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Ultraconformal Contact Transfer of Monolayer Graphene onMetal to Various SubstratesWonsuk Jung, Donghwan Kim, Mingu Lee, Soohyun Kim, Jae-Hyun Kim,and Chang-Soo Han*

W. Jung, M. Lee, S. KimDepartment of Mechanical Engineering, KAISTDaejeon 305701 , Republic of KoreaJ.-H. KimDepartment of Nano-MechanicsKorea Institute of Machinery and Materials (KIMM)Daejeon 305343 , Republic of KoreaD. Kim, C.-S. HanSchool of Mechanical EngineeringKorea UniversityAnam, SeongbukSeoul 136713, Republic of KoreaE-mail: [email protected]

DOI: 10.1002/adma.201400773

the graphene onto the target substrate [57] and includes a chem-ical etching process to remove the metal. [5,8,12] However, thisprocess requires long treatment cycles of several hours andincreases manufacturing cost because of the removal of metalfoils and the usage of an etchant. [6] PMMA is removed eithervia dissolution in acetone [17] or high-temperature annealing(350500 C) in a H 2 /Ar atmosphere [18] or in high-vacuum, [19] but is not perfectly removed, and contamination of PMMAdegrades the performance of graphene devices. [12,20] Moreover,the annealing process at high temperature leads to heavy dopingand mobility degradation of graphene [21] and is not compatiblewith polymer substrates such as polydimethylsilozane (PDMS)and polyethylene terephthalate (PET). [12] Additionally, the wettransfer induces tearing, cracking, and folding of the graphenesurface [5,13] and consistent quality control of the transferred gra-phene after the transfer process is not possible.

Approaches such as the use of a thermal release tape (TRT), [8] an additional metal layer of Au or Pd [9,10] and an epoxy [11] layeras a carrier material or an adhesive layer for the transfer pro-cess, respectively, have been explored in efforts to address theproblems noted above. Nevertheless, these methods similarlycause impurity residues on the graphene surface from theadditional layer and also require an additional cleaning pro-

cess. Therefore, successful transfer of large area monolayergraphene by direct detachment from the metal foil without anycarrier material is thus highly desirable and potentially valu-able, particularly with respect to reducing contamination andmanufacturing cost. Herein, we report the direct transfer oflarge area monolayer CVD graphene from Cu foil to varioussubstrates such as PET, PDMS, and glass without any metaletching process or additional carrier layers in a solid-state pro-cess. The key challenge of direct transfer is to achieve highlyconformal contact between the graphene and target substrateto obtain adhesion energy that is larger than that between thegraphene and Cu foil. Using a mechano-electro-thermal (MET)process, we successfully achieved strong and ultraconformalcontact between the graphene and the target substrate.

Figure 1 a schematically illustrates the newly suggestedtransfer method, where graphene grown on Cu foil is placedon the target substrate and then mechanical pressing over thecontacting area and electrostatic force across the substrate areapplied simultaneously under moderate thermal heating in alow vacuum environment. After maintaining this state for sev-eral tens of minutes, we take out the integrated sample (thesubstrate with adhered graphene on Cu foil) from the appa-ratus after the temperature decreased till 90 C. Immediatelyafter taking out the sample, we then slightly bend the targetsubstrate and carefully pull the edge of Cu foil using tweezersby hand. Finally, we can separate the Cu foil from the target

A high quality large graphene sheet has been synthesized onmetal foils based on a chemical vapor deposition process [1,2] and extensively investigated for research and industrial pur-poses as a promising material having outstanding proper-ties such as high intrinsic carrier mobility [3] and high opticaltransparency. [4] In order to fabricate graphene-based devices,the rst step is the transfer of graphene on metal to the targetsubstrate. However, the transfer technologies [311] used forgraphene require improvement in terms of speed, [6] amountof defects, and realizing a clean and easy transfer process forlarge size graphene. [5,12,13] Here, we demonstrate clean anddry transfer of monolayer graphene of 7 7 cm2 by ultracon-formal contact of the graphene with the target substrate using amechano-electro-thermal process, which enables direct detach-ment of the graphene from a Cu substrate. We directly trans-ferred the monolayer graphene from Cu foils onto PET, PDMS,and glass, respectively, without any additional carrier layers ormetal etching process. In addition, the transferred graphenestrongly adhered to the substrate and was free from physicalcontaminants and damage over its entire area. To demon-strate the strengths of the proposed method, we characterizedthe mechanical and electrical stability of a graphene lm elec-trode using a reliability test under elevated temperature and

humidity.Conventional transfer methods have been explored forthe transfer of graphene including exfoliation of several gra-phene layers from highly ordered pyrolytic graphite (HOPG)by mechanical 3M tape [3,4] or electrostatic force [14] and wettransfer methods for chemical vapor deposition (CVD) growngraphene. [15,16] Among these, the wet transfer of CVD grapheneis generally used because the physical properties of graphenecan be moderately preserved after the transfer and large areaapplication is possible. [1,2] This method typically uses polymethyl methacrylate (PMMA) as a carrier material to transfer

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substrate in air. No further processes are required to transferthe monolayer graphene and this step takes less than oneminute. Moreover, we can concomitantly acquire an undam-aged Cu foil that can be reused to synthesize CVD graphene.We achieved successful transfer of large area (7 cm 7 cm)monolayer graphene on a PET substrate using our METmethod (Figure 1 b). Figures 1 c and 1d show similar results forthe transferred graphene on the glass and PDMS substrates,respectively. Figure 1 e shows the bent image of this exible PET

lm with graphene. The boundary line of the graphene area isclearly seen in the enlarged image. To avoid distortion and hazeof the substrate due to the heating and mechanical pressing, wecarefully selected suitable transfer conditions, as listed in Table 1 .For example, the PET substrate requires a heating temperaturerange of 140 200 C, under which the PET lm is in a viscoe-lastic state and nally in a rubbery region. [22] Additionally, weanalyzed the optical haze of PET lms whether polymers weredamaged or not during the MET process and we observed theappropriate haze values for the transparent electrode lms andsolar cell panels (see Supporting Information). In some casesof high temperature and high voltage listed in Table 1 , a hazedor thermally deformed substrate was observed. Except for these

conditions, both high temperature and high voltage are bene-cial for the successful transfer of the graphene. Next, we suc-cessfully transferred the graphene to a solid-state PDMS lmaccording to the conditions listed in Table 1 . The method toprepare the PDMS substrate is explained in Supporting Infor-mation. In the case of the glass substrate, the transfer condi-tions are quite different from those used for polymer substratesbut similar to an anodic bonding process, and we anticipatethe creation of chemical bonds between oxygen atoms of the

glass and carbon atoms of graphene.[23]

Under high tempera-ture of the bonding process of 360 420 C, Na2 O and K2 O inthe glass substrate (for example, Pyrex 7740 from Corning orBoro 33from i-Nexus), which is in a conductive solid electro-lyte state, are decomposed to Na + , K+ , and O 2 ions. After highvoltage is applied across the substrates, decomposed alkali ions,Na+ and K+ , migrate to the cathode plate and the remainingO2 ions generate high electrostatic forces at the interfacebetween the glass and the graphene, which results in C-O cova-lent bonding between the graphene and the glass substrate. [23] This chemical bond, having higher adhesion energy than thatbetween graphene and Cu foil, facilitates the direct transfer ofgraphene from Cu foil to the glass substrate. Under appropriate



a

b c Glass

PDMSd

ePET

Physical pressure

High temperature High voltage

Vacuum

Figure 1. (a) Schematic description of the newly suggested transfer method. Monolayer graphene grown on Cu foil is aligned with the target substrate.Physical pressure, high temperature, and voltage are applied across the sample under a vacuum of 10mTorr. After the MET process, the graphene onthe target substrate is physically exfoliated from the Cu foil. (b) Graphene of 7 7 cm, transferred on a PET substrate. Various substrates such as (c)Glass and (d) PDMS after graphene transfer. (e) Demonstration of a graphene-based exible transparent PET lm.

Table 1. Direct transfer condition of monolayer graphene onto various substrates at physical pressure 1kgf/cm 2

Target substrate PET PDMS Glass

Completely exfoliated 185 ( C) and 900 (V) 160 ( C) and 900 (V) 360 ( C) and 600 (V)

185 200 ( C) and 600 900 (V) 420 ( C) and 200 (V)

Partially exfoliated 140 170 ( C) and 600 900 (V) 130 160 ( C) and 900 (V) 260 360 ( C) and 600 (V)

170 200 ( C) and 300 600 (V)

Not exfoliated 140 ( C) 130 ( C) and 0 (V) 260 ( C) and 0 (V)

140 170 ( C) and 300 (V)

Hazed/Thermal deection 185 ( C) or 110 ( C) 180 ( C) 420 ( C)

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conditions to transfer the graphene, the Cu foil is well pre-served and is similar to the state before the transfer (Figure S1).However, harsh condition such as 450 and 900V used for theglass substrate might result in a burned and damaged surface,as shown in Figure S2.

To characterize the properties of the transferred graphene,we used an optical microscope, eld-emission-scanning elec-tron microscope (FE-SEM), and Raman spectroscopy. Raman

spectra, which can be used to characterize the grapheneproperties in terms of doping, [1] defects, and the number oflayers,[24] were obtained using a High-Resolution DispersiveRaman Microscope (ARAMIS, Horiba Jobin Yvon) at an exci-tation wavelength of 514.5 nm. The surface of graphene onthis lm is very clean, and has no contaminants or cracks inSEM and optical images, as shown in Figures 2 a and b. Opticalimages of PDMS and PET in Figures 2 b and S3a present tracesof Cu grain boundaries, and the size of the traces on the lmmatched that of the Cu foil, as shown in the inset of Figures 2 band S3a. These traces provide evidence that the substrates aremolded into the morphology of the graphene on the Cu sur-face for ultraconformal contact. [10,22] As depicted in Figure 2 c,after the transfer process, we can conrm the existence of gra-phene on the surface of the substrate by the G peak and 2Dpeak. The graphene on PDMS exhibits a monolayer structure;the intensity of the 2D peak at 2681 cm 1 with a full width athalf maximum (FWHM) of 30.4 cm 1 is about three times theintensity of the G peak at 1584 cm 1 . The D peak, indicatingdefects of graphene, at 1350 cm 1 is quite small. Figure 2 dshows the Raman mapping and Gauss distribution of intensityratios I 2D /I G and I D /I G for the graphene on PDMS, calculatedby integrating the range of the intensity ratio for 5,000 spectrawith mapping of width and height of 20 20 m2 . The distri-butions of the I 2D /I G and I D /I G intensity ratios are respectivelycentered at values of 2.43 and 0.20. These results show that the

graphene transferred by the MET process is a uniform mon-olayer and of high quality without additional defects over theentire area. In the case of PET, the 2D peak is distinct, but theG peak is not distinguishable due to the high intensity of thePET peak at around 1610 cm 1 in Figure 2 e. As shown in theinset of Figure 2 e, we found a peak at around 2700 cm 1 on thePET substrate after the graphene transfer, and it is ascribed tothe 2D peak of the graphene. In addition, we investigated the

Raman spectra of the Cu foil before and after the MET process(Figure 2f). Before the MET process, Raman spectra of the Cufoil exhibited additional distinct G and 2D peaks of graphene,corresponding with typical Raman spectra for graphene onCu. On the contrary, there were no distinct Raman peaks ofgraphene on the metal foil after the MET process. More sup-porting data for the successful transfer of the graphene wereprovided by the FE-SEM and energy dispersive X-ray (EDX)analyses, as presented in Figures S4 and S5. The SEM imagesin Figure S4 clearly show the boundary line of graphene on thePET substrate. Pyrex glass, meanwhile, is typically composed ofseveral elements such as O, Na, Si and K. After the graphenetransfer, we found the addition of C atoms of 2.60 weight (%)but no Cu atoms on the glass surface (Figure S5). These resultsdemonstrate that graphene was successfully transferred fromthe Cu foil to each substrate.

In principle, similar to mechanical exfoliation, delamina-tion of monolayer graphene from Cu foil by exerting externalforces appears to be feasible [4,6] (Figure S6) because the interac-tion between graphene and the Cu foil involves a low bindingenergy of 33 meV per carbon atom, [25] comparable to the inter-planar coupling strength of graphene (25 meV per carbonatom). [6,26] Therefore, this direct transfer of graphene can beachieved by the increased adhesion energy between grapheneand the target substrate. Several factors determining adhe-sion energy such as surface energy as a function of the broken



1200 1800 2400

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. u . )

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. u . )


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1200 1800 2400


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10 um

Cu

I(D)/I(G)

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0.0 0.2 0.4 0.6 0.8 1.0

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1.0

5 m

C o u

n t s

Raman peak Ratio (a.u.)

5 m

0.5 1.0 1.5 2.0 2.5 3.0

GP on PET

100um

Figure 2. (a) SEM image of graphene surface on PET substrate, showing a clean surface and no contamination residues, similar to PMMA. (b) Opticalimage of graphene on PDMS. The inset gure 4 Cu foil after transfer. Traces of Cu boundaries are observed on the transferred substrate. (c) Ramanspectra of graphene on PDMS and bare PDMS surface. The black and red lines correspond to bare PDMS and graphene on PDMS. We can observedistinct peaks of G and 2D peaks from original PDMS peaks. (d) Raman mapping of intensity ratio of 2D peak over G peak and D peak over G peakof graphene on PDMS substrate for overlayer and defect analysis. The mapping area is 20 20 um, respectively. (e) The G peak of graphene is buriedby a high intensity peak of PET at 1600 cm 1 but the 2D peak of graphene could be distinctly observed. (f) After graphene transfer, there is no Ramanpeak of graphene on the Cu foil although the Cu foil showed G and 2D peaks before the MET process.

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bond energy of the exposed atoms, viscoelastic effects of thetarget substrate, and electrostatic force and van der Waals forcebetween graphene and the target substrate can be considered inthis process ( Figure 3 a).

The adhesion energy of graphene to the PET lm could beroughly estimated using only the surface energy, [27] given inTable S1 as 2( ) 0.160.2Jm1/2 2 = = GP PET GP PET . Thisvalue is less than the directly measured adhesion energy of gra-phene as-grown on Cu foil of 0.70 Jm 2 .[11] Therefore, we simu-lated the mechanical deformation and the electric eld of thesubstrates under MET process using the rubbery conditions ofPET, as shown in Figures 3 b-d by the ANSYS modeling pro-gram, which is commercial FEM software. In this simulation,we introduced measured surface roughness of Cu and PET viaAFM (equation S5 and S6 as well as roughness before transferin Figures 3 e and h). The nal state of the substrates under theMET conditions achieved fully ultraconformal contact. Thissimulation for conformal contact of PET and Cu foil, where thephysical and electrical properties are listed in Table S2 and S3,was performed under physical pressure (1kgf/cm 2 ), vacuum(10 mTorr), temperature (180 C), and applied DC voltage(950 V), which could induce more uniform attachment betweengraphene and the target substrate. [10,12,22] Figure 3b presents

the initial state and maximum deformation of 0.3 m after theMET process. Additionally, the electrostatic eld intensity andcurrent density toward the y direction on the contact surface ofPET are depicted in Figures 3 c and 3d. The maximum electriceld of 8.23 MV/cm, which is larger than the reported electriceld that is sufcient to exfoliate graphene from HOPG, [14] isconcentrated at the contact vertex points along with quite smallcurrent density, as shown in Figure 3 c. These factors induceultraconformal contact and increase the adhesion energybetween the two surfaces. Moreover, the negative charges at thesurface of PET made by electrostatic force are remained evenafter the PET process, which pull the graphene and enhancethe adhesion energy. [28] Additionally, this adhesive energy couldbe dramatically enhanced by the viscoelastic state [29] of the PETlm at MET temperature, which induces energy dissipation atthe fracture interface [3032] (See Supporting Information). More-over, the van der Waals force of the PET lm to the grapheneis maximized at this ultraconformal contact; this contact couldbe modeled as plane-to-plane although the initial state wasmodeled as cylinder-to-plane contact, as shown in Figure S7,equation (S5 and S6). Thus, the increased adhesion energy bythese several factors such as viscoelastic effect, electrostaticforce and ultraconformal contact, achieves the direct transfer of



Figure 3. (a) Conceptual illustration of effective forces between two contact surfaces for graphene transfer. (b) Surface roughness of PET and Cu foil issimulated at the rubbery region of PET as sine curves that have height of 40 nm and 100 nm, respectively, and different periods, which are the actualmeasured roughness via AFM. The initial state and deformation results after the MET process are expressed in the upper and lower gures, respec-tively. (c) The total current density of the contact surface of the PET substrate. (d) Electrostatic eld intensity to y direction of the contact surface ofPET. The maximum eld intensity occurs at a contact vertex point. Tapping mode AFM images of surface of Cu foil (e) before and (f) after graphenetransfer. (g) These AFM images show similar roughness proles of about 100 nm measured with red and green lines. Additionally, the PET substratehas similar roughness values to Cu roughness of about 120 nm, indicated by the yellow line, after graphene transfer (i), although the initial state ofthe PET substrate before the MET process shows low roughness of about 36 41 nm (h). (j) In contrast, the surface of graphene on the glass substratehas minimal roughness of under 0.25 nm and RMS 0.07 nm values.

4 um

Cu Before

Grainboundary

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Line 1

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PET 7.4nA/um 2

0

PET

CuV=950V, P=1kgf/cm 2, T=180 oC

0.327um

8.23MV/cm

0

PET

PET

Cu

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transformed infrared (ATR-FTIR) spectrum to analyze whetherthere are additional chemical bonds between the graphene andPET (Figures 4c and S10). The transmittance spectrum haspeaks at 1712cm 1 (C = O bonds), 1340 cm 1 (bending vibra-tion of CH 2 groups), 872 cm

1 (vibration of aromatic ring), and1124 cm 1 (stretching vibration of C-O bonds) of the PET sub-

strate.[34]

Wet and MET graphene samples show identical FTIRspectra, indicating that there are no additional chemical bondsat the graphene after the MET process. Next, we tested the fea-sibility of applying the transferred graphene as an electrode tolight a LED device, as shown in Figures 4 d and S9 and movieclips. For this, we performed repeated contact tests and mechan-ical adhesion tests during several attaching/detaching cycles of3M polyimide lm tape. Unlike the wet transferred graphene,the MET transferred graphene showed more stable mechanicaladhesion on the PET lm after several attaching/detachingtrials. These results demonstrate that the MET process inducesstronger adhesion of graphene to the substrate.

In summary, we have demonstrated the use of a MET pro-cess for clean, fast, inexpensive, and eco-friendly transfer oflarge area monolayer CVD graphene from Cu foil to varioussubstrates, PET, PDMS, and glass. FEM simulation suggeststhat ultraconformal contact of graphene can be achieved bythe MET process. Our experimental results for the transfer ofgraphene present both excellent quality (with no residues, fewdefects, or no folding) and remarkable mechanical and elec-trical stability even in a high temperature and humidity envi-ronment. Moreover, this process could reduce the processingtime less than 30 min as well as the manufacturing cost. Theconventional wet transfer method could be replaced by the pro-posed method, which is expected to be benecial to developwide-ranging applications such as graphene-based devices andtransparent electrodes with high quality.

Experimental SectionGrowth Method of Graphene : Monolayer graphene was synthesized on

25 m-thick copper foils (99.8%, Alfa Aesar, No.13382) using a lab-builtthermal CVD system with a quartz tube of 2 inch diameter. After loadingcopper foils in the quartz tube, the air was evacuated down to 5 10 4 torr,and then the temperature was raised up to 1000 C at a rate of 20 C/min.Before synthesis of graphene, the copper foils were annealed at 1000 Cwith gas ows of 50 sccm Ar and 20 sccm H 2 for 20 min. The synthesiswas performed at 1000 C with the mixture ow of CH4 (30 sccm) / H 2 (30 sccm) for 20 min, and then the samples were cool down to roomtemperature by natural convection under Ar environment.

Graphene Wet Transfer Process : After the synthesis, one side of the

copper foil specimen was spin-coated with poly-methyl methacrylate(PMMA) to mechanically support the graphene layer during the transferprocedure to a substrate. The graphene layer on the other side wasremoved by oxygen plasma treatment. The specimen was oated on thesurface of 0.1 M ammonium persulfate to separate the graphene/PMMAlayer from the specimen by etching the Cu foil. The oating graphene/PMMA layer was subsequently transferred onto a silicon oxide substrate.The PMMA layer was removed by immersing the substrate in chloroform.

Supporting InformationSupporting Information is available from the Wiley Online Library orfrom the author

AcknowledgementsThis work was supported by Global Frontier Research Center forAdvanced Soft Electronics and Nano Material Fundamental Research ofMSIP in Korea.

Received: February 17, 2014Revised: June 1, 2014Published online:

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