Fabrication of sub-micrometer graphene ribbon using electrospunnanofiber

Won Mook Choi • Tran Van Tam •

Nguyen Bao Trung

Received: 2 September 2013 / Accepted: 5 October 2013 / Published online: 19 October 2013

� Springer Science+Business Media New York 2013

Abstract Here we demonstrate a simple and effective

method to fabricate graphene ribbon with a sub-micrometer

width down to 260 nm by using an electrospun polymer

nanofiber as a physical etch mask. Our method involves

electrospinning polystyrene nanofiber onto chemical vapor

deposition-grown graphene film, followed the oxygen

plasma etching to remove the exposed graphene without

disturbing the underlying graphene. This work shows that the

width of the resulting graphene ribbons can be engineered by

controlling the nanofiber size and etch conditions. Based on

this graphene ribbon, we fabricated a graphene-field effect

transistor with a bottom-gated geometry, which shows an

ambipolar characteristic with a hole and electron mobility of

1636 and 134 cm2/(V s), respectively. Our approach here

may allow the fabrication of sub-micrometer graphene rib-

bon for graphene-based electronics.

Introduction

Graphene, a single sheet of sp2-hybridized carbon atoms in

a honeycomb lattice, has been attracting enormous atten-

tion owing to its outstanding optical, mechanical, electri-

cal, and thermal properties for numerous potential

applications. Among these properties, high optical trans-

parency of single layer graphene (*97 %) and remarkably

high carrier mobility (200000 cm2/V s) have received the

most attention as the next generation electronic materials in

electronics, optoelectronics, and energy-related devices

[1–6]. After the mechanical isolation of a single graphene

sheet from graphite in 2004 [1], significant progress has

been made to realize potential graphene-based applica-

tions, which is associated with large-area production of

high-quality graphene and integrating graphene into elec-

tronics devices. One promising method of preparing large-

area graphene is by chemical vapor deposition (CVD) [7–

10], and its potential use in various electronics applica-

tions, such as field effect transistor (FET) [11–14], sensor

[15–18], and photonic devices [19–22] has been addressed

in several previous studies.

For the successful implementation of such graphene-

based electronic devices, it is essentially required to

develop a precise and scalable patterning technique over a

large area. Conventional photolithography has been widely

used to produce the micrometer-scale patterns of graphene

for electronics devices. Several techniques based on the

unconventional lithography techniques have been demon-

strated to fabricate the sub-micrometer and nanometer

patterns of graphene, i.e., nanoimprint lithography, colloi-

dal lithography, micro contact printing, and direct writing

[23, 24]. Recent works demonstrated that block copolymer

lithography and colloidal lithography were utilized to

produce the nanopatterned structures of CVD-grown

graphene [11], which is known as graphene nanomesh.

Other approaches that used a self-assembly method [25]

and inkjet printing technique [26, 27] were investigated as

alternative ways of producing micrometer-scale patterns of

graphene. Although such lithography techniques fabricated

well-defined graphene patterns, there are several limita-

tions for the fabrication of sub-micrometer- and nanome-

ter-sized graphene patterns that include low-throughput

and multiple processing steps for graphene-based elec-

tronic devices.

Here, we report a simple approach to fabricate sub-

micrometer graphene ribbons using an electrospun polymeric

W. M. Choi (&) � T. Van Tam � N. B. Trung

School of Chemical Engineering and Bioengineering, University

of Ulsan, Ulsan 680-749, Republic of Korea

e-mail: wmchoi98@ulsan.ac.kr

123

J Mater Sci (2014) 49:1240–1245

DOI 10.1007/s10853-013-7807-6

nanofiber as an etch mask. Specifically, the electrospun

nanofibers can be produced in the simple and scalable way,

and these nanofibers were placed on the top of CVD-grown

graphene, which was then followed by the graphene etch

and the nanofiber removal process. Such nanofiber on

graphene protect the underlying graphene as physical etch

masks during the etch process, such as oxygen plasma

etching, and hence the graphene ribbon under the nanofiber

is produced with the sub-micrometer width. We have

demonstrated the fabrication of graphene ribbons with

widths smaller than that of the diameter of nanofiber owing

to its round shape, and controlled the graphene ribbon

width by varying the etch conditions. With this approach,

graphene ribbon was obtained with a width down to

260 nm, and graphene FET has also been fabricated with

the resulting graphene ribbon. The approach presented here

provides a simple and low cost method to fabricate the sub-

micrometer graphene ribbon without expensive equipment

and complex process in other techniques, such as photoli-

thography and nanoimprint.

Experimentals

Preparation of CVD-grown graphene

Graphene films were synthesized on copper foil (Wacopa,

purity 99.9 %, 75-lm-thick) by CVD in a tube furnace.

The Cu foil was placed in the center of the tube, and

annealed at 1000 �C under a gas flow of Ar (10 sccm) and

H2 (10 sccm) at 500 mTorr for 30 min in order to remove

organic residue and oxides from the Cu surfaces. A gas

mixture of CH4 (5 sccm) and H2 (20 sccm) at 700 mTorr

was then flowed into the tube at 1000 �C for 25 min to

grow the graphene film. After graphene growth, the system

was cooled to room temperature under an Ar atmosphere.

The graphene layer grown on the Cu foil was transferred

onto a 300-nm-thick SiO2/Si wafer with the aid of the

polymeric supporting layer of polymethyl methacrylate

(PMMA). The crystallinity of the graphene layers was

analyzed by Raman spectroscopy (Renishaw, RM-1000

Invia) with an Ar-ion laser of 514 nm.

Fabrication of graphene ribbons using the electrospun

nanofiber

The electrospun polystyrene (PS) nanofibers were used as a

physical mask for the fabrication of graphene ribbon. PS

(Mw = 350000, Sigma Aldrich) was dissolved in tetrahy-

drofuran (THF; Sigma Aldrich) at room temperature with

vigorous stirring for 8 h to ensure a homogenous solution,

and the concentration of PS solution was 25 wt%. PS

solution was inserted into a 1 mL syringe attached to a

steel needle with a 0.8 mm inner diameter. A steel sub-

strate covered with aluminum foil was placed 15 cm below

the tip of the syringe needle as a counter electrode. Elec-

trospinning was carried out at room temperature in a ver-

tical spinning configuration by applying a positive high

voltage of 20 kV, driven by a high voltage power supply.

The flow rate of PS solution was maintained at 1 mL/h.

The electrospun PS nanofibers were collected directly on

the graphene film substrate which was placed on the

counter electrode of aluminum foil. The graphene film with

PS nanofiber was exposed to conventional oxygen plasma

at 100 W to etch the graphene, and the PS nanofiber used

as an etch mask was then removed using THF solvent. For

the electrical characterization of the fabricated graphene

ribbon, the bottom-gated transistor was fabricated on a

SiO2/highly doped n-type Si substrate.

Results and discussions

Figure 1 schematically illustrates the fabrication procedure

of sub-micrometer graphene ribbon using the electrospun PS

nanofiber. First, CVD-grown graphene first was transferred

onto a 300-nm-thick SiO2/Si substrate, and the transferred

graphene film was then annealed at 400 �C with an Ar–H2

mixture (50:50) to remove the residue of PMMA supporting

layer and other organic impurities [28]. A scanning electron

microscopy (SEM; Hitachi S-4700) image of the transferred

CVD-grown graphene on SiO2/Si substrate in Fig. 1b shows

that some wrinkles and multilayer domains are observed in

graphene, which are typical features of CVD-grown graph-

ene using Cu foil. Raman spectra of the prepared CVD-

grown graphene was taken as shown in Fig. 1c in which a

negligible D-peak at 1350 cm-1 is detected with distinct

G-peak at 1560 cm-1 and 2D-peak at 2700 cm-1, indicating

the high quality of the CVD-grown graphene [10]. Moreover,

the photograph of the transferred graphene on a SiO2/Si

substrate in the inset of Fig. 1c confirms that the graphene

film was successfully transferred with an area of about

1.5 cm 9 1.5 cm. With this CVD-grown graphene film, the

electrospinning of PS nanofiber was then performed and the

nanofiber was placed onto the graphene film.

In order to etch away the unprotected graphene film, the

graphene with PS nanofiber was taken with the oxygen

plasma process, resulting in the formation of the graphene

ribbons underneath the PS nanofiber. After the oxygen

plasma etch process, the nanofiber was then easily removed

with THF solvent. The PS nanofiber was clearly observed

on the graphene film in the microscopy image of the

sample before the oxygen plasma process (Fig. 2a). Raman

spectra were taken from the nanofiber on graphene and the

unprotected graphene region, which exhibited only PS

spectra and typical spectra of CVD-grown graphene,

J Mater Sci (2014) 49:1240–1245 1241

123

respectively (Fig. 2b). After the oxygen plasma process for

typically 20 s at 100 W power, graphene underneath PS

nanofiber was survived and the graphene ribbon was

subsequently obtained after nanofiber removal. The bright

blue line was observed In the microscopy image of Fig. 2c,

indicating that the graphene ribbon was remained and other

graphene

nanofiber

oxygen plasma etching

nanofiberremoval

(a)

1 um

(b)

1500 2000 2500 3000

Inte

nsi

ty (

a.u

.)

Raman shift (cm-1)

(c)

Fig. 1 a Schematic illustration

for fabricating the graphene

ribbon with an electrospun

nanofiber etch mask. b SEM

image of CVD-grown graphene

transferred on a SiO2/Si

substrate and c its Raman

spectra. The inset shows a

photograph of CVD-grown

graphene

Fig. 2 a Microscope image and

b Raman spectra of the

electrospun nanofiber on

graphene film. c Microscope

image and d Raman spectra of

the resulting graphene ribbon

after oxygen plasma etch and

nanofiber removal

1242 J Mater Sci (2014) 49:1240–1245

123

graphene regions were perfectly etched away, thus the SiO2

substrate was exposed. These results were confirmed from

Raman spectra (Fig. 2d). A Raman spectrum of graphene

was detected only on the blue line region and Raman signal

of the SiO2 substrate was only detected other region.

Moreover, a Raman spectra of the fabricated graphene

ribbon was identical to that of as-prepared graphene before

the oxygen plasma etch, which indicates the graphene

ribbon was prepared without the deterioration of graphene

quality. The width of the graphene ribbon obtained in this

manner was principally determined by that of the nanofi-

ber, since a vertical etch was predominantly applied to

graphene film for the short etching time, resulting in a

graphene ribbon width similar to that of the PS nanofiber.

The width of graphene ribbon can be further reduced by

increasing the etching time, since undercutting of the

graphene below the nanofiber occurred with a lateral etch

owing to the cylindrical shape of nanofiber (Fig. 3a). Thus,

graphene ribbon with a controllable width down to sub-

micrometer scale can be fabricated by varying the etching

time. SEM images of Fig. 3b–e show the electrospun PS

nanofiber on graphene film and the fabricated graphene

ribbons under controlled oxygen plasma etch time. The

width of the as-prepared nanofiber in Fig. 3b is 740 nm

and, after the short plasma etch of 20 s, the width of the

resulting graphene fiber was 730 nm, which indicates that

the width of the fabricated graphene ribbon is similar to

that of the nanofiber (Fig. 3c). Increasing the etch time to

60 s resulted in a reduced width of 500 nm by undercutting

and the slight reduction of the nanofiber (Fig. 3d). Fur-

thermore, the prolonged etch time of 150 s led to a further

size reduction of graphene ribbon down to 260 nm, which

was almost one-third of the width of original nanofiber

(Fig. 3e). With our proposed method, the controllable

width of graphene ribbon can be fabricated by the variation

of etching conditions. Although our results in this work

demonstrated the sub-micrometer width of graphene rib-

bon, our approach can lead to the further size reduction of

graphene ribbon by applying a smaller nanofiber with

optimization of etching conditions.

To characterize the transport properties of the prepared

graphene ribbon, we built bottom-gated graphene FET

devices, as illustrated in Fig. 4a. With the proposed

method, graphene ribbon (width 260 nm) as an active

channel was fabricated on thermal SiO2 (300-nm-thick)

and highly doped n-type Si substrate which were used as

the gate dielectrics and back gate, respectively. The source

(S) and drain (D) electrodes of Cr/Au (5/50 nm) were

formed using photolithography and lift-off process. The

electrical characterization of the graphene FET was

(b) (c)

(d) (e)

Oxygen plasma etch for 20 s

graphene

nanofiber

for 60 s for 150 s(a)Fig. 3 a Schematic illustration

for controlling the graphene

ribbon width by the variation of

etch time. SEM images of b the

electrospun nanofiber on

graphene film before the oxygen

plasma etch, and c–e the

graphene ribbons fabricated

with the oxygen plasma etch for

20, 60, and 150 s, respectively.

The scale bar is 500 nm

J Mater Sci (2014) 49:1240–1245 1243

123

performed at room temperature under vacuum condition

(*1 9 10-6 Torr) using a Keithley semiconductor param-

eter analyzer (model 4200-SCS). Figure 4b presents typical

drain current–gate voltage (ID–VG) characteristics. The

fabricated graphene FET exhibits a typical ambipolar

transport behavior and an asymmetric transport curve with

a Dirac point shifted approximately at ?16 V. Such a large

positive shift in the Dirac point indicates that the graphene

is highly p-doped even in vacuum. The p-doping of

graphene FET can be explained by the charged impurities

of the underlying SiO2, such as H2O and O2 [29, 30], and

by the charge scattering sites on the graphene surface due

to polymer residue, which was formed from the PMMA of

the graphene transfer and PS nanofiber of the graphene etch

process [31]. The hole and the electron mobility (l) were

also calculated from the linear regime of the transfer

characteristics using

l ¼ 1

VDCg

L

W

dID

dVG

� �;

where L and W are channel length and width, respectively,

Cg is the specific capacitance of the gate dielectric, and VD,

VG, and ID denote drain–source voltage, gate voltage, and

drain–source current, respectively. From the ID–VG curve,

the calculated hole and electron mobility were 1636 and

134 cm2/(V s) at VD = 0.1 V, respectively, and these

values are comparable to those of devices fabricated using

a photolithographic process [14]. Moreover, the asymmetry

of hole and electron mobility is attributed to factors such as

scattering defects by charged impurities and chemical

contamination during transfer and device fabrication pro-

cess [32]. The inset of Fig. 4b shows the output charac-

teristics (ID–VD) of the device at four different gate

voltages from -30 to 30 V. The device shows a clear

increase of conductance induced by the gate voltage and

linear behavior, which is a typical feature of metal/zero

bandgap semiconductor junctions.

Conclusions

In summary, we have demonstrated a template-directed

approach to fabricate sub-micrometer graphene ribbons

using an electrospun PS nanofiber as a physical etch mask.

The width of graphene ribbon can be controlled with the

diameter of nanofiber and the etch conditions, and here we

have fabricated graphene ribbon with a width down to

260 nm. Using the as-fabricated graphene ribbon, we built a

bottom-gated graphene FET device on a SiO2/Si wafer, and

the obtained hole and electron mobility of device were 1636

and 134 cm2/(V s), respectively. The proposed method here

presents the promising strategy to fabricate sub-micrometer

graphene ribbon for graphene-based electronics, including

thin film transistors and chemical sensors.

Acknowledgements This work was supported by Basic Science

Research Program through the National Research Foundation of

Korea (NRF) funded by the Ministry of Education, Science and

Technology (NRF-2012R1A1A1015124).

References

1. Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y,

Dubbonos SV, Grigorieva IV, Firsov AA (2004) Science 306:666

2. Novoselov KS, Geim AK, Morozov SV, Jiang D, Katsnelson MI,

Grigorieva IV, Dubonos SV, Firsov AA (2005) Nature 438:197

3. Heersche HB, Jarillo-Herrero P, Oostinga JB, Vandersypen

LMK, Morpurgo A (2007) Nature 446:56

4. Huang X, Yin Z, Wu S, Qi X, He Q, Zhang Q, Yan Q, Boey F,

Zhang H (2011) Small 7:1876

5. Qian Y, Lu S, Gao F (2011) J Mater Sci 46:3517. doi:10.1007/

s10853-011-5260-y

6. Sumboja A, Foo CY, Wang X, Lee PS (2013) Adv Mater 25:2809

7. Reina A, Jia X, Ho J, Nezich D, Son H, Bulovic V, Dresselhaus

MS, Kong J (2008) Nano Lett 9:30

8. Kim KS, Zhao Y, Jang H, Lee SY, Kim JM, Kim KS, Ahn J-H,

Kim P, Choi J-Y, Hong BH (2009) Nature 457:706

9. Bae S, Kim H, Lee Y, Xu X, Park J-S, Zheng Y, Balakrishnan J,

Lei T, Kim HR, Song YI, Kim Y-J, Kim KS, Ozyilmaz B, Ahn

J-H, Hong BH, Iijima S (2010) Nat Nanotechnol 5:574

10. Li X, Cai W, An J, Kim S, Nah J, Yang D, Piner R, Velamakanni

A, Jung I, Tutuc E, Banerjee SK, Colombo L, Ruoff RS (2009)

Science 324:1312

(a)

-30 -20 -10 0 10 20 300

2

4

6

8

10

12

Dra

in c

urr

ent

(µA

)

Gate voltage (V)

VD= 0.1 V

-1.0 -0.5 0.0 0.5 1.0-10

-5

0

5

10

I D(µ

A)

VD (V)

(b)

Fig. 4 a Schematic illustration of the bottom-gated graphene ribbon

FET. b Current–voltage transfer characteristics of graphene FET at

VD = 0.1 V. The inset shows the output characteristics of the devices

at four different gate voltages

1244 J Mater Sci (2014) 49:1240–1245

123

11. Bai JW, Zhong X, Jiang S, Huang Y, Duan XF (2010) Nat

Nanotechnol 5:190

12. Lee S, Kim BJ, Jang H, Yoon SC, Lee C, Hong BH, Rogers JA,

Cho JH, Ahn A-H (2011) Nano Lett 11:4642

13. Nezich D, Reina A, Kong J (2012) Nanotechnology

23:015701

14. Shin H-J, Yoon S-M, Choi WM, Park S, Lee D, Song IY, Woo

YS, Choi J-Y (2013) Appl Phys Lett 102:163102

15. Lu CH, Yang HH, Zhu CL, Chen X, Chen GN (2009) Angew

Chem 48:4785

16. Ohno Y, Maehashi K, Yamashiro Y, Matsumoto YK (2009) Nano

Lett 9:3318

17. Pumera M (2011) Mater Today 14:308

18. Kwak YH, Choi DS, Kim YN, Kim H, Yoon DH, Ahn SS, Yang

JW, Yang WS, Seo S (2012) Biosens Bioelectron 37:82

19. Xia F, Mueller T, Lin Y-M, Valdes-Garcia A, Avouris Ph (2009)

Nat Nanotechnol 4:839

20. Bonaccorso F, Sun Z, Hasan T, Ferrari AC (2010) Nat Photonics

4:611

21. Han T-H, Lee Y, Choi M-R, Woo S-H, Bae S-H, Hong BH, Ahn

J-H, Lee TW (2012) Nat Photon 6:105

22. Hwang J, Choi HK, Moon J, Kim TY, Shin J, Joo CW, Han J,

Cho D, Huh JW, Choi S, Lee J, Chu HY (2012) Appl Phys Lett

100:133304

23. Zhou Y, Loh KP (2010) Adv Mater 22:3615

24. Hong J-Y, Jang J (2012) J Mater Chem 22:8179

25. Kim TY, Kwon SW, Park SJ, Yoon DH, Suh KS, Yang WS

(2011) Adv Mater 23:2734

26. Shin S-Y, Hong J-Y, Jang J (2011) Adv Mater 23:2113

27. Torrisi F, Hasan T, Wu W, Sun Z, Lombardo A, Kulmala TS,

Hsieh G-W, Jung S, Bonaccorso F, Paul PJ, Chu D, Ferrari AC

(2012) ACS Nano 6:2992

28. Lin Y-C, Lu C-C, Yeh C-H, Jin C, Suenaga K, Chiu P-W (2012)

Nano Lett 12:414

29. Chen F, Xia J, Tao N (2009) Nano Lett 9:1621

30. Wang S, Ang PK, Wang ZQ, Tang ALL, Thong JTL, Loh KP

(2010) Nano Lett 10:92

31. Pirkle A, Chan J, Venugopal A, Hinojos D, Magnuson CW,

McDonnell S, Colombo L, Vogel EM, Ruoff RS, Wallace RM

(2011) Appl Phys Lett 99:122108

32. Petrone N, Dean CR, Meric I, van der Zande AM, Huang PY, Wang

L, Muller D, Shepard KL, Hone J (2012) Nano Lett 12:2751

J Mater Sci (2014) 49:1240–1245 1245

123