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Synthesis of multi-layer graphene films on copper tape by atmospheric pressure chemical

vapor deposition method

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2013 Adv. Nat. Sci: Nanosci. Nanotechnol. 4 035012

(http://iopscience.iop.org/2043-6262/4/3/035012)

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Adv. Nat. Sci.: Nanosci. Nanotechnol. 4 (2013) 035012 (5pp) doi:10.1088/2043-6262/4/3/035012

Synthesis of multi-layer graphene films oncopper tape by atmospheric pressurechemical vapor deposition methodVan Tu Nguyen1, Huu Doan Le2, Van Chuc Nguyen1,Thi Thanh Tam Ngo1, Dinh Quang Le1, Xuan Nghia Nguyen1

and Ngoc Minh Phan1

1 Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc VietRoad, Cau Giay District, Hanoi, Vietnam2 Hanoi National University of Education, 136 Xuan Thuy Road, Cau Giay District, Hanoi, Vietnam

E-mail: chucnv@ims.vast.ac.vn

Received 7 January 2013Accepted for publication 4 June 2013Published 25 June 2013Online at stacks.iop.org/ANSN/4/035012

AbstractGraphene films were successfully synthesized by atmospheric pressure chemical vapordeposition (APCVD) method. Methane (CH4) gas and copper (Cu) tapes were used as acarbon source and a catalyst, respectively. The CVD temperature and time were in the range of800–1000 ◦C and 10 s to 45 min, respectively. The role of the CVD temperature and time onthe growth of graphene films was investigated in detail via scanning electron microscopy(SEM) and Raman spectroscopy techniques. The results of SEM images and Raman spectrashow that the quality of the graphene films was improved with increasing of CVD temperaturedue to the increase of catalytic activity.

Keywords: graphene, atmospheric pressure chemical vapor deposition, Cu tape

Classification number: 5.15

1. Introduction

Graphene is a two-dimensional material composed of carbonatoms arranged in a hexagonal atomic structure. Its theoreticalthickness is about 0.34 nm [1, 2]. However, graphene hasa number of properties which make it interesting forseveral different applications. It is the thinnest, mechanicallyvery strong, transparent and flexible conductor [3]. Theelectron mobility of graphene is very high at roomtemperature (15000 cm2 V−1 s−1) which makes this materialvery interesting for electronic high- frequency applicationssuch as high-frequency transistor and modulator [4]. Sincegraphene is the transparent electrode, it may be widely usedin liquid crystal display (LCD), organic light-emitting display(OLED), where it can replace the rather fragile and expensiveindium–tin–oxide (ITO) [5]. Established on large specific

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surface area, graphene is also used in gas detectors andbiosensors [6, 7]. Besides, new types of composite materialbased on graphene with great strength and low weight couldalso become interesting for use in satellites and aircraft [8].

Most of these promising applications require the growthof graphene films in large areas on a suitable substrate.Graphene can be prepared by many methods. The firstmethod is micromechanical cleavage from graphite. Thismethod can only produce very small graphene films [9].Recently, graphene has been produced by exfoliation anddispersion of graphite in organic solvents but the yield is verylow [10]. The chemical exfoliation is an effective methodto produce graphene film in large quantities [11]. However,chemical exfoliation involves complex chemical processesusing heavily functionalized organic groups. Another way isepitaxial growth of graphene on silicon carbide (SiC) [12].However, graphene grown on SiC substrates is difficult totransfer to other substrates. At present, thermal chemicalvapor deposition (CVD) is one of the major methods tofabricate graphene [13]. With this method, the area and shapeof graphene films are variable. In the thermal CVD process, at

2043-6262/13/035012+05$33.00 1 © 2013 Vietnam Academy of Science & Technology

Adv. Nat. Sci.: Nanosci. Nanotechnol. 4 (2013) 035012 V T Nguyen et al

Figure 1. Plot of growth parameters for synthesis of graphene onCu substrate.

high temperature, hydrocarbon gases are decomposed, whichleads to the formation of carbon atoms on the metal surface.These carbon atoms segregate and form graphene film.

In this work we report on APCVD method to growlarge-area graphene films on copper (Cu) tape. There are afew reasons for the selection of Cu metal as a substrate:(i) carbon has a low solubility in Cu substrate, (ii) absenceof a copper carbide and (iii) chemical etchant selectivityto graphene. Several parameters of APCVD such as timeand temperature, which play important roles in grapheneformation, were studied.

2. Experiment method

The graphene films were synthesized by thermal CVDmethod under high temperature (850–1000 ◦C) in argon (Ar)environment (1000 sccm). The Cu tapes with a thicknessof 35 µm and a size of 0.5 cm × 0.5 cm were used assubstrates for graphene-films synthesis process. To reducethe native copper oxide and to facilitate Cu grain growthon the Cu tape surface, the samples were annealed at CVDtemperature for 30 min in a flow of Ar and hydrogen (H2,300 sccm). After 30 min, a flow of methane (CH4, 30 sccm)was introduced for growth process (figure 1). The time forthe CVD process ranged from 10 s to 45 min. After a presetgraphene growth time, the samples were cooled rapidly undera flow of Ar (1000 sccm). The grown graphene films were thencharacterized using SEM, Raman spectroscopy and atomicforce microscopy (AFM) techniques.

3. Results of synthesis of graphene

3.1. Effect of growth temperature

One of the critical factors in synthesis of graphene isgrowth temperature because high energy is necessary todissociate methane so that it reacts with the catalyst. Tostudy the influence of the growth temperature on the synthesisof graphene, we executed the CVD process at differenttemperatures 850, 900, 950 and 1000 ◦C for the same time andgas flow (30 min and Ar/H2/CH4 = 1000/300/30 sccm flowrates). The shape and the size of graphene patches referredto as domains can be imaged by SEM. Figure 2 is the SEMimages which shows the effect of growth temperature on theformation of the graphene while keeping time and gas flowrates unchanged. Figure 2 shows that the density and sizeof graphene domains increase with increasing temperature.

Figure 2. SEM images of the Cu tape’s surface after CVD processfor 30 min at different temperatures: (a) 850 ◦C, (b) 900 ◦C,(c) 950 ◦C, and (d) 1000 ◦C.

Figure 3. Raman spectra of the graphene film grown on the surfaceof the Cu substrate at different temperatures: (a) 950 ◦C and (b)1000 ◦C.

The graphene grown at 850 ◦C has domains of about100–200 nm size. Meanwhile, the graphene grown at900 ◦C produces domains of about 1–3 µm size. At highertemperature the domains spread and at growth temperatures of950 and 1000 ◦C (figures 2(c) and (d)) graphene films coveredthe whole surface of the Cu tape. Borders of the graphenefilms appear clearly on the surface of the Cu tape. Theseresults are due to the increase of the precursor activity at hightemperature.

It is known that Raman spectroscopy is a powerful, yetrelatively simple method to characterize the thicknessand crystalline quality of graphene layers [14, 15].Raman spectroscopy was performed under excitationlaser (λ = 632.8 nm) on the CVD grown graphene. Figure 3shows Raman spectra of the graphene film grown on theCu substrate surface at CVD temperatures of 950 and1000 ◦C. Raman spectra of two samples grown at 950 and1000 ◦C show three Raman peaks at 1333 cm−1 (D band),at 1582 cm−1 (G band) and at 2660 cm−1 (2D band). TheD band indicates the defective and impure structure ofgraphite sheets. The G band indicates the order and purity

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Adv. Nat. Sci.: Nanosci. Nanotechnol. 4 (2013) 035012 V T Nguyen et al

(a) (b) (c)

(d) (e) (f)

Figure 4. SEM images of graphene films grown on the Cu tape’s surface after CVD process for (a) 10 s, (b) 30 s, (c) 1 min, (d) 5 min,(e) 30 min and (f) 45 min.

structure of graphite sheets. The 2D band is the characteristicpeak of graphene structure. It is known that the ratioI2D/IG of the intensity I2D and IG of the bands 2D and Gis dependent on the number of graphene layers [14, 15].The ratio I2D/IG ∼ 2–3 is for monolayer graphene,2 > I2D/IG > 1 for bilayer graphene and I2D/IG < 1for multilayer one. Based on the ratio I2D/IG of Ramanspectra, it can be concluded that the graphene films grown onthe Cu tape at 950 and 1000 ◦C are multilayers. The graphenefilms grown at 1000 ◦C have higher ratio I2D/IG and lowerratio ID/IG compared to those of the graphene films grownat 950 ◦C. These results show that the CVD temperature of1000 ◦C is the optimum temperature for growing high-qualitygraphene films on the Cu tape surface.

3.2. Effect of growth time

Figure 4 shows SEM images of graphene films grown onCu tape surface with the growth time ranging from 10 s to45 min at 1000 ◦C under flow of CH4 and H2 (CH4/H2 =

30/300 sccm). The dark features in the SEM images infigures 4(a)–(c) correspond to the graphene domains and thebright regions are the bare Cu. It is clear that the graphenedomains grown during 10 s are smaller than that grown during30 s (figures 4(a) and (b)). The formed graphene area increaseswith the growth time increasing. Figure 4(d) indicates thatafter 5 min of growing, the entire surface of the copper foilis covered with graphene. This shows that the carbon atomsactively move in and out of the edges of the graphene sheet tillall the growth domains are linked, forming a solid and stablegraphene sheet.

Figure 5 shows a comparison of the Raman spectra ofthe graphite and the graphene films grown on the surface ofthe Cu with the growth time ranging from 5 min to 45 min.The Raman spectra show that the 2D band of the graphenefilms shifts to short wavelength (2660 cm−1) compared towavelength (2725 cm−1) of the bulk graphite. The ratio ofI2D/IG does not change much. This value changes from 0.42

1000 1500 2000 2500 3000

ID/IG = 0.28

ID/IG = 0.29

ID/I

G = 0.45

ID/IG = 0.16

I2D/ IG = 0.45

I2D/IG = 0.44

I2D/IG = 0.43

I2D/IG = 0.42

1333 1582 2725

2660

spectra of graphite

45 min.

30 min.

15 min.

5 min.

Inte

nsi

ty (

arb

. un

its

)

Raman shift (cm-1)

Figure 5. Comparison of the Raman spectra of the graphene filmsgrown on the surface of the Cu and graphite measured at 632.8 nm.

(for 5 min of growing) to 0.45 (for 45 min of growing). But theratio of ID/IG changes clearly. For growing time of 30 min, theratio ID/IG is lowest (with the value of 0.16). It indicates thatthe quality of the graphene is the best. These results show thatthe number of layers of graphene seemingly do not changeclearly over growth time and the growth time of 30 min isoptimum.

Although the growth time does not affect much thenumber of layers of graphene, the number of layers ofgraphene depends on the morphology of the Cu substrate.At a constant temperature, the CH4 decomposition rate onan ideal flat Cu surface should be uniform. However, the Cutape used for graphene growth is not single crystal, there aremany atomic steps and grain boundaries even after annealing.These sites usually have much higher chemical activationenergy than those of the flat regions of Cu. Therefore,hydrocarbons prefer to decompose on the sites with highactivation energies (such as grain boundaries and surface

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Adv. Nat. Sci.: Nanosci. Nanotechnol. 4 (2013) 035012 V T Nguyen et al

Figure 6. SEM image (a) and Raman spectra (b) of the graphene domains on the surface of the Cu tape after CVD process at 1000 ◦C for30 min.

Figure 7. (a) Photo and (b) AFM image of the graphene film after transferring from the Cu tape to the glass substrate.

steps) to form nucleation centers. Detailed description on theeffect of Cu surface to the growth of high-quality graphenewas also published in [16].

Figure 6(a) shows the SEM image of the graphenedomains on the surface of the Cu tape after CVD process at1000 ◦C for 30 min. There are two different visibly contrastingregions in the SEM image. The regions with A circle and withB circles are few-layer and multilayer structures, respectively.Figure 6(b) shows the Raman spectra measured on twodifferent points of graphene film. The ratio of I2D/IG is 0.84and 0.44 for the few-layer and multilayer graphene film,respectively. The 2D band of the few-layers graphene shiftsto short wavelength (at 2650 cm−1) compared to wavelength(at 2660 cm−1) of the multilayer graphene.

To estimate clearly the thickness of the graphene film,we transferred the graphene film from the Cu tape to glasssubstrate (figure 7(a)) and measured the difference betweenthe height of the graphene film and the glass substrate usingAFM. Figure 7(b) shows the AFM image of the graphenefilm after transferring from the Cu tape to silicon substrate.AFM image indicates that the thickness of the graphene filmwas about 5 nm; this means that the grown graphene films aremultilayer (about 15 layers). The thickness of the grapheneis only 5 nm, so the transmission of the graphene film is very

high. We can see clearly the word of IMS-VAST covered bythe graphene film (figure 7(a)).

4. Conclusion

Multilayer graphene films were successfully synthesized onthe Cu tapes by APCVD method. The quality of the graphenefilms was improved with increasing CVD temperature. Thegrowth time does not much affect the number of layers ofgraphene films. CVD temperature of 1000 ◦C and CVD timeof 30 min are the optimum temperature and time for growinghigh-quality graphene films on the Cu tape, respectively. Thegraphene films were successfully transferred from the Cutape to other substrates by wet etching Cu with a solution ofiron(III) nitrate. This opens a promising way for applicationof graphene films in LCD, OLED electronic devices, in gasdetectors and biosensors.

Acknowledgments

The authors acknowledge financial support from projects ofthe Key Laboratory for Electronic Materials and Devices,IMS (grant HTTD 01.12), Vietnam National Foundation

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Adv. Nat. Sci.: Nanosci. Nanotechnol. 4 (2013) 035012 V T Nguyen et al

for Science and Technology Development (NAFOSTED,code 103.99–2012.15). Besides, this work is supported fromprojects of Director of IMS and Young scientist of VAST2012.

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