Delivered by Publishing Technology to: Sung Kyun Kwan UniversityIP: 115.145.196.100 On: Wed, 19 Feb 2014 05:55:32

Copyright: American Scientific Publishers

RESEARCH

ARTIC

LE

Copyright © 2013 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol. 13, 8090–8094, 2013

Plasma Treatment of Thin Film Coated with GrapheneFlakes for the Reduction of Sheet Resistance

Sung Hee Kim1, Jong Sik Oh1, Kyong Nam Kim1, Jin Seok Seo1,Min Hwan Jeon2, Kyung Chae Yang1, and Geun Young Yeom1�2�∗

1Department of Materials Science and Engineering, Sungkyunkwan University,Suwon, Gyeonggi-do 440-746, South Korea

2SKKU Advanced Institute of Nano Technology (SAINT), Sungkyunkwan University,Suwon, Gyeonggi-do 440-746, South Korea

We investigated the effects of plasma treatment on the sheet resistance of thin films spray-coated with graphene flakes on polyethylene terephthalate (PET) substrates. Thin films coated withgraphene flakes show high sheet resistance due to defects within graphene edges, domains, andresidual oxygen content. Cl2 plasma treatment led to decreased sheet resistance when treatmenttime was increased, but when thin films were treated for too long the sheet resistance increasedagain. Optimum treatment time was related to film thickness. The reduction of sheet resistancemay be explained by the donation of holes due to forming �-type covalent bonds of Cl with carbonatoms on graphene surfaces, or by C Cl bonding at the sites of graphene defects. However, dueto radiation damage caused by plasma treatment, sheet resistance increased with increased treat-ment time. We found that the sheet resistance of PET film coated with graphene flakes could bedecreased by 50% under optimum conditions.

Keywords: Plasma Doping, Cl2, Sheet Resistance, Graphene, Thin Film, Flexible Substrate.

1. INTRODUCTION

Many researchers are investigating graphene as a trans-parent electrode material to replace transparent conduc-tive oxides using rare-earth materials such as indium tinoxide (ITO) due to its high transmittance and structuralflexibility.1–4 Graphene, a type of few-monolayer graphitewith two-dimensional honeycomb crystal structure, is char-acterized by high mobility, thermal conductivity, a highYoung’s modulus, good conductivity, chemical stability,and optical transmittance.5–7 Chemical vapor deposition(CVD) is a well-known method for the fabrication ofhigh quality, large graphene sheets.8–10 However, the CVDmethod has many limitations for mass production andwide application, requiring metal substrates such as nickelor copper and high temperature processes over 1000 �C.In addition, after deposition, chemical etching of the sub-strate metal is required, and the resulting graphene thinfilms must be transferred to flexible target substrates suchas SiO2. This complicated process results in low produc-tivity and poor reliability.11

∗Author to whom correspondence should be addressed.

Recently, thin films coated with graphene flakes havebeen investigated as replacements for CVD graphene trans-parent conductive electrode films due to advantages suchas high productivity, scalability, and the ability to usesubstrate materials of a variety of shapes.12–16 However,graphene flakes exhibit high sheet resistance due to resid-ual oxygen content and defects within the graphene flakesthemselves.17–19 Various doping techniques are applied todecrease sheet resistance, such as wet treatment, evapora-tion of dopant materials, and plasma treatment. These tech-niques have been applied to doping carbon nanotubes20�21

and have been also investigated for the doping of graphene,especially CVD grown graphene.22–25

In this study, we applied Cl2 plasma treatments to thinfilms coated with graphene flakes and investigated theeffects of the plasma treatment on the electrical proper-ties of the flakes. The effects of plasma treatment on sheetresistance and physical damage to graphene flakes wereespecially closely investigated.

2. EXPERIMENTAL DETAILS

Figure 1(a) shows a schematic diagram of the spray coat-ing system used in our experiments. Layers of graphene

8090 J. Nanosci. Nanotechnol. 2013, Vol. 13, No. 12 1533-4880/2013/13/8090/005 doi:10.1166/jnn.2013.8191

Delivered by Publishing Technology to: Sung Kyun Kwan UniversityIP: 115.145.196.100 On: Wed, 19 Feb 2014 05:55:32

Copyright: American Scientific Publishers

RESEARCH

ARTIC

LE

Kim et al. Plasma Treatment of Thin Film Coated with Graphene Flakes for the Reduction of Sheet Resistance

(a)

(b)

Fig. 1. (a) Spray coating system and (b) plasma doping system used inexperiments.

flake were coated on polyethylene terephthalate (PET)films measuring 1 cm×1 cm. The graphene flake solution(Angstron Materials, N002-PDR, 4 milligram graphene in50 ml isopropanol) was prepared using a sonication pro-cess for 1 hour. The size of the stage was 15 cm×15 cm,and the stage was moved continuously in the x–y direc-tion to promote uniform coating with graphene flakes. Thestage temperature was maintained at 50 �C in a N2 condi-tion while spraying graphene flakes, and PET films coatedwith graphene flakes were annealed for 10 minutes toremove the solvent.Figure 1(b) shows the inductively coupled plasma (ICP)

system (STS Inc.) used for plasma treatment. The ICPsource was connected to a 13.56 MHz of rf power, and thetreatment conditions were 4 Watts and 5 mTorr Cl2. Nobias power was applied during plasma treatment and thesubstrate was maintained at room temperature (25 �C).After depositing silver electrodes at the four corners of

each graphene flake-coated PET sample, the sheet resis-tances of the samples were measured using a four-pointprobe (Keithley 2000 multimeter). A UV-VIS-NIR spec-trophotometer (UV-3600, SHIMADZU) was used to assesstransmittance and characterize the optical properties of

PET films coated with graphene flakes. Scanning electronmicroscopy (SEM, Hitachi S-4700) was used to assess sur-face morphology, and the degree of surface damage sus-tained during plasma treatment was investigated by Ramanspectroscopy (ALPHA300, WITec).

3. RESULTS AND DISCUSSION

The optical transmittances of graphene-coated PET filmswere measured as a function of wavelength for PET sub-strates with different numbers of graphene spray coatingsusing UV-visible spectroscopy (Fig. 2). The optical trans-mittances of the graphene-coated PET films did not varysignificantly with wavelength. The addition of one coatingdecreased the optical transmittance by about 4–5% fromthe value of 91.5% determined for the first coating. Mono-layer graphene is known to decrease optical transmittanceby about 2.3%.26 Therefore, one spray coating of grapheneflakes corresponded to an average coating of two graphenelayers on PET substrate in our experiments.Figure 3 shows SEM images of graphene flakes coated

on PET (a) after the first spray coating and (b) after fivespray coatings. The surface of the PET substrate was uni-formly coated with graphene flakes without any gaps, andthe graphene flakes were laid flat on the PET substrate.Even after 5 coatings, no significant changes of graphenesurface morphology were observed, due to the uniformcoating (Fig. 3(b)).Figure 4 shows the sheet resistances of two graphene-

coated PET thin films with optical transmittances of (a)77.6% (sample A, four spray coatings) and (b) 73.8%(sample B, five spray coatings). Multiple spray coatingsof graphene flakes were measured as a function of Cl2plasma treatment time. The sheet resistances were 803 and8.7 k�/sq., respectively, before plasma treatment. There-fore, the sample with higher optical transmittance also hadhigher sheet resistance. Graphene-coated PET films weretreated with an ICP at 13.56 MHz 4 Watts and 5 mTorr Cl2.

400 500 600 900 100060

70

80

90

100

5 Spray coatings

4 Spray coatings

3 Spray coatings

2 Spray coatings

1 Spray coating

Tra

nsm

ittan

ce (

%)

Wavelength (nm)

Fig. 2. Optical transmittances of graphene-coated PET films measuredas a function of wavelength using UV-visible spectroscopy for differentnumbers of graphene spray coatings.

J. Nanosci. Nanotechnol. 13, 8090–8094, 2013 8091

Delivered by Publishing Technology to: Sung Kyun Kwan UniversityIP: 115.145.196.100 On: Wed, 19 Feb 2014 05:55:32

Copyright: American Scientific Publishers

RESEARCH

ARTIC

LE

Plasma Treatment of Thin Film Coated with Graphene Flakes for the Reduction of Sheet Resistance Kim et al.

(a)

(b)

Fig. 3. SEM images of graphene-coated PET surfaces (a) after onespray coating and (b) after five spray coatings.

An increase in plasma treatment time initially decreasedsheet resistance for all samples. This decrease of sheetresistance with increased Cl2 plasma treatment time maybe related to charge transfer from Cl atoms to graphene.Among the four outermost electrons in a graphene car-bon atom, three of the electrons form sp2 hybrid orbitalsand have strong �-type covalent bonds with neighboringcarbon atoms, but the remaining electron forms a weaker�-type covalent bond with other carbon atoms on thegraphene surface. During plasma treatment, Cl atoms canbe attached to the graphene surface and transfer holesto the graphene by forming �-type covalent bonds withgraphene carbon atoms. In addition, Cl atoms can attach todefects in graphene and transfer holes by forming C Clbonds with carbon. Therefore, increasing the plasma treat-ment time increases the attachment and bonding of Clatoms to graphene, and also increases the charge trans-fer to graphene, which may be related to the decrease ofsheet resistance observed with increases of plasma treat-ment time.For sample A, the sheet resistance was lowest at about

20 sec when it had decreased about 50% from 803 to404 k�/sq (Fig. 4(a)). However, further increase of theplasma treatment time led to a rapid increase of sheetresistance. After a plasma treatment of 50 sec, the sheetresistance was higher than it had been before the plasmatreatment. For sample B the sheet resistance also decreasedwith plasma treatment time, but the sheet resistance didnot reach its minimum until after 60 sec of plasma treat-ment, after which it had decreased from 8.7 to 6.7 k�/sq.Therefore, the sheet resistance was decreased by about23% (Fig. 4(b)). The increase of sheet resistance observed

0 10 20 30 40 50 60200

400

600

800

1000

1200

1400(a)

Shee

t res

ista

nce

(kΩ

/sq)

Time (sec)

Transmittance 77.6%

0 10 20 30 40 50 605

6

7

8

9

10

11(b)

Shee

t res

ista

nce

(kΩ

/sq)

Time (sec)

Transmittance 73.8%

Fig. 4. The sheet resistances of two graphene-coated PET thin filmswith optical transmittances of (a) 77.6% (sample A) and (b) 73.8% (sam-ple B) after multiple spray-coating with graphene flakes.

with increased the plasma treatment time after reachingminimum sheet resistance (Fig. 4(a)) may be related toradiation damage on the graphene surface caused by Cl2plasma exposure. The density of defects on the graphenesurface increases and sheet resistance also increases dueto Cl ion bombardment of the graphene surface duringplasma treatment. Therefore, the minimum sheet resistanceshown in Figure 4(a) is affected by the decrease of sheetresistance caused by charge transfer from increased num-bers of Cl atoms on the graphene surface, and also by theincrease of sheet resistance caused by the radiation dam-age due to Cl2 plasma treatment. For sample B, due tothe presence of thicker graphene layers on the PET, thedamage to the graphene surface was not significant andthe number of Cl atoms bonded to the graphene was notsufficient (Fig. 4(b)). Therefore, the sheet resistance didnot reach the minimum value possible during the plasmatreatment times used in our experiments, and the optimumtreatment time was related to the thickness of films coatedwith graphene flakes.Figure 5 shows Raman spectra observed on PET films

coated with graphene flakes (a) before Cl2 plasma treat-ment and (b) after Cl2 plasma treatment of 60 sec underthe conditions shown in Figure 5(a). The peak observed at

8092 J. Nanosci. Nanotechnol. 13, 8090–8094, 2013

Delivered by Publishing Technology to: Sung Kyun Kwan UniversityIP: 115.145.196.100 On: Wed, 19 Feb 2014 05:55:32

Copyright: American Scientific Publishers

RESEARCH

ARTIC

LE

Kim et al. Plasma Treatment of Thin Film Coated with Graphene Flakes for the Reduction of Sheet Resistance

800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800

0.0

0.5

1.0

C==O bond

1700 cm–1

D-peak

1350 cm–1

G-peak

1580 cm–1

Nor

mal

ized

inte

nsity

(a.

u.)

Raman shift (cm–1)

60 sec plasma treated: PET peaks

(b)

800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800

0.0

0.5

1.0

C==O bond

1700 cm–1

Nor

mal

ized

inte

nsity

(a.

u.)

Raman shift (cm–1)

As-is : PET peaks

(a)G-peak

1580 cm–1

D-peak

1350 cm–1

2D-peak–1

2700 cm–1

2D-peak

2700 cm–1

Fig. 5. Raman spectra observed for the PET films coated with grapheneflakes (a) before Cl2 plasma treatment and (b) after Cl2 plasma treatmentfor 60 sec for the conditions shown in Figure 4 (a).

∼1350 cm−1 (D peak) is related to defects in the graphene,and the peaks at ∼1580 (G peak) and ∼2700 cm−1 (2Dpeak) are related to the graphene itself.27 In addition tothe peaks related to the graphene, sharp peaks (arrows)that originated from the PET film were also observed. TheG-peak observed 1580 cm−1 from the graphene was super-imposed on the peak caused by the PET. Even withoutplasma treatment, PET films coated with graphene flakesexhibited D peaks due to defects in the graphene flakesthemselves. However, after 60 sec plasma treatment theintensity of the D peak (and the ratio of the D peak tothe G peak) increased, possibly due to radiation damageto graphene flakes caused by Cl ion bombardment dur-ing plasma treatment. Therefore, sheet resistance increasedwith increased plasma treatment time, as long as the treat-ment time was long enough (Fig. 5(b)). To decrease thesheet resistance further using plasma treatment, radiationdamage caused by Cl ion bombardment must be reducedby controlling the plasma parameters.

4. CONCLUSION

PET films that were spray coated with graphene flakeswere treated with Cl2 plasma, and changes of sheet

resistance were observed when the plasma treatment timewas increased. The optical transmittances of the graphene-coated thin films decreased with decreased sheet resis-tance, due to the increased thicknesses of the grapheneflake coatings. Cl2 plasma treatment decreased the sheetresistances of PET films coated with graphene flakes withincreased plasma treatment time due to increased chargetransfer to the graphene. However, due to the radiationdamage on the graphene surface sustained during plasmatreatment, the sheet resistance again increased when thethin film was treated for too long. The optimum treatmenttimes were related to the thicknesses of thin films coatedwith graphene flakes. The reduction of sheet resistanceduring Cl2 plasma treatment may be related to hole dona-tion due to �-type covalent bonding of Cl on graphenesurfaces with carbon atoms, or to the formation of C Clbonds at graphene defect sites. Under optimum condi-tions, the sheet resistances of graphene flake films couldbe decreased by about 50%.

Acknowledgments: This work was supported by theIndustrial Strategic Technology Development Program(10041926, Development of high density plasma tech-nologies for thin film deposition of nanoscale semicon-ductor and flexible display processing) funded by theMinistry of Knowledge Economy (MKE, Korea), NanoMaterial Technology Development Program through theNational Research Foundation of Korea (NRF) fundedby the Ministry of Education, Science and Technology(2012M3A7B4035323), and the World Class Universityprogram of National Research Foundation of Korea (GrantNo. R32-10124).

References and Notes

1. S. K. Bae, H. K. Kim, Y. B. Lee, Xiangfan Xu, J. S. Park, andY. Zheng, Nature Nanotech. 5, 132 (2010).

2. X. Wang, L. Zhi, and K. Müllen, Nano Lett. 8, 1 (2008).3. X. Li, Y. Zhu, W. Cai, M. Borysiak, B. Han, D. Chen, R. D. Piner,

L. Colombo, and R. S. Ruoff, Nano Lett. 9, 12 (2009).4. Y. Zhang, Y. W. Tan, H. L. Stormer, and P. Kim, Nature 438, 04235

(2005).5. K. S. Novoselov and A. K. Geim, Nature Mater. 6, 1849 (2007).6. A. H. C. Neto, F. Guinea, N. M. R. Peres, K. S. Novoselov, and

A. K. Geim, Rev. Mod. Phys. 81, 109 (2009).7. X. Li, C. W. Magnuson, A. Venugopal, J. An, J. W. Suk, B. Han,

and M. Borysiak, Nano Lett. 10, 11 (2009).8. A. Reina, X. Jia, J. Ho, D. Nezich, H. Son, V. Bulovic, M. S.

Dresselhaus, and J. Kong, Nano Lett. 9, 1 (2009).9. X. Li, W. Cai, J. An, S. Kim, J. Nah, and D. Yang, Science 324, 1312

(2009).10. W. Wang, S. Guo, M. Penchev, J. Zhong, J. Lin, and D. Bao,

J. Nanosci. Nanotechnol. 12, 9 (2012).11. Q. He, H. G. Sudibya, Z. Yin, S. Wu, H. Li, F. Boey, W. Huang,

P. Chen, and H. Zhang, ACS Nano 4, 6 (2010).12. O. C. Compton and S. T. Nguyen, Small 6, 6 (2010).13. H. A. Becerril, J. Mao, Z. Liu, R. M. Stoltenberg, Z. Bao, and

Y. Chen, ACS Nano 2, 3 (2008).14. L. Kavan, J. H. Yum, and M. Grätzel, ACS Nano 5, 1 (2011).15. G.Eda,G. Fanchini, andM.Chhowalla,NatureNanotech. 3, 83 (2010).16. L. Kavan, J. H. Yum, and M. Grätzel, Nano Lett. 11, 12 (2009).

J. Nanosci. Nanotechnol. 13, 8090–8094, 2013 8093

Delivered by Publishing Technology to: Sung Kyun Kwan UniversityIP: 115.145.196.100 On: Wed, 19 Feb 2014 05:55:32

Copyright: American Scientific Publishers

RESEARCH

ARTIC

LE

Plasma Treatment of Thin Film Coated with Graphene Flakes for the Reduction of Sheet Resistance Kim et al.

17. H. J. Shin, K. K. Kim, A. Benayad, S. M. Yoon, and H. K. Park,Adv. Funct. Mater. 19, 12 (2009).

18. T. Wu, S. Liu, H. Li, L. Wang, and X. Sun, J. Nanosci. Nanotechnol.11, 11 (2011).

19. Z. Wei, D. Wang, S. Kim, S. Y. Kim, and Y. Hu, Science 328, 1373(2010).

20. S. Maldonado, S. Morin, and K. J. Stevenson, Carbon 44, 8 (2006).21. J. H. Jang, S. C. Lim, D. L. Duong, and G. Kim, J. Nanosci. Nano-

technol. 10, 6 (2010).22. T. O. Wehling, K. S. Novoselov, S. V. Morozov, E. E. Vdovin, M. I.

Katsnelson, A. K. Geim, and A. I. Lichtenstein, Nano Lett. 8, 1(2008).

23. F. Günes, H. J. Shin, C. Biswas, G. H. Han, E. S. Kim, S. J. Chae,J. Y. Choi, and Y. H. Lee, ACS Nano 4, 8 (2010).

24. X. Miao, S. Tongay, M. K. Petterson, and K. Berke, Nano Lett. 12, 6(2012).

25. Y. C. Lin, C. Y. Lin, and P. W. Chiu, Appl. Phys. Lett. 96, 133110(2010).

26. R. R. Nair, P. Blake, A. N. Grigorenko, K S. Novoselov, T. J. Booth,T. Stauber, N. M. R. Peres, and A. K. Geim, Science 320, 5881(2008).

27. A. C. Ferrari, J. C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri,F. Mauri, S. Piscanec, D. Jiang, K. S. Novoselov, S. Roth, and A. K.Geim, Phys. Rev. Lett. 97, 187401 (2006).

Received: 13 January 2013. Accepted: 14 May 2013.

8094 J. Nanosci. Nanotechnol. 13, 8090–8094, 2013