A review of chemical vapour deposition of graphene on copper †
Cecilia Mattevi,*a Hokwon Kim a and Manish Chhowalla *ab
Received 5th July 2010,Accepted 4th October 2010DOI:10.1039/c0jm02126a
The discovery of uniform deposition of high-quality single layered graphene on copper has generated significant interest.That interest has been translated into rapid progress in terms of large area
deposition of thin films via transfer onto plastic and glass substrates.The opto-electronic properties of the graphene thin films reveal that they are of very high quality with transmittance and conductance values of >90%and 30U /sq,both are comparable to the current state-of-the-art indium tin oxide transparent conductor.In this Feature Article,we provide a detailed and up to date description of the literature on the subject as well as highlighting challenges that must be overcome for the utilization of graphene deposited on copper substrates by chemical vapour deposition.
Introduction
The unique properties of graphene have triggered numerous fundamental and technological studies.
It is well known that graphene is a mimetal where the charge carriers behave as Dirac fermions (zero effective mass),1which gives ri to extraordinary effects such as mobilities up to 200000cm 2V À1s À1,2ballistic transport distances of up to a micron at room temper-ature,3half-integer quantum Hall effect,3,4and absorption of only 2.3%of visible light.5The large carrier mobilities also make it potentially uful for high frequency electronic devices 6while the low absorbance complemented with its mi-metallic nature makes it an ideal transparent conductor where transparency and low resistance are required.7Integrated devices will require wafer scale deposition that can be procesd using existing or post complementary metal oxide miconductor (CMOS)fabrication techniques.Implementation as a transparent conductor will require uniform deposition over large areas with controllable number of graphene layers.The requirements have led to the development of a rapidly evolving rearch thrust within the field of graphene bad on deposition of high quality and uniform thin films over large areas with controllable thickness.
The best quality graphene,in terms of structural integrity,is obtained by mechanical cleavage of highly oriented pyrolytic graphite.8Thus the efficacy of any new deposition methods is determined by comparison with properties of pristine mechan-ically exfoliated graphene.Although pristine graphene has very low concentration of structural defects,which makes it inter-esting for fundamental studies,t
he flake thickness,size and location are largely uncontrollable.Several strategies are pres-ently being pursued to achieve reproducible and scalable graphene on substrates.One example is covalent 9,10or non-covalent 11exfoliation of graphite in liquids.The methods however can introduce structural and electronic disorder in the graphene.12–14Another example is the conversion of SiC(0001)to
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graphene via sublimation of silicon atoms at high temperatures.15High quality wafer scale graphene with switching speeds of up to 100GHz 6has been demonstrated using this technique.Although the price of the initial SiC wafer is relatively high compared to that of silicon,the technique maybe suitable for radio and THz frequency electronics where the excellent performance of the devices could offt the cost of the wafers.
The most promising,inexpensive and readily accessible approach for deposition of reasonably high quality graphene is chemical vapor deposition (CVD)onto transition metal substrates such Ni,16Pd,17Ru,18Ir 19or Cu.20In particular,recent developments on uniform single layer deposition of graphene on copper foils over large areas have allowed access to high quality material.20,21Although CVD of graphene on copper is relatively new,veral groups around the world have already reported excellent device characteristics such as mobilities of up to 7350cm 2V À
1s À1at low temperature and large area growth (up to 30inches).21In this article,we provide a detailed review of the most important aspects of graphene growth on copper by CVD.
Graphene on transition metals
宽带上行速度The formation of few layered graphene resulting from prepara-tion of transition metal surfaces and in industrial heterogeneous catalysis 22has been known for nearly 50years.22In fact,the concept of combining carbon with other materials and then dissociating it to form graphite was first propod in 1896.23,24Layers of graphite were first obrved on Ni 22,25,26surfaces that were expod to carbon sources in the form of hydrocarbons or evaporated carbon.At about the same time,the formation of thin graphite layers on single crystal Pt 27,28substrates was obrved in catalysis experiments.It was surmid that the formation of graphite was the conquence of diffusion and gregation of carbon impurities from the bulk to the surface during the annealing and cooling stages.The interest in graphene has led to the revaluation of the methods for controllable deposition.Indeed,graphene growth has been demonstrated on a variety of transition metals [Ru,18,29Ir,19,30Co,31Re,32Ni,33–35Pt,31,36Pd 31,37]via simple thermal decomposition of hydrocar-bons on the surface or surface gregation of carbon upon cooling from a metastable carbon–metal solid solution.The
a
Materials Department,Imperial College,London,SW72AZ,UK.E-mail:c.mattevi@imperial.ac.uk b
Materials Science and Engineering,Rutgers University,Piscataway,NJ,08854,USA.E-mail:manish1@rci.rutgers.edu
†This paper is part of a Journal of Materials Chemistry themed issue on Chemically Modified Graphenes.Guest editor:Rod Ruoff.
FEATURE ARTICLE /materials |Journal of Materials Chemistry
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carbon solubility in the metal and the growth conditions deter-mine the deposition mechanism which ultimately also defines the morphology and thickness of the graphene films.
Graphene can grow on veral hexagonal or other crystallo-graphic surfaces.Growth on hexagonal substrates is frequently referred to as epitaxial even if significant lattice match is abnt between the graphene and substrate.For lattice mismatch of less than 1%19as on Co(0001)31and Ni (111)33surfaces,graphene growth is commensurate with the substrate lattice.In contrast,lattice mismatch between graphene and Pt(111),38Pd(111),38Ru(111)29and Ir(111)30is >1%and therefore growth is incom-mensurate as indicated by the obrvation of Moiere patterns.The growth on Ir(111)39,40is particularly interesting becau the Moiere patterns indicate a template on the graphene surface for spar adsorption of hydrogen,resulting in a superlattice struc-ture of graphane islands that induce a band gap of 0.5–0.73eV 40at the Fermi level.The appearance of a band gap is particularly appealing for graphene TFTs capable of exhibiting large on/off ratios.
Recent results of growth on relatively inexpensive poly-crystalline Ni 16,34,35,41and Cu 20substrates have triggered interest in optimizing CVD conditions for large area deposition and transfer.Graphene deposited on polycrystalline Ni and trans-ferred onto insulating substrates exhibit mobility values 16of up to 3650cm 2V À1s À1and half-integer quantum Hall effect.16However,the fundamental limitation of utilizing Ni as the catalyst is that single and few layered graphene is obtained over few to tens of microns regions and not homogeneously over the entire substrate.34Th
e lack of control over the number of layers is partially attributed to the fact that the gregation of carbon from the metal carbide upon cooling occurs rapidly within the Ni grains and heterogeneously at the grain boundaries.
In contrast to Ni,exceptional results in terms of uniform deposition of high quality single layered graphene over large areas have been recently achieved on polycrystalline copper foils.20The initial 20and subquent follow-on 21,42–50studies have demonstrated the growth of single layered graphene over areas as large as 30-inches.Detailed imaging and spectroscopic analys have revealed that over 95%of the copper surface is covered by single layered graphene while the remain
ing graphene is 2-3layers thick,independent of growth time or heating and cooling rates.20The growth on copper is simple and straightforward,making high quality graphene over large area readily accessible.Furthermore,thin copper foils are inexpensive and can be easily etched with solvents available in most laboratories so that transfer onto desired substrates can be readily achieved.The features as well as the fact that copper is inexpensive,make it an appealing process for the deposition of graphene.
Graphene from silicon carbide and transfer onto insulating substrates
In addition to CVD methods,epitaxial growth of graphene is also achievable on insulating SiC(0001)6,15substrates via subli-mation of silicon atoms and graphitization of remaining C atoms by annealing at high temperature (1000–1600 C).Epitaxial graphene on SiC(0001)has been demonstrated to exhibit high mobilities,especially multilayered films.Recently single layered
SiC converted graphene over a large area has been reported and shown to exhibit outstanding electrical properties.51
For electronic applications,graphene on insulating substrates such as plastic foils,glass or SiO 2/Si wafers is required.Prently,transfer of the as-grown graphene from metallic surfaces onto desired in
sulating substrates using various different methods is performed.16,21,52The most straightforward method for trans-ferring graphene grown on metals is to chemically etch the metal away to obtain free floating graphene membranes that can be scooped onto desired substrates.16Wet etching of substrates such as Ni and Cu 53are feasible but is challenging for metals such as Ru,Ir,Pd,Pt.54Dry transfer methods such as peeling of mm sized flakes of epitaxial graphene from SiC(0001)using a bilayer of Au/Polyamide 55stamp has been demonstrated.
In the remainder of the feature article,we describe the state-of-the-art of graphene growth on copper along with recent advancements towards transfer and direct deposition onto insulating substrates.We highlight possible mechanisms for graphene growth on copper,which may be helpful in depositing the material directly onto insulating substrates.We also describe the state-of-the-art in terms of opto-electronic properties of the transferred graphene thin films and end with conclusions and outlook for graphene on copper.
Substrate requirements for graphene growth
Copper has been shown to cataly the growth of veral carbon allotropes such as graphite,56diamond,57carbon nanotubes 58,59and most recently graphene 20as shown in Fig.1.Th
初学者打领带e growth of graphite on copper was unintentionally achieved in 199156,60in experiments designed to catalyze the growth of diamond by CVD.In the initial experiments,single and multi layered graphene were produced on (100),56,60(110),56,60(111),56,60and (210)60copper surfaces via carbon implantation at elevated
temperatures
Fig.1SEM images of carbon allotropes grown on Cu:(a)random network of single walled carbon nanotubes.58Reprinted with permission from Ref.58,copyright 2006,American Chemical Society.(b)Diamond on Cu(111).57Reprinted with permission from Ref.57,copyright 1997,Elvier.(c)Single layer graphene,80the int shows STM atomic reso-lution images of graphene;(d)as received copper foil ud as the substrate.In panel (c)dark regions reprent nucleation sites.Terraces on the copper surface are also visible.
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and subquent out-diffusion through carbon dissolution-precipitation mechanism.Specifically,carbon implantation into Cu was achieved by bombarding at 70–200keV (with dos of up to 1018ions/cm 2)at 800–1000 C.After bombardment,the implanted copper was held at 800–1000 C for veral hours to allow carbon to diffu to the surface where it precipitated into graphitic planes.Bad on the implantation study obrvations which was designed to grow diamond,the rearchers switched to hot filament CVD (HFCVD)57to deposit diamond but obrved thin graphite on copper substrates under some conditions.
The requirement of high quality graphite as moderators in nuclear reactors in the early 1960s 61led to substantial knowledge of crystalline sp 2carbon formation on hot transition metal surfaces.25The precipitation of graphite via formation of tran-sition metal and carbon solid or liquid solution (either by exposure to hydrocarbons 25,26,36,38or deposition of amorphous carbon on the hot metal surface 62,63)has been widely studied and the mechanisms have been verified for all known catalysts for graphite (i.e.the transition metals belonging to the VIII group).Although many different experimental conditions have been found to be important,the graphite properties have been shown to be very nsitive to cooling rate and exposure time to carbon source.In addition to pure transition metals,carbides of tran-sition metals such as TaC,64WC,64TiC,64HfC 64and LaB 365that have high
coordination numbers and are highly reactive can also rve as catalysts for graphite precipitation.54,66
Of the various transition metals,graphitic carbon formation on Ni has been intensively studied owing to its suitability as catalyst for high quality graphite 25as well as nanotubes.67Here we briefly describe the formation of highly crystalline sp 2carbon on Ni to contrast it with a different formation mechanism on copper.The pha diagram of Ni and C (Ref.68)reveals that the solubility of carbon in nickel at high temperature (above $800 C)forms a solid solution and lowering the temperature decreas the solubility,allowing carbon to diffu out of Ni (Fig.2a).It is well known from metallurgical studies that the formation of metastable Ni 3C pha promotes the precipitation of carbon out of Ni.Carbon preferentially precipitates out at the grain boundaries of polycrystalline Ni substrates so that the thickness of the graphite at the grain boundaries is substantially larger than within the grains.Thus,the number of graphene layers can significantly vary along the surface of Ni.Co and Fe show similar catalytic behaviour as can be surmid from the pha diagrams [Fig.2b and 2c,respectively 68],which show carbon solubility at 850–1000 C while carbide and graphitic phas are stable at lower temperatures.63Like Ni,Co forms a metastable carbide at high temperature which parates into pure metal and graphite during cooling of the cobalt-carbon solid s
olution (Fig.2b).In the ca of Fe (Fig.2c),cementite (Fe 3C)is a stable carbide and therefore graphite precipitation from Fe can be obtained only under very specific cooling rates.63The ability to form sp 2crystalline carbon from solid solutions of various transition metals is dependent on their carbon affinity.That is,in the ca of Fe there is competition between the formation of graphitic carbon and carbide owing to the high affinity between Fe and C.
The catalytic power of transition metals and some of their compounds is well known 54,66and aris from partially filled d-orbitals or from the formation of intermediate compounds that
adsorb and activate the reacting substances.Catalysis by metals results from their ability to provide low energy pathways for reactions either by facile change of oxidation states or by formation of appropriate intermediates.In light of this,Fe has asymmetrical distribution of electrons in the d-shell {[Ar]3d 64s 2},leading to mutual repulsion which may explain its higher affinity towards carbon 54with respect to Co,Ni and Cu where the 3d shell is progressively filled,suggesting less reactive configurations (Table 1).Copper has the lowest affinity to carbon as reflected by the fact that it does not form any carbide phas 69,70(Fig.2d)and has very low carbon solubility compared to Co and Ni (0.001–0.008weight %at $1084 C for Cu,66,70$0.6weight %for Ni at $1326 C,and $0.9%weight for Co at $1320 C)68(Fig.2).The low reactivity with carbon can be attributed to the fact that copper has
a filled 3d-electron shell {[Ar]3d 104s 1},the most stable configuration (along with the half filling 3d 5)becau the electron distribution is symmetrical which minimizes reciprocal repul-sions.As a result,Cu can form only soft bonds with carbon via charge transfer from the p electrons in the sp 2hybridized carbon to the empty 4s states of copper.54,71Hence this peculiar combi-nation of very low affinity between carbon and copper along with the ability to form intermediate soft bonds makes copper a true catalyst,as defined by textbooks,for graphitic carbon formation.The 3d 7and 3d 8orbitals of Co and Ni are between the most unstable electronic configuration (Fe)and the most stable one (Cu).Bad on this,it emerges that the most suitable catalysts for graphitic carbon formation are tho transition metals that have low affinity towards carbon but that are still able to stabilize carbon on their surfaces by forming weak bonds.
An interesting example of a metal that has carbon solubility at high temperature between that of nickel and copper and does not form a carbide is ruthenium.Growth of single layer graphene over large area has been achieved on polycrystalline ruthenium thin films (50–500nm)72as well as on Ru(0001)single crystals.18The growth on Ru is carried out by enrichment of interstitial carbon via exposure of the substrate to ethylene (5Â10À7Torr at 950 C),followed by slow cooling in UHV to 550 C.The carbon solubility in Ru,which is lower than Ni but higher than Cu,can be lowered by applying a
gradual decrea of the temperature to obtain uniform graphene nucleation and growth.Some parallels between graphene growth on Cu and unexpected single walled carbon nanotube growth on noble metals such as Ag and Au can also be made,73suggesting that graphene growth on Ag and Au should be possible as recently demonstrated 74,75for Ag.Graphene growth on copper
Graphene on copper is in principle straightforward,involving the decomposition of methane gas over a copper substrate typically held at 1000 C.Growth of predominantly monolayer graphene on copper foil has recently been reported using hexane 47at 950 C to explore the possibility of using liquids precursors that could facilitate the doping of graphene during synthesis by using nitrogen and boron containing organic solvents.47The specific growth parameters that have been utilized for achieving the best graphene films on Cu are summarized in Table 2.Most of the depositions have been performed on copper foils with thickness ranging from 25–50m m.20,21,46–48,76Recently graphene deposition on e-beam 43,45and thermally evaporated 42,44copper thin films
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(thickness >500nm)on SiO 2/Si substrates have been demon-strated.For thin film Cu catalysts,the thickness must be controlled to ensure that de-wetting does not occur.However,de-wetting has been ud to directly deposit graphene onto insulating SiO 2/Si substrates.More specifically,recently it has been demonstrated that the copper thin films can be evaporated off after graphene growth so that the as-grown graphene rests on the insulating substrate.43If done controllably and reproducibly,this method could be uful for direct deposition of high quality
graphene onto insulating substrates without the need for trans-fer.Although the most commonly ud deposition temperature is 1000 C,growth at temperatures ranging from 800–950 C 44,50,76have also been reported.The CVD of graphene on copper is done under low (0.5–50Torr)20,42or atmospheric pressure 45of methane and hydrogen gas mixture at various ratios as indicated in Table 2.
Copper substrate pre-treatment
Thus far,experiments have indicated that there is little influence of deposition parameters on the physical and electrical properties of as-grown graphene on copper.However,the pre-treatment of the copper foils has been found to be important in obtaining large graphene domains in the as-deposited
product.20,21The copper substrate pre-treatment rves veral important func-tions that ensure high quality graphene deposition.First,as-received Cu is covered by native oxide (CuO,Cu 2O),77which
Table 1Carbon affinity to different transition metals is reported.The affinity decreas moving from Fe to Cu.Noble metals are indicated in the last column and listed in decreasing ‘‘noble’’character from top to bottom Fe Co Ni Cu Ru Rh Pt Ag Os
Ir
Pd
Au
Fig.2Binary pha diagrams of transition metals and carbon:68,70(a)Ni–C;(b)Co–C;(c)Fe–C;(d)Cu–C.Reprinted with permission of ASM International Ò.All rights rerved,The low carbon solubility in Cu,of $0.008weight %at $1084 C as reported in Ref.70,is highlighted in the int of panel (d)70for the temperature and composition of interest for graphene growth.Reprinted with permission from Ref.70,copyright 2004,Elvier.
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reduces its catalytic activity.Therefore prior to deposition the Cu substrate must be annealed in a hydrogen reducing atmosphere at 1000 C.78Wet chemical pre-treatment by dipping in acetic acid 79has also been demonstrated to partially remove Cu 2O.The annealing stage prior to deposition is also important for increasing the Cu grain size and rearranging the surface morphology (introduction of atomic steps,elimination of surface structural defects)to facilitate growth of graphene flakes.Typi-cally the Cu foils are annealed for 30min 20,21,45,76while shorter treatment has been reported for s
ub-micron Cu thin films.42–44A systematic correlation between the homogeneity of graphene domains and Cu grain size and crystallographic orientation has yet to be elucidated.
In Fig.3,taken from Ref.20,the growth of single layer gra-phene at different times is shown.The scanning electron microscopy (SEM)images of graphene on copper can be difficult to interpret due to the atomic thickness of the film.Here we briefly explain the evolution of the graphene film deposited on copper by describing the SEM images in Fig.3.In Fig.3a,gra-phene of finite size (one is indicated by the larger oval)in the form of dark irregularly shaped flakes can be en.The nucle-ation site of one of the flakes is indicated by the smaller oval in Fig.3a.As the growth time is incread,the graphene domains
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1000
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Fig.3SEM images of graphene on Cu for different growth times:20(a)1min;(b)2.5min,(c)1min from Ref.80for comparison;and (d)10min.In panel (a)the smaller circle reprents a possible nucleation site and a Cu grain boundary is also indicated.The larger circle indicates a gra-phene domain.In pan
el (b)the highlighted region is a void where the graphene domains have yet to join to form a continuous layer.The curved lines reprent terraces on the copper surface.Image in (c)is provided to highlight the differences in the nucleation density and initial domain sizes by changing the Cu pre-treatment conditions and the CH 4pressure.The average graphene domain size is (4.8m m Æ1.2m m)in the image shown in panel (a)while in (c)it is (0.84Æ0.25m m).In panel (d),SEM image of a continuous graphene film on Cu is shown.Wrinkles where the graphene domains have presumably joined along with a dark patch indicated by the circle reprenting a double layer are highlighted.Panel (a),(b),(d)are reprinted with permission from Ref.20,copyright 2009,American Association for the Advancement in Science.
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