Gate-voltage effects on photoluminescence spectra of suspended single-walled
carbon nanotubes are investigated. Photoluminescence microscopy and excitation
spectroscopy are used to identify individual nanotubes and to determine their
chiralities. Under an application of gate voltage, we observe slight blueshifts
in the emission energy and strong quenching of photoluminescence. The
blueshifts are similar for different chiralities investigated, suggesting
extrinsic mechanisms. In addition, we find that the photoluminescence intensity
quenches exponentially with gate voltage.Comment: 4 pages, 4 figure

Chiashi, S.

Kato, Y. K.

Maruyama, S.

Moritsubo, S.

Murai, T.

Shimada, T.

Yasukochi, S.

English

arXiv

S. Yasukochi

T. Murai

S. Moritsubo

T. Shimada

S. Chiashi

S. Maruyama

Y. K. Kato

Crossref

Physical Review B

Gate-induced blueshift and quenching of photoluminescence in suspended single-walled carbon nanotubes

Gate-voltage effects on photoluminescence spectra of suspended single-walled carbon nanotubes are investigated. Photoluminescence microscopy and excitation spectroscopy are used to identify individual nanotubes and to determine their chiralities. Under an application of gate voltage, we observe slight blueshifts in the emission energy. In addition, we find that the photoluminescence intensity decreases exponentially with gate voltage. Quenching of the K-momentum exciton emission is also observed

Yasukochi S.

Murai T.

Moritsubo S.

Shimada T.

Chiashi S.

Maruyama S.

Kato Y. K.

UTokyo Repository

RAPID COMMUNICATIONS
PHYSICAL REVIEW B 84, 121409(R) (2011)
Gate-induced blueshift and quenching of photoluminescence in suspended
single-walled carbon nanotubes
S. Yasukochi,1 T. Murai,1 S. Moritsubo,1 T. Shimada,1 S. Chiashi,2 S. Maruyama,2 and Y. K. Kato1,*
1Institute of Engineering Innovation, The University of Tokyo, Tokyo 113-8656, Japan
2Department of Mechanical Engineering, The University of Tokyo, Tokyo 113-8656, Japan
(Received 25 April 2011; published 22 September 2011)
Gate-voltage effects on photoluminescence spectra of suspended single-walled carbon nanotubes are
investigated. Photoluminescence microscopy and excitation spectroscopy are used to identify individual
nanotubes and to determine their chiralities. Under an application of gate voltage, we observe slight blueshifts in
the emission energy. In addition, we find that the photoluminescence intensity decreases exponentially with gate
voltage. Quenching of the K-momentum exciton emission is also observed.
DOI: 10.1103/PhysRevB.84.121409 PACS number(s): 78.67.Ch, 85.35.Kt, 71.35.−y, 78.55.−m
Understanding of electric-field effects on optical emission
from single-walled carbon nanotubes (SWCNTs) is a key to
the development of carbon-based nanoscale optoelectronics.1
It has been shown that electric fields can drive light emis-
sion in SWCNTs.2,3 In comparison, photoluminescence (PL)
is quenched by an application of electric fields. Micelle-
encapsulated SWCNTs show a reduction of PL intensity
by electric fields along the tube axis.4 Similar quenching
occurs in suspended nanotubes within field-effect transistor
(FET) structures.5–7 In contrast to absorption measurements
where redshifts due to the Stark effect8 and doping-induced
screening6 have been observed, such strong quenching of PL
has made it difficult to unambiguously resolve shifts in the
emission spectra.
Here we report on gate-induced blueshift and quenching
of PL in individual suspended SWCNTs with determined
chirality. As-grown nanotubes within FET structures are
identified by PL imaging using a laser scanning confocal
microscope. Excitation spectroscopy is used to determine their
chirality, and PL spectra are collected as a function of gate
voltage. Surprisingly, we find that the emission blueshifts
when the gate voltages are applied, contrary to the expectation
that screening causes redshifts. Furthermore, the PL intensity
decreases exponentially with gate voltage, and a comparison
with a model assuming doping-induced exciton relaxation
proportional to carrier density9,10 shows that it cannot account
for all of the quenching observed. We also observe quenching
of the K-momentum exciton emission by the gate field.
The suspended nanotube FETs [Fig. 1(a)] are fabricated
on p-type Si substrates with 100-nm-thick oxide. We begin
by etching 1-μm wide trenches with a depth of ∼5 μm. The
wafer is then annealed at 900 ◦C in oxygen for an hour to
form an oxide layer inside the trenches. 1-nm Ti and 15-nm Pt
are deposited for source and drain contacts, and then catalyst
areas are defined on the drain electrodes. Co acetate and
fumed silica are dissolved in ethanol and deposited on the
wafer. Finally, carbon nanotubes are grown by chemical vapor
deposition using ethanol as a carbon source.11 Typical device
characteristics are shown in Fig. 1(b), and we note that these
devices show hysteresis due to water adsorption.12
PL spectra are collected with a home-built laser scanning
confocal microscope.13 An output of a continuous-wave
Ti:sapphire laser is focused onto the sample with an objective
lens, and a steering mirror allows scanning of the laser
spot. PL is detected with a liquid-nitrogen-cooled InGaAs
photodiode array attached to a single-grating spectrometer.
All measurements are done in air at room temperature.
Typical PL spectra are shown in Fig. 1(c). The sharp peak
at 0.9 eV originates from a suspended SWCNT, while PL from
the Si substrate shows a broad emission at 1.1 eV. By scanning
0.890 eV
1.098 eV
 
Emission Energy (eV)P
L 
In
te
ns
ity
 
(ar
b. 
un
its
)
(c)
0.8 0.9 1.0 1.1 1.2
0
10
20
30
(a)
Si
SiO2
Ti/Pt SWCNT
5
PL Intensity (arb. units)
0 1 2 3 4
(e)
2 µm
2 µm
250
PL Intensity (arb. units)
5 10 15 20
(f )
 
D
ra
in
 C
ur
re
nt
 (n
A)
Gate Voltage (V)
−5 0 5
(b)
0
20
40
10
30 Vd = 100 mV
Drain
Source 5 µm
(d)
FIG. 1. (Color) (a) A schematic of a device. (b) Typical elec-
trical characteristics. Drain voltage Vd = 100 mV is used. (c) PL
spectra for a (9,8) carbon nanotube (blue) and Si substrate (red).
(d) Optical microscope image of a device. The scale bar is
5 μm. The blue and red dots indicate the positions of the laser
spot where the blue and red curves in (b) are taken, respec-
tively. The black box shows the scan area for the PL images.
(e) and (f) are PL images at emission energies of 1.098 eV and
0.890 eV, respectively. Spectral integration windows are 4 meV wide,
and the scale bars are 2 μm. For (c), (e), and (f), excitation energy of
1.653 eV and excitation power of 0.65 mW are used.
121409-11098-0121/2011/84(12)/121409(4) ©2011 American Physical Society
RAPID COMMUNICATIONS
S. YASUKOCHI et al. PHYSICAL REVIEW B 84, 121409(R) (2011)
Emission Energy (eV)
0.8 0.9 1.0
G
at
e 
Vo
lta
ge
 (V
)
−1
0
PL Intensity (arb. units)
1 10
1.1 1.2
1
2
−2
(a)
PL
 In
te
ns
ity
 
(ar
b. 
un
its
)
0
5
10
15
20
0.92 0.940.93
(b) 0 V
−1.2 V
−1.5 V
Emission Energy (eV)
FIG. 2. (Color) (a) PL intensity as a function of gate voltage
and emission energy for a (10,6) nanotube. Excitation energy of
1.653 eV and excitation power of 126 μW are used. The source and
drain contacts are grounded during these measurements. (b) Typical
spectra at selected gate voltages. Open circles are data, and lines are
Lorentzian fits. Red, blue, and green correspond to Vg = 0, −1.2, and
−1.5 V, respectively.
the laser spot and collecting PL spectra in the area shown in
Fig. 1(d), PL images for the Si substrate and the nanotube are
constructed [Figs. 1(e) and 1(f)]. Using these images, we check
that the PL comes from a fully-suspended nanotube in between
source and drain electrodes. Next, the chirality of the nanotube
is identified by PL excitation spectroscopy,14 using tabulated
data for suspended SWCNTs.15 We look for nanotubes with a
sharp single peak in the PL excitation map to exclude bundled
tubes.
Following such procedures, we have investigated gate-field
dependences of PL in seven individual SWCNTs. Figure 2(a)
shows a series of PL spectra taken as a function of the gate
voltage Vg for a (10,6) nanotube. The PL intensity is largest
near Vg = 0 V, and decreases dramatically with an application
of gate voltage, as observed previously.5,6 The PL intensity
maximum is not exactly at Vg = 0 V, because of the hysteretic
behavior of the devices. PL spectra at representative gate
voltages are shown in Fig. 2(b). In addition to the quenching,
we find that the emission blueshifts slightly as the gate voltage
is applied. In order to quantitatively characterize the blueshift,
we fit the PL peak with a Lorentzian function. The peak
position and the peak area are extracted from the fitting
parameters, and they are used as a measure of the emission
energy and PL intensity, respectively.
Such analysis is performed on all nanotubes investigated,
and in Fig. 3 we present data from three nanotubes of different
chirality. For all three nanotubes, we observe the blueshift as
well as quenching. The blueshift can be as much as 7 meV, and
is limited by the loss of signal quality due to the quenching at
higher gate voltages. We do not observe significant changes in
the line widths within the accuracy of the fits.
As a possible cause of the blueshift, we consider the
effects of strain. Gate voltage may exert electrostatic forces
on the nanotube and induce strain, resulting in changes in the
band-gap energy that reverses its sign depending on the type
of the nanotube.16,17 If strain is responsible for the observed
energy shifts, we expect the sign of the energy shift for a
(10,6) nanotube [Fig. 3(a)] to be different from that of a (10,5)
 
Emission Energy (eV)
Ex
ci
ta
tio
n 
En
er
gy
 
(eV
)
1.55
1.65
0.8 0.9
1.70
1.60
1.0 1.1 1.2
 
 PL Intensity (arb. units)
1 2 3 4
(a) (10,6)
0
2
4
0
1
2
3
PL
 P
ea
k 
Ar
ea
 (a
rb.
 un
its
)
Bl
ue
sh
ift
(m
eV
)
6
8
10
12
Gate Voltage (V)
0 1 2−2 −1
(d) 4
 
 
 
0 105
PL Intensity (arb. units)
(b) (9,8)
Emission Energy (eV)
Ex
ci
ta
tio
n 
En
er
gy
 
(eV
)
1.55
1.65
0.8 0.9
1.70
1.60
1.0 1.1 1.2
 
 
0 105
PL Intensity (arb. units)
(c) (10,5)
Emission Energy (eV)
Ex
ci
ta
tio
n 
En
er
gy
 
(eV
)
1.55
1.65
0.8 0.9
1.70
1.60
1.0 1.1 1.2
 
PL
 P
ea
k 
Ar
ea
 (a
rb.
 un
its
)
Bl
ue
sh
ift
(m
eV
)
0
2
4
8
6
0
1
2
3
0 2−2 4−4
Gate Voltage (V)
(e) 4
 
PL
 P
ea
k 
Ar
ea
 (a
rb.
 un
its
)
Bl
ue
sh
ift
(m
eV
)
0
3
6
0
2
4
8
6
3 4 5 6
Gate Voltage (V)
(f)
910
FIG. 3. (Color) (a–c) PL excitation maps for (10,6), (9,8), and (10,5) nanotubes, respectively. The spectra are not corrected for the changes
in excitation power as it does not vary by more than a factor of two within the tuning range. The nanotubes shown in (a) and (b) are the same as
those shown in Fig. 2 and Figs. 1(c)–1(f), respectively. (d–f) Gate-voltage dependences of PL peak area (red line) and blueshift (blue squares)
for the nanotubes shown in (a–c), respectively. In these measurements, the nanotubes are resonantly excited. Excitation powers of 126, 79, and
212 μW, and Vg sweep rates of 18, 20, and 6 mV s−1 are used for (d–f), respectively. The gate voltage is scanned from negative to positive in
panels (d) and (e), while it is scanned from positive to negative in panel (f). The blueshifts are measured from 930.4, 889.2, and 1008.2 meV
in (d–f), respectively.
121409-2
RAPID COMMUNICATIONS
GATE-INDUCED BLUESHIFT AND QUENCHING OF . . . PHYSICAL REVIEW B 84, 121409(R) (2011)
N
or
m
al
iz
ed
 P
L 
Pe
ak
 A
re
a
1
−0.10
0.05
−0.05
0.00
0.10
ρ
(nm
-
1 )
E F
(eV
)0.5
Gate Voltage (V)
−0.5
0.0
0 1 2−2 −1
0.1
0.01
Gate Voltage (V)
0 1 2−1
FIG. 4. (Color) Normalized PL peak area as a function of Vg. The
data (circles) are the same as in Fig. 3(d), but an offset of 0.36 V
have been added to Vg to fit the simulation. Calculations assuming
relaxation rates linear (blue curve) and exponential (red curve) in
carrier density are plotted. α = 1.30 nm ps−1 is used for the blue
curve, while β = 1.94 × 10−2 ps−1 and ρ0 = 2.48 × 10−2 nm−1 are
used for the red curve. Inset shows calculated carrier density (red)
and Fermi energy (blue) as a function of Vg. The hole densities are
obtained by negating Vg, EF, and ρ in the calculations for electrons.
nanotube [Fig. 3(c)]. Since we observe blueshifts regardless
of the nanotube type, strain-induced changes in the band gap
cannot explain our data.
The observation of blueshift is quite surprising as the Stark
effect and screening by gate-induced carriers have shown
redshifts.6,8 It is reasonable that the Stark effect does not
play a role, because it requires significantly higher fields.8
Regarding the screening effect, redshifts for E33 absorption
at somewhat larger gate voltages have been observed using
excitation energy dependence of Raman scattering.6 However,
a theoretical calculation shows that doping induces a blueshift
as a result of renormalization of band-gap and exciton
binding energies.18 We note that chemically-doped SWCNTs
in solution do not show much PL energy shifts either,19 and
it may be the case that E11 responds differently to doping
compared to higher bands. This is reasonable since the exciton
binding energy is a significant fraction of the band gap
for the E11 transition, and also because it is the transition
between the bands that are actually being occupied by
carriers.
We now turn our attention to the quenching of PL with gate
voltage. Interpretation by carrier extraction5 has difficulties
explaining exciton dissociation and carrier drift from the
center of the trench to the contacts. Since fields perpendicular
to the nanotube axis do not cause much quenching,4 it is
likely that electrostatic doping plays an important role. Phase-
space filling and doping-induced exciton relaxation have been
suggested as possible mechanisms.6
We model these doping-induced effects quantitatively and
compare with the data for the (10,6) nanotube which has
comparatively smaller hysteresis. We begin by computing
the carrier density ρ for a Fermi energy EF from ρ(EF) =∫ ∞
E1
dE [g(E)f (E,EF)], where g(E) = 4πh¯vF E√E2−E21 is the
density of states for the first conduction band at an electronic
energy E, h¯ is the Planck constant, vF = 9.5 × 105 m s−1
is the Fermi velocity in graphene, E1 = (E11 + Eb)/2 is
half the band-gap energy, E11 = 0.930 eV is the excitonic
transition energy, Eb = 0.34/d eV is the exciton binding
energy,20 d = 1.1 nm is the nanotube diameter, f (E,EF) =
[1 + exp(E−EF
kBT
)]−1 is the Fermi-Dirac distribution function, kB
is the Boltzmann constant, and T = 300 K is the temperature.
Taking into account the quantum capacitance,21 we obtain the
relation between Vg and EF from Vg(EF) = EFe + eρ(EF)Cg , where
e is the electronic charge and Cg is the geometric capacitance
of the gate. We use Cg = 10 aF μm−1, based on estimates in
similar devices.6,22 The results of the calculation are shown in
the inset of Fig. 4.
To simulate the changes in the PL efficiency, we calculate
γ0/[γ0 + γd(ρ)] where γ0 = 0.01 ps−1 is the exciton relaxation
rate in the absence of doping23 and γd(ρ) is the doping-induced
relaxation rate. Assuming γd proportional to ρ,9,10,19 we have
performed a least-squares fit to normalized data with γd = αρ
where α is a proportionality constant used as a fitting parameter
(Fig. 4, blue curve).
There are several issues with this doping-induced relaxation
model. First, we obtain α = 1.30 nm ps−1, which is about an
order of magnitude larger than the theoretical calculation.9
Second, it does not reproduce the quenching exponential with
Vg. In addition, at higher gate voltages, the model deviates
from the experimental data by factors of 2–5.
As a possible reason for this discrepancy, we consider
the phase-space filling effect.6,24 For an exciton with zero
center-of-mass momentum, the wave function  in k space can
be written as (k) = exp(−k2σ 2/2), where k is the relative
momentum and σ is the exciton radius.24 We assume that
electron and hole masses are the same, so that the relative
momentum of the exciton is equal to the electron momentum.
Using the electron dispersion E2 = E21 + h¯2v2Fk2, the fraction
of the filled states is given by
∫ ∞
E1
dE
[
exp
(
− E2−E212h¯2v2F σ
2
)
g(E)f (E,EF)
]
∫ ∞
E1
dE
[
exp
(
− E2−E212h¯2v2F σ
2
)
g(E)
] ,
but it can only account for ∼10% reduction in the PL
intensity at Vg = 2 V for σ = 2 nm. As another possibility,
E22 may also shift due to the gate voltage and change the
absorption efficiency, but E22 linewidth of ∼40 meV requires
shifts that are significantly larger than the observed E11
shifts.
To better describe the observed results, we have introduced
a phenomenological expression for doping-induced relaxation
given by γd = β[exp(|ρ|/ρ0) − 1] and performed a least-
squares fit with β and ρ0 as fitting parameters (red curve
in Fig. 4). Apart from the asymmetry between positive and
negative voltages caused by the hysteresis, this model provides
a good fit to the data. Such an expression implies exponential
increase of relaxation rate with carrier density. Since ρ
increases almost linearly with Vg, mechanisms that scale
exponentially with gate voltages or fields are also plausible.
Yet another possibility is the role of carriers in higher bands,
which would scale exponentially with Vg because of the tail of
the Fermi-Dirac distribution.
121409-3
RAPID COMMUNICATIONS
S. YASUKOCHI et al. PHYSICAL REVIEW B 84, 121409(R) (2011)
In addition to the bright exciton emission that we have
discussed, there also exists a weak emission at a lower energy.
In Fig. 2(a), there is a small peak around 0.8 eV due to
K-momentum excitons.25–27 This peak also disappears upon
application of the gate voltage, consistent with the prediction
that K-momentum excitons also experience doping-induced
relaxation.9
In summary, we have investigated the gate-field effect on
PL spectra of individual suspended SWCNTs with determined
chiralities. Slight blueshifts due to applied gate fields have
been observed for three different chiralities. PL quenching
increases exponentially with gate voltage, suggesting that
doping-induced exciton relaxation may be stronger than
previously thought. The K-momentum excitons emission is
also found to be quenched by the gate fields.
We thank T. Kan and I. Shimoyama for the use of the
electron beam evaporator, and acknowledge support from
KAKENHI (23104704, 21684016), Mizuho Foundation for
the Promotion of Sciences, Research Foundation for Opto-
Science and Technology, SCAT, SCOPE, TEPCO Memorial
Foundation, The Japan Prize Foundation, and Photon Frontier
Network Program of MEXT, Japan. The devices were fabri-
cated at the Center for Nano Lithography & Analysis at The
University of Tokyo.
*Corresponding author: ykato@sogo.t.u-tokyo.ac.jp
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121409-4


Gate-induced blueshift and quenching of photoluminescence in suspended
  single-walled carbon nanotubes

Gate-induced blueshift and quenching of photoluminescence in suspended single-walled carbon nanotubes

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