Transferred wrinkled Al2O3 for highly stretchable and transparent graphene–carbon nanotube transistors

Transferred wrinkled Al2O3 for highly stretchable and transparent graphene–carbon nanotube transistors

Transferred wrinkled Al 2 O 3 for highly

stretchable and transparent graphene–carbon

nanotube transistors

Sang Hoon Chae1, Woo Jong Yu2,3, Jung Jun Bae1, Dinh Loc Duong1, David Perello4, Hye Yun Jeong1, Quang Huy Ta1, Thuc Hue Ly1, Quoc An Vu1, Minhee Yun4, Xiangfeng Duan2and Young Hee Lee1*

Despite recent progress in producing transparent and bendable

thin-film transistors using graphene and carbon nanotubes 1,2 ,

the development of stretchable devices remains limited either

by fragile inorganic oxides or polymer dielectrics with high

leakage current 3,4 Here we report the fabrication of highly

stretchable and transparent field-effect transistors combining

graphene/single-walled carbon nanotube (SWCNT) electrodes

and a SWCNT-network channel with a geometrically wrinkled

inorganic dielectric layer The wrinkled Al 2 O 3 layer contained

effective built-in air gaps with a small gate leakage current

of 10−13 A The resulting devices exhibited an excellent on/off

ratio of ∼10 5 , a high mobility of ∼40 cm 2 V−1 s−1 and a low

operating voltage of less than 1 V Importantly, because of the

wrinkled dielectric layer, the transistors retained performance

under strains as high as 20% without appreciable leakage

current increases or physical degradation No significant

performance loss was observed after stretching and releasing

the devices for over 1,000 times The sustainability and

performance advances demonstrated here are promising for

the adoption of stretchable electronics in a wide variety of

future applications.

In contrast to rigid electrical devices, the successful fabrication

of stretchable thin-film transistors (TFT) requires that the active

channel, electrodes and gate dielectric be engineered to withstand

high levels of strain without degradation of the electrical properties

In such TFTs, carbon materials are a promising candidate for both

the conducting electrodes and the semiconducting channel Highly

transparent, flexible semiconducting SWCNTs with a high mobility

of ∼40 cm2V−1s−1 and an on/off ratio of ∼105 are ideal for use

as an active-channel material for stretchable devices5–9 Likewise,

the low sheet resistance (∼300 ), high transmittance (∼97.7%)

and high fracture strain resistance (>20%) of monolayer graphene

can make it an excellent complementary electrode material10–12

However, the limited tensile strength of dielectric gate materials

is the primary challenge for producing stretchable devices Typical

inorganic gate oxides are fragile and easily degrade in both

bendable and stretchable devices, and polymer dielectrics have high

leakage current despite their excellent bendability4,13 Therefore,

to maximize the performance of the oxide without compromising

the ability to stretch and bend, we propose a new approach for

preparing a wrinkled gate dielectric using a transfer method

1 Centre for Integrated Nanostructure Physics (CINAP), Institute of Basic Science (IBS), Department of Energy Science, Department of Physics,

Sungkyunkwan University, Suwon 440-746, Republic of Korea, 2 Department of Chemistry and Biochemistry, California Nanosystems Institute, University of California, Los Angeles, California 90095, USA,3Department of Electronic and Electrical Engineering, Sungkyunkwan University, Suwon 440-746, Republic

of Korea, 4 Department of Electrical and Computer Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, USA *e-mail: leeyoung@skku.edu

Figure 1 shows a schematic representation of the fabrication of stretchable graphene/SWCNT TFTs on a thin polydimethylsiloxane (PDMS) film Initially, a 200-nm-thick sacrificial Al layer was deposited onto a SiO2/Si substrate A thin polyimide (PI) layer was formed on the Al film by spin-coating and annealing polyamic acid14 (PAA) (see Supplementary Information S1) Monolayer graphene was then transferred onto the PI film and patterned as source, drain and gate electrodes using photolithography and O2 plasma etching The monolayer graphene was grown on copper foil using atmospheric pressure chemical vapour deposition (APCVD) and had a sheet resistance of 366  per square and a transmittance

of 98% at a wavelength of 550 nm (ref 15) Our main challenge was to fabricate the stretchable gate dielectric using a thin layer of aluminium oxide (Al2O3) A 50-nm-thick Al2O3layer was deposited onto rough Cu foil using atomic layer deposition (ALD) followed

by edge coating with poly(methyl methacrylate) (PMMA) This selective PMMA edge-coating was necessary to prevent fracture

of the Al2O3 layer When the PMMA was coated on the whole oxide layer area, the Al2O3 was fractured easily during acetone cleaning owing to the strong adhesion between the PMMA and the

Al2O3 layer (Supplementary Fig S3) The Cu foil was chemically etched with a Cu etchant (CE-100), and the Al2O3layer was then transferred onto the graphene electrodes The transferred Al2O3 layer was patterned using photolithography and chemically etched with hydrofluoric acid (HF) The resulting Al2O3layer was wrinkled with a wavy structure, as shown in the scanning electron microscopy (SEM) image in Fig 1c The wrinkled structure imparts stability to the gate dielectric under high tensile strain16–19, which is remarkably different from a flat structure It is also noted that our wrinkled oxide is naturally formed during the transfer process and randomly oriented, in good contrast with uniaxially grooved structures16,17, allowing the realization of biaxial stretchability Next, the SWCNT network was prepared using APCVD (ref 20), transferred onto the device and then patterned using photolithography and O2 plasma etching After transferring the APCVD-grown SWCNT network, photoresist was patterned to cover both the active-channel region and graphene electrodes (S, D, G) to prevent graphene damage (Supplementary Fig S5) Using this process, SWCNTs remained on the graphene electrode surfaces after photoresist removal Finally, the resulting TFT-patterned PI film was detached from the Si wafer

by etching the sacrificial Al layer and transferred onto a PDMS

Device fabrication

PI

Al/wafer

PI coating on wafer Transfer PI on PDMS

PDMS

Stretching

HF

Cu foil

Al2O3deposition

on Cu foil

Cu etching PMMA

PMMA coating

PMMA

GR

100 nm

a

b

c

i) Graphene transfer and pattern

O2 plasma

ii) Wrinkled oxide transfer and pattern iii) SWCNT transfer and pattern

O2 plasma

Transfer on graphene and PMMA removal

Figure 1|The fabrication scheme of the device array a, Stretchable device fabrication procedure PI was coated onto an Al-deposited SiO2/Si wafer Once

a monolayer of graphene was patterned for the electrodes, and the active-channel SWCNT film was deposited, the aluminium layer was removed by etching, and the PI/devices were then transferred onto a stretchable PDMS substrate The photograph illustrates the stretchable device array.

b, Graphene/SWCNT TFT fabrication procedure The transferred monolayer of graphene was patterned by O2plasma to form source, drain and gate electrodes The wrinkled oxide was transferred directly onto the prepared substrate The patterned gate oxide was formed on the channel region by HF

etching The APCVD-grown SWCNT random network was transferred and patterned to form an active-channel area c, The wrinkled Al2 O 3 layer transfer process A 50 nm-thick Al 2 O 3 layer was deposited onto copper using ALD, followed by PMMA edge-coating The copper layer was removed with Cu etchant (CE-100) and then the PMMA-attached wrinkled Al 2 O 3 layer was transferred onto the prepared substrate, and the PMMA was removed The wrinkled aluminium oxide layer is clearly visible in the SEM image.

substrate The photograph in the last panel of Fig 1a shows the

highly stretchable graphene/SWCNT TFT on PDMS

Figure 2a,b shows a schematic representation of the graphene/

SWCNT electrodes and an optical image of the fabricated transistor,

respectively The transfer characteristics of the graphene/SWCNT

TFT were measured within a ±1 V gate bias, which was applied

through a gate electrode placed underneath the wrinkled gate

oxide Well-defined on-state and off-state currents were observed

with a high on/off ratio of ∼105 (Fig 2c) Typical of carbon

nanotube (CNT)-based devices, a gate-range- and

sweep-rate-dependent hysteresis was also observed (see Supplementary

Information S5) It is of note that the on-current level was smaller in

the device with the wrinkled oxide than that with a flat oxide2,21(see

Supplementary Information S6) In field-effect transistor theory,

IDS=µ ·C i·Wch·(1/Lch) · (VGS−VT) · VDS, where µ,C i,Wch,Lch

and VT are the mobility, specific gate capacitance, channel width,

channel length and threshold voltage, respectively22; that is, the

source–drain current is proportional to the gate capacitance

The measured specific gate capacitance of the wrinkled Al2O3

was measured to be 1.49 nF cm−2 at 100 kHz, which is 80 times

lower than that of flat Al2O3 (120 nF cm−2; see Supplementary

Information S8) This observation is explained by the formation

of an air gap, which was inevitably introduced during the transfer

of the oxide layer The ALD-deposited Al2O3 layer on copper

has a microscopic roughness of ∼50 nm, which is similar to

the roughness of the copper foil (Fig 2d) This roughness was

retained even after the transfer of the foil Macroscopic wrinkles

with a roughness of ∼600 nm were additionally introduced during

the oxide transfer process (Fig 2d, last panel) To examine the effect of the Cu morphology on wrinkle formation, micro-sized square patterns with a height of 50 nm were etched into the foil After deposition and transfer of the Al2O3 according to the above procedures, no appreciable change in the capacitance was observed (Supplementary Fig S8) Interestingly, the height of the wrinkles was found to be dependent primarily on the thickness

of the oxide layer (Supplementary Fig S9) The small magnitude

of the measured capacitance can be described by modelling the wrinkled oxide as a series connection of the oxide layer and the air gap (Fig 2e) The overall decrease of the capacitance produces

an effective dielectric constant eight times smaller and an effective oxide thickness 10 times greater than those of flat Al2O3 The calculated effective thickness of the air gap is ∼500 nm, which

is in agreement with the measured height of the macroscopic wrinkles In spite of the small gate capacitance, a high on/off ratio of 105and a high mobility of 40.2 cm2V−1s−1(parallel-plate model) were achieved at a low operating gate bias (±1 V) The field-effect mobility remained high in the wrinkled oxide devices because of the decrease in the capacitance that was induced by the air gap Although low capacitance values often suggest weak coupling between the gate and the active channel, the fabricated devices exhibit no appreciable decreases in performance This is explained by the inhomogeneous field distribution of the wrinkled oxide (see also Supplementary Information S11) A strong electric field is localized at the valley of the wrinkle, allowing strict control over the carriers in the SWCNT channel in this region This field localization gives rise to good device performance The

Wrinkled Al2O3

lDS

lDS

¬1.0

¬50

25

¬1.0¬0.5 0.0

VDS (V)

VGS = 1 V

0.5 1.0

¬0.5 0.0

VGS (V)

VDS

0.5 1.0

500 mV

50 mV

5 mV

10 ¬13

10 ¬15

10 ¬11

10 ¬9

10 ¬7

50 µm

a

Insulator Channel

VT = 170 mV

g

e

Cox

Cair

Graphene electrode

Ctot

tair

Air gap model

Air

CNT channel

Al2O3 CNT channel

Graphene electrode

c

h

f

d

~600 nm

5 µ m

Drain

Cylindrical model

Parallel-plate model

10 3

10

0 5 10

Density (SWCNTs µm ¬1 )

0.0 0.2

10 ¬9

10 ¬7

lDS

0.6 0.8 1.0 1.2

¬0.5 0.0

VGS (V) 0.5 1.0 400

0 10 20 30 40 50 60 70 80 90 100

450 500

Device with PI

PI Device

550 Wavelength (nm)

600 650 700 20

s¬1

250 500 750 1,000 1,250

10 5

10 7

SS = 98

mV dec ¬1

Figure 2|Device structure and characteristics of graphene/SWCNT TFT using wrinkled Al2O3 for gate dielectrics a, Schematic illustration of the

graphene/SWCNT TFT with a wrinkled Al 2 O 3insulator b, Magnified optical image of the device with the wrinkled Al2 O 3 , which is indicated by the white

dashed line The inset shows a single TFT (scale bar, 100 µm) c, Typical transfer characteristics of a transistor The inset shows the IDS–VDS in terms of

various VGS(−1 V to 1 V) d, SEM images of the copper foil, the Al2 O 3 on the copper foil, and the transferred Al 2 O 3 layer The inset shows the macroscopic

wrinkle (scale bar, 1 µm) e, The schematic of the air-gap model with a series connection of the Al2 O 3layer and air gap f, The on/off ratio and mobility in

terms of the SWCNT density (scale bar, 1 µm) g, I1/2DS–VGSand IDS–VGScharacteristics (VDS = 500 mV) The red dashed line serves as a visual guide to

extract the threshold voltage The black dashed line shows the subthreshold swing h, Transmittance of device The inset shows an optical image of TFT on

a PI/PDMS as marked by the red dashed lines (scale bar, 1 cm).

presence of air gaps resulted in an order of magnitude decrease

in the gate leakage current compared with the flat oxide layers

(Supplementary Fig S13)

The device characteristics strongly relied on the density of

the SWCNTs, which was controlled by the concentration of

the catalyst By decreasing the SWCNT density, the on/off ratio

increased, whereas the mobility was degraded (Fig 2f) This

trade-off results from the presence of metallic SWCNTs in the

channel6 The parallel-plate model is widely used for calculating

the mobility of common TFTs such as silicon, organic and other

semiconductor TFTs In the case of the CNTs, the CNTs are sparsely

distributed in the channel (Fig 2f, insets) particularly at a low

CNT density, and the parallel-plate capacitor model overestimates

the gate capacitance and underestimates the mobility A realistic

cylindrical model that considers the electrostatic coupling between

the CNTs (refs 23,24) was also used to calculate the mobility

(see Supplementary Table S1 and Fig S14) The cylindrical model

yielded a high mobility of 624.8 cm2V−1s−1 at a low SWCNT

density of 0.7 SWCNTs µm−1 However, at the high CNT density

limit, both models approached a similar value of ∼800 cm2V−1s−1

The subthreshold swing was 98 mV dec−1, which is similar to that

of an individual CNT TFT (ref 25) Our graphene/SWCNT TFT

array with PI showed a high transmittance of 78% (device itself

shows 84%) at the wavelength of 550 nm, because of the highly

transparent graphene electrodes, the SWCNT channel and the

Al2O3dielectric as shown in Fig 2h The inset presents an optical image that illustrates the transparency of a graphene/SWCNT TFT array on the PI/PDMS substrate

To provide a proof of concept for the feasibility of the wrinkled oxide for stretchable electronics, the device was stretched along both the channel length and width axis Optical images of the devices stretched along the length direction (16% strain) and along the width direction (20% strain) are shown in Fig 3a,b (see also Supplementary Fig S17) The graphene/SWCNT electrodes, the SWCNTs channel and the dielectric layer were simultaneously stretched The transfer characteristics were obtained in terms

of strain (Fig 3c,d) The on-current value was linearly reduced

by 4% per strain in the length direction and 2% per strain in the width direction with respect to the unstrained current value because of the increase in the contact resistance between the CNTs (refs 26,27; see Supplementary Information, S14) This result contrasts with previous studies in which the resistance increased exponentially with strain in high-density CNT films28,29 The device failed at strains above 20% for both directions when the leakage current of the oxide layer surged (see Supplementary Information, S15) Furthermore, the large built-in air gap in our wrinkled oxide can act as a secondary dielectric layer to prevent leakage current in spite of the presence of local Al2O3

cracks Nevertheless, the on/off ratio fluctuated but did not degrade as the strain increased (Fig 3g,h) The graphene/SWCNT

Ion

Ion

Ion

Ioff

Ioff

Ioff

Ioff

c

10 ¬8

0%

2%

4%

Width direction Length direction

Width direction Length direction

Width direction Length direction

10 ¬7 0%

4%

6%

d

a Length direction stretching b Width direction stretching

0.00

0.05

0.10

¬5.0 ¬2.5 0.0 2.5 5.0

10 ¬14

10 ¬12

10 ¬10

IDS

IDS

8%

10%

12%

14%

16%

0.04 0.08

0.12

¬5.0 ¬2.5 0.0 2.5 5.0

10 ¬11

10 ¬9 10%

12%

14%

16%

18%

20%

¬5.0

¬2.5 0.0 2.5 5.0 0

4 8

12 16

0.00

0

4 8

12 16 20

e

1.0

1.5

f

10 ¬11

10 ¬9

10 ¬7

0 10 20 30 40

0.5 R /Ro gm/gmo R /Ro

gm/gmo

h g

µ

µ

µ

µ

10 ¬13

10 ¬14

10 ¬12

10 ¬10

10 ¬8

10 ¬12

10 ¬10

10 ¬8

Strain (%)

0 2 4 6 8 10 12 14 16 18 20

Strain (%)

0 200 400 600 800 1,000

Stretching counts

0 200 400 600 800 1,000

Stretching counts

IDS

V

GS (V)

¬5.0

¬2.5 0.0 2.5 5.0

V

GS (V)

IDS

1.0

1.5

0.5

1.0 1.5

0.5

1.0 1.5

10 ¬11

10 ¬9

10 ¬7

0.5

10 ¬13

s¬1

0 10 20 30

40 Mobility (c

s¬1

0 10 20 30

40 Mobility (c

s¬1

0 10 20 30

40 Mobility (c

s¬1

Figure 3|The device performance changes with tensile strain, and fatigue testing when stretching and releasing 1,000 times a,b, Schematic illustrations

and optical images of the stretchable graphene/SWCNT TFTs on PI/PDMS stretching along the channel length direction up to 16% (a) and stretching

along the channel width direction up to 20% (b) c,d, Transfer characteristics (VDS =1 V) with tensile strain applied along the length direction (c) and width direction (d) The insets show log-scale characteristics The devices exhibit stable operation for stretching up to 16% in the length direction and 20%

in the width direction e,f, Normalized on/off ratio, transconductance, current levels (on-current (blue circle), off-current (red circle)) and mobility variation (black square) with respect to values at zero strain along the length direction (e) and the width direction (f) g,h, Normalized on/off ratio,

transconductance, current levels (on-current (blue circle), off-current (red circle)), and mobility variation (black square) with 10% stretching and releasing cycles.

Randomly wrinkled Al2O3

GPa

Al2O3 thickness (tox) = 50 nm

R = P /H

H P

a

b

0.0475%

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

1.8 1.6

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

1.8 1.6

0

0 10 20 30

R

40 ∞

10

20

30

Figure 4|Stress–strain simulation of wrinkled Al2O3 based on elasticity a, The stretchability of uniaxially wrinkled Al2O 3as a function of a given ratio R, and the respective stress distributions for a given elongation (10% and 20%) with tox =50 nm and R = 2 The negative curvature area of the wrinkle is

more severely stressed, as shown in the inset b, Mapping of the stress distribution for a randomly wrinkled oxide under several elongations: 0 (pristine), 10

and 20% As for the uniaxially wrinkled Al 2 O 3 , the negative curvature area of the wrinkle is more severely stressed (insets).

hybrid electrode helps to improve connectivity and hence the

stretchability of electrodes The mobility fluctuation precisely

resembled the change in the transconductance, but the magnitude

of the fluctuation was higher because of the incorporation of

strain in the channel length and width (Fig 3e,f) A fatigue test

was also performed by repeating 10% stretching and releasing for

both directions Although the on/off ratio, transconductance and

mobility of the devices fluctuated to some degree, the devices were

stretched and released up to a maximum of 1,000 times without

being deteriorated We emphasize that the long-term stability was

maintained with minimum leakage current because of the highly

stretchable wrinkled gate dielectric

To support the robustness of the wrinkled oxide for stretchable

devices, a finite-element method was used to model the

elongation-induced stress of a thin Al2O3layer Two models were considered:

uniaxially wrinkled structures and randomly wrinkled structures

Figure 4a shows the stretchability of uniaxially wrinkled Al2O3as

a function of a ratio R (R = P/H , where P is the period and H is

the height of the wrinkles) When R = 2, maximum stretchability

of almost 30% was achieved at the given ultimate tensile strength

of Al2O3(1.9 GPa; ref 30) This is in contrast to flat Al2O3, where

the stretchability reaches only 0.475% at ultimate tensile strength

Also shown in Fig 4a are the respective stress distributions for

a given elongation (10% and 20%) with 50-nm-thick Al2O3 and

R = 2 It is intriguing to note that the negative curvature area of the

wrinkle is stressed more severely, as shown in the inset A randomly

wrinkled structure was generated from atomic force microscopy

(AFM) topography scans of the transferred oxide Figure 4b shows

a three-dimensional mapping of the stress distribution on the

randomly wrinkled oxide under elongations of 0 (pristine), 10 and

20% At 20%, the maximum stresses (up to the ultimate tensile

strength) are distributed randomly but are still localized in a wavy

shape It is again emphasized here that the highest stress is located

in the negative curvature regions, similar to the uniaxial stress case

(see also Supplementary Information S16)

Figure 5 demonstrates the compatibility of the devices on various stretchable media These stretchable media include human skin, rubber, a toothpaste tube, aluminium foil, a plastic heart and a light-bulb surface (Supplementary Information S19) The technical relevance of our transparent devices is widespread, encompassing the areas of flexible, bendable, twistable and stretchable electronics

Methods

Synthesis of the transferred wrinkled Al2O3 A 50-nm-thick alumina (Al2 O 3 ) layer was deposited on 3 cm × 3 cm Cu foil using ALD at 10 −3 torr and 200 ◦ C Scotch tape covered the centre part of the Cu foil during spin-coating of PMMA (1,000 r.p.m., 1 min) On removing the scotch tape, only the edge of the Cu foil

is covered by PMMA After edge-coating of PMMA, Al 2 O 3 that deposited on the bottom side of the Cu foil was etched by HF, and then the Cu was etched by a wet etching process (CE-100) The PMMA-coated Al 2 O 3 was rinsed four times using deionized water for ∼10 min and transferred onto the target substrate The sample was dried in a dry oven at 70 ◦ C for 10 min The PMMA was removed using acetone, and then the sample was baked at 150 ◦ C for 3 h to give good adhesion between the Al 2 O 3 and the substrate.

Transfer process of a graphene and SWCNT network PMMA was spin-coated

onto the graphene-grown Cu foil at 500 r.p.m for 5 s and then 1,000 r.p.m for

1 min A Cu etchant (CE-100) was used to dissolve the copper The SWCNT network grown on a Si wafer was PMMA-coated in a manner similar to the graphene transfer process The SiO 2 layer was rapidly removed using HF After rinsing (deionized water, ∼10 min) four times, the PMMA-coated graphene/SWCNT was transferred onto the target substrate This sample was dried in a dry oven at 70 ◦ C Finally, the PMMA was removed with acetone.

Fabrication of stretchable transistor array on PI/PDMS substrate The 200 nm Al

layer was deposited onto the SiO 2 /Si layer using a thermal evaporator at 10 −6 torr PAA was coated onto the Al/SiO 2 /Si, which was followed by a heat treatment (300 ◦ C) at 10 −3 torr The PI layer converted from PAA has a thickness of 50 µm Graphene was transferred onto the PI-coated Si wafer with an Al scarifying layer and patterned by photolithography and O 2 plasma etching (480 mtorr, 20 W, 10 s) to transparent source, drain and gate electrodes The wrinkled Al 2 O 3 was transferred and patterned by photolithography and HF wet etching on the patterned graphene

as the dielectric layer Finally, the APCVD-grown SWCNT-network channel was transferred and the photoresist was patterned on both the active-channel region and the graphene electrodes (S, D, G) to prevent graphene damage The

b

c

5 10 15 20

IDS

Before bending After bending

Before bending After bending

Before bending After bending

VDS = 0.5 V

VDS = 0.5 V

VDS = 0.5 V

¬1.0 ¬0.5 0.0 0.5 1.0 0

5 10 15 20

IDS

0

5 10 15 20

IDS

0

VGS (V)

¬1.0 ¬0.5 0.0 0.5 1.0

VGS (V)

¬1.0 ¬0.5 0.0 0.5 1.0

VGS (V)

Rubber tube

Toothpaste

Aluminum foil

Figure 5|Photographs of stretchable graphene/SWCNT TFT arrays transferred onto various substrates and the related transfer characteristics

a–c, Our devices were transferred onto a cylindrical rubber tube (outer diameter is 2 cm; a), a polypropylene toothpaste tube (b) and aluminium foil (c).

Transfer characteristics of stretchable graphene/SWCNT TFTs were measured before and after bending, where the estimated peak strains of each bending

are 1.5% (a), 2% (b) and 1% (c), respectively I–V results in linear scale show no appreciable difference even after bending.

unnecessary SWCNTs were selectively etched away by O 2 plasma After finishing

the device fabrication on the PI, the device on the PI was transferred to PDMS of

with a 0.5 mm thickness using Al layer etching.

Device characterization and stretching tests SEM images were recorded on a

JEOL7600F, and a SPA400 (SEIKO) was used to record the AFM images Raman

spectroscopy (RM1000 microprobe; Renishaw) was used to characterize the

SWCNTs and graphene with a wavelength of 514 nm (2.41 eV) and a Rayleigh line

rejection filter Ultraviolet–visible–near-infrared absorption spectroscopy (Varian,

Cary 5,000) was used to analyse the transmittance of graphene The device array

was placed in a uniaxial stretch machine One side of the PI/PDMS substrate was

fixed, and the other side was pulled to stretch Each fatigue test was performed for

up to 1,000 cycles to 10% strain Electrical measurements were performed using a

probe station (Keithley 4200) while stretched.

Received 28 May 2012; accepted 21 January 2013; published online

3 March 2013; corrected online 8 March 2013

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Acknowledgements

This work was supported by the Research Centre Program of IBS (Institute for Basic Science) in Korea, the International Research & Development Program (2011-00242) of the NRF of Korea funded by MEST, and the Human Resources Development programme (No 20124010203270) of the KETEP funded by the Korea government Ministry of Knowledge Economy.

Author contributions

S.H.C contributed to the experimental planning, experimental measurements, data analysis and manuscript preparation W.J.Y and X.D performed the experimental planning D.L.D performed the AFM measurement, J.J.B and Q.A.V performed the finite-element method simulation, and H.Y.J took the photographic images D.P and M.J.Y contributed to the theoretical calculations Q.H.T and T.H.L prepared the graphene samples for the experiments Y.H.L contributed to the experimental planning, data analysis and manuscript preparation.

Additional information

Supplementary information is available in the online version of the paper Reprints and permissions information is available online at www.nature.com/reprints Correspondence and requests for materials should be addressed to Y.H.L.

Competing financial interests

The authors declare no competing financial interests.

In the version of this Letter originally published online, in the left panel in Fig. 3a, the schematic of the device was missing This error has been corrected in all versions of the Letter

nanotube transistors

Sang Hoon Chae, Woo Jong Yu, Jung Jun Bae, Dinh Loc Duong, David Perello, Hye Yun Jeong, Quang Huy Ta,

Thuc Hue Ly, Quoc An Vu, Minhee Yun, Xiangfeng Duan and Young Hee Lee

Nature Materialshttp://dx.doi.org/10.1038/nmat3572 (2013); published online 3 March 2013; corrected online 8 March 2013