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Soft-lithographic processed soluble micropatterns of reduced graphene oxidefor wafer-scale thin film transistors and gas sensors†

Jia Zhang,a PingAn Hu,*a Rongfu Zhang,a Xiaona Wang,a Bin Yang,b Wenwu Cao,b Yibin Li,c Xiaodong He,c

Zhenlong Wanga and William O’Neilld

Received 20th August 2011, Accepted 6th October 2011

DOI: 10.1039/c1jm14071j

PDMS based imprinting is firstly developed for patterning of rGO on a large area. High quality stripe

and square shaped rGO patterns are obtained and the electrical properties of the rGO film can be

adjusted by the concentration of GO suspension. The arrays of rGO electronics are fabricated from the

patterned film by a simple shadow mask method. Gas sensors, which are based on these rGO

electronics, show high sensitivity and recyclable usage in sensing NH3.

Introduction

Patterning of nanostructure materials to generate a functional

microstructure on a large area is promising for mass production

of micro/nanoscale devices. Graphene discovered in 2004 is a two

dimensional material of sp2 bonded carbon atoms and has

aroused huge research interest in a wide field of physics, material

science, biology and electronics.1–3 Graphene exhibits excellent

electrical properties and great chemical stability, and may be

considered as the building block of future electronics and

sensors. Graphene transistors show carrier mobilities between

3000 and 27 000 cm2 V�1 s�1,4 which makes graphene a promising

material for nanoelectronics. Mechanically exfoliated graphene

has been demonstrated to have the ability to detect gases down to

a single molecular level.5 For these microelectroncis and sensor

applications, large area and selective patterning of graphene on

a substrate is urgently in need. The most popular method to

produce graphene for research purposes is through ‘‘scotch tape’’

or mechanical cleavage of graphite.1 However, this technique has

a very low yield, in which monolayer graphenes have to be dis-

cerned from a mixture of thick graphene flakes. Therefore, the

‘‘scotch tape’’ process is unsuitable for large-scale production of

devices. Patterned graphene can be produced by a chemical

vapor deposition method (CVD) on the patterned metallic film

from a hydrocarbon source.6 The device array can be made of

aKey Lab of Microsystem and Microstructure, Harbin Institute ofTechnology, Ministry of Education, No. 2 YiKuang Street, Harbin,150080, P.R. China. E-mail: hupa@hit.edu.cnbDepartment of Physics, Harbin Institute of Technology, Harbin, 150080,P.R.ChinacCenter for Composite Materials, School of Astronautics, Harbin Instituteof Technology, 150080, P.R.ChinadCentre for Industrial Photonics, Institute for Manufacturing, Departmentof Engineering, University of Cambridge, 17 Charles Babbage Road,Cambridge, CB3 0FS, UK

† Electronic supplementary information (ESI) available. See DOI:10.1039/c1jm14071j

714 | J. Mater. Chem., 2012, 22, 714–718

those grown graphene patterns. However the patterned metallic

films for graphene growth are made using a complex photolith-

ographic technique; and a complex procedure is required to

transfer patterned graphenes to the insulating substrate prior to

device fabrication6–8 since metallic substrates are unsuitable for

electrical devices application.

Reduced graphene oxide (rGO) produced from chemical or

thermal reduction of graphene oxide (GO) is competitively

alternative material for graphene. GO prepared from graphite via

a chemical exfoliation method emerges as single- or few-layered

graphene with high yield production dispersions (�4 mg ml�1).9

The solution processability of GO offers unique advantages since

it is readily amenable for spin-coating, casting, transferring,

imprinting onto substrates for large-scale production of gra-

phene electronic circuits. Especially, residual chemically active

defect sites render rGO as promising material for functional

electronic sensors.10–14 Some chemical/biological sensors made

from single-layer rGO or few-layers rGO have been demon-

strated.10–14 Up to date, most of those studies are focused on

a single electronic device or sensor. For real-life applications, it is

essential to produce highly integrated electronics and sensors, for

which research into patterning of continuous rGO films is

urgently needed.

Patterning GO have been performed using micromolding in

capillaries (MIMIC).15 This method mainly exploits a weak

capillary force which produces the limit area of micropatterns

(<20 cm � 20 cm). In addition, a limit geometric structure of

stripe type can be made by the MIMIC method. Nano-

imprinting, which is a most widely used soft-lithography tech-

nique, allows fabrication of two- or three-dimensional structures

with micro/nanoscale resolution.16–20 Polydimethylsiloxane

(PDMS) is the most popular soft elastomeric and conformable

stamp for nanoimprinting. In the imprinting procedure, pressure

is applied to make the PDMS stamp contact with the solution (or

soft solid materials) coated substrate and various geometric

patterns can be replicated to the substrate from the PDMS stamp

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(shown in Fig.1a). Compared to MIMIC, the advantages of this

technique include simplicity, large area (depending on the size of

stamp) and possible production of various geometric micro-

patterns. To our best knowledge, up to date, there is no report

about exploitation of this imprinting technique for GO

patterning. In this paper, we present PDMS based imprinting to

fabricate wafer-scale continuously conductive rGO micro-

patterns. Highly integrated thin film transistors (TFTs) are made

based on these rGO micropatterns. And the applications of our

rGO TFT for gas sensors are also demonstrated.

Experimental details

Graphite oxide was synthesized from natural graphite flakes

using a modified Hummer’s method. Graphite oxide was then

fully exfoliated in water to produce suspension of graphene oxide

(GO) sheet for imprinting. The glassy carbon toolsets were

prepared via a previously reported laser machining.15 A 10 : 1

(PDMS : curing agent) mixture of the PDMS and silicon elas-

tomer kit (Dow Corning) was poured onto the glassy carbon

mold. Then, the PDMS was cured at 80 �C for 2 h. After curing,

the PDMS stamp was carefully lifted off the glassy carbon

master. To fabricate the rGO pattern (shown in the schematic

drawing of Fig. 1): a GO suspension was dispersed onto the

substrate, then the PDMS stamp (length � width � thichness:

10 � 10 � 3 mm; depth of pattern structure: 1 mm) was pressed

onto the GO suspension and pressure of 0.2 N was used to ensure

close contact between the PDMS and the substrate. After water

was evaporated by heating at 80 �C for 1 h, the PDMS stamp was

carefully lifted off the substrate, leaving the GO patterns on the

substrate intact. Patterned GO films were annealed in H2/Ar (10/

40 sccm) under ambient pressure at 800 �C for 30 min. The

patterned GO and rGO films were characterized by optical

microscopy (Leica DM4500P), scanning electron microscopy

(SEM, Hitachi S-4200), atomic force microscopy (Nanoscope

IIIa Vecco) and Raman spectroscopy (LabRAM XploRA,

power 0.15 mW, excitement wavelength 532 nm). The element

components and structure analysis was performed using X-ray

photoelectron spectroscopy (XPS, Thermo Sentific K-Alpha

XPS, using Al (Ka) radiation as a probe). Cr/Au electrodes on

Fig. 1 Schematic illustration of (a) a soft-lithographic procedure for

patterned rGO film, (b) an rGO electronics array fabricated using

a shadow mask.

This journal is ª The Royal Society of Chemistry 2012

the patterned film were fabricated using film-plating machines

(ZHD-300) by a shadow mask. Electrical measurements of these

devices were performed using a semiconductor analyzer (Keith-

ley 4200 SCS) combined with a Lakeshore probe station.

Results and discussion

The significance of our work is in demonstrating the fabrication

of continuous patterned thin film consisting of single or few

layered rGO by an imprinting procedure of soft-lithography and

the simplicity to make the array of graphene electronics from

these patterns. The imprinting procedure is schematically shown

in Fig.1. Graphene oxide for imprinting is synthesized from

natural graphite flakes using modified Hummer’s method.21 We

can prepare single-layer GO with different concentrations

(Fig. S1).† A high quality and large area rGO pattern (depending

on the size of PDMS mold) can be made on solid or flexible

substrates by this PDMS imprinting procedure. Stripe and

square shaped micropatterns of GO film are generated from

PDMS replication. The morphologies of patterned films are

characterized using optical microscopy, SEM, AFM. A series of

typical images of the rGO pattern that were made using 0.3 mg

ml�1 GO suspension are shown in Fig. 2. Fig. 2a gives a low

magnification view of stripe shaped rGO patterns. The stripe

structure has a width of�15 mm and length up to 5 cm. The array

of the square shaped rGO pattern is shown in Fig. 2b. Each

microscale square has a size 30 � 30 mm. A magnified image

(Fig. 2c) shows that the patterned film is continuous and

composed of stacked individual rGO flakes with a size range of

Fig. 2 Optical image of patterned rGO films: (a) striped shape, (b)

square shape. (c) A magnified SEM image indicates that rGO films are

composed of one- to a few-layer randomly stacked rGO sheets. (d) AFM

image of the film edges. (e) Section analysis indicates that the thickness of

rGO sheets is �3 nm. (f) Raman spectra of GO and rGO film.

J. Mater. Chem., 2012, 22, 714–718 | 715

Fig. 3 (a) Thickness of patterned rGO film as a function of GO

suspension concentration. (b) Conductance of patterned rGO film as

a function of GO suspension concentration. SEM images of the patterned

rGO film made from GO suspension of (c) 0.1 mg ml�1, (d) 0.5 mg ml�1,

(e) 1 mg ml�1.

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1�15 mm. The AFM images of the film edge (Fig. 2d, Fig. 2e)

indicate that the film thickness is �3 nm, which corresponds to

2�3 layers of rGO. These patterned films exhibit nonuniformity

in colours under optical microscopy, which indicates nonuni-

formity in the film thickness (shown in Fig. 2a and Fig. 2b).

Deeper colours mostly observed at the film edges reflect a little

bigger thickness at the rim. GO pattern are formed in the

patterned PDMS stamp, in which GO suspension is evaporated.

The generation of nonuniformity in GO film thickness may be

accounted for by the coffee ring effect that occurred in the

microscale chamber which formed by compact contact of PDMS

stamp with substrate. GO suspension evaporates on a solid

surface. As the three-phase contact line (CL) of the GO

suspension droplet is pinned on the surface in the microscale

chamber due to physical roughness or surface heterogeneities,

the evaporation induces a capillary flow within the droplet which

drives the suspended GO flakes toward the CL and leaves a little

thicker structure at the rim (shown in Fig. 2a, b.).22,23 In addition,

there are no GO residues left on the surface of PDMS stamp after

imprinting (shown in Fig. S2).† This is attributed to the low

surface energy of the PDMS interface (19.8 mJ m�2). Therefore,

the low surface energy of the PDMS interface enhances the

formation of a high quality GO pattern.

The structure of GO and rGO is characterized using Raman

spectroscopy. Raman spectra taken from a GO film and the

corresponding chemically reduced GO film are shown in Fig. 2f.

Compared to GO, the G band (�1600 cm�1) of rGO is sharper

and more symmetrical and has an up-shift of �2 cm�1. The

intensity ratio of D (�1350 cm�1) and G band (ID/IG), which is

utilized to determine the extent of defects in graphene, decreases

from 1.2 to 0.92. This implies an increase in the size of sp2

domains in rGO.24,25 This can be explained by the removal of

epoxide and hydroxyl groups on the basal plane and carbonyl

and carboxyl groups on edges of GO via chemical reduction,

leading to the restoration of sp2-hybridized carbons. The struc-

ture and components of GO and rGO are further analyzed by

XPS. The analytic results are shown in Fig. S2:† the oxygen

content in GO is more than 24.9% as compared with 17.11% in

rGO (calculated from Fig. S3a). The high-resolution XPS spectra

of C1s of as-prepared GO sheets (Fig. S3b) shows that oxidized

carbon atoms exist in form of different functional groups:

aromatic or conjugated C (284.5 eV), the C in C–O bonds (286.8

eV), the carbonyl C (C]O, 288.3 eV), and the carboxylate

carbon (O–C]O, 289.0 eV).24,26 These oxygen containing groups

are produced from harsh oxidation, which destroyed the sp2

atomic structure of graphene. Those oxygen containing groups

are also observed from rGO (Fig. S3c), but the intensities of

those oxidized carbon groups are much weaker than that in the

GO, proving considerable de-oxygenation and restoration of the

sp2 carbon sites by the reduction treatment.

In previous reports on the rGO film prepared by filtration or

transfer printing,18,26 the thickness of the as-made rGO film,

which can be controlled and adjusted by the concentration of GO

suspension, is the key for the field effect properties. We perform

the experiments in the thickness and conductance of the rGO film

as a function of the concentration of GO suspension (shown in

Fig. 3). The thickness shows a linear increase with concentration

of the GO suspension (Fig. 3a). The rGO films fabricated from

GO suspension with concentrations 0.5 mg ml�1 and 1 mg ml�1

716 | J. Mater. Chem., 2012, 22, 714–718

are continuous and exhibits respective average thicknesses of 6

nm and 10 nm (shown in Fig. 3a, Fig. 3d, Fig. 3e), while

a discontinuous film with an average thickness of 2 nm is

obtained when the concentration of GO suspension is below 0.1

mg ml�1 (shown in Fig. 3a, Fig. 3c). From Fig. 3b, the conduc-

tivity of rGO increases from 1.2 � 10�11 to 1 � 104 S m�1 in the

GO concentration range of 0.05�0.5 mg ml�1, and higher

concentrations (>0.5 mg ml�1) lead to a nearly constant

conductivity of �7 � 3 � 104 S m�1.

Multilayered graphene reaches the 3D limit of graphite in

terms of its electronic and dielectric properties at about 10

layers,27,28 which theoretically corresponds to a thickness below

3.4 nm for the pristine graphene since a single graphene has

a thickness of 0.34 nm, while owing to the oxygen functional

groups in both sides, single layer GO and rGO show a much

larger thickness of �1 nm, which is reported in the previous

theoretical and experimental research.29–31 Our experimental

results (Fig. 3a) show that a dense rGO flake film with a stacked

layer number above 10 can be fabricated using a high concen-

tration (1 or 2 mg ml�1) solution, which has the characteristics of

a good conductor. And a rGO film made using GO solution with

concentration of below 0.5 mg ml�1 can be used for transistor

application since its average stacked layer number is estimated to

be below 5.

The parallel array of graphene electronics for the gas sensor is

made from patterned film using a shielding mask (shown in

Fig.1b and Fig. 4a). This provides a simple and efficient

approach for scalable production of rGO electronics and sensors.

The devices with a channel length (L) of 60 mm and a width (W)

This journal is ª The Royal Society of Chemistry 2012

Fig. 4 (a) SEM images of the array of rGO TFT, the insert SEM image

in (a) is a magnified TFT device, bar¼ 15 mm. (b) Transfer curves of rGO

TFT measured in air and in vacuum. (c) Output curve of rGO TFT in

vacuum. (d) Current variance of rGO TFT versus exposal to 1000 ppm

NH3. (e) Resistance variance of rGO TFT as a function of NH3

concentration. (f) Adsorption and desorption of NH3 on the rGO sensor.

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of 15 mm are made on a doped silicon substrate with a 300 nm

thermally grown SiO2 layer. The silicon wafer layer is used as the

back gate. The conductivity of the patterned rGO film is in the

range of 100 � 1000 S m�1. The electrical transport properties of

rGO film TFT in air and in vacuum are measured. The transfer

characteristics (Ids�Vgs) of a typical rGO device are shown in

Fig. 4b. It has high conductance under a negative gate bias and

almost no conduction under a positive gate bias when the device

is measured in air, indicating that the carriers in this condition

are holes and the device behaves as p-type semiconductor.32–35

This strong p-type behaviour is resulted from the adsorption of

oxygen or water from the ambient or hydroxyl group of the

substrate. In order to exclude such ambient impact, the device is

placed inside a high vacuum chamber (<5 � 10�5 Torr) for 24 h

before electrical measurement. Ambipolar behavior, which has

been reported in single-layer reduced GO transistor,34,35 is then

observed from the rGO film transistor (shown in Fig. 4b). The

output characteristics (Ids�Vds) exhibits a large linear region in

Fig. 4c, implying a good ohmic contact between the Cr/Au

electrodes and the rGO. The mobility (m) of hole or electron can

be extracted from the two branches of the transfer characteristics

using the following equation:36

m ¼�DIdsDVgs

L

W

��ðVds CoxÞ (1)

where Cox is the silicon oxide gate capacitance (which is 1.15 �10�8 F cm�2 for a gate oxide thickness of 300 nm). The room

temperature-field effect mobilities of as-made rGO TFTs are

This journal is ª The Royal Society of Chemistry 2012

�2.3 cm2 V�1 s�1 for holes and �0.84 cm2 V�1 s�1 for electrons.

Compared to the pristine graphene, rGO exhibits very low

mobilities, which may result not only from the scattering by

doping defects, growth defects, and other imperfections or

adsorbates,35,37 but also from the scattering at junctions of two or

more rGO sheets overlapped.

Current interest in rGO sensors can be attributed to two main

reasons. First, its every constituted atom is at surface which

makes its electrical transport properties sensitive to the envi-

ronment, which is similar to pristine graphene. Second, the

existence of chemically active defects such as vacancies or small

holes may render more adsorption sites for gaseous molecules,

which enhances the sensing capability.10 To test the sensing

performance of as-fabricated rGO TFTs, we use NH3 gas as the

sensing target in this study. Sensing performance of the devices is

characterized against the low concentration of NH3 diluted in N2

gas. The rGO sensor shows a strong response to NH3 and

exhibits a decrease of about 10% in conductance with the expo-

sure to 1000 ppm NH3. This is verified by the measurement of

Ids�Vds curve of the devices at the gate voltage of 0.1 V as shown

in Fig. 4d. This conductance variance is due to the depletion of

holes in rGO by the NH3 adsorption.37 Resistance variance of

rGO film as a function of NH3 concentration is given in Fig. 4e,

the resistance of rGO film increases linearly with the NH3

concentration. The lowest detection concentration of 400 ppm

has been achieved in our experiments, which is not even opti-

mized. Compared to other NH3 sensors such as oxide sensor

which are operated at high temperature of 100�300 �C,38 the

rGO film sensors work at room temperature, which greatly

facilitates the operation.

To test the stability, the adsorption and desorption of NH3 on

rGO film device are performed several times (shown in Fig. 4f). A

flow of pure N2 is applied to clean the device after each run. The

sensor demonstrates stable sensing capability after several circles

of adsorption and desorption of NH3. The adsorption behavior

of the molecules on graphene is proposed to be closely related to

the structure.39 Adsorption on sp2-bonded carbon (the low-

energy binding sites) occurs through weak dispersive forces,

which is a recoverable process; while adsorption onto higher

energy binding sites (e.g. vacancies, structural defects, and

oxygen functional groups) is usually nonrecoverable without

moderate thermal treatment.39 For example, a carboxylic acid

group with single- and double-hydrogen bonding allows binding

energies of at least several hundred meV molecule�1 for adsorp-

tion of a molecule.40 Our recoverable sensor is here attributed to

the fully reduced treatment, which increases the number of sp2-

bonded carbons.13 This fully reduced treatment can be evidenced

by the analysis of Raman spectra and XPS (shown in Fig. 2f and

Fig. S3†). As for the sensing mechanism, the sensing behavior

can be attributed to the effective adsorption of NH3 on the

surface of p-type rGO. NH3 is an electron donator because of

the existence of the lone pair of electrons. Electron transfer from

the adsorbed NH3 to the rGO leads to depleted hole concen-

tration and decreased electrical conduction in rGO film.37

Conclusions

In summary, we demonstrate that the PDMS based imprinting

enables large area patterning of rGO film, which makes scalable

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fabrication of rGO electronics a reality. Microscale features with

stripe and square geometries are successfully fabricated by the

PDMS based imprinting. An array of rGO TFTs are made using

the patterned film following a simple mask shielding process and

a gas sensor is then fabricated. The sensor is quite sensitive even

at the non-optimized stage, and the sensing capability is very

stable. Our results are significant because this PDMS based

imprinting method make scalable fabrication of electronics and

sensors of rGO an easy task.

Acknowledgements

This work is supported by the National Natural Science Foun-

dation of China (NSFC, No. 61172001), the Scientific Research

Foundation for Returned Overseas Chinese Scholars, State

Education Ministry and the Fundamental Research Funds for

Central Universities and Chinese Program for New Century

Excellent Talents in University.

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