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Current Applied Physics 12 (2012) 1017e1022

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Current Applied Physics

journal homepage: www.elsevier .com/locate/cap

Chemical vapor sensing properties of graphene based on geometrical evaluation

Sukju Hwang a, Juhwan Lim a, Hyung Goo Park a, Whan Kyun Kim a, Duck-Hwan Kimb, In Sang Song b,Jae Hun Kim c, Seok Lee c, Deok Ha Woo c, Seong Chan Jun a,*

a School of Mechanical Engineering, Yonsei University, Seoul 120 749, Republic of Koreab Samsung Advanced Institute of Technology, Yongin 446 712, Republic of KoreacKorea Institute of Science and Technology, Seoul 130 650, Republic of Korea

a r t i c l e i n f o

Article history:Received 30 August 2011Received in revised form18 December 2011Accepted 26 December 2011Available online 17 January 2012

Keywords:GrapheneMechanical cleavageGas sensor

* Corresponding author. Tel.: þ82 2 2123 5817; faxE-mail address: scj@yonsei.ac.kr (S. Chan Jun).

1567-1739/$ e see front matter � 2012 Elsevier B.V.doi:10.1016/j.cap.2011.12.021

a b s t r a c t

Graphene is a promising candidate for chemical vapor sensing. We prepared graphene sheets from highlyoriented pyrolytic graphite through mechanical cleavage in order to investigate their responses to NH3

and NO2 as electron donors and acceptors, respectively. We investigated how the geometric character-istics of graphene, such as length-to-width (L/w) ratio and number of layers, affect chemical sensingproperties at room temperature and ambient atmosphere. In this study, the L/w ratio of an individualgraphene sheet, which is related to graphene conductivity, dominated the NH3 sensing characteristics,while the number of graphene layers had no significant effect. We also studied the effects of variousthermal treatments on graphene sensitivity and recovery time in an ambient atmosphere. This studyconfirms the effects of geometry, operation temperature and gas concentration on the NH3 and NO2

sensing performances of graphene.� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Graphene is a two-dimensional monolayer of carbon atoms [1,2]that has superior mechanical [3], thermal [4], and electrical prop-erties [5] than other materials. Graphene is a possible option for useas a high-quality vapor chemical sensor due to its long mean-freepath [6], minimal electron scattering [5], and high electronmobility [7]. The operational principle of graphene sensing is basedon changes in the electrical conductivity induced by the absorbanceor desorbance of a gas molecule on the surface.

A chemical vapor sensor should resolve various chemicals undermoderate conditions with detection sensitivities up to the quantumlimit. The large surface area of two-dimensional graphene impartssufficient sensitivity to detect a single electron [8]. The two-dimensional structure also permits a rapid response, allowinggraphene to detect electron donor or acceptor gas moleculesadsorbed onto its surface [8], a characteristic which makes it suit-able for use as a solid-state sensor [9]. Adsorbed gas moleculeschange the local carrier concentration in graphene, causing step-like changes in the resistance [2] or the capacitance [10]. Further-more, graphene has high electrical conductivity and an sp2-bondedhexagonal structure with almost no defects [11]. Graphene has lowthermal noise due to fluctuations, and the thermal motions of

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All rights reserved.

charges make it an exceptional gas sensor material with formidablestability. These characteristics can initialize chemical sensing,which involves heating, annealing, and recycling. Initialization isa pivotal performance factor for commercial applications.

Previously, the chemical sensing ability of carbon nanotube(CNT) networks has been used to detect chemical molecules[12e15]. Furthermore, a nano-sheet [16] and a graphene oxide (GO)[17,18] sensor were shown to be able to detect various chemicals.The GO sensing device performed better than did the CNT networkdevices [17] because the oxides in GO induce more electron scat-tering and diffusive transport, so thermal treatment effects are lessaffected for electron transport in quantized nano-scaled devices. Inour study, detectionwith graphene is based on changes in electricalconductivity induced by absorbance or desorbance of a gas mole-cule on the surface. We present the effects of the number of layersand the length-to-width (L/w) ratio of graphene on its sensingproperties for target gases at room temperature and atmosphericpressure. Mono-, bi-, and tri-layers of graphene with almost thesame L/w ratio showed consistent responses to NH3. Decreasing theL/w ratio improved the sensing characteristics of graphene bychanging the conductivity. The effects of various thermal treat-ments on graphene were also investigated. The graphene sensordescribed in this study had excellent sensing capability due to lownoise, high signal and a strong electrical response to chemicalvapor. In this study, we verified that graphene is a superior candi-date for use in chemical vapor sensing.

S. Hwang et al. / Current Applied Physics 12 (2012) 1017e10221018

2. Experimental

2.1. Sensor device fabrication

Methods to produce graphene include micromechanicalcleavage [5], graphite oxide exfoliation [19], and chemical vapordeposition growth [20]. We used mechanical cleavage from highlyoriented pyrolytic graphite (HOPG) to produce graphene withhigh crystal quality. The graphene was then transferred to anoxidized silicon substrate (500 nm oxide layer) with a pre-deposited alignment through mechanical exfoliation. Once thetarget graphene was selected, metal electrodes for the chemicalsensor were fabricated. To precisely define the two graphene ends,standard electron beam lithography methods were applied usingpolymethylmethacrylate as the electron beam resist (MicrochemCorp., C4 950). After development, Au was deposited carefully toa 30 nm thickness to achieve tight binding between the carbon andAu molecules and to minimize the contact resistance. Fig. 1(a)shows graphene with Au electrodes at both ends. To preparedcontact electrodes with a chip-holder, larger metal electrodescomposed of a Cr/Au bilayer (10 nm/400 nm) were patterned usingphotolithography.

Fig. 1(b) shows an AFM image of graphene. The total graphenethickness was measured to determine the number of layers. Toachieve amore precisemeasurement, we used Raman spectroscopyat 633 nm as shown in Fig. 1(c). Raman spectra are unique in thatthe 2D peak can distinguish between one, two, and several layers.We can define the layers based on the G (blue line) and 2D (greenline) peaks. A linear 2D peak for a monolayer becomes nonlinearand is shifted as the thickness increases [21]. This observationconfirmed the number of layers in our graphene samples.

Fig. 1. (a) SEM image of graphene embedded on a silicon wafer with Au electrodes. (b)AFM image of graphene before electrode deposition. The inset shows the graphenethickness. (c) Raman spectra of mono- (black), bi- (violet), and tri- (orange) layergraphene obtained through mechanical cleavage from HOPG. All spectra weremeasured at 633 nm. (For interpretation of the references to colour in this figurelegend, the reader is referred to the web version of this article.)

2.2. Measurements

The graphene device described above was connected to thefabricated chip-holder, guaranteeing accuracy and reliability, by Alwires in order to decrease the resistance. This assembly was loadedinto a gas chamber for exposure to the target gases.

Fig. 2 shows photographs of the chip-holder with a graphenedevice and a schematic of the graphene device on the SiO2/Sisubstrate connected to the Au electrodes on the chip-holder. Theinset shows an enlargement of the graphene device. Variations inresistance for the samples were measured using a semiconductorparameter analyzer (Keithley 4200-SCS). Gas flow was controlledby a mass flow controller (MFC), and the summed N2 (used as thepurging gas) and target gas (also called carrier gas) flow rates werelimited to 300 ml/min. NH3 gas with a purity of 99.999% and NO2gas with a purity of 99% were used in measurement. The gas wasflowed in quartz tube for measurement which was opened in bothsides as Fig. 2. Though N2 was used for carrier gas, the open systemmeasurement presented far similar condition as in an air system.Fig. 3(a) shows the response to 200 ml/min NH3 and 200 ml/minNO2. The resistance change is defined as [(R � R0)/R0]*100 (%),where R0 is the original sensor resistance and R is the resistancewhen exposed to the target gas.

The resistance remained constant in the presence of N2,increased with NH3, and decreased with NO2. In the presence ofeither target gas, the resistance changed rapidly due to absorptiononto the graphene [9]. Upon exposure to NH3, the gas acted as anelectron donor, shifting the valence band of graphene away fromthe Fermi level, resulting in hole depletion and increased resis-tance. NO2, on the other hand, accepted electrons from graphene,created holes, and decreased the resistance. Based on this mecha-nism, gas sensors can identify gases using the conductance change.

Graphene is a comparative carbon material for gas sensorsbecause it has excellent conductivity and experiences largeconductance changes in the presence of only a few electrons. Theinset graph of Fig. 3(a) shows measured I/V curves after joining thegraphene devices and chip-holder. The relationship betweencurrent and voltage at saturation are clearly represented for NH3and NO2.

Fig. 2. Photographs of a chip-holder with graphene, a schematic of the SiO2/Si substrate, an optical image of the graphene device, and a schematic of the measurement system.

Fig. 3. (a) Resistance changes when NH3 and NO2 gases were absorbed into graphene.(Inset) I/V curves of N2, NH3 and NO2 after joining the graphene and the chip. (b) NH3

gas sensing results for a graphene gas sensor. The experiment was performed byalternately flowing N2 and NH3 gases through the chamber. (c) Results of graphene gassensing in alternate flows of N2 and NO2 gases.

S. Hwang et al. / Current Applied Physics 12 (2012) 1017e1022 1019

3. Results and discussion

3.1. Comparison of the sensing properties for NH3 and NO2

Alternating the flow of target and purge gases determined thesensing signal of the graphene gas sensor. As shown in Fig. 3(b),flowing NH3, a target gas, into the chamber increased the grapheneresistance, whereas N2, the purge gas, decreased the grapheneresistance. The experiment consisted of the introduction of N2 for300 s, the target gas for 300 s, and purging for 300 s. The timeintervals of all sensing signals were not clearly distinguished at200 ml/min and 100 ml/min flow rates. Fig. 3(c) shows the sensingsignal when the experiment was performed as above using NO2 asthe target gas. The graphene resistance returned to baseline wheneither target gas was purged.

3.2. Geometrical effects of graphene on the sensing properties

Fig. 4 compares the responses of mono-, bi- and tri-layers ofgraphene sheets with similar L/w ratios for NH3 at 200 ml/min. Theaverage samples lengths andwidths were 2.96 mmand 10.43 mm formonolayers, 3.73 mm and 12.89 mm for bilayers and 2.88 mm and10.25 mm for tri-layers, respectively. The responses of all sheets toNH3 were similar, as shown Fig. 4(a). Fig. 4(b) shows the responsetime and the saturation rate of resistance change for graphenesheets with different layers. Response time was defined as R reachto 63.2% of Ro, following the response curve R ¼ Ro*[1-exp(-t/tau)].In response curve for t ¼ tau, the sensor response level reach to63.2% of initial resistance. The response time for monolayer gra-phenewas 20 s, and the timewas less than 13 s for bi- and tri- layergraphene. The rate of resistance change increased slightly with anincreasing number of layers (monolayer: 5.56%, bilayer: 5.98%,trilayer: 6.16%).

Fig. 5(a) shows changes in response of NH3 at 200 ml/min uponby varying the L/w ratio and surface area.We used bilayer grapheneto measure changes in response due to geometry. The S #1, #2, #3,#4 and #5 had similar areas, as shown in Table 1; however, the L/wratio increased with increasing sample number. Although graphenesamples measured in this study had almost the same amount ofarea for gas absorbance, S #1 responded better than did the othersamples. Therefore, the graphene response properties to NH3 weredominated by the ratio of length-to-width rather than by thenumber of layers. In this measurement, graphene with lower L/wratio showed better sensitivity to target gases. This result may be

Fig. 4. (a) Comparison of responses among mono-, bi- and tri-layers of graphene withNH3 flowing at 200 ml/min. (b) Response time and the rate of resistance change due tothe number of layers.

Fig. 5. (a) The change in response due to L/w ratio and surface area of graphene withNH3 flowing at 200 ml/min. (b) Response time and the saturation degree of resistancevariation within measured samples.

Table 1Geometries of bilayer graphene samples including width, length, L/w ratio andsurface area.

S # 1 S # 2 S # 3 S # 4 S # 5

Width (mm) 12.04 12.89 14.77 5.49 4.65Length (mm) 3.76 3.73 3.11 9.04 10.04L/w 0.21 0.29 0.31 1.65 2.16Surface area (mm2) 45.95 48.14 45.23 49.59 46.69

S. Hwang et al. / Current Applied Physics 12 (2012) 1017e10221020

explained by graphene’s conductivity, which was predicted quan-titatively that it will increases as the L/w ratio decreases [22e24].Fig. 5(b) shows the response time and the rate of resistance changein our graphene devices. Even though the rate of resistance changedecreased linearly as the L/w ratio increased, response time wasless affected.

Fig. 6 shows that adsorbed NH3 gas onto a graphene sensorchanged the resistance, or the degree of response. As the target gasflow rate increased from 100 to 300 ml/min, the ratio of resistancechange increased. The ratios increased to 2.5% at 100 ml/min, 5% at200 ml/min and 8% at 300 ml/min, as shown Fig. 6(a). The flow rateof 300 ml/min had a higher resistance change than those of thelower flow rates. The flow rate also determined the resistivitychange. Fig. 6(b) shows the response time and the rate of resistancechange due to NH3 flow rate. Response time decreased linearly withincreasing target gas concentration.

3.3. The effects of thermal treatments on sensitivity and recovery

Fig. 7 shows graphene’s sensor performance under thermaltreatment with various temperatures up to 550 �C. Fig. 7(a) showsthe temperature-dependent recovery of graphene sensor fromRT(room temperature) to 150 �C. The recovery for NH3 at roomtemperature need longer time with low signal-to-noise ratio, butdidn’t recover to initial state. Heating the graphene device in N2 at100 �C and 150 �C decreased the recovery time, and the signal-to-noise ratio of the conductance increased with increasing temper-ature. This is probably due to a desorption increase with thermaltreatment. The inset of Fig. 7(a) shows that the recovery timedecreases as temperature increases up to 550 �C, almost linearly.

Thermal treatments reduce graphene resistance. Fig. 7(b) showsthe response change with increasing temperature from RT to150 �C, where there is a clear trend toward larger responses at hightemperatures. We observed a 5% change at RT and an almost 45%change at 150 �C. Increased responses seem to be related to elec-tron transfer and the nature of the surface interaction between thegas and graphene. The initial graphene resistance used in this studywas 224 kU. After annealing in N2 at 50 �C and 300 ml/min for30 min, the resistance of the graphene device decreased slightly.The resistance showed a larger decrease during annealing at 150 �C,where it decreased dramatically to 29 kU, as shown in Fig. 7(c).Fig. 7(d) presents the sensitivity changes due to thermal treatment.The sensitivity of measurement 1 with low L/w ratio and on PCBchip was presented by green dot and follows the scale bar on left,while that of measurement 2 with high L/w ratio and not on PCBchip was indicated by green dot and follows the scale bar on right.

Fig. 6. (a) Response of graphene to flow rates. (b) Response time and the saturationrate of resistance change within NH3 flow rate.

Fig. 7. (a) The response change with increasing temperature from at 300 ml/min. (b) Tmeasurements of a graphene device with successive multi-step heating. (d) The sensitivity

S. Hwang et al. / Current Applied Physics 12 (2012) 1017e1022 1021

The two measurements showed different sensitivity due todifferent L/w ratio and different chip contact. While the optimalPCB chip with better contact starts to melt at 180 �C, so measure-ment with higher temperature cannot be conducted with it. Thesensitivity of sensor linearly increased with higher thermal treat-ment as shown in Fig. 7(d). In addition, blunt and slow response ofthe sensor on 100 �C gets sharper and faster in 550 �C, as shown intwo insets of Fig. 7(d). The high temperature measurement is themost important factor governing gas sensing performance. Fasterresponse and recovery time, high signal-to-noise ratio, highersensitivity and lower residue for NH3 with higher temperaturesuggest the high potential of graphene for use in sensingapplications.

4. Conclusions

In this study, we confirmed that graphene is a promisingcandidate for use in chemical vapor sensors. Gas sensing wasmeasured by flowing NH3 and NO2 over the graphene at atmo-spheric pressure and room temperature. The electrical resistivity ofgraphene increased or decreased in response to NH3 and NO2adsorbances on the surface and its ability to act as an electrondonor or acceptor, respectively. The response to NH3 was domi-nated by L/w ratio rather than the number of graphene layers. Thesaturation rate of resistance change increased as the L/w ratiodecreased. The on-off response of graphenewas sensitive to the gasflow rate, and the sensitivity increased and recovery timedecreased as temperature increased. We also observed bettersensor performance as temperature increases up to 550 �C, such aslinear increasing of sensitivity and linear decreasing of recoverytime. Future research will focus on the gas sensing ability ofchemically derived graphene to contribute to commercial

he response change with increasing temperature from at 300 ml/min. (c) Ids � Vds

change with increasing temperature from RT to 550 �C at 300 ml/min.

S. Hwang et al. / Current Applied Physics 12 (2012) 1017e10221022

applications and on increasing the understanding of the geomet-rical effects on the chemical sensing properties.

Acknowledgments

Sukju Hwang and Juhwan Lim have equally contributed to thiswork. This work was partially supported by Samsung AdvancedInstitute of Technology (2010-8-2239), the Priority ResearchCenters Program (2009-0093823), the Pioneer Research CenterProgram (2010-0019313), and the Basic Science Research Program(2011-8-0856) through the National Research Foundation of Korea(NRF) funded by the Ministry of Education, Science and Technology(MEST) of the Korean government.

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