Synthesis of large-area graphene on molybdenum foils by

Synthesis of large-area graphene on molybdenum foils by chemical vapor deposition

Yuanwen Wu a ,Guanghui Yu a ,*,Haomin Wang a ,Bin Wang a ,b ,Zhiying Chen a ,Yanhui Zhang a ,Bin Wang a ,Xiaoping Shi a ,Xiaoming Xie a ,Zhi Jin c ,Xinyu Liu

State Key Laboratory of Functional Materials for Informatics,Shanghai Institute of Microsystem and Information Technology,Chinese Academy of Sciences,865Changning Road,Shanghai 200050,People’s Republic of China b

School of Physics and Optoelectronic Technology,Dalian University of Technology,2Linggong Road,Dalian 116024,People’s Republic of China c

Microwave Devices and Integrated Circuits Department,Institute of Microelectronics,Chinese Academy of Sciences,3West Beitucheng Road,Beijing 100029,People’s Republic of China

A R T I C L E I N F O Article history:

Received 5January 2012Accepted 2July 2012Available online 7July 2012

A B S T R A C T

We report the synthesis of large-area graphene ?lms on Mo foils by chemical vapor depo-sition.X-ray diffraction indicates that the dissolution and segregation process governs the growth of graphene on Mo foils.Among all processing parameters investigated,the cooling rate is the key one to precisely control the thickness of graphene ?lm.By optimizing the cooling rate between 1.5and 10°C/s,we managed to achieve graphene ?lms ranging from mono-to tri-layer.Their uniformity and thickness were con?rmed by Raman spectroscopy and optical measurements.The carrier mobility of ?lms reaches as high as 193cm 2V à1s à1.Our experiments show that the Mo substrate has the similar simplicity and large tolerance to processing conditions as Cu.

1.Introduction

Graphene,a two-dimensional hexagonal network of sp 2hybridized carbon atoms,offers many remarkable properties such as high carrier mobility,current capability,mechanical strength,and in-plane thermal conductivity [1].It is one of the most promising materials for applications in ?exible trans-parent conductive thin ?lm [2,3],high frequency transistors [4],hydrogen storage cells [5],and integrated circuit [6].There are different synthesis methods including mechanical exfolia-tion of highly oriented pyrolytic graphite [7],thermal evapora-tion on SiC [8],reduction of graphene oxide [9],unzipping of carbon nanotubes (CNTs)[10]and chemical vapor deposition (CVD)[11].The CVD growth of graphene on transition metals like Cu [11,12],Ni [13,14],etc.is considered the most ef?cient

way for large-scale fabrication.Graphene was deposited on Cu in a surface adsorption process [15]while graphene on Ni grows with the dissolution and segregation process [16].More recently,CVD on Cu foils has been applied to produce large-scale homogeneous mono-[11]or bi-layer [12]graphene by the self-limiting growth owing to Cu’s low carbon solubility (0.006weight%at 1000°C).In comparison,CVD on Ni with the high carbon solubility (0.28weight%at 1000°C)has been studied for a long time and it was found that the cooling rate is critical in suppressing formation of multi-layer graphene on Ni [2,16],but it still remains a challenge to control the car-bon precipitation precisely and obtain uniform graphene.In order to boost graphene research toward different industrial applications,new methods or substrates for growing large-size

*Corresponding author:Fax:+862152419931.E-mail address:ghyu@https://www.360docs.net/doc/6c1951638.html, (G.Yu).

uniform graphene with precise layer control are urgently required.

So far,very few experiments have been done on Mo,while early studies show that Mo might act as an effective catalyst for CNTs [17].Recently,Dai et al.reported the design of a bin-ary metal alloy for mono-layer graphene [18].The key is to uti-lize a Mo component in a binary alloy to control the carbon precipitation.As such,we think that Mo could be an alterna-tive toward well-controlled large-scale graphene.According to the binary phase diagram of Mo and C [19](see Fig.S1in Supplementary data),the solubility of C in Mo at 1000°C is about 0.0026weight%.This value is smaller than that in Cu and Ni.The ultra-low C solubility in bulk Mo could lead to the self-limiting growth of graphene ?lms.In Mo,carbon sol-ubility exhibits a linear relationship with temperature rang-ing from 0to 1000°C.The sharp slope indicates that the C solubility in Mo varies a little within this temperature range.A ?nite amount of carbon can segregate from the Mo bulk during cooling,and could not form thicker ?lm.Therefore in principle,we can control the number of graphene layers on Mo precisely by modifying the cooling rate.

In this research,we demonstrate that Mo is another suit-able substrate for the CVD growth of graphene.The thickness of as-grown graphene ?lms can be conveniently controlled by adjusting the cooling rate.This work presents an ef?cient and well-controllable way to synthesize homogeneous graphene in different thicknesses.

2.Experimental

The growth experiments were carried out in a vertical quartz-tube furnace under ambient pressure.The Mo foils (99.99%purity,100l m thick)were ?rstly annealed under a ?ow of Ar (300sccm)/H 2(100sccm)for 10min at 1000°C in order to remove the surface oxides.Methane gas in ?ow rates (from 50to 200sccm)was feeding into the reaction chamber during the growth stage.The samples were then cooled down from 1000to 500°C with controlled cooling rates.In the graphene transfer process,a thin layer of poly(methyl methacrylate)(PMMA)was spin-coated onto the as-grown graphene and the Mo foil was etched away in 1M FeCl 3aqueous solution.After the PMMA/graphene layer was manually moved onto arbitrary substrates such as a SiO 2/Si substrate and a quartz plate,the PMMA layer was removed by exposure to liquid ace-

tone.Finally,the graphene samples were rinsed in isopropyl alcohol and de-ionized water,and then dried in nitrogen ?ow.Further analysis was performed using X-ray diffraction (XRD)(D/MAX-2200/PC,Rigaku X-ray Diffractometer,Cu K a radia-tion,k =1.54A

?),Raman spectroscopy (Raman Microscope,Thermo DXR,532nm),optical transmittance and optical microscopy (Leica DM4000M).

3.Results and discussions

Fig.1a shows an XRD result of the Mo foil before any process-ing.The pattern reveals a polycrystalline structure on Mo sur-face.T wo diffraction peaks located at 58°and 73°correspond to the two crystalline planes of (200)and (211)of the body centered cubic Mo (PDF 42-1120,a =0.3147nm),respectively.The formation of peak near 2h =29°is due to the harmonics of the strongest peak Mo (200)at 2h =58°.Fig.1b shows a typ-ical XRD pattern of the Mo foil after growth.The appearance of Mo (110)peak and the disappearance of Mo (211)indicate a structural reconstruction,while the Mo 2C peaks prove that C-atoms have diffused into the metal.The existence of car-bide phase Mo 2C can slowdown the precipitation of C onto Mo surface during synthesis process and it helps the forma-tion of thinner and uniform graphene.

Fig.2(a–c)shows the Raman spectra of graphene synthe-sized under different growth conditions corresponding to methane ?ow rates,growth time and cooling rates,respec-tively.The characteristic bands of graphene at $1350cm à1(D-band),$1590cm à1(G-band)and $2690cm à1(2D-band)can be observed from all samples.It is observed that the band intensity ratio of I 2D /I G is inversely proportional to the number of graphene layer.It can be seen from Fig.2a and b that the methane ?ow rate and the growth time have very little in?u-ence on the graphene formation,and the ratio of I 2D /I G %1in each spectrum may indicate bi-or tri-layer graphene was syn-thesized.The I 2D /I G ratio in Fig.2c varies from 1to 2when the cooling rate was adjusted from 1.5to 10°C/s,suggesting that thinner graphene ?lm was synthesized.The I 2D /I G ratio in-creases with the cooling rate,and no graphene signal was ob-served for the cooling rate 60.5°C/s,which indicates that the cooling rate is directly related to the number of as-grown graphene layers.These results were subsequently con?rmed by their optical transmittance.As shown in Fig.2d,their 350–750nm transparencies are 97.41%(10°C/s),95.65%(3°C/

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s)and 92.17%(1.5°C/s)corresponding to mono-,bi-and tri-layer graphene ?lm,respectively.The slow cooling rate below 0.5°C/s trapped the whole C-atoms in Mo bulk with suf?cient time to form stable Mo 2C and caused not enough C-atoms segregated from inside;while the medium cooling rate of 1.5or 3°C/s resulted in a considerable amount of C-atoms to move onto the surface and assemble graphene.In our experiment,only the comparatively high cooling rate of 10°C/s allowed a ?nite amount of C-atoms segregation and ?-nally led to thickness self-limitation.Therefore,the existence of carbide phase Mo 2C and the observed major effect of cool-ing rates proved that the growth mechanism of graphene on Mo substrate is the dissolution and segregation process,which is similar to the case of Ni.It is worth mentioning that some literatures reported that the growth on Cu depends on the surface adsorption and catalyst because of its low carbon solubility [11,14].The explanation cannot be applied to the case of Mo,even though Mo has the lowest carbon solubility.The possible reason is mainly related to the different carbon af?nity [20].The much lower carbon af?nity in Cu restrains the carbon atoms to recompose and form graphene only on the surface,as re?ected by the fact that no carbide phases in copper was detected [21].

Generally,D-peak is always utilized to analyze the qual-ity of graphene ?lms.The D-peak is present in all samples,as shown in Fig.2(a–c).The phenomenon implies the pres-ence of either structural defect (e.g.grain boundary [22],vacancies [23])or strongly-bound adatoms [24]in graphene samples.The defects serve as the inter-valley scatters in electron–phonon interaction.In order to analyze the amount of defects,we calculated the integrated intensity ratio between D and G band (I D /I G ),which probes the in-

600

700

Wavelength (nm)

varying from 50to 2005228

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ter-valley scattering rate of defects and boundaries.In Fig.3,it is found that the I D /I G changes little through meth-ane ?ow rates,growth time and cooling rates.This suggests that defects and boundaries are not sensitive to the pro-cessing conditions.The scattering centers for D-band may not be spots defects,as they can be healed by annealing [25,26].The scattering centers in these samples are almost grain boundary.The I D /I G value varies from 0.8to 0.9in the CVD graphene samples.We can estimate the average scatter distance by the empirical relation established for determining the crystallite size L a of nanographite from Ra-man spectroscopy [27,28]:

L a ?e2:4?10à10Tk 4L eI D =I G Tà1

;

e1T

where k L is the wavelength of the excitation laser.The for-mula yields L a =22.6nm for I D /I G =0.85and k L =532nm.The value of L a contains information about the average scattering distance.In nanographitic materials,L a refers to the crystal size.While in disordered graphene,it represents the average distance of the randomly distributed defects.Although Ra-man spectroscopy at k L =532nm probes scattering rates at energies one order of magnitude larger than typical Fermi le-vel in transport experiments,one can expect that L a is equal to the transport mean free path l for carriers in the large-an-

(c)

(a)

100015002000I n t e n s i t y (a . u .)

G 2D

(b)

20 μm

5 μm

2600

2700

2800

EXP.FIT

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gle scattering[29].In this way,one could ef?ciently estimate the carrier mobility through Raman peaks.For Dirac fermi-ons,one can write carrier mobility:

l?láe m F

E F

;e2T

where v F%106m/s is the Fermi velocity and e%1.6·10à19C is the elementary charge.Assuming the electrons and holes are generated at k L=532nm,their energies locate at E F=1.17eV with l%L a=22.6nm.The carrier mobility can be derived: l%193cm2Và1sà1.It is necessary to point out that the carrier mobility in graphene?lms strongly depends on the dimension of graphene crystal domain,whose continuous growth is lim-ited by the grain size of Mo substrate.To obtain large Mo grains and high quality of graphene,temperature as high as2623°C (melting point of Mo)is necessary.Limited by the furnace de-sign,we can only anneal the samples below1100°C and no obvious change of Mo grain was observed.In our recent work, higher quality graphene?lms were obtained on epitaxial sin-gle-crystal Mo(110)thin?lms on basal-plane sapphire via CVD(Fig.4).The results proved that removing Mo grain boundaries would signi?cantly improve the quality of graph-ene on Mo.We also noticed that defect-free graphene could be obtained on other single-crystal metal?lms such as Cu [30]and Pt[31],which revealed that there still exists the possi-bility of further improving the quality of graphene on Mo.

Raman mapping was conducted to evaluate the uniformity of graphene?lm on Mo foil.A representative Raman map of the I2D/I G ratios over a bi-layer graphene?lm is given in Fig.5a.The data clearly demonstrate the high uniformity of graphene with the most area in green color indicating the I2D/I G values from0.9to1.1.Fig.5b is the optical image of the bi-layer graphene transferred onto a SiO2(100nm)/Si sub-strate,which also con?rms the?lm uniformity.(The investi-gations of mono-and tri-layer graphene are shown in Fig.S2in Supplementary data).Fig.5c shows the Raman spec-tra for the bi-layer graphene in Fig.5b.The D,G,D0and2D peak locates at1348,1590,1625and2686cmà1,respectively. The intense D-peak and the appearance of D0-peak reveal that some defects were introduced into graphene during the trans-fer process.

4.Summary

Graphene?lms on Mo foils were successfully fabricated by CVD method.The graphene?lms were deposited at1000°C for5–30min at different CH4?ow rates and treated at differ-ent cooling speeds.We found that the number of graphene layer is only sensitive to the cooling rate.The carrier mobility of these?lms could reach about193cm2Và1sà1.XRD reveals that the growth is governed by the dissolution and segrega-tion mechanism.This work provides an ef?cient and well-controllable method to synthesize homogeneous large-scale graphene for electronic and optical applications.

Acknowledgements

This work was supported by National Science and Technology Major Project(Grant No.2011ZX02707)and National Natural Science Foundation of China(No.61136005).The authors would like to thank Dr.Hui Bi and Chuanzheng Yang for assis-tance in transmittance and XRD measurements.

Appendix A.Supplementary data

Supplementary data associated with this article can be found, in the online version,at https://www.360docs.net/doc/6c1951638.html,/10.1016/j.carbon. 2012.07.007.

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