Electron field emission from reduced graphene oxide on polymer filmI. Sameera, Ravi Bhatia, Jianyong Ouyang, V. Prasad, and R. Menon Citation: Applied Physics Letters 102, 033102 (2013); doi: 10.1063/1.4788738 View online: http://dx.doi.org/10.1063/1.4788738 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/102/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Ultra low field electron emission of graphene exfoliated from carbon cloth Appl. Phys. Lett. 101, 153104 (2012); 10.1063/1.4758291 Free standing graphene-diamond hybrid films and their electron emission properties J. Appl. Phys. 110, 044324 (2011); 10.1063/1.3627370 Klein tunnelling model of low energy electron field emission from single-layer graphene sheet Appl. Phys. Lett. 99, 013112 (2011); 10.1063/1.3609781 Temporal field emission current stability and fluctuations from graphene films Appl. Phys. Lett. 97, 062106 (2010); 10.1063/1.3474800 Field emission from polymer-converted carbon films by ultraviolet radiation Appl. Phys. Lett. 78, 2009 (2001); 10.1063/1.1360233

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Electron field emission from reduced graphene oxide on polymer film

I. Sameera,1,a) Ravi Bhatia,2 Jianyong Ouyang,2 V. Prasad,1 and R. Menon1

1Department of Physics, Indian Institute of Science, Bangalore, India2Department of Materials Science and Engineering, National University of Singapore, Singapore

(Received 10 November 2012; accepted 4 January 2013; published online 22 January 2013)

Field emission of reduced graphene oxide coated on polystyrene film is studied in both parallel

and perpendicular configurations. Low turn-on field of 0.6 V/lm and high emission current

density of 200 mA/cm2 are observed in perpendicular configuration (along the cross section),

whereas a turn-on field of 6 V/lm and current density of 20 lA/cm2 are obtained in parallel

configuration (top surface). The emission characteristics follow Fowler–Nordheim (FN)

tunneling and the values of enhancement factor estimated from FN plots are 5818

(perpendicular) and 741 (parallel). Furthermore, stability and repeatability of the field emission

characteristics in perpendicular configuration are presented. VC 2013 American Institute of Physics.

[http://dx.doi.org/10.1063/1.4788738]

Graphene based systems are quite attractive for several

applications in electronics due to high intrinsic carrier mobil-

ity and current carrying capacity. Although significant pro-

gress in fabricating devices like transistors, detectors, etc.,

with improved performance has been reported,1–3 the investi-

gations of field emission properties are in an early stage.

Among carbon based materials, carbon nanotubes

(CNTs) have shown a great deal of potential in field emitting

devices.4–6 Although CNTs have shown very good field

emitting features, a consistent scenario in terms of structural

and emitting properties is yet to be established due to the dif-

ficulties involved in controlling the spacing among the tubes

and their aspect ratios. Whereas, in graphene based systems,

the 2D structure offers a facile route to make large area field

emission devices.

Field emission of graphene has two major difficulties,

one is large-scale synthesis of graphene and other is fabrica-

tion of field emission cathode with graphene laying flat on

the substrate, resulting in lack of sufficient field enhance-

ment. Graphene in simplest solution processable form can be

obtained by reducing graphene oxide (GO) in solvents like

alcohol, deionized water, etc.7–10 The resulting reduced gra-

phene oxide (rGO) is nearly similar to that of few layer gra-

phene, and the main advantage is that rGO can be drop-

casted or spin-coated on any substrate and is scalable over

large area to fabricate field emission cathodes. Also, solution

processing route does not involve high temperature process

unlike chemical vapor deposition (CVD), hence viable for

flexible electronics. Several efforts have been made to assure

the potential applications of rGO in transparent conducting

electrodes, transistors, sensors, supercapacitors, etc.11–15

However, number of reports in the field emission of

rGO is less compared to CNTs, and detailed studies are

required to improve the field emission from the former.

Recently, a few groups have reported on the field emission

of graphene by using, hydrogen exfoliation, exfoliation from

carbon cloth, electrophoresis, screen printing, etc.16–20 But

in all these methods, the field emission capabilities are

restricted by the random protrusions of the graphene films.

In order to achieve large field enhancement and high emis-

sion current densities, the sharp edges of graphene must be

grown vertically on the substrate. There are a few reports on

the field emission of vertically aligned few layer graphene

synthesized by CVD.21,22 But the high temperature growth

approach is complicated since control of process parameters

with precision is quite difficult. Even though solution meth-

ods are less complicated and scalable, field emission is lim-

ited by randomness of graphene protrusions. In the present

study, field emission of rGO coated on polystyrene (PS) is

presented by utilizing the cross section of the sample. Here,

we have combined the advantage of solution process, and

also the vertically orienting rGO by utilizing the cross sec-

tion of the sample.

Natural graphite powder of high-purity (SP-1graphite,

purity> 99.99% purchased from the Bay Carbon Inc.) was

oxidized by using modified Hummers’ process as described

elsewhere to produce GO.23 Then, rGO is obtained from as-

prepared GO by its rapid reduction with zinc powder at room

temperature under ultrasonication. Thereafter, rGO was col-

lected by vacuum filtration. It was rinsed with copious

amounts of deionized water and successively desiccated with

the freeze-drier for 2 days.23

As-prepared rGO can be readily suspended in methanol.

PS beads are dissolved in toluene and the resulting solution

is allowed to dry in a flat bottom beaker. The PS film (thick-

ness �50 lm) in the beaker served as a substrate for coating

rGO. The rGO suspension in methanol was sonicated for

15 min to achieve better dispersion and then drop-casted

over the PS film. The rGO suspension was allowed to get

dried overnight, resulting in uniform coating over PS film.

The average thickness of the coated rGO is �500 nm. The

rGO coated PS film was readily peeled off from glass beaker

and used for the field emission measurement.

FEI Quanta 200 scanning electron microscope (SEM),

equipped with a tungsten electron source and energy disper-

sive X-ray analysis (EDS), is used for the morphological

studies of rGO coated PS film. Raman characterization is

carried out by using HORIBA JOBINYVON Lab RAM HR

Raman spectrometer with 514.5 nm laser. Field emission

a)Author to whom correspondence should be addressed. Electronic mail:

sameeraivaturi@gmail.com.

0003-6951/2013/102(3)/033102/5/$30.00 VC 2013 American Institute of Physics102, 033102-1

APPLIED PHYSICS LETTERS 102, 033102 (2013)

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measurement is carried out in a diode configuration in which

cathode (copper plate) and anode (stainless steel plate of di-

ameter 6 mm) are separated by an adjustable gap (typically

�200 lm) using a micrometer. Distance between sample

and anode is measured from the top surface of the sample.

Field emission measurement is carried out in a chamber

maintained at a pressure of 4� 10�6 mbar at room tempera-

ture. Keithley 2410 source meter is used to record the field

emission current-voltage characteristics by sweeping the

voltage up to 1100 V.

SEM images of the top surface of rGO coated PS film

are shown in Fig. 1. The images show that rGO is uniformly

coated over PS film. SEM images depict the presence of ran-

dom edges of the rGO on PS film, at different magnifica-

tions. EDS spectrum contains two intense peaks

corresponding to C and O [see Fig. 2(a)], originated from

rGO, and this also confirms the absence of any impurity like

Zn which is used in the reduction process.

Raman spectra of rGO on PS film recorded for the top

surface and at the edges are presented in Fig. 2(b). Raman

spectra of as-prepared rGO-PS film show the two intense

bands: D and G bands. The D-band is disorder induced band

centered at �1350 cm�1 and is caused by phonon scattering

at defect sites and impurities, whereas the G-band at

�1580 cm�1 is E2g mode at the C-point of graphene which is

related to phonon vibrations in sp2 bonded carbon atoms. For

qualitative analysis of graphitic structures, the intensity ratio

of the D-band to the G-band [I(D)/I(G)] is measured, which

is inversely proportional to the quality and ideally is zero for

highly ordered single crystal graphite. For rGO-PS film,

I(D)/I(G) is �0.98, indicating the presence of defects on top

surface. At the cut edge of the rGO, this ratio slightly

increases to 1.09. Two more peaks of feeble intensity can be

noticed at �2680 cm�1 and �2950 cm�1, which can be

attributed to 2D and D þ G modes, respectively.24

The field emission characteristics of rGO-PS film

(� 6 mm� 6 mm) in parallel and perpendicular configura-

tions are presented in Fig. 3(a). In parallel configuration, as-

prepared rGO-PS film lies flat on the cathode (copper plate)

and its top surface faces the anode. In perpendicular configu-

ration, the rGO-PS film was cut by stainless steel scissors

and the cross section of the film with a thickness of 500 nm

FIG. 1. (a)-(d) SEM micrographs at different magnifications displaying edges of rGO on PS film.

033102-2 Sameera et al. Appl. Phys. Lett. 102, 033102 (2013)

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of rGO was utilized for field emission. The obtained I-E

graphs in both the configurations are compared in Fig. 3(a).

The sharp increase of emission current in perpendicular con-

figuration can be attributed to large number of rGO edges

acting as active emitters, which are available for electron

emission at the cross section of the rGO-PS film. The slow

rise of emission current in parallel configuration indicates

the limited number of active emitters available for emission.

More precisely, the normalized value of current density can

be compared and here, the total area under the anode for

emission is considered for calculating current density. The

emission current densities (J) vs. applied electric field (E)

data in both parallel and perpendicular configurations are

presented in Figs. 3(b) and 3(c), respectively. Maximum val-

ues of J obtained for parallel and perpendicular configura-

tions are �20 lA/cm2 (at 10.5 V/lm) and �200 mA/cm2 (at

1.65 V/lm), respectively.

The emission characteristics are analyzed by using

Fowler–Nordheim (FN) equation for field emission,25

lnðI=E2Þ ¼ lnðAab2=uÞ � Bu3=2=bE; (1)

where I is the emission current, A¼ 1.56� 10�6 AeV/V2,

B¼ 6.83� 109 V/eV3/2 m, b is a field enhancement factor, uis the work function, a is effective emission area, E¼V/d is

the applied field, d is the distance between the anode and the

cathode, and V is the applied voltage.

The turn-on fields (for emission current of 0.1 lA) of

rGO-PS film in parallel and perpendicular configurations

are 6 V/lm and 0.6 V/lm, respectively. The field emission

of the rGO-PS film originates from the protruded edges of

the rGO.15 In perpendicular configuration, large number of

sharp edges created at the cross section of rGO-PS film

localize and enhance the applied electric field, thereby

allowing electrons to tunnel at very low electric fields,

whereas the number of these edges is limited in parallel

configuration due to the randomness in orientation of the

edges of rGO. This effect can be clearly seen from the field

enhancement factor (b). The values of b are calculated by

FIG. 3. (a) Field emission I-E curves in

parallel (black) and perpendicular (red)

configurations of rGO-PS film; field

emission J-E curves of rGO-PS film (b)

parallel configuration, and (c) perpendic-

ular configurations. Insets: correspond-

ing FN plots, the red lines show the

linear fits to Eq. (1); (d) repeatability of

field emission J-E curves of rGO-PS film

in perpendicular configuration. Inset:

emission current vs. time showing the

stability for 25 min.

FIG. 2. (a) EDS spectrum of rGO coated

PS film (b) Raman spectra of rGO on PS

film on the top surface (black) and near

the cross section (red).

033102-3 Sameera et al. Appl. Phys. Lett. 102, 033102 (2013)

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assuming the work function of graphene as 5.0 eV, from the

slope of linear regions in FN plots, which are shown as

insets in Figs. 3(b) and 3(c), respectively. The values of bin parallel and perpendicular configurations are 741 and

5818, respectively.

Figure 3(d) presents the repeatability of the field emis-

sion J-E characteristics in perpendicular configuration;

hardly any shift in turn on field was noticed even after sev-

eral voltage sweeps unlike the previous studies.22 The

reproducibility of the field emission J-E characteristics

indicates the excellent capability of the rGO film to emit

high current densities. Stability of emission current is also

an important parameter to be investigated for a field emit-

ter. The emission stability for rGO-PS film is shown in the

inset of Fig. 3(d). Emission current density of 17 mA/cm2 is

observed for a period of 25 min by applying a constant elec-

tric field of 0.8 V/lm.

A comparison of the results obtained in the present study

with earlier works on field emission studies of graphene and

CNTs26 is shown in Table I. The field emission efficiency of

graphene predominantly depends on creating the sharp edges

of graphene sheets parallel to the direction of electric field.

To achieve this, vertical growth of graphene by CVD,

plasma treatment, etc., has been reported.19,20,27 Composite

routes, screen printing, electrophoresis, exfoliation methods,

etc., have been adopted by different studies,16–20 which are

summarized in the Table I. In order to create the sharp edges,

we have utilized the cross section of the rGO-PS film which

has both the advantage of flexibility and efficient emission

from rGO protrusions along the direction of applied field. It

can be observed that the values of turn-on and threshold

fields are low in the present study compared to that of previ-

ous reports [see Table I]. The field emission parameters of

rGO-PS film are quite comparable to the values of MWCNT-

PS composite indicating the efficiency of rGO to be a poten-

tial field emitter.

In summary, the field emission of rGO on a flexible

polymer substrate is shown to have high current densities

at low voltages. The rGO is drop-casted on PS substrate.

We have presented a simple and robust way of utilizing

rGO-PS film for field emission, by using the cross section

which enables the sample to be an efficient electron field

emitter. Low turn on field of 0.6 V/lm and high current

density of 200 mA/cm2 (at 1.65 V/lm), with high emission

stability and repeatability of field emission characteristics

for rGO-PS film in perpendicular configuration are

demonstrated.

The authors are grateful to Institute Nanoscience Initia-

tive (INI), MNCF Indian Institute of Science, Bangalore for

SEM, Raman characterizations. I. Sameera would like to

acknowledge CSIR, New Delhi for financial support.

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TABLE I. A comparison of the parameters describing the field emission characteristics in the present study with earlier works on field emission studies of gra-

phene and CNTs.

Sample name

Turn-on field

(V/lm)

Threshold field

(V/lm)

Enhancement

factor

Max. current

density (mA/cm2) Ref no.

rGO coated on PS film 0.6 (3 mA/cm2) 1.65 (200 mA/cm2) 5818 200 …

Graphene-PS thin film … 4 (10�8 A/cm2) 1200 1 19

Hydrogen exfoliated graphene 1.18 (10 lA/cm2) 1.43 (0.2 mA/cm2) 4907 0.5 16

Graphene from exfoliation of carbon cloth 0.4 (10 lA/cm2) 0.7 (1 mA/cm2) 13 000 3 17

Graphene films by electrophoretic deposition 2.3 (10 lA/cm2) 5.2 (10 mA/cm2) 3700 23 18

Screen printed graphene 1.5 (1 lA/cm2) 3.5 (1 mA/cm2) 4539 2.5 20

Vertical few layer graphene 1.8 (10 lA/cm2) … 6795 7 21

MWCNT-PS composite in vertical configuration 1.28 (0.1 mA/cm2) 1.55 (1 mA/cm2) 6494 100 26

Plasma treatment of vertically aligned few layer graphene 2.23 (10 lA/cm2) 4.4 (1.33 mA/cm2) 5130 1.33 27

033102-4 Sameera et al. Appl. Phys. Lett. 102, 033102 (2013)

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