full-layer controlled synthesis and transfer of large-scale monolayer graphene for nitrogen dioxide...

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Full-Layer Controlled Synthesis and Transfer of Large-Scale Monolayer Graphene for Nitrogen Dioxide andAmmonia SensingVu Van Quang a , Ngo Si Trong a , Nguyen Ngoc Trung b , Nguyen Duc Hoa a , Nguyen Van Duy a

& Nguyen Van Hieu aa International Training Institute for Material Science (ITIMS), Hanoi University of Scienceand Technology (HUST) , Hai Ba Trung Dist. , Hanoi , Vietnamb School of Engineering Physics , HUSTAccepted author version posted online: 12 Sep 2013.

To cite this article: Analytical Letters (2013): Full-Layer Controlled Synthesis and Transfer of Large-Scale Monolayer Graphenefor Nitrogen Dioxide and Ammonia Sensing, Analytical Letters, DOI: 10.1080/00032719.2013.832270

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ACCEPTED MANUSCRIPT 1

SENSORS

FULL-LAYER CONTROLLED SYNTHESIS AND TRANSFER OF LARGE-SCALE MONOLAYER GRAPHENE FOR NITROGEN DIOXIDE AND AMMONIA

SENSING

Vu Van Quang1, Ngo Si Trong1, Nguyen Ngoc Trung2,Nguyen Duc Hoa1, Nguyen Van Duy1, Nguyen Van Hieu1

1International Training Institute for Material Science (ITIMS), Hanoi University of Science and Technology (HUST), Hai Ba Trung Dist., Hanoi, Vietnam.2School of

Engineering Physics, HUST

International Training Institute for Materials Science (ITIMS), Hanoi University of Science and Technology (HUST) No.1, Dai Co Viet Road, Hanoi, Vietnam. Phone: 84 4

38680787, Fax: 84 4 38692963, E-mail: [email protected], vn/[email protected]

Received: 2013-06-04; accepted: 2013-07-22

Abstract

The controlled synthesis of large-scale monolayer graphene to be used in the

development of cost-effective, mass-produced, and easy-to-use gas sensors for the real

time monitoring of toxic gases is an important issue. In this study, scalable monolayer

graphene was controllably synthesized by chemical vapor deposition on a copper

substrate and effectively transferred on a silica (insulator) substrate for gas sensing. A

high-quality graphene layer was obtained by changing the growth conditions. Raman

measurements indicated that large-area (1 ×1 cm2) monolayer graphene dominated the

transferred films. Gas sensing characterizations demonstrated that monolayer graphene

can effectively detect NO2 and NH3 within the temperature range of 100 °C to 200 °C.

Keywords: Full-layer; graphene; Raman; CVD; gas sensor

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INTRODUCTION

Theoretical prediction of monolayer sp2-bonded carbon atoms with a honeycomb

structure, the so-called isolated graphene, was expected to be thermodynamically

unstable(Peierls 1935) and was thus considered impossible to exist naturally until its

discovery in 2004(Novoselov et al. 2004). Since then, graphene has been attracting

considerable attention worldwide because of its exceptional electronic and mechanical

properties (e.g., superb charge carrier mobility and flexibility) (Eda, Fanchini, and

Chhowalla 2008; Sire et al. 2012), as well as potential applications in various fields (e.g.,

field-effect transistor(Meric et al. 2008; Schwierz 2010), flexible and stretchable

electronic devices(Yan, Cho, and Ahn 2012), touch screens in mobile phones(Bae et al.

2010), transparent electrodes used in organic solar cells(Wang et al. 2011), and gas

sensors(Chung et al. 2012; Nomani et al. 2010)).

The potential applications of graphene depend on their size, quality, and number of layers.

Extensive focus has been given to the controllable synthesis and applications of graphene,

and several successful techniques have been developed(Wei and Liu 2010). For instance,

graphene can be fabricated by inductively coupled plasma chemical vapor

deposition(Nang and Kim 2012), chemical vapor deposition (CVD)(Mattevi, Chhowalla,

and Kim 2011), reduction from graphene oxide(Song et al. 2012), annealing silicon

carbide(Sutter 2009), and mechanical exfoliation (Novoselov et al. 2004). Graphene can

be seen as a mono-graphitic layer where each carbon atom coordinates with five

neighboring atoms in a plane to form a hexagonal structure; thus monolayer graphene can

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be easily fabricated by mechanical exfoliation method because the bond of carbon atoms

at different graphitic layers is very weak. This method can enable the synthesis of high-

quality graphene sheets, but difficulties are encountered in size control, creation of a

large area, or mass production. Controlling the number of graphene layers and handling

for device fabrication are also of concern. The tiny size of exfoliated graphene film also

limits its practical utility in electronic applications. A controlled vapor depositionmethod

using inexpensive and readily accessible nickel and copper substrates and catalysts to

grow large-area graphene layers has been recently performed(Li, Cai, An, et al. 2009;

Thiele et al. 2010). The synthesis of graphene on copper is effective for monolayer

growth, and several research groups have already reported success in this area and

excellent device characteristics(Bae et al. 2010). The successful fabrication and transfer

of large-scale monolayer graphene have opened a new strategy for its electronic

applications(Lee et al. 2010).

Efforts have also been made in the detection of hazardous gas such as NO2 and NH3 in

various fields involving chemical processing, agriculture, environmental monitoring, gas

exhaust, and explosives to reduce dangerous and toxic situations(Yavari et al. 2012;

Trong et al. 2012; Joshi et al. 2010). The characteristic of graphene as a mono-graphitic

layer,i.e., carbon atoms are boundtogether in a plane, causes every atom in the layer to act

as an activation site for gaseous interaction (Schedin et al. 2007). With its tuneable

electrical conduction and coupling ability with dipole molecules, one electron in orbital

pz in graphene readily interacts with charged materials. Thus, even slight modulations in

the surface state through gas adsorption and desorption can result in variations in

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conductance of graphene and provide output signals(Leenaerts, Partoens, and Peeters

2008; Bradley et al. 2003). Indeed, soon after its realization, grapheme has been proven

as an excellent candidate for gas and vapor sensors because of its ultrafast response, low

Johnson noise, and long-term durability. These features are attributed to the large surface-

to-volume ratio, ultrafast electron transport, metallic conductivity, and fewcrystal defects

of graphene(Schedin et al. 2007). The gas-sensing properties of pristine and

functionalized graphene have been investigated for carbon dioxide and ammonia

gases(Gautam and Jayatissa 2012). Some studies have confirmed the gas-sensing abilities

of graphene at room temperature. Schedin et al.(Schedin et al. 2007)first reported on the

use of exfoliated graphene to detectvery low concentrations of NO2 gas. Highly sensitive

NO2 and NH3 sensors at ambient temperature have been fabricated using chemical vapor

deposition – grown graphene(Yavari et al. 2012). Epitaxial graphene grown on SiC

substrates has also been investigated for gas sensing, and results demonstratethat

graphene layers offer very high sensitivity and selectivity, as well as fast response for

NO2 detection(Nomani et al. 2010). The sensing characteristics of few-layer graphenes

prepared by the thermal exfoliation of graphitic oxide for NO2 and under humidity have

likewise been investigated(Ghosh et al. 2009). Treatment of graphene by ozone has been

reported to remarkablyenhance sensing performance, including percentage response,

detection limit, and response time(Chung et al. 2012). However, to fabricate a simple

sensor with practical applications, the method should be easy, low cost, and be mass

produced. Thus, the controlled and reproducible synthesis of high-quality large-area

graphene films is necessary for the successful development of graphene-based gas

sensors.

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In this study, high-quality monolayer graphene was prepared by chemical vapor

deposition and transferred onto an inter digitated Pt electrode substrate for gas-sensor

characterization. Optimization of the synthesis process for the full layer growth of

monolayer graphene was completed by observing the oxidized grains of copper foil and

Raman characterization. The transfer processes were controlled to avoid damaging the

graphene film and ensuring the fewest crystal defects. The grapheme was also used to

detect NH3 and NO2 at the ppm level in air at moderate temperatures.

EXPERIMENTAL

Synthesis of Graphene

Graphene films were grown on copper foils at 1000 °C in a chemical vapor deposition

system composed of a 70 mm-diameter quartz tube heated in a split hot-wall tube furnace

(a three-zoneheater system, as shown in Figure 1(A,B)). The heating process was

automatically controlled with a program that enabled all graphene films to be reproduced

with the same recipe. A typical growth process involved inserting a flat copper substrate

on top of an alumina ceramic holder into a quartz tube, followed by evacuation and back

filling with H2 (g) to avoid copper oxidation at high temperatures. The furnace

temperature was increased to 1000 °C within 1 h while maintaining an H2 (g) pressure of

200 mTorr under a 20 sccm flow. Next, a 5 sccm flow of CH4(g) was introduced for a

desired period (typically 10 min) under a total pressure of 300 mTorr. After CH4

exposure, the furnace was cooled to room temperatureat a cooling rate of 50 °C/min.

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Figure 1(C) shows photos of an as-received copper foil (left) and a copper foil after

graphene growth (right).

Transfer of Graphene Layer

In order to investigate the gas-sensing properties of the graphene, the as-grown graphene

films on copper substrates were transferred onto the silicon dioxide substrate that

supported bar-type platinum electrodes. Efficient transfer techniques must minimize film

damage and preserve the excellent properties of graphene. For this purpose,

graphene/copper was coated with polymethylmethacrylate (PMMA), which “held” the

graphene onto the surface of a copper etchant solution and water. The graphene/copper

sample was cut to 1×1 cm2 in size, and then attached on a flat surface of a spin coater.

Then the polymethylmethacrylate was dropped (20 µl), and spun at a rate of 1000 rpm to

generate a polymer layer with a thickness of ~1 µm. This layer covered the entire of

graphene side, thus kept it floating on the etchant solution when the copper substrate was

completely etched away. The substrate was etched in aqueousiron(III) chloride solution.

The etching time depended on the thickness of the copper foils, area, and concentration of

etching solution. The copper foils had a thickness of 25 µm and an area of 1×1 cm2 and

were completely removed after dipping in 1 g/mL FeCl3solution for 25 min. The floating

polymethylmethacrylate/graphene in etchant solution was pulled up by a plastic plate and

rinsed several times in deionized water to remove all residual etchants. Finally, the

polymethylmethacrylate/graphene layer (floated on water) was transferred carefully onto

a silica wafer with 300 nm thermally grown SiO2. The polymethylmethacrylate was

removed by repeatedly rinsing the wafer with acetone. More details about transferring

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processes can be found in supporting information (Fig. S1). The remaining monolayer

graphene on the substrate can be observed with the naked eye because grapheme on the

300nm-thick SiO2/Sislightly changes the reflective light wavelength. The synthesized

materials were characterized by several advanced techniques, such as micro-Raman

spectroscopy, optical imaging, and scanning electron microscopy (SEM).

Gas-Sensing Characterization

The gas-sensing properties of the graphene sensor toward NO2 and NH3were investigated

at 100, 150, and 200 °C. The gas-sensing properties of the graphene sensors were

measured with NO2 (50 ppm to 400 ppm) and NH3 (from air to gas and back to air)by a

flow-through technique with a standard flow rate of 400 sccm for both dry air balance

and analytic gases using a home-made system(Thong, Loan, and Hieu 2010). During the

sensing measurements, the resistance of the sensors was continuously measured using a

Keithley Instrument (model 2700) interfaced with a computer while the dried air and

analytic gases were switched on/off each cycle for evaluating the response and recovery

of the device. Standard gas (0.1%) balanced with high-purity (99.99%)N2was purchased

from Air Liquide Group. Details of the high-speed gas-switching system are described in

(Thong, Loan, and Hieu 2010). The gas concentration was calculated as

C(ppm)=Cstd(ppm)� f/(f+F), where f and F are the flow rates of the analytical gas and

dry air, respectively, andCstd(ppm) is the concentration of the standard gas used. By

varying the flow rate of dry air and standard gas, the analytical gases NO2 and NH3were

generated in a concentration range of 50-400 ppm, and 250-1000 ppm, respectively.

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RESULTS AND DISCUSSION

Material Characterization

Optical imaging is a fast and effective technique for rapidly observing graphene grains

grown on copper substrates. However, graphene grains can be difficult to visualize

because of the homogeneous contrast of graphene on copper. Thus, partial oxidation of

the parts of the copper substrate that the graphene film does not cover is necessary to

visualize the graphene grains, and such oxidation can be achieved by exploiting the

excellent oxidation-resistant properties of graphene. The present study aimed to evaluate

the quality of graphene layer grown under different conditions by observing the optical

image of fabricated samples. Prior to observation, the samples were partially oxidized for

several minutes in air on a 150 °C hot plate to ensure oxidation of copper where graphene

was not grown. The graphene layer was a superior protective coating for materials and

was easily oxidizedupon exposure to air; thus, only regions that were not covered by

graphene were oxidized and resulted in different contrasts in the optical image. The

bright region observed in the optical image comprised graphene grains, whereas the dark

region was bare copper oxide (Fig. 2). Figures 2 (A to D) show that under growth

conditions (1) to (3), graphene was not fully grown on the entire copper substrate.

Graphene grains were clearly observed in the optical image as fern leafs or flakes(Li et al.

2011). However, under growth condition (4), graphene was grown fully. Notably, the full

layer growth of graphene on the substrate was very important in transfer processes to

ensure success in obtaining large-scale monolayer graphene without distortion. If a non-

full layer growth of graphene was obtained on the substrate, then the graphene layer was

considered damaged and a large-scale monolayer graphene was not obtained after

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transferring. The growth mechanism of graphene on copper by chemical vapor deposition

has been proposed by Li et al.(Li, Cai, Colombo, et al. 2009). This mechanism

involvesthe predominantsurface nucleation of carbon on the surface followed by a two-

dimensional growth process. During growth, methane was first adsorbed onto the copper

and dehydrogenated into carbon and hydrogen atoms on the surface. Graphene growth

startedupon nucleation of carbon atoms on copper followed by expansion to form grains

before fully covering the remaining surface(Duong et al. 2012).

An SEM image of full-layer graphene grown on the copper substrate is shown in Figure

3(A). The copper grains had an average size of about 20 µm. However, the graphene

grains were difficult to observe in the SEM image because of the homogeneous contrast

of full-layer graphene. Debris were also observed as scratches on the graphene layer

during sampling. Figure 3 (B) shows the optical microscopy images of full-layer

graphene grown after transferring onto silicon substrate. The sample was successfully

transferred without damaging the layer. The image differed from the non-full-layer

growth sample, in which the layer was damaged after transfer (inset of Figure 3(B)). The

SEM image of the transferred graphene had a perfect layer without any significant

damage apart from some wrinkles, which can be due to the poor contact between

graphene and silicon dioxide substrate, as shown in Figure 3(B). The perfect transfer of

graphene onto silicon dioxide substrate was efficient and can be used for electronic

device and sensor applications(Lee et al. 2010).

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Raman spectroscopy was used to characterize the quality of graphene at room

temperature. This technique enables the evaluation of graphene thickness (layers)

because its properties critically depend on the number of layers(Malard et al. 2009;

Ferrari et al. 2006). A typical Raman spectrum of graphene has three major features: (i)

the G-band at ~1584 cm-1 that arises out of the in-plane vibrations of sp2-hybridized

carbon atoms close to the Г point, (ii) the 2D-band (otherwise known as the G’-band) at

~2700 cm-1 related to the double resonance Raman scattering process, and (iii) the D-

band at ~ 1300 cm-1. However, the D-band is not Raman active for perfect graphene

because of its general defects and the misalignment of graphene domains (grains) when

they fully cover the copper surface during synthesis. In our study, the Raman

characteristics of the copper foil before and after graphene growth, as well as after

transferonto silicon substrate, were investigated for comparison. The results are presented

in Figure 4(A to C), respectively. Figure 4(A) shows the Raman spectra of copper foil,

and no associated active Raman mode of carbon was observed within the measured range.

Two active Raman modes of G-band and 2D-band were observed in the sample after

graphene growth (Figure (B)). The appearance of G-band and 2D-band indicated the

successful growth of graphene on the copper substrate. The D-band of graphene

corresponded to the first-order edge or indicated that the defect-induced zone boundary

phonons were low and can be ignored from the spectra, thereby suggesting that the

defects and misalignment of inter-domains were negligible. In our measurement, the

micro-Raman sampling area was about 20 µm in size, which was smaller than the grain

size of copper. Thus, the D-band intensity decreased to ensure few defects in the

boundary. After transferring onto silicon dioxide substrate, a sharp G-band peak at 1593

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cm-1 with a full width at half-maximum estimated from the single-Lorentzian fit of 15.8

cm-1was observed. The number of graphene layer (thickness) could be estimated by

comparing the intensity of G-band and 2D-band. The ratio IG/I2Dwas reported to be about

0.5 for a monolayergraphene and this value increased with increasing of layer

numbers(Malard et al. 2009) .In our study, the ratio of peak intensities IG/I2D was about

0.57, indicating that the graphene films remained a monolayer even after transferonto the

SiO2/Si substrate. The D-band was also observed to increase in the transferred graphene,

indicating the existence of grain defects.

Gas-Sensing Characteristics

NO2 and NH3 are highly toxic and cause severe damage to human respiration systems and

lung tissues even at concentration levels <1 ppm in air(Yavari et al. 2012). Thus,

monitoring NO2 and NH3is extremely important. The use of graphene for sensing NO2

and NH3 gases at room temperature has been reported(Singh et al. 2013). However, NO2

molecules can tightly bind onto the surface of graphene, leading to a long response time

(τ90%≈30 min). The recovery of a graphene sensor can be enhanced by heating the sensor

at a high temperature of about 200 °C (Schedin et al. 2007). Accordingly, the gas-sensing

performanceof the synthesized graphene sensor towardNO2 and NH3 at 100, 150, and

200 °C was characterized in this study. The morphology of the graphene sensor chip and

its electrical properties are shown in Figure 5(A, B). Variations in the resistance of the

sensor to temperature (Figure 5(B)) demonstrated that graphene had a positive

temperature coefficient resistance similar to the behavior of metallic materials, where the

sensor resistance significantly increasedwith increased temperature from 30 °C to 300 °C.

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This result suggested that residual electron–phonon scattering was part of the scattering

mechanism in the conduction of synthesized graphene(Tan et al. 2007).

The gas-sensing characterization was performed by measuring variations in sensor

resistance upon exposure to different concentrations of analytical gases. The responses to

50, 100, and 400 ppm NO2 gas measured at 100, 150, and 200 °C are shown in Figure 6

(A to C), respectively. The sensor resistance markedly decreased at all measured

temperatures compared with that value in air as NO2 gas was introduced. After stopping

gas exposure, graphene resistance returned to its original value, suggesting that the sensor

can be reversibly operated at all measured temperatures. This sensor differed from the

sensor operated at room temperature, in which the response time of the sensor was

inversely associated with the NO2 concentration because NO2 molecules strongly

adsorbed on graphene surface(Ghosh et al. 2009). The responses of the graphene sensor

to NH3 measured at different temperatures are shown in Figure 6 (D to F). In contrast to

NO2, the sensor showed increasing resistance upon exposure to NH3 gas because of the

reducing behavior of NH3 gas. Upon adsorption onto the graphene surface, NH3 gas

tended to donate free electrons to graphene, thereby increasing sensor resistance(Yavari

et al. 2012).

The response and recovery characteristics of graphene sensors dependon working

temperatures. Figures 7(A) and (B) show the sensor response and recovery times as a

function of working temperatures for different NO2 concentrations, respectively. Sensor

response is defined as RNO2/RAir and the recovery time for graphene resistance to

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recover90% of the steady-state value. Typically, for graphene sensor soperated at high

temperatures, sensors have a low response and require a short recovery time because the

adsorbed molecules can easily detach through the assistance of thermal vibration of

molecules. However, our observations indicated that the sensor showed the highest

response and longest recovery time at a measured temperature of 150 °C, indicating the

strongest bending of NO2 molecule on the surface of graphene. The discrepant

observation in this study can possibly be due to the effect of humidity on sensitivity. The

sensor showed good sensing characteristics to NO2 and NH3 at temperature of 150oC;

however, the sensitivity to other concentrations and the long-term stability of the devices

should be carefully checked for actual applications.

The stability and reusability of a device are important parameters affecting the practical

application of a sensor. Thus, the stability of the sensor measured for NO2 at an operating

temperature of 100 °C was investigated for three cycles, and the results are shown Figure

7(C). The sensor had a good response rate and stability after turning on and off from air

to NO2 and back to air. Graphene maintained higher stability than metal oxide-based

devices because of the high crystallinity and absence of grain size growth during the

sensing characterization of the synthesized monolayer graphene(Korotcenkov and Cho

2011).

It is believed that the gas sensing properties of the graphene are dependent on the

synthesis methods, measurement conditions, and design of the sensors. For evaluation of

the gas-sensing performances of the synthesized monolayer graphene, we compared our

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results with other reports, as shown in Table 1. The grapheme films obtained in this study

exhibited a similar response time with those of other graphene samples. However, the

recovery time was shorter than those values measured at room temperature. This is

consistent with Nomani’s report (Nomani et al. 2010), where the increase in working

temperature leads to a significantly improvement of the response/recovery characteristic.

In addition, working conditions affect strongly on the responsivity of the sensors.

The sensing mechanism of a graphene-based gas sensor can be explained based on the

conduction model(Meric et al. 2008). According to a theoretical calculation(Leenaerts,

Partoens, and Peeters 2009), considering that the lowest unoccupied molecular orbital of

NO2 is 0.3 eV below the Dirac point of graphene, each NO2 molecule adsorbed onto the

surface of the graphene layer attracts electrons from graphene, thereby resulting in

lowered graphene resistance. The charge transfer from graphene to the NO2 molecule

depends on adsorption sites and orientations and varies from 0.099 to 0.102 electrons. In

addition, in our experiments, dry air was used as a reference during gas-sensing

measurement, and the graphene under this condition displays p-type behavior due to the

electron withdrawing nature of adsorbed oxygen moieties(Yavari et al. 2012). Therefore,

the attraction of electrons from graphene leads to an increase in the number of conducting

holes, thereby shifting the Fermi level closer to the valence band and resulting in

decreased graphene resistance compared with that value in air(Ko et al. 2010). The

response to NH3 is the inverse of the response to NO2 because NH3 is a reducing gas.

Upon adsorption onto the graphene surface, NH3 molecules tend to donate electrons to

graphene, resulting in increased graphene resistance compared with that value in air.

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CONCLUSIONS

An effective process of synthesizing large-area monolayer graphene and creating

effective transferring processes that can enable its application in sensor devices for the

sensitive detection of highly toxic environmental analytes such as NO2 and NH3 was

demonstrated. The graphene-based gas sensor retained its sensitivity to NO2 after cycling

from air to gas, indicating the reusability of the sensor. Monolayer graphene effectively

detected NO2 and NH3within the moderate temperature range of 100 °C to 200 °C

ACKNOWLEDGEMENT

This work was supported by the National Foundation for Science and Technology

Development (NAFOSTED) research program (code 103.02-2011.42).

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Table 1. Nitrogen dioxide and ammonia sensing characteristics of graphene synthesized

by different methods.

Graphene

synthesis

method

Response

time

Recovery time Sensitivity Working

conditio

ns NO2 NH3 NO2 NH3 NO2 NH3

(Schedin et

al. 2007)

Mechanic

al

exfoliatio

n

~ 400

s

~50

0 s

# # Single

molecul

e

Single

molecul

e

RT,

Vacuum

(Chen, et al.

2012)

CVD ~ 400

s

~40

0 s

Partially

recover

after 400

s, under

UV

illuminatio

n

Partially

recover

after 400

s.

3% @

800 ppt

2% @ 4

ppm

RT, UV,

N2 as

reference

gas

(Khai, et al.

2012)

Chemical

reduction

~100

0 s

# Partially

recover

after 1500

s for 50

ppm, at

200°C

# 5% @

50 ppm

# RT,

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(Ghosh et al.

2009)

Thermal

exfoliatio

n

~60

mn

# Partially

recover

after 40

mn for

1000 ppm

# 25% @

500

ppm

# RT,

(Nomani et

al. 2010)

Epitaxial

graphene

on 6H-

SiC

~50 s # ~100 s Partially

recover

after 300 s

for 550

ppm

10% @

18 ppm

1.5% @

550

ppm

300°C

Present work CVD ~ 400

s

~

500

s

Completel

y recover

after 300-

1200 s

Completel

y recover

after

about

600s

5% @

50 ppm

2 % @

250

ppm

100-

200°C,

dry air as

reference

gas

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Figure 1. (A) Photo of the chemical vapor deposition system used for graphene growth.

(B, C) Photos of bare copper (left) and graphene on copper (right).

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Figure 2. Optical images of graphene grown on copper substrate under different

conditions. The images show increased graphene grain sunder varied growth conditions

and exposure times to CH4 gas: (A) 30 s, (B) 1 min, (C) 3 min, and (D) 15 min.

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Figure 3. (A) SEM image of full-layer grapheme grown on copper foil. (B, C) Optical

and SEM images, respectively, of a graphene layer after transfer onto silicon substrate.

(Inset of (B))Photo of a non-full-layer graphene grown after transfer.

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Figure 4. Raman spectra of copper foil (A) before and (B) after graphene growth. (C)

Graphene layer after transfer onto silicon dioxide substrate.

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Figure 5. (A) Optical image of graphene based gas sensor. (B) Temperature dependence

of sensor resistance. (Inset of B) Design sensor.

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Figure 6. Response to different NO2concentrations measured at (A) 100, (B) 150, and (C)

200 °C. Response to different concentrations of NH3 measured at (D) 100, (E) 150, and

(F) 200 °C.

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Figure 7. (A) Sensor response and (B) recovery time as a function of working

temperatures for different NO2 concentrations. (C) Stability and reusability of graphene-

based sensor.

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full-layer controlled synthesis and transfer of large-scale monolayer graphene for nitrogen dioxide...

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