initial stage of graphene growth on a cu substrate-temperatura crescimento.pdf

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Published October 10 2011

r 2011 American Chemical Society 22369 dxdoiorg101021jp205980d | J Phys Chem C 2011 115 22369ndash

22374

ARTICLE

pubsacsorgJPCC

Initial Stage of Graphene Growth on a Cu Substrate

Chanyong Hwang dagger K Yoodagger S J Kimdagger E K Seodagger H YuDagger and L P Birosect

daggerCenter for Nano-imaging Technology Korea Research Institute of Standards and Science 267 Gajeong-Ro YuseongDaejeon 305-340 KoreaDaggerCenter for Nano-Bio Technology Korea Research Institute of Standards and Science 267 Gajeong-Ro YuseongDaejeon 305-340 KoreasectResearch Institute for Technical Physics and Materials Science H-1525 Budapest POB 49 Hungary

rsquo INTRODUCTION

Ever since the 1047297rst discovery of mechanically exfoliated

graphene from the bulk graphite1 graphene research has drawna lot of attention Graphenersquos unique properties distinguish itfrom other normal conductors these include the unusualquantum Hall eff ect the charge carriers behaving like Diracfermion and Klein tunneling24 These fundamental issuesrelated to graphene have been studied both experimentally andtheoretically for the last 7 years5 However the problems of growth are still under debate The detailed growth process isnot clearly known especially in the submonolayer regionMoreover the growth of graphene on insulating layers is oneof the most critical issues for its application toward solid-statedevices but was not within the scope of this studyIn this report

we present more details on the initial growth of graphene on a

Cu foil The growth of graphene on Cu foil was 1047297rst reported in2009 by Ruoff 6 and it has drawn a great deal of attention due toits pure surface chemical process character in contrast withother transition metals such as Ni where the growth involves

bulk diff usion processes too Though many eff orts have beenmade for graphene growth on Cu foil79 microscopic detailsfor the growth on the Cu surface are not clearly understoodThus a recipe to form high-quality graphene has not yet beenestablished10 Recently nonlinearity of the graphene growthrate with C-adatom density suggests that growth proceeds by the addition of C atom clusters to the graphene edge11 Howeverthere is no experimental support for this cluster behavior

This nonlinearity in growth rate can have other origins sincethe rebonding process is more critical for the completion of

the graphene growth One advantage of using Cu foil forgraphene growth is the low solubility of carbon in Cu Inaddition Cu does not form an alloy with carbon which meansno carbide formation This characteristic is attributed to thealmost 1047297 lled 3d bands the bonding of graphene with the Cusubstrate is through hybridization of the orbital of grapheneand the unoccupied state of Cu Thus the bonding betweengraphene and the Cu substrate is the smallest among 3dtransition metals These characteristics are quite essentialfor the catalyst for Cu to lower the dehydrogenation barrierof the hydrocarbon used in graphene growth Because theformation of sp2 honeycomb-chained graphene with fewerdefects needs a high-temperature process during growth

hydrocarbon cracking at a lower temperature using suchtechniques as the plasma method would not be advantageousIn most cases the growth time for completion of a single layeris less than 1 min if the pressure and 1047298ow rate of thehydrocarbon are not extremely low Moreover energeticcarbon atoms are more likely to form an sp3 bonding Theuse of acetylene C2H2 or ethylene C2H4 cannot be asolution for the growth of high-quality graphene Howeverthis annealing temperature is still much higher than the

Received June 25 2011Revised September 29 2011

ABSTRACT Thegrowth of graphene on copper foil has attracted attention in thelast two years due to its feasibility for a controllable growth process One of the key issues remaining for practical application of graphene in solid-state devices isgrowth with a large grain size Because the CC bond in graphene is strongenough to prevent the evaporationcondensation process Smoluchowski ripen-ing is expected to be the dominant process for coalescence In this article we

present the initial growth process of graphene on a Cu foil via the chemical vapordeposition method by using secondary electron microscopy and Raman micro-scopy In contrast to the other transition-metal substrates such as Ir and Rh thecenter of graphene islands binds to the substrate more rigidly than the edge Forthe growth with a large grain size the graphene should be grown on a substrate

with a low diff usion barrier for the carbon clusters (or islands) with low 1047298ux this isthe controlling parameter for the grain size In addition high-temperature growth(or annealing) generally becomes a dominant condition for the completion of graphene growth with large grains after thecoalescence



22370 dxdoiorg101021jp205980d | J Phys Chem C 2011 115 22369ndash22374

The Journal of Physical Chemistry C ARTICLE

temperature for thermal decomposition of methane Methanestarts to decompose at temperatures above 800 K12 In additionthe sticking coefficient of methane on copper is extremely small

When the substrate temperature increases the methane caughton it can decompose into successive hydrocarbon intermediatesthat is CH3 (methyl) CH2 (methylene) and CH (methylidyne)These hydrocarbon intermediates are more reactive thanmethane so they evolve to a thermodynamically more stablecarbon atom on the Cu surface

rsquoEXPERIMENTAL SECTION

The graphene was synthesized on a polycrystalline Cu foil(25 μm thick) by the thermal CVD method Initially Cu foils

were heated inside a chamber covered by the furnace under H2

gas pressure (012 sccm) upto 1263 K for 30min After 40minannealing of Cu foils for cleaning at a temperature of 1263 KCH4 andH2 mixture gases were simultaneously transferred to thefurnacefor a desiredgrowthtime The temperature of thefurnace

was immediately lowered to RT at a cooling rate of 50 Kmin We changed all of the growth parameters to optimize thegraphene growth on this Cu foil Scanning electron microscopy and Raman spectroscopy measurements were carried out afterthe graphene samples were exposed to the air

rsquoRESULTS AND DISCUSSIONFigure 1 shows a series of SEM images for the initial growth of

graphene on Cu foil at a temperature of 1263 K As the growthtime increased the size of the graphene island started to increaseThe size of the grain was in the range of 2030 μm in (c) Thegrowth temperature was maintained at 1263 K and the gasmixture ratio between H2 and CH4 was 021 The growth time

was varied at (a) 15 (b) 30 (c) 60 and (d) 120 s with a totalpressure of 780 mTorr The number of nucleation sites increased

by more than 100 between 15 and 30 s growth and decreased between 30 and 60 s growth Here the 1047298ow rate of methaneincluding theH2 carrier gas waslowsothe growth timerequired wasrelatively high when comparedtocases ofhigh1047298ow rates of methane

As the growth time increased the size of islands began to grow to form bigger islands (bd) (Figure 1) At this stage the islanddensity is big enough so that the carbon clusters (or atoms)can easily reach the island in the diff usion process Because the

bond strength between carbon atoms is quite high two adjacentislands cannot evolve to coalescevia edge diff usion Instead if thetemperature is relatively low (below 1273 K) then the totalnumber of nucleation sites increases while the size of already established islands increases with the carbon atoms (or smallclusters) nearby Here the island shape was more or lessdendrimer-like instead of fractal The lobes of the islands have

a backbone with the orientation determined by the crystallinestructure of the substrate Usually the transition from fractal todendritic growth comes when the 1047298ux of the constituent of the1047297lm increases13 Because the temperature of the substrate and thediff usionbarrier are also aff ecting the eff ective 1047298ux forthe growthit is not easy to conclude whether it is a fractal or a dendrimer

based on the 1047298ux only Two factors including the high tempera-ture and the strong bond strength between carbon atoms play animportant role on the shape of islands We will discuss this inmore detail later Figure 2 shows an SEM image of the graphenegrowth depending on the growth temperature with H2CH4 =235 growth time = 20 s The formation of a new nucleus ispossible only if a certain supersaturation is reached The super-saturation increases with increasing temperature as the decom-

position rate of the precursor also increases with increasingtemperature This is what happens from Figure 2a toFigure 2b Also the smaller number of growing islands at highertemperatures is an indication of the coalescenceof smaller islands

with the larger ones as shown in Figure 2c Because theevaporationcondensation process due to the diff erence in

vapor pressures between two (small and big) islands is notallowed in graphene growth at this temperature Ostwald ripen-ing is not possible14 The bond strength between carbon atoms isfairly high (36 eV) so that the edge diff usion is not allowed hereThe shape of the island grown at higher temperatures is muchmore complicated than that of the islands grown at lowertemperatures This indicates that the smaller islands coalescing

Figure 1 SEM images for the initial growth of graphene on Cu foil at the temperature of 1263 K





with the larger ones do not lose their individual crystallographic

orientation completely In this way after a coalescence processtakes place the larger island which incorporated the smallerones will have new lobes Figure 3 shows the island density vs1000T in log scale for several diff erent growth conditionsBelow 1253 K the variations in the density of the grapheneislands are small Above this temperature the slope of the

variation of the island density becomes large This means thatthe coalescence of the islands does not follow the Ostwaldripening process but rather the Smoluchowski ripening processthat is the ripening via island diff usion and aggregation From theslope in the Arrhenius plot in Figure 3 the migration barrier of the carbon islands is dependent on the 1047298ow rate of methane15

Below 1253 K it ranges from 16 to 34 eV whereas above

1253 K it ranges from 40 to 89 eV depending on the 1047298owrateof carbon This type of process was suggested in the graphenegrowth on an Ir(111) surface16 Small graphene islands candiff use more easily so they can coalesce with big islands Oncethey form a single island the rebonding process between carbon

Figure 3 Island density of graphene islands vs 1000T where T is thegrowth temperature (a) H2CH4 = 15 growth time = 10 s (square)(b) H2CH4 = 27 growth time = 10 s (circle) and (c) H2CH4 = 235growth time = 20 s (diamond)

Figure 4 SEM image that shows the onset of the coalescence of twoislands

Figure 2 SEM images of graphene growth depending on the growth temperature with the H2CH4 = 235 and a growth time of 20 s (a) 1213 (b)1253 (c) 1273 and (d) 1303 K





atoms takes placeto eventually forma well-characterized graphenelayer Here the temperature is again the most critical parameterfor forming a uniform graphene layer We present an SEM imagein Figure 4 that shows the onset of the coalescence of two bigpolygonally shaped islands in several locations on the samesample Smoluchowski ripening also involves the edge diff usionin the growth of the metal overlayer However due to the strongCC bond here the rebonding process between carbon atomsplays an essential role in graphene growth

Because the 1047297lm growth is basically a nonequilibrium processthe kinetics of surface diff usion plays an important role in

determining the morphology of the 1047297lm On the basis of a kineticMonte Carlo simulation of the 1047297lm growth there are threeimportant factors for the morphology of the island to form in themonolayer regime the temperature diff usion barrier on top of the terraces and Schwoebel barrier across the step edge Thesethree parameters contribute to the overall shape of the 1047297lmmorphology The most distinctive features are the fractal (ordendrimer)-like or polygon-type islands based on the substratestructure Many edges show hexagon-shaped structures whichmeans that there exists a substrate eff ect Figure 5 shows theimage of the partially covered graphene overlayer on the Cu foilThe grain boundary of the Cu foil is clearly seen to be near thecenter which crosses from the top left to the bottom right Thesecondary electron yield for copper is far larger than 1 whereas

that of graphic carbon is slightly above 1 Thus if the graphenelayer covers the top of the copper foil the number of secondary electrons decreases compared with that of copperThe graphene-covered area which was con1047297rmed by the Raman spectroscopyis the dark part The grain boundary or step edge of the Cusubstrate cannot be a nucleation or pinning site for graphenegrowth This means that the diff erence between the Schwoebel

barrier and the diff usion barrier should be very small Theconstituent carbon atoms (or clusters) involved in nucleationdid not seem very diff erent on the terrace or across the steps

Some regions on the graphene island are less dark (gray)compared with the black areas in the SEM measurement These

blacker areas are easily found near the edge of the graphene island

which is shown in Figure 6 This area with enhanced contrast isclearly distinguished from the dark lines originating from the stepsat the copper substrate which are similar to wood grain We show the optical and Raman mapping of these partially coveredgraphene islands in Figure 7 The bright area on the grapheneisland (identi1047297ed by SEM and at the center of island) yields ahigher intensity at Raman spectroscopy (2685 cm1) while thedarkerarea yields a higherintensity at (2660 cm1) This 2D bandstems from the second-order double resonant Raman scattering

near thezone boundary The shape of thespectrum shows that it iscomposedof a singlelayer and thewavenumber is very close to thegraphene Thepeak shift of the 2D state wasstudied quite recentlyand many controlling parameters (eg number of layers tem-perature excitation wavelengths) are present Recently a smallstrain of less than 1 has been found to induce a shift in the 2Dstateof20cm117 This can bedueto phononsoftening due to thelesser interaction of graphene with the substrate near the edgeSometimes thisphonon softening (lessbounding to the substrate)happens inside the island but this has been attributed to theinhomogeneous substrate eff ect Peculiar domelike shapes of graphene clusters have been reported to form on Ir(111) andRh(111) surfaces1819This meansthat at the edge theinteraction

Figure 5 SEM image of a partly covered graphene overlayer on Cu foil magni 1047297ed in series where the scale bar is marked

Figure 6 SEM image of a graphene island on the Cu foil The edge of each island shows dark contrast compared with that of the center





between graphene and the Ir or the Rh substrates is more rigidthan that at the central region which is opposite to the case of graphene islands on a Cu substrate The diff erence between Ir andCu may be attributed to the diff erent growth paths of carbon

atoms (or clusters) Nucleation of thesecond layer of graphene onIr(111) may take place under the 1047297rstlayer of graphene20 which isnot possible in Cu(111) Another possibility is the relatively largelattice mismatch between graphene and Ir (or Rh) compared withCu The nearest-neighbor distances in Ir(111) and Rh(111) are271 and 261 Aring respectively whereas it is 254 Aring in Cu(111) Thelattice constant in graphene is 246 Aring This means that the size of the superstructure formed is smallest in Cu and more Cu atomscan be located near the local minimum energy position Consider-ing the lattice constant and the interaction energy betweengraphene and substrate Au will be a better substrate to grow graphene with a large grain size at a shorter time Practically it isnoteasyto 1047297nd the smooth gold foil since it canbe easilyscratched

rsquoCONCLUSIONS

In conclusion the initial growth process of graphene on a Cufoil was studied by using SEM and Raman microscopy Wesummarize the initial growth of graphene islandson Cuas shown

in Figure 8 Once the nucleation of graphene starts (a) thenumber of nucleation sites increases (b) with the distribution of diff erent sizes of islands If the temperature is high enough so thatthe island diff usion is possible then they start to coalesce to forma bigger island via island diff usion (c d) that is the so-calledSmoluchowski ripening process In the meantime the smallclusters (or islands) can be attached to the big island to grow since the smaller clusters diff use easily However the edgediff usion is quite limited due to the strong bonding betweencarbon atoms Because the temperature for the growth of graphene is high the rebonding process between carbon atomsdominates to control the overall shape of the island This edgepart binds to the substrate loosely compared with the central part

Figure 7 Optical and micro-Raman image plot of graphene islands on Cu foil The Raman intensity scale bar is shown below the image

Figure 8 Schematic drawing of the graphene growth on Cu





since the edge part keeps rebonding process during the growthThis has been con1047297rmed by Raman microscopy that the edge of the island was identi1047297ed to have a lower 2D wavenumber thanthat of the central part of the island Once the island becomes bigenough so that the island diff usion is not easily accessible then atthis stage they are more or less percolated so that until thecompletion of the single layer the small carbon clusters or atoms

can be attached to the edge to extend their sizes (e f) Thereforeone recipe to grow the graphene with large grains is to usethe substrate with the small diff usion barrier Also high tem-perature and the low 1047298ux is the essential step toward the growthof large grains Actually the Au substrate seems to be a goodcandidate for the graphene growth if it can maintain the 1047298atterrace with low surface roughness

rsquoAUTHOR INFORMATION

Corresponding AuthorE-mail cyhwangkrissrekr

rsquoACKNOWLEDGMENT

This research was supported by the Converging ResearchCenter Program through the Ministry of Education Science andTechnology (2010K000980) and Korean-Hungarian Joint La-

boratory Program by the Korea Research Council of Funda-mental Scienceand Technology We wish to thankProf J S Kimfor his critical reading

rsquoREFERENCES

(1) Novoselov K S Geim A K Morozov S V Jiang D Zhang Y Dubonos S V Grigorieva I V Firsov A A Science 2004 306 666ndash669

(2) Zhang Y Tan Y-W Stormer H L Kim P Nature 2005 438 201ndash204

(3) Zhou S Y Gweon G-H Graf J Fedorov A V SpataruC D Diehl R D Kopelevich Y Lee D-H Louie S G Lanzara A Nat Phys 2006 2 595ndash599

(4) Katsnelson M I Novoselov K S Geim A K Nat Phys 2006 2 620ndash625

(5) Neto A H C Guinea F Peres N M R Novoselov K SGeim A K Rev Mod Phys 2009 81 109ndash162

(6) Li X Cai W An J Kim S Nah J Yang D Piner R Velamakanni A Jung I Tutuc E Banerjee S K Colombo L Ruoff R S Science 2009 324 1312ndash1314

(7) Li X Magnuson C W Venugopal A Tromp R M Hannon J B Vogel E M Colombo L Ruoff R S J Am Chem Soc 2011 133 2816ndash2819

(8) Woff ord J M Nie S McCarty K F Bartelt N C DubonO D Nano Lett 2010 10 4890ndash4890

(9) Xuesong Li Magnuson C W Venugopal A An J Suk J WHan B Borysiak M Cai W Velamakanni A Zhu Y Fu L VogelE M Voelkl E Colombo L Ruoff R S Nano Lett 2010 10 4328ndash4334

(10) Rao C N R Sood A K Voggu R Subrahmanyam K S J Phys Chem Lett 2010 1 572

(11) Loginova E Bartelt N C Feibelman P J McCarty K F New J Phys 2009 11 063046

(12) Chan C-J Back M H Back R A Can J Chem 1975 53 3580

(13) Brune H Romainczyk C Roder H Kern K Nature 1994 369 469ndash471

(14) Ratsch C Venables J A J Vac Sci Technol A 2003 A21 S96ndashS109

(15) Fichthorn K A Scheffler M Phys Rev Lett 2000 84 5371ndash5374

(16) Coraux J N rsquoDiaye A T Engler M Busse C Wall DBuckanie N zu Heringdorf F-J M van Gastel R Poelsema BMichely T New J Phys 2009 11 023006

(17) YuV WhitewayE Maassen JHilkeM arXiv11011884v1(18) Lacovig P Pozzo M Dario A Vilmercati P Baraldi A

Lizzit S Phys Rev Lett 2009 103 166101(19) Wang B Ma X Caffio M Schaub R Li W-X Nano Lett

2011 11 424ndash430

(20) Nie S Walter A L Bartelt N C Starodub E Bostwick ARotenberg E McCarty K F ACS Nano 2011 5 2298ndash2306





temperature for thermal decomposition of methane Methanestarts to decompose at temperatures above 800 K12 In additionthe sticking coefficient of methane on copper is extremely small

When the substrate temperature increases the methane caughton it can decompose into successive hydrocarbon intermediatesthat is CH3 (methyl) CH2 (methylene) and CH (methylidyne)These hydrocarbon intermediates are more reactive thanmethane so they evolve to a thermodynamically more stablecarbon atom on the Cu surface

rsquoEXPERIMENTAL SECTION

The graphene was synthesized on a polycrystalline Cu foil(25 μm thick) by the thermal CVD method Initially Cu foils

were heated inside a chamber covered by the furnace under H2

gas pressure (012 sccm) upto 1263 K for 30min After 40minannealing of Cu foils for cleaning at a temperature of 1263 KCH4 andH2 mixture gases were simultaneously transferred to thefurnacefor a desiredgrowthtime The temperature of thefurnace

was immediately lowered to RT at a cooling rate of 50 Kmin We changed all of the growth parameters to optimize thegraphene growth on this Cu foil Scanning electron microscopy and Raman spectroscopy measurements were carried out afterthe graphene samples were exposed to the air

rsquoRESULTS AND DISCUSSIONFigure 1 shows a series of SEM images for the initial growth of

graphene on Cu foil at a temperature of 1263 K As the growthtime increased the size of the graphene island started to increaseThe size of the grain was in the range of 2030 μm in (c) Thegrowth temperature was maintained at 1263 K and the gasmixture ratio between H2 and CH4 was 021 The growth time

was varied at (a) 15 (b) 30 (c) 60 and (d) 120 s with a totalpressure of 780 mTorr The number of nucleation sites increased

by more than 100 between 15 and 30 s growth and decreased between 30 and 60 s growth Here the 1047298ow rate of methaneincluding theH2 carrier gas waslowsothe growth timerequired wasrelatively high when comparedtocases ofhigh1047298ow rates of methane

As the growth time increased the size of islands began to grow to form bigger islands (bd) (Figure 1) At this stage the islanddensity is big enough so that the carbon clusters (or atoms)can easily reach the island in the diff usion process Because the

bond strength between carbon atoms is quite high two adjacentislands cannot evolve to coalescevia edge diff usion Instead if thetemperature is relatively low (below 1273 K) then the totalnumber of nucleation sites increases while the size of already established islands increases with the carbon atoms (or smallclusters) nearby Here the island shape was more or lessdendrimer-like instead of fractal The lobes of the islands have

a backbone with the orientation determined by the crystallinestructure of the substrate Usually the transition from fractal todendritic growth comes when the 1047298ux of the constituent of the1047297lm increases13 Because the temperature of the substrate and thediff usionbarrier are also aff ecting the eff ective 1047298ux forthe growthit is not easy to conclude whether it is a fractal or a dendrimer

based on the 1047298ux only Two factors including the high tempera-ture and the strong bond strength between carbon atoms play animportant role on the shape of islands We will discuss this inmore detail later Figure 2 shows an SEM image of the graphenegrowth depending on the growth temperature with H2CH4 =235 growth time = 20 s The formation of a new nucleus ispossible only if a certain supersaturation is reached The super-saturation increases with increasing temperature as the decom-

position rate of the precursor also increases with increasingtemperature This is what happens from Figure 2a toFigure 2b Also the smaller number of growing islands at highertemperatures is an indication of the coalescenceof smaller islands

with the larger ones as shown in Figure 2c Because theevaporationcondensation process due to the diff erence in

vapor pressures between two (small and big) islands is notallowed in graphene growth at this temperature Ostwald ripen-ing is not possible14 The bond strength between carbon atoms isfairly high (36 eV) so that the edge diff usion is not allowed hereThe shape of the island grown at higher temperatures is muchmore complicated than that of the islands grown at lowertemperatures This indicates that the smaller islands coalescing

Figure 1 SEM images for the initial growth of graphene on Cu foil at the temperature of 1263 K


































rsquoCONCLUSIONS













rsquoACKNOWLEDGMENT



rsquoREFERENCES



















2011 11 424ndash430



































rsquoCONCLUSIONS













rsquoACKNOWLEDGMENT



rsquoREFERENCES



















2011 11 424ndash430










rsquoACKNOWLEDGMENT



rsquoREFERENCES



















2011 11 424ndash430


initial stage of graphene growth on a cu substrate-temperatura crescimento.pdf

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