Red, green, and blue points are the results for CuPc adsorbed states,
air-molecule adsorbed states, and pristine states without molecularadsorption, respectively. Note that the data points for the air-molecule
adsorbed and pristine nanotubes are obtained from ref 28, where thetrion emission for both states have been investigated by using ﬁeld
eﬀect transistor structures. The error bars are smaller than thesymbols. (d) Schematic diagram for tuning of the many-bodyinteraction energies with the dielectric constant. (e) Coeﬃcient forthe correlation is shown as a function of the diameter.
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without air-molecule adsorption from ref 28 as green and blue
points, respectively. These data points also follow the trend,suggesting that there is a universal relation applicable tonanotubes under diﬀerent surface conditions. Similar correlation is observed for (13,2) and (11,3) nanotubes (Figure 5b,c,
respectively), as well as for the other chiralities. We note that
CuPc adsorption can shift the E11 energy by ∼100 meVcompared to the pristine state, oﬀering ﬂexible tunability overthe emission energy.
The correlation can be understood in terms of the moleculardielectric screening (Figure 5d). In general, the many-bodyinteractions determining the T and E11 exciton energies are
sensitive to environment, which can simply be parameterized
as the environmental dielectric constant ε. Assuming thatinteraction energies scale as ε−α with α being a constant,29,38
−α
−α
−α
−α
ΔEX−T = Eε=1
+ Eε=1and E11 = Esp + Eε=1
− Eε=1
tb ε
st ε
se ε
eb ε ,
ε=1
ε=1
ε=1
where Eε=1
,
E
,
E
,
and
E
are
the
trion
bindingenergy,tb
stseeb
singlet−triplet splitting energy, self-energy, and exciton binding
energy at ε = 1, respectively. The slope for the correlation isthen given by

ArticleAUTHOR INFORMATION

Corresponding Author*E-mail: yuichiro.kato@riken.jp.
ORCID

Keigo Otsuka: 0000-0002-6694-0738
Yousoo Kim: 0000-0001-7730-0704
Yuichiro K. Kato: 0000-0002-9942-1459
Notes

The authors declare no competing ﬁnancial interest.

■

ACKNOWLEDGMENTS
Work supported in part by RIKEN (Incentive Research
Project), JSPS (KAKENHI JP16H05962 and JP17H07359),
and MEXT (Nanotechnology Platform). We thank the
Advanced Manufacturing Support Team at RIKEN fortechnical assistance.

■

E ε=1 + E ε=1∂ΔE X−T= tbε=1 stε=1 , indicating a linear relation∂E11
Ese − Eeb

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V.; Roussignol, P.; Delaire, J. A.; Deleporte, E. Quantum efficiency ofenergy transfer in noncovalent carbon nanotube/porphyrin compounds. Appl. Phys. Lett. 2010, 97, 141918.
(12) Correa, J. D.; Orellana, W. Optical response of carbon
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correlation therefore implies that the many-body interactionenergies scale similarly within the experimental conditions
explored, which is reasonable because the scaling factor for theE11 exciton CX is comparable to that for the trion CT.
Using the reported values for pristine suspended nanotubes
ε=1
ε=1
ε=1
of Eε=1
tb = 117 meV, Est = 78 meV, Ese = 980 meV, and Eeb =725 meV for 1 nm diameter tubes,28,29 the proportionality
coeﬃcient is about 0.8. To compare this value with theexperimental results, we ﬁt the data points for the organic
molecule adsorbed state with a linear function. We impose theline to go through a data point for the air-molecule adsorbed
state, where the E11 emission energy is obtained from the tablein ref 25 and ΔEX−T is determined from the equation given in
ref 34. The obtained coeﬃcients range from 0.8 to 1.2 (Figure5e), which is a reasonable agreement considering the large
uncertainty in the absolute value of Eε=1
se .

■CONCLUSIONSIn summary, we have investigated adsorption eﬀects of CuPc
molecules on excitons and trions in air-suspended CNTs bymeasuring PL spectra of the CuPc/CNT hybrids. By averaging
the PLE spectra for tubes with the same chiralities, the red shiftof the E11 and E22 resonances has been observed withincreasing deposition thickness due to the molecular screening.
The E11 energy modiﬁcation from the pristine state to the
CuPc adsorbed state can reach 100 meV, which is signiﬁcantly
larger than the energy shift with organic molecule adsorption
in solutions. Furthermore, we ﬁnd that the trion emission
emerges at an energy lower than the E11 exciton emission,
which is likely due to charge transfer between CuPc molecules
and CNTs. The spectra from individual tubes show that there
is a good correlation between the exciton−trion energy
separation and the exciton emission energy. A model assuming
power law scaling of the many-body interactions with dielectric
constant can quantitatively explain the observed correlation,
which should be able to describe nanotubes with diﬀerent
surface conditions in a universal manner. Our ﬁndings showthat organic molecule adsorption signiﬁcantly aﬀects manybody interaction energies of suspended CNTs, which opens up
a pathway to CNT devices utilizing noncovalent molecularfunctionalization.
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DOI: 10.1021/acs.jpcc.8b12496
J. Phys. Chem. C 2019, 123, 5776−5781

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DOI: 10.1021/acs.jpcc.8b12496
J. Phys. Chem. C 2019, 123, 5776−5781