Carbon nanotubes-based flexible Transparent conducting film

Young Hee Lee

Department of Physics, Department of Nanoscience and Nanotechnology,

Center for Nanotubes and Nanostructured Composites,Sungkyunkwan Advanced Institute of Nanotechnology,

Sungkyunkwan University, Suwon 440-746, Korealeeyoung@skku.edu

http://nanotube.skku.ac.kr/

Outline

1. Introduction2. Material quality 3. Film quality4. Post treatment

- doping effect

Current research topics

- Mass production (t-MWCNTs): Diameter, chirality, length control,aligned nanotubes

- Thermal CVD, microwave PECVD,RF-PECVD, arc discharge

- Catalyst design- Magnetic nanoparticles (Gd, GdFe)

Synthesis- Chirality separation- Doping (Li, K, Na, N, F, Cl)- Purification- Dispersion (macro and nano)- Functionalization

Engineering properties

- Transparent conducting film - All-CNT flexible transparent TFT- Transistor (FET)- Optoelectronic devices- Gas sensor- Field emitter (FED, LCD BLU)

Electronic devices

Applications

- Hydrogen storage- Fuel cell- Hybrid capacitor- Nano refrigerator

(Peltier device)- CDI for water purification

Energy storage- Polymer (PC, PET)/CNT- Metal/CNT- Polymer nanofiber/CNT- LC/CNT

Composites

Group members

Hong Zhang GengKi Kang Kim

Properties of CNTs

- Several tube types- High aspect ratio (1000 - 100000),

diameter (~ nm), length (~ μm)- Hollowness inside , surface area up to 1500 m2/g- Mechanical strength (100 x steel, 10 x kevlar) - Elastic modulus (1-2 TPa),7 times stronger than steel- Thermal conductivity (6000 W/mK),

twice the diamond - Electric conductivity (10,000 S/cm), 1000 times larger than Cu-wire

MWCNT

DWCNT

SWCNT

Single

Mass

Random Aligned

Conducting filmsCompositesGas adsorptionFuel cellsBatteries SupercapacitorsFilters

Mass Mass productionproduction

Chemical Chemical modificationmodification

FED, BLUCathode ray tubesMicrowaveamplifiers

Structure Structure controlcontrol

Bio-technologyDDS

Controlled Controlled growthgrowth

SPMNano-fabrication

Bio-sensorsElectronic devices

LSI memoryLogic

Via-hole wiring

Vast area for CNT applications

Why flexible TCFs?

Future electronic

device

E-paperPCB board

Wearable displays

Flexible displays

Transparency

- Working transistor

- Electrodes

- transparent- conductive

ITO filmExcellent transmittance and conductivity

0 30 60 90 120 150 1800

500

1000

1500

2000

2500

ITO can be cracked easily against bending, yielding poor flexibility

SWCNT film 94% T

SWCNT film 70% T

Sh

eet r

esis

tanc

e (Ω

/sq)

Bending angle (degree)

ITO

CNT film

Flexible TCFs with CNTs

By courtesy of Paul J Glatkowiski, Eikos Inc.

Indium is a limited natural resource!

Blue diamond and gold star: Eikos, Red diamond: PEDOTYellow diamond: ITO, Yellow square: gold or silver metal

Paul J Glatkowiski, Eikos Inc.

Target values

ITO

CNT

Science 305 (2004) 1273

50 nm film was measured to be 30 Ω/sq(resistivity 1.5 X 10-4 Ωᆞcm)

Filtration method

1. The film thickness is readily controlled.2. Homogeneity of the films is guaranteed by

the process itself. 3. They tend to lie straight, gaining maximal

overlap maximal electrical conductivity

J. Am. Chem. Soc. 126, 4462 (2004)

sheet resistance (Rs) of 80 Ω/sq and %T of 80

CNT film by dipping

PEDOT film by ink-jet

Dipping method

Flexible TCFs with CNTs

Diamond and Related Materials 13 (2004) 256

CNT network

CNT-PA CNT-PPy CNT/SOCl2

Transparency (%) 80-90% 62-90% 60-80% 93-99%

resistance, R(Ω) 660 245 400 1.1X103

transparent thin CNT network

network of CNT-PA

sprayed with an air-brush pistolsprayed with

an air-brush pistol

deposit the CP, PA or PPy

on them electrochemically.

deposit the CP, PA or PPy

on them electrochemically.

Flexible TCFs with CNTs

Applied Surface Science 252 (2005) 425

CNT network on PET ITO on PET

Transparency (%) 90 % 78 %

Sheet resistance, Rs(Ω/sq)

1 kΩ/sq 15 Ω/sq

Spray method

Diam. Rel. Mater. 14 (2005) 1882 Nano Letters 6 (2006) 1880

filtration method

Flexible TCFs with CNTsbar coating

Journal Coating method CNTs dispersion polymer substrate Transmitt

ance (%)Sheet

resistance(Ω/sq)

variable

J. Am. Chem. Soc. 126 (2004) 4462 Dipping SWNTs Triton-X 100 X patterned

PET film 80 80

Applied Surface Science 252 (2005) 425 spray SWNTs

MWNTs SDS X PET film 90 1k

Nano Letters6 (2006) 677 Filtration SWNTs SDS X PET 70 1k

Nano Letters6 (2006) 1880 Filtration SWNTs SDS PEDOT PE 87 380

JAP101 (2007) 016102 Filtration SWNTs SDS PET 80 150

APL88 (2006) 123109 Filtration SWNTs SDS PET 80 120

Nano Letters4 (2004) 2513 Filtration SWNTs X X Variable 85 1k

electrode

PET

PET

Science 305 (2004) 1273 filtration SWNTs Triton-X 100 X 70 30

Diamond and Related Materials 13 (2004) 256

Sprayed/electro-chemical

SWNTsMWNTs SDS PA/PPY 70

82

80

245

Diam. Rel. Mater.14(2005) 1882 Bar coating t-MWNTs

SWNTs PEDOT PEDOT 249

APL90 (2007) 121913 Filtration SWNTs SDS 200

More systematic study is required to understand the film performance for materials and the film preparation processes.

Comparison results related to TCFs

What does determine the film performance?

Film performanceSheet resistance, transmittance, hardness (adhesion)

Material parameters

- Types of CNTs- Purity of CNTs- Diameter- Crystallinity of CNT walls

Film parameters

- Dispersion of CNTs in solvent- Film preparation method- Bundle size- Degree of entanglement of CNT network

Film preparation methods

Method Advantage Drawback

Filtration

- Easy process for thin layer (<100 nm)- Easy to remove surfactants- Excellent uniformity

- Limited to filter size- Extra labor to remove filters

Dip coating

- Easy to fabricate and scale up- Cheap cost

- Sensitive to surface functionalization- Take relatively long time - Hard to control uniform coating in large area

spray coating

- Easy to fabricate and scale up- Cheap cost- Excellent uniformity

- Need attention for nozzle blocked

Inkjet printing - Micro-pattern formation - CNT-ink preparation

- Nozzle cloaking

Filtration Dip coating Spray coating

Spray gun

Pump

Filter

Inkjet printing

Solvent

CNT

Process for TCFs

Dispersion Centrifugation

Spray coatingCharacterization

Spray gun

Rs

%T

Characteristics of TCFs using SWCNTs

Arc SWCNTs reveal the best TCF performance! But why?

40 50 60 70 80 90 100101

102

103

104

105

Arc

HiPCO

Laser

Sh

eet r

esis

tanc

e (Ω

/sq)

Transmittance at 550 nm (%)

CVD

Each curve contains several data points from films with different number of sprays by SWCNT solution dispersed in deionized water with SDS

Characteristic of SWCNTs

- Presence of carbonaceous particles- Purity (transition metals)- Crystallinity of CNT walls- Bundle size- Diameter

10 nm

100 nm

Thermogravimmetric analysis (TGA)

• CNTs start burning off in air at high temperature. Transition metals were not burned off at high temperature.

• Burning temperature relies on several parameterscatalyst amounts, bundle diameter, number of walls, crystallinity of CNTs.

0 200 400 600 800

0

20

40

60

80

100

10.02%

425oCTG

(wt%

)(a) CVD

DTG

(w

t%/o C

)

0 200 400 600 800

0

20

40

60

80

100

TG (w

t%)

604oC

4.55%

DTG

(w

t%/o C

)

(b) HiPCO

0 200 400 600 8000

20

40

60

80

100

12.12%

395oC

Temperature (oC)

TG (w

t%)

DTG

(w

t%/o C

)

(c) Laser

0 200 400 600 800

0

20

40

60

80

100

DTG

(w

t%/o C

)

3.54%

503oC

Temperature (oC)

TG (w

t%)

(d) Arc

Resonant Raman spectroscopy

π π∗

2.41 eV ± 0.1

S11

S22

M11

S33

S44

M22

S55

S66

M33

Diameter (nm)

van Hove singularities: diameter & chirality-dependent

Material parameters for SWCNTs: G’-band

• G’-band: double resonance or overtone of D-band.• Metallic tubes reveal larger intensity than semiconducting ones.• Electron-phonon interaction plays a dominant role here.

K. K. Kim et al. submitted

500 1000 1500 2000 2500 3000

26251589

1.52

632.8 nm (1.98 eV)

(b)M

150 180

148

179

164RBM

500 1000 1500 2000 2500 3000

2680

514 nm (2.41 eV)

1.38

1591

150 200

148

172180

RBM

164

(a) S

Inte

nsity

(arb

. uni

ts)

0 500 1000 1500 2000 2500 3000

514 nm (2.41 eV)

1591

1337NHFA-HTT

Pristine

200 250

267260

246204

RBM

183

2400 2600

2670

G'-Band

2659(a) (b) (c)

Inte

nsity

(arb

. uni

ts)

Raman shift (cm-1)

An et al. JACS 127, 5193(05)Seo et al. JACS 127, 15724(05) An et el. JEM 35, 235(06)

SM

(a) (b) (c) (d)

400

450

500

550

600 (b)

Temp. Purity

Bur

ning

tem

pera

tue (o C

)

88

90

92

94

96

CVD

HiPCO

Laser

Arc

Purity (wt%

)

(a) (b) (c) (d)0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4 ArcLaser

HiPCO

CVD

(a)

Dia

met

er (n

m)

Material parameters for SWCNTs

• For semiconductor SWCNTs, the energy gap decreases with increasing diameters.

• Thus larger diameters of SWCNTsshould give higher conductivity in TCFs.

Diameter

The burning temperature is strongly correlated to the metal content.

Burning temperature & purity

Material parameters for SWCNTs: G’-band & D-band

(a) (b) (c) (d) (a) (b (c) (d)0.0

0.2

0.4

0.6

0.8

1.0 633 nm(1.96eV)

514 nm(2.41eV)

Rat

io o

f int

ensi

ty

Types of CNTs

D/G G'/G

The intensity of D-band and G’-band/G-band of four types of SWCNTs

(a) (b) (c) (d)0

10

20

30

40

Inte

nsity

rat

io o

f G'/D

Types of CNTs

514 nm 633 nm

CVD

HiPCO

Laser

Arc

The intensity of G’-band/D-band of four types of SWCNTs at excitation energy of

514 nm and 633 nm.

Material parameters for SWCNTs: G’-band & D-band

• G’-band represents metallic composition in the CNT powder.• The higher the G’-band, the more metallic components.• D-band represents the presence of defects on the tube wall.

(a) (b) (c) (d)0

10

20

Ave

r. in

tens

ity r

atio

of G

'/DTypes of CNTs

Semiconductor Metalic

CVD

HiPCO

Laser

Arc

160 180 200 220 240 260 280 300 320

AM

S-SWCNTs

Inte

nsity

(Arb

. uni

ts)

Raman shift (cm-1)

M-SWCNTsHiPCO SWCNTs

AS

SM

SMDGSM AA

AAIII+

×=)()( /' ),( 633)(514)()( SMSMSM IIAverageI =∑

The sheet conductance of TCFs with different type SWCNT at transmittance of 70 % and 80 %.

Sheet conductivity of TCFs

CVD HiPCO Laser Arc0.000

0.002

0.004

0.006

0.008

0.010

Shee

t con

duct

ance

(S.s

q)Type of SWCNT

70 % T 80 % T

40 50 60 70 80 90 100101

102

103

104

105

Arc

HiPCO

Laser

Sh

eet r

esia

tanc

e (Ω

/sq)

Transmittance at 550 nm (%)

CVD

Can we define any appropriate parameter?

Material quality factorfor film performance

0.8 1.0 1.2 1.40.000

0.002

0.004

0.006

0.008

0.010

80 %T

Shee

t con

duct

ance

(S.s

q)

Diameter (nm)

CVD SWCNT HiPCO SWCNT Laser SWCNT Arc SWCNT

(b)70 %T

80 85 90 95 1000.000

0.002

0.004

0.006

0.008

0.010

80 %T

Purity (wt%)

Shee

t con

duct

ance

(S.s

q)

CVD SWCNT HiPCO SWCNT Laser SWCNT Arc SWCNT

(a) 70 %T

0 5 10 15 200.000

0.002

0.004

0.006

0.008

0.010

Metallic

Aver. intensity ratio of G'/Dfro metallic SWCNT

80 %TSh

eet c

ondu

ctan

ce (S

.sq)

CVD SWCNT HiPCO SWCNT Laser SWCNT Arc SWCNT

(d)70 %T

0 5 10 150.000

0.002

0.004

0.006

0.008

0.010(c)

Aver. intensity ratio of G'/Dfor semiconductor SWCNT

80 %T

Shee

t con

duct

ance

(S.s

q)

CVD SWCNT HiPCO SWCNT Laser SWCNT Arc SWCNT

Semiconductor 70 %T

Conductivity of SWCNTs

)2/exp(0 TkEnn Bgi −=

)exp()2

exp(TkEE

TkE

pB

fi

B

g −−∝

]/)exp[( TkEEnp Bfii −= ]/)exp[( TkEEnn Bifi −=

The intrinsic carrier concentration:

where kB and T are the Boltzmann constant and temperature.

The conductivity can be generally expressed as pn pene μμσ +=

For semiconductor SWCNT For metallic SWCNT

where n and p are n-type (electron) and p-type (holes) carrier concentrations, respectively, and μn and μp are the respective electron and hole mobility. The mobility is dominated by random scattering from collisions with lattice atoms, impurity atoms, other scattering centers.

)exp()2

exp(Tk

EETk

En

B

if

B

pg −−∝

The p-type and n-type carrier concentrations:

Filled DOSat Ef metal

Band gap

Metallic

Semiconducting

Band gap of SWCNTs

Band gap of SWCNTs

)eV( /82.0 DEg =

)eV( /2 0 DaE ccg γ−=

)eV( /105.0 DEpg =

Odom, Teri; et. al. Nature 391 p59 1998

Dresselhaus; et. al. Physics reports 409, 2005, 27

eV 9.2 nm, 142.0 0 ==− γcca

First-principles calculations, pseudogap ~ 0.1 eVP. Delaney, et al, Nature, 39, 1998, 466

M. Ouyang, et al, Science 292, 702 (2001)

Metallic

By fitting experimental data:

(10,10)

(8,8)

STM

Semiconducting

Material quality factor for SWCNTs

Here we define a material quality factor that represents the metallicity of the SWCNTs

D: diameter of SWCNTs; P: purity of SWCNT powder; : average intensity ratio of G’/D-band for semiconductor SWCNTs;: average intensity ratio of G’/D-band for metallic SWCNTs.

One may say that arc SWCNT is the best candidate for TCFs from material point of views

(a) (b) (c) (d)0

2

4

Arc

LaserHiPCO

CVDMat

eria

l qua

lity

fact

or

Types of CNTs

)( 052.0105.0

052.082.0

MD

SD

m IeIePQ ∑×+∑××=−−

)( 22M

TkE

STkEE

TkE

m IeIeePQ B

pg

B

fi

B

g

∑×+∑×××=−−−

MI∑SI∑

)semi( )eV( /82.0 DEg =)metal( )eV( /105.0 DEpg =

fi EE ≈ :SWCNT Intrinsic

nMI μ~∑pSI μ~∑

eV 0.026 :@RT ≈TkB

pn pene μμσ +=

0 1 2 3 4 50.000

0.002

0.004

0.006

0.008

0.010

80 %T

Shee

t con

duct

ance

(S.

sq)

Material quality factor

CVD SWCNT HiPCO SWCNT Laser SWCNT Arc SWCNT

70 %T

The sheet conductance at 70 %T & 80 %T vs material quality factors High material quality factor leads to high conductivity of TCFs.

Arc SWCNTs reveal the best TCF performance in good agreement with the prediction of the material quality factor!

Geng et al. NANO 2, July (07)

Relation of conductance with material quality factor

(a) CVD (b) HiPCO

(c) Laser (d) Arc

Morphology of the SWCNT film

Key parameters for film: Small bundle size, sparseness of CNT network, and entangled random network.

1 μm

50 60 70 80 90 1000

200

400

600

800

1000

1200

FPD

Arc-I

Arc-II

Shee

t res

ista

nce (Ω

/sq)

Transmittance at 550 nm (%)

TS

(b)

200 400 600 800 1000 1200 14000.0

0.5

1.0

1.5

2.0

2.5

Solution-IAbs

orba

nce

(a.u

.)

Wavelength (nm)

Solution-II

(a)

Degree of dispersion of SWCNTs

Key parameters for film: Long, small bundle size or individual SWCNTs give high performance for TCFs.

500 nm500 nm

500 nm500 nm

Illustration of the TCFs

Rs= 100 Ω/sq @ 75 %T

Rs= 250 Ω/sq @ 86 %T

Rs= 400 Ω/sq @ 91 %T

Touch panel

Electrodes

We proposed the material quality factor for CNTs that is directly correlated to the performance of the transparent conducting films.

SummarySummaryTransparent conducting film preparation method

Spray method

SWCNT dispersionDI water dispersion with SDS: sonication and centrifuge

Strong material dependenceArc > Laser > HiPCO > CVD SWCNTs

Film performance* Material quality factor Qm: High purity, large diameter, high IG’/D

* Film preparing condition:Better dispersion, small bundle size, entanglement of CNT networks

Post treatment to improve conductivity

×100,000 ×50,000 ×20,000

×10,000 ×2,000

The properties of TCFs

50 60 70 80 90 1000

200

400

600

800

1000

FPD

Arc SWCNTs

Shee

t res

ista

nce (Ω

/sq)

Transmittance at 550 nm (%)

TS

Sheet resistance vs transmittance curve is obtained by the optimized dispersion condition for arc discharge SWCNTs.

0 20 40 60 80 100 1200

200

400

600

800

1000

1200

1400

Sheet resistance Transmittance

Thickness (nm)

Shee

t res

ista

nce (Ω

/sq)

50

60

70

80

90

100

Transmittance (%

)

Sheet resistance and transmittance as a function of SWCNT film thickness measured by AFM step edges.

(a) Effect of acid treatment for the sheet resistance changed by various acids. Nitric acid shows the best effect to reduce the sheet resistance of TCFs.

HCl HNO3 H2SO4 HClO40.0

0.2

0.4

0.6

0.8

1.0

Rs/R

s0

Acid types

Concentration 6 M 12 M

0.0 0.2 0.4 0.6 0.8 1.0 6 12 18 240.0

0.2

0.4

0.6

0.8

1.0

Rs/R

s0

Nitric acid treatment time (hours)

Pristine

Enhancement conductivity by acid treatment

(b) Sheet resistance change with nitric acid treatment time. Sheet resistance reduced significantly in a short time and fluctuated with long treatment time.

Properties after acid treatment

(a) Transmittance in visible range did not obviously change after nitric acid treatment.

400 500 600 700 8000

20

40

60

80

100

Tran

smitt

ance

(%T)

Wavelength (nm)

Pristine

Acid treated

0 3 6 9 120.0

0.2

0.4

0.6

0.8

1.0

Rs/R

s0

After acid treatment (days)

Rs0=180 Ω/sq Rs0=150 Ω/sq

30 days

(b) Stability of TCFs after nitric acid treatment, sheet resistance did not significantly increased at long time.

286 285 284 2830

5000

10000284.1

C

ount

rate

(arb

. uni

ts)

Binding energy (eV)

Pristine SWCNTHNO3 treatment

30 min 12 hrs

284.5

284.0

C1s

(at%) C1s O1s N1sPristine SWCNTs 96.04 3.96 0.0

HNO3 treatment 30 min 90.13 9.35 0.52HNO3 treatment 12 hours 82.18 15.89 1.94

Table 1 Atomic concentration of the SWCNT samples

XPS of pristine SWCNT powder before and after acid treatment.

Black: pristine SWCNTs, preheat treated at 200 oCfor 20 min to remove humidity;

Red: nitric acid treatment for 30 min;

Blue: nitric acid treatment for 12 hours.

XPS analysis

Sheet resistance as a function of transmittance at 550 nm before and after acid treatment compared with other published results

50 60 70 80 90 1000

200

400

600

800

1000

FPD

Transmittance at 550 nm (%)

Shee

t res

ista

nce (Ω

/sq) As-prepared

Acid treatedRinzlerManoherZhouGrünerYooChhowallaRoth

TS

(d)(c)

Comparison with other’s resultsGeng et al. JACS Comm. 129, 7758 (07)

TCF morphology

taken at 60 degree angle

BeforeSDSor

impuritieson the

surface.

After acid treatment:

Impurities were removed and the protruded

nanotubeswere wet on the surface.

295 290 285 280

C1s (a)

410 405 400 395

iii

ii

SWCNT film HNO3 1 h H2SO4 1 h

N1sNO2

HNO3

-C=N

(c)

i

1080 1075 1070

-SO3Na

iiiii

Na1s (b)

i

175 170 165

-SO3Na

ii

iii

2P 1/2

S2p 2P 3/2

H2SO4

(d)

i

294 292 290 288 286

iiiii

C-O, C=O, -COO...

i

Cou

nt ra

te (a

rb. U

nits

)

Binding energy (eV)

285 284

iiiii

C1s i

540 535 530

Nitrites oxide R-SO3Na

(e)

O1s

Element C1s N1s O1s Na1s S2p

SW film 93.56 0.00 5.69 0.34 0.41

HNO3 88.66 2.22 9.12 0.00 0.00

H2SO4 88.40 0.00 10.44 0.00 1.16

Atomic concentration

XPS analysis

SDS can be efficiently removed by acid treatment and add a little oxide to the surface. The C1s core level was not obviously shifted by both nitric and sulfuric acid.

2600 2700 2800

ii

iii

G'-band

i

1400 1500 1600

iii

ii

i

Treatment time Pristine SWCNT SWCNT film 20 min 40 min 60 min 80 min 100 min

G-band

D

150 200 250

i

ii

iii EM11

633 nm RMB

2600 2700

i

ii

iii

G'-band

Raman shift (cm-1)

150 200

iiiii

ES33

514 nm RBM

i

1400 1500 1600

iiiii

i

Treatment time Pristine SWCNT SWCNT film 20 min 40 min 60 min 80 min 100 min

G-band

D

Raman spectroscopy analysis

After removal of SDS, the metallicity of carbon nanotubes was partially recovered.

Inte

nsity

(arb

. uni

ts)

Dispersion and functionalization of SWCNTs

Sodium Dodecyl Sulfate (SDS)

O’Connell et al. Science 297, 593 (2002)

Diazonium Salts

Modification of Electronic Structure via Functionalization

Chhowalla et al, APL 90, 121913 (2007)

C OOH

HNO3

Strano et al. Science 301, 1519 (2003)

1) Adhesion of SDS in solvent

4a) Physical absorption

2) Wetting of SDS on CNT

3) Removal of SDS from CNT4b) Functionalization

Model for acid treatment process

film

In acidW

ash

and

dr

y

Before acid:

After acid:

Densification of film

Thickness densified ratio = 55 / 74 ≈ 75 %

Pristine SWCNTswith clean edges

and bundles

Dry SWCNT/SDS film without

washing covered by SDS

SWCNT film after rinsing in water

with some residual SDS

Bundles and edges become clean again after acid treatment

SDSSDS

SDSSDS

SDSSDS

SDSSDS

20 nm20 nm

10 nm10 nm

10 nm10 nm 10 nm10 nm5 nm5 nm

10 nm10 nm 10 nm10 nm 10 nm10 nm

TEM images of SWCNT/SDS

Enhancing Conductivity of TCFs

0 30 60 90 1200

1k

2k

3k

4k

5k

6k

Acid treated

Con

duct

ivity

(S/c

m)

Thinckness (nm)

As prepared

(b)

- After acid treatment, the CNTs on PET become densified and the thickness decreased, together with the recovery of metallicity of SWCNTs.- This leads to enhance the conductivity of CNTs by a factor of ~ 4.

SummarySummary

Transparent conducting films were fabricated by a spray method on PET films using SDS-dispersed SWCNT solution.

TCFs were further treated by acid to reduce the sheet resistance without affecting the transmittance and stability for a long time.

XPS analysis indicated that SDS can be efficiently removed by acid treatment, resulting the low contact resistance among CNT networks.

All these effects by acid treatment lead to enhance the conductivity of carbon nanotubes by a factor of ~ 4.

Material quality factor that determines the conductivity of film was determined using diameter, purity, metallicity, and carrier concentration.

Structure of carbon nanotubes

armchair

zigzag

chiral

SWCNT

Classification of carbon nanotubes- Single-wall CNT, double-wall CNT, multi-wall CNTs- Armchair, zigzag, and chiral nanotubes

Carbon nanotubes

SWCNT DWCNT Thin-MWCNT MWCNT

Zigzag

Armchair

Filled circles denote n-m divisible by 3 which give metallicnanotubes.

Electronic structure of SWCNTs