RAPID COMMUNICATIONSPHYSICAL 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,*
2

1
Institute of Engineering Innovation, The University of Tokyo, Tokyo 113-8656, Japan
Department 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 areinvestigated. 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 inthe 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.
PACS number(s): 78.67.Ch, 85.35.Kt, 71.35.−y, 78.55.−m

1098-0121/2011/84(12)/121409(4)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 fromthe Si substrate shows a broad emission at 1.1 eV. By scanning(a)Ti/Pt SWCNT

(d)

Drain

SiO2
Si
Drain Current (nA)

Understanding of electric-field effects on optical emission
from single-walled carbon nanotubes (SWCNTs) is a key tothe development of carbon-based nanoscale optoelectronics.1
It has been shown that electric fields can drive light emission in SWCNTs.2,3 In comparison, photoluminescence (PL)is quenched by an application of electric fields. Micelleencapsulated SWCNTs show a reduction of PL intensity
by electric fields along the tube axis.4 Similar quenchingoccurs 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 theemission spectra.
Here we report on gate-induced blueshift and quenchingof PL in individual suspended SWCNTs with determinedchirality. As-grown nanotubes within FET structures are
identified by PL imaging using a laser scanning confocal
microscope. Excitation spectroscopy is used to determine theirchirality, 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 expectationthat 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 accountfor all of the quenching observed. We also observe quenchingof the K-momentum exciton emission by the gate field.
The suspended nanotube FETs [Fig. 1(a)] are fabricatedon 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 toform 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 andfumed silica are dissolved in ethanol and deposited on thewafer. 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 thesedevices show hysteresis due to water adsorption.12
PL spectra are collected with a home-built laser scanningconfocal microscope.13 An output of a continuous-wave
Ti:sapphire laser is focused onto the sample with an objective

Source
40
30
20
10
0

(b)

Vd = 100 mV

(e)

5 µm

1.098 eV

2 µm
−5
0
5
Gate Voltage (V)PL Intensity (arb. units)

DOI: 10.1103/PhysRevB.84.121409

0

1
2
3
4
5
PL Intensity (arb. units)

30

(c)
20

(f )

10
0
0.8 0.9 1.0 1.1 1.2
Emission Energy (eV)0.890 eV2 µm

05
10 15 20 25
PL Intensity (arb. units)

FIG. 1. (Color) (a) A schematic of a device. (b) Typical electrical 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, respectively. 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.

121409-1©2011 American Physical Society

RAPID COMMUNICATIONSS. YASUKOCHI et al.

−1
−2
0.8

0.9
1.0
1.1
Emission Energy (eV)

10

1.2

−1.2 V5
0−1.5 V

0.92 0.93 0.94
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 anddrain 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 areconstructed [Figs. 1(e) and 1(f)]. Using these images, we checkthat 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 asharp single peak in the PL excitation map to exclude bundled
tubes.

(10,6)

1.65
1.60
1.55
1.0
1.1
0.9
Emission Energy (eV)

12
(d)
10
8
6
4
2
0
−2
−1
0
1
Gate Voltage (V)

1.2

4
3
2
1
0
2

Blueshift (meV)

0.8

Excitation Energy (eV)(a)

PL Intensity (arb. units)
5
10

0

4

(b)

1.70

0

(9,8)

1.65
1.60
1.55
0.8
PL Peak Area (arb. units)

1.70

PL Peak Area (arb. units)

Excitation Energy (eV)PL Intensity (arb. units)
1
2
3

1.0
1.1
0.9
Emission Energy (eV)

(e)

8

1.2

4
3

6
2

4

1
2
0
0

−4

−2 0
2
4
Gate Voltage (V)1.70

PL Intensity (arb. units)
5
10

(c)

(10,5)

1.65
1.60
1.55
1.0
1.1
0.9
Emission Energy (eV)

0.8

9

10
8

(f)
6

6
4

3

2
0

1.2

0
3
4
5
Gate Voltage (V)Blueshift (meV)0

15

Excitation Energy (eV)1

Following such procedures, we have investigated gate-fielddependences 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 applicationof 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, andis 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 theeffects of strain. Gate voltage may exert electrostatic forceson the nanotube and induce strain, resulting in changes in the
band-gap energy that reverses its sign depending on the typeof 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)

0V

(b)

PL Peak Area (arb. units)

2

20

Blueshift (meV)

PL Intensity (arb. units)

PL Intensity (arb. units)
1
10

(a)

Gate Voltage (V)PHYSICAL REVIEW B 84, 121409(R) (2011)

6

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, and212 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 inpanels (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 meVin (d–f), respectively.
121409-2

RAPID COMMUNICATIONSGATE-INDUCED BLUESHIFT AND QUENCHING OF . . .

PHYSICAL REVIEW B 84, 121409(R) (2011)

0.1

0.10
0.0

0.00
−0.05
−0.10
−2

0.01

0.5

0.05

−1

EF (eV)ρ (nm-1)

Normalized PL Peak Area

1

−0.5
−1 0
1
2
Gate Voltage (V)01Gate Voltage (V)2

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 areused 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 regardlessof 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 notplay 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 andit 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 bycarriers.
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 thecenter of the trench to the contacts. Since fields perpendicular
to the nanotube axis do not cause much quenching,4 it islikely that electrostatic doping plays an important role. Phasespace filling and doping-induced exciton relaxation have beensuggested as possible mechanisms.6
We model these doping-induced effects quantitatively andcompare with the data for the (10,6) nanotube which has
comparatively smaller hysteresis. We begin by computing
theF from ρ(EF ) = ∞ carrier density ρ for a Fermi energy E
4 √ E
is theE1 dE [g(E)f (E,EF )], where g(E) = πh̄vF
2
2
E −E1

density of states for the first conduction band at an electronicenergy E, h̄ is the Planck constant, vF = 9.5 × 105 m s−1
is the Fermi velocity in graphene, E1 = (E11 + Eb )/2 ishalf 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 ) =F)]−1 is the Fermi-Dirac distribution function, kB[1 + exp( E−EkB T
is the Boltzmann constant, and T = 300 K is the temperature.
Taking into account the quantum capacitance,21 we obtain theF)relation between Vg and EF from Vg (EF ) = EeF + eρ(E, whereCg
e is the electronic charge and Cg is the geometric capacitanceof the gate. We use Cg = 10 aF μm−1 , based on estimates insimilar 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 = αρwhere 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 anorder 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 deviatesfrom 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 zerocenter-of-mass momentum, the wave function  in k space canbe written as (k) = exp(−k 2 σ 2 /2), where k is the relative
momentum and σ is the exciton radius.24 We assume thatelectron and hole masses are the same, so that the relative
momentum of the exciton is equal to the electron momentum.
Using the electron dispersion E 2 = E12 + h̄2 vF2 k 2 , the fractionof the filled states is given by




∞
E 2 −E12 2
g(E)f
(E,E
dE
exp
−
σ
)
F
E1
2h̄2 v 2
 F 2 2 ,∞E −E1 2g(E)E1 dE exp − 2h̄2 v 2 σF

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 theabsorption 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 introduceda phenomenological expression for doping-induced relaxation
given by γd = β[exp(|ρ|/ρ0 ) − 1] and performed a leastsquares fit with β and ρ0 as fitting parameters (red curvein Fig. 4). Apart from the asymmetry between positive andnegative 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 COMMUNICATIONSS. YASUKOCHI et al.

PHYSICAL REVIEW B 84, 121409(R) (2011)

In addition to the bright exciton emission that we havediscussed, there also exists a weak emission at a lower energy.
In Fig. 2(a), there is a small peak around 0.8 eV due toK-momentum excitons.25–27 This peak also disappears upon
application of the gate voltage, consistent with the predictionthat 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 determinedchiralities. Slight blueshifts due to applied gate fields havebeen observed for three different chiralities. PL quenching
increases exponentially with gate voltage, suggesting that
doping-induced exciton relaxation may be stronger thanpreviously thought. The K-momentum excitons emission is
also found to be quenched by the gate fields.

*

14

Corresponding author: ykato@sogo.t.u-tokyo.ac.jp
P. Avouris, M. Freitag, and V. Perebeinos, Nature Photon. 2, 341(2008).
2
J. A. Misewich, R. Martel, P. Avouris, J. C. Tsang, S. Heinze, and
J. Tersoff, Science 300, 783 (2003).
3
D. Mann, Y. K. Kato, A. Kinkhabwala, E. Pop, J. Cao, X. Wang,
L. Zhang, Q. Wang, J. Guo, and H. Dai, Nature Nanotech. 2, 33(2007).
4
A. V. Naumov, S. M. Bachilo, D. A. Tsyboulski, and R. B. Weisman,
Nano Lett. 8, 1527 (2008).
5
Y. Ohno, S. Kishimoto, and T. Mizutani, Nanotechnology 17, 549(2006).
6
M. Steiner, M. Freitag, V. Perebeinos, A. Naumov, J. P. Small,A. A. Bol, and P. Avouris, Nano Lett. 9, 3477 (2009).
7
M. Freitag, M. Steiner, A. Naumov, J. P. Small, A. A. Bol,
V. Perebeinos, and P. Avouris, ACS Nano 3, 3744 (2009).
8
A. D. Mohite, P. Gopinath, H. M. Shah, and B. W. Alphenaar, Nano
Lett. 8, 142 (2008).
9
V. Perebeinos and P. Avouris, Phys. Rev. Lett. 101, 057401 (2008).
10
J. M. Kinder and E. J. Mele, Phys. Rev. B 78, 155429 (2008).
11
S. Maruyama, R. Kojima, Y. Miyauchi, S. Chiashi, and M. Kohno,
Chem. Phys. Lett. 360, 229 (2002).
12
W. Kim, A. Javey, O. Vermesh, Q. Wang, Y. Li, and H. Dai, Nano
Lett. 3, 193 (2003).
13
S. Moritsubo, T. Murai, T. Shimada, Y. Murakami, S. Chiashi,
S. Maruyama, and Y. K. Kato, Phys. Rev. Lett. 104, 247402 (2010).
1

We thank T. Kan and I. Shimoyama for the use of theelectron beam evaporator, and acknowledge support fromKAKENHI (23104704, 21684016), Mizuho Foundation for
the Promotion of Sciences, Research Foundation for OptoScience and Technology, SCAT, SCOPE, TEPCO Memorial
Foundation, The Japan Prize Foundation, and Photon Frontier
Network Program of MEXT, Japan. The devices were fabricated at the Center for Nano Lithography & Analysis at The
University of Tokyo.

S. M. Bachilo, M. S. Strano, C. Kittrell, R. H. Hauge, R. E. Smalley,
and R. B. Weisman, Science 298, 2361 (2002).
15
Y. Ohno, S. Iwasaki, Y. Murakami, S. Kishimoto, S. Maruyama,
and T. Mizutani, Phys. Rev. B 73, 235427 (2006).
16
L. Yang and J. Han, Phys. Rev. Lett. 85, 154 (2000).
17
M. Huang, Y. Wu, B. Chandra, H. Yan, Y. Shan, T. F. Heinz, andJ. Hone, Phys. Rev. Lett. 100, 136803 (2008).
18
C. D. Spataru and F. Léonard, Phys. Rev. Lett. 104, 177402 (2010).
19
K. Matsuda, Y. Miyauchi, T. Sakashita, and Y. Kanemitsu, Phys.
Rev. B 81, 033409 (2010).
20
G. Dukovic, F. Wang, D. Song, M. Y. Sfeir, T. F. Heinz, and L. E.
Brus, Nano Lett. 5, 2314 (2005).
21
S. Ilani, L. A. K. Donev, M. Kindermann, and P. L. McEuen, Nature
Phys. 2, 687 (2006).
22
A. W. Bushmaker, V. V. Deshpande, S. Hsieh, M. W. Bockrath, andS. B. Cronin, Phys. Rev. Lett. 103, 067401 (2009).
23
Y.-F. Xiao, T. Q. Nhan, M. W. B. Wilson, and J. M. Fraser, Phys.
Rev. Lett. 104, 017401 (2010).
24
L. Lüer, S. Hoseinkhani, D. Polli, J. Crochet, T. Hertel, and
G. Lanzani, Nature Phys. 5, 54 (2009).
25
O. N. Torrens, M. Zheng, and J. M. Kikkawa, Phys. Rev. Lett. 101,
157401 (2008).
26
Y. Murakami, B. Lu, S. Kazaoui, N. Minami, T. Okubo, and
S. Maruyama, Phys. Rev. B 79, 195407 (2009).
27
R. Matsunaga, K. Matsuda, and Y. Kanemitsu, Phys. Rev. B 81,
033401 (2010).

121409-4