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Crack-Free Growth and Transfer of Continuous Monolayer GrapheneGrown on Melted Copper

Ye Fan, Kuang He, Haijie Tan, Susannah Speller, and Jamie H. Warner*,

Department of Materials, University of Oxford, Parks Road, Oxford, OX1 3PH, United Kingdom

ABSTRACT: Monolayer graphene with large domain sizescan be grown by chemical vapor deposition using a Cu catalystin its molten state. However, extending this to fully continuoussheets of graphene on the centimeter scale is challenging,

because of cracks, rips, and tears that are induced upon rapidcooling. The various issues that prohibit fully continuousgraphene sheets are identied and solutions presented. Theseinclude (i) developing a novel two-stage CVD growth processthat lls in the cracks and holes formed upon cooling; (ii)appropriate choice of underlying wetting substrate of W,instead of Mo, which causes holes; and (iii) a new electrochemical transfer method that removes W and then Cu to enable theefficient transfer of crack-free graphene sheets onto silicon wafers. Our results provide important solutions to challenges relatedto the synthesis and transfer of high-quality monolayer graphene grown on molten Cu catalysts for electronic applications.

INTRODUCTION

Graphene is expected to play versatile roles in various electronicand spintronic applications including, but not limited to, radio-frequencyeld-effect transistors,14 spin valves,5,6 solar cells,7,8

and organic light-emitting diodes (OLEDs).9,10 Despite itshuge potential, graphene-based devices remain far fromindustrialization, due in part to challenges associated with the

synthesis and transfer processing of uniform large-areamonolayer graphene. Physical methods such as mechanicalexfoliation were the rst approach that successfully isolatedsingle-layer graphene from graphite.11 Although mechanicalexfoliated graphene exhibits good electronic performance,1214

mechanical exfoliation is too is labor-intensive and low yield tosupply graphene for industrial-scale device fabrication. Anotherroute to isolate graphene is via wet-chemical methods.Chemical exfoliation15 or chemical reduction of graphiteoxide16,17 produce large numbers of small graphene fragments18

that are favored for certain applications such as molecularsensing19,20 and pollutant processing,21but are not desirable forelectronic applications, because of their small size and poorconductivity. Obtaining graphene from graphite is a top-down

approach, whereas fabricating graphene from carbon-basedprecursors provides a bottom-up approach that is scalable.The bottom-upmethod mainly includes thermal annealing ofsilicon carbide22 and chemical vapor deposition (CVD).Epitaxial graphene on thermal annealed silicon carbide exhibitsexcellent electronic properties,1,23 but its applications arelimited by the challenges in controlling graphene layer numberand transferring it to alternative substrates. On the other hand,CVD graphene can match epitaxiallygrown graphene on siliconcarbide in its electronic properties,2426 while being easier totransfer onto an arbitrary substrate.

Although the record size of individual isolated graphenedomains continues to increase (now in the millimeter

scale),2730 it is not easy to extend these same growth recipesto produce large-area (centimeter-scale) uniform continuousgraphene sheets with concomitant millimeter domain sizes,

because of various challenges in the growth approaches. Large-area continuous graphene sheets have been extensivelysynthesized, but with smaller domain sizes on the order of 20m.3133 Multilayer regions of graphenecan be found even ingraphene synthesized under low pressure.34 Impurities, surfaceroughness, and grain boundaries of Cu are suspected to beresponsible for the multilayered regions.3537 In contrast, large-area single-layer graphene domains are easily synthesized whenthe Cu catalyst is in a molten state rather than solid form.3841

With longer growth time, graphene domains oating on meltedCu are expected to grow larger and nally meet and match witheach other, forming a continuous lm. Nevertheless, synthesisand transfer of graphene lm on melted Cu is not trivial. Ahigh-melting-point metal is used to wetthe liquid Cu duringsynthesis to prevent balling, but it may induce unwantedalloying. In addition, the chemical inertia of the supportingmetal makes conventional wet-etching transfer methodsdifficult. Furthermore, graphene stores a huge amount ofelastic energy during Cu solidication and maynally result incracks.

In this paper, we study the growth and transfer process offully continuous graphene sheets on melted Cu by CVD, asopposed to prior work that focused on just growing largesingle-crystal domains on the order of few hundreds ofmicrometers. We nd that macrodefects (e.g., cracks andholes) arise from tensions introduced in the growth andtransfer processes of graphene. To address that problem, we

Received: May 26, 2014Revised: July 19, 2014

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developed a novel two-stage regrowthmethod. A tension-freeelectrochemical transfer method is also investigated for theefficient transfer of centimeter-scale continuous graphene

without introducing any major cracks.

RESULTS AND DISCUSSIONGraphene is grown by ambient pressure chemical vapor

deposition, as reported in the Methods section. The catalystsubstrate, cooling rate, and transfer method are found to be thekey factors in determining the quality of the nal grapheneproduct. Refractory metals like W or Mo are used as substrates,

because they have higher melting points than that of Cu andalso wet the molten Cu, producing at Cu surfaces forgraphene growth. Although W and Mo share similar chemicaland mechanical properties, they have different effects on themorphology of the CVD-grown graphene. Hexagonal particlesdecorate the as-grown graphene on Mo-supported Cu, asshown in Figure 1. Despite their similar shape with graphene

domains, Raman spectroscopy shows no peaks associated withgraphene, while energy-dispersive X-ray spectroscopy (EDX)reveals that the particles are actually composed of MoCO(see Figures1cf).

The density of MoCO particles increases with reactiontime (i.e., the time when the sample is exposed to CH4), butdoes not change with preannealing time (i.e., the time when thesample is annealed in hydrogen before introducing CH4).Therefore, the MoCO particles are likely to be formed bymetal-catalyzed carbonization. In contrast to graphene grownon Cu supported by Mo, graphene grown on Cu supported by

W achieves continuity. No secondary phase particles are foundover the entire sample, as shown in Figure1h.

The second key factor determining the continuity ofgraphene is the cooling process. We compare two coolingprocesses: the conventional fast cooling, and a novelregrowth cooling. Fast cooling refers to a cooling process

where the sample is quickly removed from the heating zoneimmediately after graphene growth to rapidly cool to roomtemperature. Regrowth cooling refers to a cooling process

where the temperature is gradually reduced from 1090 C to1060 C for the last 30 min of graphene growth beforeremoving the sample out of the heating zone and cooling toroom temperature.

Numerous cracks are found in the fast-cooled graphene andcan be classied into three types. The rst type of cracks are thelong ones that originate from the edge of the entire graphenesheet, propagating up to a millimeter across until reaching a Cugrain boundary, as shown in Figure2a. This type of crack is theresult of the stress between graphene and Cu during Cusolidication, as illustrated in Figure2l. Notches in the edge ofthe graphene lm, which may be etched by the nano-particles,42,43 are commonly found near these long cracks, asshown in Figure 2b. Stress between graphene and Cuconcentrates around the notches and leads to some of themfurther developing into larger cracks, as illustrated in Figure 2l.Smaller cracks were found propagating out sideways from thelong main cracks, as shown in Figures 2j and2k. These shorteroff-shoot side cracks are of higher density but are narrower thanthe main long cracks. The second type of crack includes thecrack that spreads across the dimple in Cu along the thermalgrooves. This type of crack spreads hundreds of micrometersand has widths up to several tens of micrometers, as shown inFigure2d. The drastic deformation of Cu during solidicationintroduces a huge amount of stress on the graphene lm andnally tears this type of crack in the lm, as illustrated in Figure2e. Secondary cracks running parallel with the thermal groovingalso commonly appear, as pointed out by yellow arrows inFigure2d. The third type of cracks form around the Cu grain

boundary and we call them boundary tears. They are parallelwith each other and intersect the Cu grain boundary at 45, asshown in Figures2f and2h. In contrast with the stress-inducedcracks from thermal grooving, shear stress is responsible forboundary tears. Cu grains rotate to accommodate stressduring solidication,44which shears graphene near the Cu grain

boundaries. Cracks then form on the Cu grain boundary torelease the tensile stress. Occasionally, some cracks of this typecan develop into larger lightning-shaped cracks, as shown inFigure2g.

In order to eliminate these cracks, graphene was grown by anovel regrowth process, hereafter referred to as regrowngraphene. This resulted in substantially better continuity thanfast-cooledgraphene. Slowly decreasing the temperature from

1090C to 1060C enables enough time for Cu to solidify andrelease the thermal stress between graphene and Cu. Fewercracks are found as a result (see Figures 3a and 3e).Furthermore, by continuing to supply CH4during this period,it allows graphene to continue to grow and ll the vacant spaceleft by cracks in the lm, actively repairing the cracks. Theregrowngraphene is more continuous, and when the sampleis baked in air, there is no sign of oxidation of the Cu in themain central region (see Figures3a and3e).

Although the growth of graphene by CVD can lead to high-quality material, it is the transfer stage that is a major rate-limiting step in the progress of graphene for electronics.Transferring a single graphene layer from one substrate to

Figure 1.SEM and EDX images of graphene grown by CVD on Cuwith a Mo or W wetting substrate: (a) SEM image of graphene grownon molten Cu with a Mo wetting substrate by CVD, where ahexagonal domain comprised of MoCO is observed to disrupt thegraphene in the local neighborhood, as noted by the arrow; (b) SEMimage of a region of graphene grown by CVD on Cu:Mo used forenergy-dispersive X-ray spectroscopy (EDX) maps in panels cf; (c)Mo L1EDX map; (d) Cu K1EDX map; (e) O K1EDX map; (f) CK1_2EDX map; (g) SEM image showing a large number of holes ingraphene caused by the MoCO domains; and (h) continuousgraphene lm obtained by CVD on molten Cu with a W substrate(inset shows a typical high-magnication image of graphene on Cusupported by W).

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another without inducing cracks is not trivial. Although severaltransfermethods for graphene grown on Cu foil,34,45 or metalthin lm46,47 have been reported, there is still a lack of detailedstudies on the transfer process of graphene grown on meltedCu to silicon substrates with an oxide surface layer. Here, wecompare the effect of different transfer methods on the qualityof graphene, in terms of continuity and surface residues.PMMA is spin-coated onto graphene as a protective supportinglayer for all methods discussed. Figure 4 schematicallyillustrates the three transfer methods, and two Cu etchantsused, studied in this report. In the side etching method, thesample oats on the FeCl3 solution. Because of the chemicalinertness of W, the etchant can only gradually dissolve Cu fromthe narrow space between graphene and W substrate accessible

through the side of the sample. When Cu is totally consumed,the W substrate sinks, leaving the graphenePMMA lmoating on the etchant. In the bubbling method, hydrogen

bubbles lift the graphenePMMAlm offthe Cu substrate.48,49

These hydrogen bubbles come from the electrolysis of a 1 Msodium hydroxide solution with the sample used as the cathode.

Apart from the two methods above, we have developed a novel2-step transfer method specically for graphene grown onmolten Cu with a W substrate. First, the W substrate is etched

by the electrochemical method. This etching step is referred asanodic etchinghereafter. Following the anodic etching of W,Cu is dissolved with either FeCl3 or ammonium persulfate,followed by successive rinsing in water to nish the transfer.

The transfer process inuences the quality of graphene onthe silicon substrate, in terms of the continuity and surfacecontamination. In our experiment, transferring CVD graphenegrown on molten Cu by the bubbling method destroys itsintegrity, as shown in Figure 5a5c. Turbulence caused byhydrogen bubbles twists and cracks the graphenePMMAlmduring the transfer process. However, we have noticed thatsome isolated hexagonal domains of monolayer graphene with

widths up to 100 m retain better continuity than thecontinuouslm after transfer by the bubbling method, becauseof their smaller size and reduced tension. A substantial amountof surface contamination remains on the graphene even afterthe cleaning process, described in the Methods section. Theroot-mean-square surface roughness of graphene transferred by

the bubbling method reaches as high as 7.2 nm, which is thehighest among all methods we examine here. Compared to the

bubbling method, the side etching method retains betterintegrity of the graphene. Only few cracks are found across thegraphene, as shown in Figures5e and5f. However, because ofthe chemical inertness of W, common etchants such as FeCl3orammonium persulfate cannot dissolve it efficiently. In this way,the etchant can only approach Cu from the space betweengraphene and the W substrate, which results in a slow etchingspeed. The etchant process generally takes 35 days tocomplete. Moreover, W anchors to the bottom of the liquid anddistorts the graphenePMMA lm during etching causingfurther cracks. The bright blue contrast in Figure 5d indicates

Figure 2. Cracks in fast-cooled graphene samples: (a) SEM image of long cracks across graphene; originating from graphene edges, crackspropagate through graphene until reaching a Cu grain boundary. (b) SEM image of nanoparticles etching tracks in graphene edges. (c) SEM imageof the cracks in graphene resulting from thermal grooving of the Cu; a dimple, indicated by an arrow, shows the deformation of Cu duringsolidication, which tears the graphene lm. (d) High-magnication SEM image of the tail of the crack in graphene induced by thermal grooving;secondary cracks running parallel to the main crack are indicated. (e) Schematic illustration of the formation of cracks in graphene from thermalgrooving and its secondary cracks. (f) SEM image of boundary tearsin graphene from a Cu grain boundary. (g) SEM image of a millimeter-scalelightning-shaped crack. Some cracks among the boundary tearsoccasionally develop into large lightning-shaped cracks. (h) Histogram of theintersection angles between boundary tears and the copper grain boundary. 95 cracks over 6 different Cu grain boundaries are counted. (i)Schematic illustration of the formation of boundary tears. (j) SEM image of the large cracks which reach the edge of graphene (as in panel (a)),along with the offshoot side cracks. (k) Higher-magnication SEM image of the offshoot side cracks stemming from a main crack. (l) Schematicillustration of the formation of long cracks that reach the edge of graphene and its offshoot side cracks.

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the FeCl3PMMA residue on graphene. The AFM measure-

ment (Figure 5f) con

rms the discontinuity of graphene bynding cracks with widths of 600 nm. The surface roughnessis 2.4 nm, which is similar to previous reported values oftransferred CVD graphene.

Both side etchingand bubblingtransfer methods stretchor distort the graphenePMMAlm at a certain stage. The 2-step etching method, on the other hand, is stress-free andtherefore preserves the continuity of graphene. The rst step oftransfer involves oxidizing and dissolving W by anodic etchingin a sodium hydroxide solution. The reaction requires a highpH solution environment, which is provided by the 2 mol/Lsodium hydroxide solution in our experiment. W with a lateralsize of 1 cm2 and thickness of 100 m is found to dissolve

within 30 min by anodic etching. We further study the effect of

two Cu etchants

FeCl3 and ammonium persulfate

on thegraphene. When Cu is dissolved by FeCl3, a similar amount ofFeCl3PMMA residue is found on the graphene, as shown inFigure5g. On the other hand, not much surface residue appearson the graphene when it is transferred by ammonium persulfate(see Figure 5j). Regardless of which etchant is used, thegraphene remains crack-free over a centimeter-scale region. Wealso used AFM to measure the surface roughness of the 2-steptransferred graphene. When Cu is etched by FeCl3, the surfaceroughness is 3.4 nm. When Cu is etched by ammoniumpersulfate, a atter surface is observed, with a root-mean-squareroughness as low as 1.6 nm. It is clear that using ammonium

Figure 3.Morphology of graphene treated by different cooling processes: (a) Optical microscopy image of fast-cooled graphene on Cu (sample isbaked in air at 150 C for 15 min to oxidize uncovered Cu, in order to increase the optical contrast); oxidized Cu appears dark red, while grapheneprotected Cu appears light orange. (b) SEM image of a large crack in fast-cooled graphene from thermal grooving. (c) SEM image of boundarytearson the fast-cooled graphene. (d) SEM image of long cracks from the graphene edge. (e) Optical microscopy image of regrowngraphene onCu; sample is partially oxidized in order to increase the image contrast. (f) SEM image of crack-free regrowngraphene covering the Cu grainboundary. (g) SEM image of crack-free regrowngraphene edge; no long cracks develop from the edge of graphene. (h) Higher-magnication SEMimage of the fast-cooled sample, showing a large crack. (i) Higher-magnication SEM image of regrown graphene showing no cracks.

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persulfate leads to a cleaner graphene surface than FeCl3as theCu etchant.

To further quantify the graphene grown and transferredfollowing our procedure, we examined its electronic properties

within different length scales. Graphene is patterned andtrimmed into 2-m ribbons by a combination of electron beamlithography and oxygen plasma etching. Arrays of Cr/Aucontacts are then deposited by evaporation onto the ribbon, sothat the channel length is 200 m N, whereNis an integer,as shown in Figure6a. The large aspect ratio of the graphene

ribbon ensures the conductivity is sensitive to the continuity ofthe graphene. Raman mapping indicates a 2D/G ratio of 2,conrming the single layer nature of graphene, as shown inFigure6c. A weak D peak is also observed, which is from theedges of the 2-m ribbon sampled by the Raman spectroscopymeasurement. The conductivity of graphene is measured overdifferent channel lengths. The plot of resistance versus lengthintercepts they-axis at 240 , as shown in Figure6d, which isfrom the contact resistance according to previous reports.50,51

The resistance of the graphene ribbons in our geometry is422.5/m; thus, we can derive the sheet resistance as 825/. We also measured the sheet resistance directly on othertransferred samples using the Van der Pauw geometry and

obtained similar sheet resistance values. The eld-effectmobility of the graphene measured in air with this geometry

was 1600 cm2/(V s), as shown in Figure 6e. With a smallerlength-to-width ratio, we nd an average FET mobility of3000 cm2/(V s). Further cleaning processes, such as ultrahigh

vacuum annealing, using BN as a substrate, and performingmeasurements under vacuum should help to further increasethe mobility values in our devices.

CONCLUSION

The results in this paper provide a solution for large-area single-layer graphene grown on a liquid Cu catalyst and its transferonto silicon substrates. Our ndings clarify the inuence ofeach step in the process on the nal quality of graphene. Threecritical factors, e.g., metal substrate, cooling process, andtransfer method, are used to determine the quality of thegraphene. The metal substrate should be carefully chosen so asto balance its wettability by the melted Cu and chemicalinertness. Secondary phase particles caused by the Mo substrate

break holes in graphene, while a W substrate provided acontinuous graphene sheet. Different cooling processes changethe thermal stresses in graphene. A conventional fast-coolingprocess tears graphene, because of drastic deformation of Cu

Figure 4.Schematic illustrations of different transfer processes: (a) Side etching and H2bubbling methods to remove graphenePMMAlm fromthe CuW substrate. In the side etchingmethod, Cu is gradually etched by FeCl3solution from sample side. In the bubblingmethod, graphene isdetached from the Cu by delamination by hydrogen bubbles. (b) Anodic etching method; W is etched by an electrochemical process, and,subsequently, the remaining Cu is dissolved by either FeCl3or ammonium persulfate solution.

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during solidication. We have addressed that problem bydeveloping the regrowth cooling process, during whichgraphene continues to grow from the edge of the cracks andstitches them. In addition to the graphene growth, the choice oftransfer process also inuences the continuity of graphene.Distortion caused by bubbles or heavy substrates sinking tearscracks into graphene grown on Cu. A stress-free wet transfer

method is developed to minimize the transfer-induced cracks ingraphene. These results will help the further development oflarge-area continuous sheets of high-quality monolayergraphene for electronic applications.

METHODS

Graphene Preparation. Graphene is grown via chemical vapordeposition (CVD). Both the substrate (W or Mo) and the catalyst(Cu) are cut into 1 cm2 pieces and then thoroughly cleaned in acetoneand IPA. An addition step of rinsing Cu in hydrochloric acid (HCl) isapplied to eliminate the oxide layer. Cu is then mounted on W andplaced into the furnace. Samples are melted and annealed at 1090 Cwith a ow of 100 sccm of hydrogen (25%) and 200 sccm of argon for

30 min before growth. Graphene is grown on liquid Cu with a ow of200 sccm of argon, 80 sccm of hydrogen (25%), and 10 sccm ofmethane (1%) for 90 min. Two cooling processes are studied in thiswork. In the cooling process referred as fast cooling, samples arequickly moved out of the heating zone and cooled to roomtemperature directly after the synthesis. In the cooling processreferred as regrowth cooling, a secondary 30-min growth process ofgraphene at 1060 C is carried out without changing the ow rate ofeach gas. The sample then is removed from the heating zone andcooled to room temperature.

Transfer.A 500-nm PMMAlm is spin-coated onto graphene as aprotection layer right after synthesis. Three transfer methods (e.g.,bubbling transfer, side etching, 2-step etching) and two etchants(e.g., iron chloride (FeCl3) and ammonia persulfate) are compared inthis work. In the bubblingmethod, the sample is used as the cathodein the electrolysis of a 1 M sodium hydroxide solution. The current ismaintained as 0.1 A, to provide a stable generation of hydrogenbubbles from the sample. Hydrogen bubbles peel the graphenePMMAlm offfrom the Cu substrate gradually. In the side etchingmethod, the sample is oated on the FeCl3 solution. Cu is slowlyconsumed by the 1 M FeCl3 solution from the side until thegraphenePMMA lm separates from the substrate. The underlying

Figure 5.Continuity and surface roughness of transferred graphene: (a) Optical microscope, (b) SEM, and (c) AFM images of graphene transferredby bubblingmethod (graphene is shredded and large amounts of surface contamination are found); (d) Optical microscope, (e) SEM, and (f)AFM images of graphene transferred by the side etchingmethod (some cracks and holes are found); (g) Optical microscope, (h) SEM, and (i)AFM images of graphene transferred by the 2-stepmethod with Cu etched by FeCl3(graphene maintains continuity over a large region but somesurface residues are found; and (j) Optical microscope, (k) SEM, and (l) AFM map of graphene transferred by the 2-stepmethod with Cu etchedby ammonia persulfate (graphene is clean and continuous over a large region). The brightness/contrast of the optical microscope images areadjusted to reveal any surface contamination that might be present.

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W substrate prevents the access of FeCl3to the Cu from beneath andonly permits etching from the side. In the 2-step etchingmethod, theW substrate is electrochemically etched in a 2 M sodium hydroxide

solution. The sample is linked with the anode, and a 2.4 V voltagedrop is applied between the sample and the cathode (Cu foil).Hydrogen bubbles are generated from the cathode while no gas isreleased from the sample. Cu is then etched by a 1 M solution of eitherFeCl3 or ammonium persulfate.

For graphene transferred with all methods, cleaning steps are carriedimmediately after separating the graphenePMMA lm from thecatalyst. The lm is rst rinsed in DI water for 30 min before a secondrinse in 1 M HCl for another 15 min. Etchant residues are removed bythis step. The graphenePMMAlm is again rinsed in deionized (DI)water for three times with at least 1 h per rinse.

AUTHOR INFORMATION

Corresponding Author

*E-mail: Jamie.Warner@materials.ox.ac.uk.

Notes

The authors declare no competing nancial interest.

ACKNOWLEDGMENTS

J.H.W. thanks the Royal Society and Balliol College for support.Y.F. thanks the Clarendon Fund for support. H.T. thanks theMerdeka Scholarship for support.

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Chemistry of Materials Article

dx.doi.org/10.1021/cm501911g| Chem. Mater. XXXX, XXX, XXXXXXH