CVD growth and processing of graphene for electronic applications

Phys. Status Solidi B 248, No. 11, 2604–2608 (2011) / DOI 10.1002/pssb.201100179 p s sb

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basic solid state physics

CVD growth and processing ofgraphene for electronic applications

Shishir Kumar1,2, Niall McEvoy2, Hye-Young Kim2, Kangho Lee2, Nikos Peltekis2, Ehsan Rezvani1,2,Hugo Nolan1,2, Anne Weidlich2, Ronan Daly2, and Georg S. Duesberg*,1,2

1School of Chemistry, Trinity College Dublin, Dublin 2, Ireland2Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), Trinity College Dublin, Dublin 2, Ireland

Received 20 May 2011, revised 5 August 2011, accepted 13 September 2011

Published online 5 October 2011

Keywords carbon nanostructures, chemical vapour deposition, graphene, nanodevices

*Corresponding author: e-mail [email protected], Phone: þ353 1 896 3035, Fax: þ353 1 896 3037

The remarkable properties of graphene have potential for

numerous applications; however, their exploitation depends on

its reliable production. The chemical vapour deposition (CVD)

growth of graphene on metal surfaces has become one of the

most promising strategies for the production of high quality

graphene in a scaleable manner. Here, we discuss graphene

growth on nickel (Ni) and copper (Cu) directly from both

gaseous hydrocarbons and solid carbon precursors. Further, we

discuss in detail the transfer of graphene films to insulating

substrates, by direct and polymer supported transfer methods.

� 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction The exceptional properties of gra-phene [1] make it potentially useful for innovativeapproaches to applications such as next generationelectronics, chemical and biological sensors, displays, solarcells and electrochemical electrodes [2]. The most notablefeature of graphene is its two-dimensionality. All atoms ingraphene are accessible, making its properties highlysusceptible to external influences. This can be either a boonor a bane. On the one hand one can probe or tune somecharacteristic of graphene very easily, and on the other,defects or impurities degrade those desired properties veryrapidly. Thus, for electronic applications, where the veryhigh electrical mobility and thermal conductivity ofgraphene is needed, the minimisation of undesired impu-rities, adatoms and grain boundaries is required. As thelattice defects in graphene may enhance the sensitivity byproviding binding sites [3], a somewhat lower qualitygraphene may be optimal for sensing applications. For bulkapplications like composites, even lower quality graphenemay be used. Production methods for graphene are neededwhich are flexible enough to accommodate these demands.

Various processes for making graphene have beenreported: mechanical [4] or chemical exfoliation [5],decomposition of SiC [6] and chemical vapour deposition(CVD) using hydrocarbon precursors [7]. CVD of graphenecan be achieved on various planar metallic substrates, aprocess which is compatible with thin film technology. As

CVD is a scalable method it can fulfil the needs of industrialproduction of graphene, in particular, for electronic displays,and sensor applications. In the first part of this paper wedescribe our work on graphene growth on nickel and coppersubstrates using CVD.We discuss the mechanism of growthof graphene on these substrates, comparing it to growthof pyrolytic carbon (PyC), which forms on various inertsubstrates under similar growth conditions. We also outlineprocedures which exploit the graphitising effect of metals onsputtered carbon films.

The demand for graphene on insulating substratesrequires transferring graphene from the metals to desiredsubstrates. We discuss various methods to accomplish thistransfer. In general, transfer methods work at low tempera-tures so graphene can be put on a wide range of substratesincluding plastics. However, the transfer can degrade itsproperties by introducing impurities, lattice defects andimperfections. We point out the pros and cons of eachmethod in this regard.

2 Graphene CVD on metallic substrates In thestandard model for a CVD process, the gaseous precursordiffuses from the gas phase to the solid surface of thesubstrate through a boundary region. After adsorption on thesurface, it diffuses about the surface and interacts withsubstrates and other adsorbed species. In this process it reactsor forms intermediaries which finally yield the deposited


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material. Some products may leave the surface and arecollected at the outlet of the reactor.

Thismodel also applies to the CVDof carbon onmetallicsurfaces. A precursor like acetylene or methane can beflowed over a heated metal surface, where it breaks to formcarbon forms which are either deposited or dissolved in themetal. Unused carbon and hydrogen produced by thereaction are pumped out of the reaction chamber. Severalparameters can be tuned to produce graphene by this process.The most important parameter is the metal used as thesubstrate, as some of them have the ability to form wellgraphitised layers, as opposed to growth on inert substrateswhich yields PyC. Metals can also catalyse the cracking ofhydrocarbons, lowering the growth temperature. Nickel andcopper are among the most suitable metals for growth dueto the moderate strength of their interaction with carbonand their lattice match with graphite. The differences insolubility of carbon in these metals can be viewed as a formof strength of interaction. Apart from this interaction, theamount of deposition and its structure will depend on thediffusion of carbon atoms, the morphology of the substrateand the thermodynamics of the process, e.g. Ni yields CNTswhen it is in a nanoparticulate form and graphene when it isused as a thin film.

2.1 Growth on nickel Nickel films (200 nm) wereprepared by E-beam evaporation on thermally-grown silicondioxide (300 nm) films on Si h100i substrates. Thesesubstrates were introduced into a tube furnace and heatedto high temperatures (950 or 1000 8C). After reduction by amixture of hydrogen and argon (1:1) for 5min, the Ar flowwas shut off and acetylene was introduced, keeping theoverall pressure between 0.5 and 5Torr for various dwelltimes. The flow rate was nominally 60 sccm for acetyleneand 180 sccm for hydrogen or argon, unless otherwisespecified. The substrates were then cooled under nitrogenflow with rates exceeding 15 8C/s or as slow as 0.15 8C/s.

A photograph of a SiO2 substrate partially covered withNi and subjected to growth under acetylene at a pressure of10 Torr for 20min at 950 8C is shown in Fig. 1. Both of theregions are covered with carbon as seen by the Raman

Figure 1 (online colour at: www.pss-b.com) Raman spectra ofcarbon deposits at different sites across the boundary on a SiO2

substrate partially coated with Ni (growth at 950 8C, 20 Torr for20min). Sites 1, 2, 3, 4 and 5marked in the optical image on the leftshow signatures for graphite, FLG, defective FLG, PyC with someorder and PyC, respectively (right panel).

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spectra (Fig. 1, right panel) taken on the points 1–5, acrossthe boundary between Ni and SiO2 regions. All the Ramanspectra show a prominent G-peak (at �1580 cm�1) whichbecomes sharper in going from the SiO2 to the Ni region.Furthermore, the intense and broad disorder peak (D-peak, at�1350 cm�1) seen on deposits on SiO2 completely dis-appears on Ni coated regions. In addition, a strong 2D peak(at 2650 cm�1) is present in spectra from carbon deposits onNi. These spectra indicate that the Ni and SiO2 regions arecoated with graphitic and PyC respectively, while theboundary has multilayered graphene.

The overall thickness of both of these carbon films can becontrolled by varying the dwell time of the precursor andother process conditions. The graphitic films were trans-ferred onto SiO2 substrates by fishing graphene as describedlater and their thickness was measured with a profilometer.The thickness of PyC on SiO2 was measured by making agentle scratch and profiling it. The growth rates observed at950 8C and 5 Torr precursor pressure are shown in Fig. 2(a).The rate for graphite films onNi is about 150 nm/h in the first5min and then continues at �30 nm/h. For dwell times lessthan 5min, the Raman spectra show a decrease in the ratio ofintensity of 2D and G peaks (I2D/IG) with increasingprecursor dwell time implying an evolution from monolayerto thicker graphitic deposits. A similar effect was achieved

Figure 2 (online colour at: www.pss-b.com) (a) Thickness vs.dwell time plot for carbon deposits onNi andSiO2 substrates, grownat950 8Cand5 Torr. (b)Ramanspectra1,2,3and4ofgraphenefilmson Ni after dwell times of 0.5, 1, 3 and 5min, respectively. Thedecreasing 2D/G ratios from 1 to 4 (3.7, 0.74, 0.48 and 0.33,respectively) indicate an increase in the thickness of graphene, frommonolayer (1) to>10 layers (4). (c)Ramanspectra ofgraphenefilmsgrown at 950 8C under a pressure of 1 Torr with acetylene flow of2 sccm (1) and 20 sccm (2). Raman spectrum 3 was obtained on asample subjected to a slow cool down (0.15 8C/s) and grown understandard conditions with a dwell time of 1min. (d) Typical Ramanspectra of graphenegrownon topof copper foils. Top spectrum is for15min precursor flow and bottom one is for 30min, showingmonolayer graphene and PyC like carbon deposited on top of it,respectively.


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Figure 3 (online colour at: www.pss-b.com) (Upper row) Differ-ent arrangements for graphitising carbon using transition metals, Mdenotesmetal (Ni orCu) andC is a carbon source. (Lower row)SEMimage for samples processed from second arrangement showinggraphene and Ni clusters (scale bar is 2mm). Raman spectra atdifferent points on the sample are shown on the right, indicatingmonolayer to multilayer graphene.

by altering the flow rate of precursor as shown in Fig. 2(c).Thus, adjusting the CVD parameters allows for control overthe deposit thickness.

It has been shown that the carbon dissolved in Ni at hightemperatures segregates to the surface when the substrate iscooled down, due to the decreasing solubility of carbon,resulting in graphene [8]. Our experimental results do notstrictly conform to this growth model. Assuming that alldissolved carbon (about �1% at. at 950 8C [9]) segregatesupon cooling from a fully saturated 200 nm Ni film, amaximum of a 2 nm thick deposit can be expected (under theassumption of similar lattice constants). This situation mayoccur upon slow cooling (0.15 8C/s) of the substrate, and theRaman spectrum for this case [Fig. 2(c)] shows few layergraphene (FLG), providing an estimate for the upper boundof carbon that can segregate from our Ni films. Carbides,such as Ni3C are unstable beyond 400 8C [10] and so cannotexplain thicker deposits. Thus, the mechanism of graphenegrowth on Ni is more accurately described as a combinedprocess of segregation of carbon from the bulk anddeposition from the gas phase. The latter growth on Nimay be possible due to the availability of step edges on thinNi films and catalytic activity of Ni, both of which areconducive to graphitic growth.

2.2 Growth on copper In contrast to the graphenegrowth on Ni, the growth on Cu is thought to be solelysurface growth, as carbon has a very low solubility in coppereven at temperatures close to its melting point. Indeed,isotopic and microscopic investigation of the surface hasindicated lateral growth of graphene islands on the Cusurface [11]. As shown by Rasool et al. [12] uniformcoverage of graphene on copper is observed regardless of thecrystal orientation, step edges, etc. This stems from themuchweaker catalytic action of Cu towards precursors, such thatgraphitic growth does not necessarily start or stop at thediscontinuities on copper surfaces.

High purity copper foils were used for graphene growthin a quartz tube furnace. The foils were introduced into thefurnace and heated to 1000 8C under a 10 sccm flow ofhydrogen. After waiting 15min at that temperature, methaneat 20 sccm was introduced into the furnace keeping thepressure at 310mTorr. The methane flow was shut off after20min and the furnace was cooled with compressed airmaintaining the flow of hydrogen over the samples. Thesewere removed from the furnace at room temperature. Atypical Raman spectrum of graphene lying on top of the foilsis shown in Fig. 2d. This indicates the presence ofmonolayergraphene. With a larger methane dwell time (30min), theRaman spectrum changes to resemble a superposition ofgraphene and PyC Raman spectra (see Fig. 2). Deposition ofPyC on top of the graphene is expected as graphene iscatalytically inactive and deposition should occur in a similarmanner to that on SiO2.

It should also be noted that we observed double and triplelayer graphene also in some isolated patches on copper foils.This indicates that either the copper step edges can nucleate


growth of multiple layers or segregation is also taking placeon copper. The former seems unlikely as plenty of step edgesshould be available on copper foils used in growth but thedensity of multilayers is quite small. The latter can explainthe low coverage of thicker graphene films due to very lowsolubility of carbon in copper.

2.3 Graphene from solid precursors The growthof graphitic carbon on Ni and Cu demonstrated their role incatalytically graphitising carbon. The transition metals canalso graphitise solid carbon sources [13].We can exploit thiscatalytic activity in order to convert carbonaceous depositsinto graphite or graphene. Some such schemes are shown inFig. 3, where a carbonaceousmaterial is laid in contact with acatalytic metal and the whole arrangement is annealed tographitise the carbon source. The metal can then be etchedaway after cooling to yield highly graphitised carbon orgraphene. Polymer or evaporated aromatic carbons can bechosen as the carbon source and the metal of choice istypically Ni or Cu. A similar experiment has been reportedby Sun et al. using polymers deposited on copper [14].

An scanning electron microscope (SEM) image andRaman spectrum for a processed sample produced using thesecond arrangement is also shown in Fig. 3. A thin layer ofsputtered carbon (4 nm) was used under 60 nm of Ni, theannealing was done at 1000 8C for 10min under H2

atmosphere. The etching failed to remove Ni completelyfrom the substrate and results in graphene patches. TheRaman spectra at some points across the sample indicate thatgraphene was formed under Ni. Similar results are obtainedfrom the first and third configurations, in which multilayergraphene appears on the top surface.With optimisation theseresults could be used to deposit graphene (mono ormultilayer) on top of variety of objects, leading to interestingmaterials (e.g. beads encapsulated with graphene).

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Figure 5 (online colour at:www.pss-b.com)CVDgraphene trans-ferred onto insulating substrates. (a) AFM of few layer graphenegrown on Ni after transfer and additional cleaving (scale 2mm).(b) Graphene grown on copper transferred directly with scotch tapemethod (scale 1mm).

3 Transfer process A process for the transfer ofgraphene grown on metal surfaces to insulating substrates isrequired in order to make electrical devices. Some of thesemethods are discussed in following.

The most straightforward way to transfer graphenelocated on a metal surface to other substrates is to etch themetal, which releases graphene in the etchant, allowing it tobe fished onto substrates. This is not very reliable asmonolayer graphene will most likely disintegrate intosmaller pieces in the etching medium. The disintegrationoccurs owing to the limited size of graphene flakes andthe mechanical forces tearing apart the patchwork ofgraphene flakes. Multilayer graphene as produced on Nican survive this process due to overlapping flakes and thehigher strength of thicker films. Although this methodminimises the contamination and is suitable for most of thesubstrates, it is useful only for thicker graphitic layersproduced onNi. Figure 4a shows such layers transferred ontoquartz pieces from Ni films. Applying a similar process tographene grown on copper produces graphene floating in theetching medium which breaks into smaller pieces with theslightest movement (the flake size of graphene on oursamples is about 3mm).

To produce thinner high quality graphene from thickertransferred films an additional treatment using scotch tape isused to thin multilayer graphene down using repeatedpeeling as demonstrated in Ref. [15]. The multilayergraphene grown on Ni was transferred to SiO2 substratesand then scotch tape was used to peel it. This has theadvantage of removing contamination with the peeling andproviding higher flake density than above. The result of thiscan be seen in the atomic force microscopy (AFM) image inFig. 5a. Patchy films of stitched together graphene flakes are

Figure 4 (online colour at: www.pss-b.com) Graphene obtainedusingdifferent transfermethods.(a)Multilayergraphenefilmsfishedonto quartz pieces, (b) graphene transferred using PMMA support,(c) SEM image of graphene transferred using PMMA showingcontaminants andcracks and (d)withoutpolymer support suspendedon a carbon grid. The scale bars are 1 cm, 10mm, 1mm and 1mm,respectively.

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visible. This type of film allows for studies on theconductivity of individual flakes and interflake contacts [15].

Polymer supported transfer is the most commonly usedmethod and has also been demonstrated successfully on alarger scale [16]. Typically a thin filmof poly-methyl-metha-acrylate (PMMA) is spin-coated on top of graphene grownon Cu foils. Thermal tape is applied on top of the PMMAlayer for a rigid support and then the underlying copper isetched using FeCl3 solution (1M). The resulting films haveto be washed with deionised water and can be placed on topof suitable substrates with the graphene side in contact withthe surface. Uniform pressure and heat (150 8C) is applied tothe film, which melts the PMMA and helps it to conform tosubstrate topology. The heat also decomposes the adhesive inthe thermal tape, allowing easy removal. In the last step thePMMA layer is dissolved with hot acetone, leaving behindgraphene adhered to the substrate surface (Fig. 4b). It is notnecessary to use thermal tape in this process, however ithelps with handling. The polymer supported graphene can bedredged on to the desired substrates and polymer can bedissolved after drying the substrate thoroughly.

Polymer supported films provide almost completetransfer of graphene from metal to substrates. However,problems arise with the subsequent removal of polymer fromthe graphene surface. It is well known that polymer residuesremain even after prolonged solvent exposure (Fig. 4c). Theorganic contamination can affect the properties of grapheneaswell as contactsmade to it; therefore, formost applicationsit is necessary to remove these contaminants. Thus, whenpossible a better strategy is to use a process in which thegraphene avoids being in contact with polymers. This can beachieved by placing a grid onto graphene on copper tosupport it and then etching the copper. This has been shownby Regan et al. to directly transfer graphene onto TEM grids[17]. We adopted and optimised this method by using gridsmade from photo resists, which can have flexible sizes andmesh widths. We expose a positive photoresist using ashadow mask, followed by development and an anneal at400 8C. Annealing makes the photoresist insoluble insolvents. These photoresist grids were then left to dry ongraphene grown on copper, so that they adhered to thegraphene. The underlying copper was then etched and the


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grids with graphene were fished on top of copper TEM grids.During the process graphene in specific areas does not comeinto contact with polymer and this is seen in SEM images inFig. 4d. In this way very clean suspended graphene samplescan be obtained in a reliable manner.

Mechanical exfoliation can be used to transfer graphenefrom the metal surface to other substrates. The sticky surfaceof scotch tape was stuck on to graphene lying on copper foil,and peeled apart. The same surface was then put onto a SiO2

substrate and pressed gently on top. The scotch tape waspeeled away after leaving it in that position for 15min. Thisprocess results in some flakes getting transferred to the SiO2

substrate. Figure 5b shows an HRSEM image of the surfaceof SiO2 substrate after this process. Clean graphene flakes inthe micron range are visible. These can be easily contactedwith E-beam lithography. The advantage of this method overmechanical exfoliation of graphite is that the flakes yieldedare predominantly monolayer with just one transfer step.

4 Conclusions In summary, we described the growthof graphene on Ni and Cu substrates by CVD. Both metalscatalyse the formation of graphene and by optimising thegrowth parameters graphitic films of varying thickness canbe obtained. We further showed the graphitisation of solidprecursors to graphene using the same catalytic metals. Foreffective usage of graphene grown on metal substrates areliable method to transfer the monolayered material has tobe developed.We discussed several transfermethods to yieldhigh quality layers on insulating substrates or suspended ongrid structures.

Acknowledgements This work was supported by the SFIunder Contract No. 08/CE/I1432. S. K. acknowledges the EmbarkInitiative via IRCSETscholarships.H.-Y.K. andK.L. acknowledgeNational Research Foundation of Korea (NRF) grant.

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CVD growth and processing of graphene for electronic applications