Near room-temperature synthesis of transfer-free graphene films

Transcription

1 Received 31 Aug 2011 Accepted 15 Dec 2011 Pulished 24 Jn 2012 DOI: /ncomms1650 Ner room-temperture synthesis of trnsfer-free grphene films Jinsung Kwk 1, Je Hwn Chu 1, Je-Kyung Choi 1, Soon-Dong Prk 1, Heungseok Go 2, Sung You Kim 1, Kiog Prk 2,3, Sung-De Kim 4, Young-Woon Kim 4, Euijoon Yoon 4,5,6, Suneel Kodmk 7 & Soon-Yong Kwon 1,2,3 Lrge-re grphene films re est synthesized vi chemicl vpour nd/or solid deposition methods t elevted tempertures (~1,000 C) on polycrystlline metl surfces nd lter trnsferred onto other sustrtes for device pplictions. Here we report new method for the synthesis of grphene films directly on SiO 2 /Si sustrtes, even plstics nd glss t close to room temperture ( C). In contrst to other pproches, where grphene is deposited on top of metl sustrte, our method invokes diffusion of cron through diffusion couple mde up of cron-nickel/sustrte to form grphene underneth the nickel film t the nickel sustrte interfce. The resulting grphene lyers exhiit tunle structurl nd optoelectronic properties y nickel grin oundry engineering nd show micrometre-sized grins on SiO 2 surfces nd nnometre-sized grins on plstic nd glss surfces. The ility to synthesize grphene directly on non-conducting sustrtes t low tempertures opens up new possiilities for the friction of multiple nnoelectronic devices. 1 School of Mechnicl nd Advnced Mterils Engineering, Ulsn Ntionl Institute of Science nd Technology, Ulsn , Repulic of Kore. 2 School of Electricl nd Computer Engineering, Ulsn Ntionl Institute of Science nd Technology, Ulsn , Repulic of Kore. 3 Opto-Electronics Convergence Group, Ulsn Ntionl Institute of Science nd Technology, Ulsn , Repulic of Kore. 4 Deprtment of Mterils Science nd Engineering, Seoul Ntionl University, Seoul , Repulic of Kore. 5 Energy Semiconductor Reserch Center, Advnced Institutes of Convergence Technology, Seoul Ntionl University, Suwon , Kore. 6 Deprtment of Nno Science nd Technology, Grdute School of Convergence Science nd Technology, Seoul Ntionl University, Suwon , Kore. 7 Deprtment of Mterils Science nd Engineering, University of Cliforni Los Angeles, Los Angeles, Cliforni 90095, USA. Correspondence nd requests for mterils should e ddressed to S.-Y.K. (emil: sykwon@unist.c.kr).

2 nture communictions DOI: /ncomms1650 Grphene hs generted enormous scientific curiosity owing to its ultrthin geometry 1,2, quntum electronic trnsport 2,3, ndgp opening 4,5, excellent therml conductivity 6 nd high mechnicl strength 7. Among ll of the potentil pplictions for grphene, reliztion of grphene-sed flexile electronic disply technology ppers to e closer to relity 8. Currently, lrge-re grphene films re est synthesized vi pyrolytic crcking of hydrocron gses 8 11 nd/or solid deposition method 12 t elevted tempertures (~1,000 C) on polycrystlline metl surfces nd lter trnsferred onto other sustrtes for the friction of devices. However, the high growth tempertures impose limittions on the choice of sustrtes nd the susequent trnsferility of s-grown lyers influence device performnce. Recently, ilyer grphene hs een grown directly on insulting sustrtes t elevted tempertures (~1,000 C) using spin-coted polymer films s solid source of cron (C) with nickel (Ni) thin films on top or ottom of the polymer film 13,14. Here we descrie very low-temperture nd trnsfer-free pproch to controllly deposit grphene films onto desired sustrtes. Our synthesis methodology exploits the properties of diffusion couple, wherein Ni thin film is deposited first on the sustrte, nd solid C is then deposited on top of the Ni, nd llowed to diffuse predominntly long grin oundries (GBs) to crete thin grphene film t the Ni sustrte interfce. To tune the structurl nd optoelectronic properties of the resulting grphene lyers, we hve engineered grin sizes of the Ni films on the sustrte of choice. Our method llows for uniform nd controllle deposition of wrinkle-free grphene films with micrometre-sized grins on SiO 2 surfces, nd with nnometre-sized grins on plstic nd glss. This study suggests tht lrge-scle, device-redy grphene lyers cn e simply prepred without trnsfer process on ny ritrry sustrte t low tempertures highly desirle for electronic nd optoelectronic pplictions. Results Diffusion-ssisted synthesis (DAS) method. Figure 1 schemticlly illustrtes our process, which we refer to s DAS. We demonstrte the pplicility of this method using thermlly oxidized Si(100), poly(methyl methcrylte) (PMMA)-covered SiO 2, nd commercilly ville glss s sustrtes. First, ~100-nmthick film of polycrystlline Ni (poly-ni) is deposited vi electronem evportion t tempertures s low s ~25 C on PMMA nd glss nd t tempertures s high s ~400 C on SiO 2 /Si(100). Ni thin films deposited on SiO 2 /Si(100) were nneled t ~1,000 C Ni Si SiO 2 Grphite powder Grphene c d 1.3.u. Intensity (.u.) e 1,200 1,600 2,000 2,400 2,800 Rmn shift (cm 1 ) f g u. I DS (10 6 A) Au/Cr V G (V) Figure 1 Structurl nd electricl properties of the DAS-grown grphene films on SiO 2 /Si sustrtes t low tempertures. () Schemtic drwing of the DAS process for directly depositing grphene films on nonconducting sustrtes. The digrms represent (from left to right) the elementry steps in the DAS process, including deposition (nd nneling) of Ni thin films on desired sustrtes (SiO 2 /Si or PMMA, glss), preprtion of diffusion couple of C Ni/sustrte, nneling in Ar or ir ( C) to form C Ni/grphene/sustrte nd formtion of grphene on desired sustrtes y etching wy C Ni diffusion couple, respectively. Representtive () opticl microscopy (OM) imge, (c) Rmn spectr from red, lue nd green spots showing the presence of one, two nd three lyers of grphene, respectively, (from ottom to top) nd (d) Rmn mp imge of the G/2D nds of grphene grown t temperture T = 160 C for 5 min on SiO 2 (300 nm)/si sustrte. Scle rs, 4 µm ( nd d). (e) Representtive SEM imge of grphene grown t T = 160 C for 5 min on SiO 2 (300 nm)/si sustrte (scle r, 100 µm). (f) Higher mgnifiction SEM imge of e, showing the presence of 1 ML (red dot), 2 ML (lue dot) grphene nd multilyer grphene ridges (white dot); (scle r, 10 µm). (g) Typicl room temperture I DS -V G curve t V DS = 10 mv from DAS-grown ML grphenesed ck-gted FET device. The estimted crrier (hole) moility is ~667 cm 2 V 1 s 1 t room temperture nd it shows wek p-type ehviour. The inset shows opticl microscopy imge of this device nd the scle r is 10 µm. I DS, drin-source current; V DS, drin-source voltge; V G, gte voltge.

3 nture communictions DOI: /ncomms1650 ARTICLE in H 2 mient to yielding strong 111-texture nd 5 20 µm sized grins with smooth surfces. Using Rmn spectroscopy, we confirm tht the surfces of Ni (fter nneling) nd SiO 2 (fter removl of the metl) re free of crystlline C. In cse of Ni thin films deposited t room temperture, the verge grin size is out nm. Detils of the deposition, morphology nd crystllinity of the Ni films re presented in Methods nd Supplementry Figures S1 nd S2. The choice of Ni is motivted y the fcts tht it hs high soluility for C nd Ni(111) is nerly lttice-mtched with sl plnes of grphite, oth of which fcilitte commensurte epitxil growth of grphene 15,16. Solid C is supplied from pste composed of grphite powder (Aldrich, product ) dispersed in ethnol. (Highly oriented pyrolytic grphite cn lso e used s C source ut we otined etter results with the pste.) To ensure efficient contct t the C Ni interfce nd stimulte the C-C ond reking on ctlytic Ni(111) surfce 17,18, pressure ( < 1 MP) is uniformly pplied y mechniclly clmping the C Ni/sustrte diffusion couple using molydenum holding stge. The smples re then seled in qurtz tue filled with inert rgon gs (or with ir) nd heted for short periods (t = 1 10 min) t tempertures T etween 25 nd 260 C. Sustrte tempertures re mesured using k-type thermocouple directly connected to the smple holder nd re ccurte to within 10 C. Following grphene growth, the C-Ni diffusion couple is etched wy using n queous solution of FeCl 3 leving ehind grphene film on the desired sustrte. The detils of this process re presented in Methods. Synthesis nd chrcteriztion of grphene films on SiO 2 Si sustrtes. Figure 1 f shows typicl results of surfce morphology nd Rmn spectr, respectively of the grphene lyers on SiO 2 /Si(100) otined t T = 160 C nd t = 5 min. The three Rmn spectr, colour coded for clrity, in Figure 1c re otined from the three corresponding coloured circles. They show three primry fetures: D nd t ~1351 cm 1, G nd t ~1592 cm 1 nd 2D nd t ~2685 cm 1, ll expected pek positions for grphene. Film thicknesses re determined y mesuring the rtio of G-to-2D pek intensities (I G /I 2D ) nd the full-width t hlf-mximum (FWHM) vlues of the symmetric 2D nd. For the spectr of monolyer (ML)-re grphene, we otin (1) I G /I 2D < 0.5 nd (2) the FWHM vlue of ~38 cm 1 for the 2D nd, consistent with previous reports 9,11. The ssocited G/2D nd mp in Figure 1d illustrtes the uniformity of grphene films over lrge res (~320 µm 2 ) covered mostly with 1ML nd 2ML grphene s identified y the I G /I 2D < 0.5 nd I G /I 2D 1, respectively. There re, however, few regions with multilyer grphene (for exmple, the region highlighted y the white dot in Fig. 1f) nd we refer to them s grphene ridges. From opticl nd scnning electron microscopy (OM nd SEM) imges cquired from the smples, we note tht the s-synthesized grphene films re wrinkle-free nd smooth over lrge res. Interestingly, we find tht the morphologies of regions covered with mono- nd i-lyer grphene resemle those of the grins, nd the multilyer grphene ridges, the GBs in the Ni thin films. These oservtions suggest tht multilyer grphene growth is fvoured t defects such s GBs. This is plusile s GB diffusion is typiclly fster thn ulk diffusion nd GBs cn serve s nucletion sites 19,20. The electricl properties of grphene lyers on SiO 2 /Si(100) otined t T = 160 C nd t = 5 min hve een evluted with ckgted grphene-sed field-effect trnsistor (FET) devices nd typicl dt for the FET devices re shown in Figure 1g nd Supplementry Figure S3. The V shpe of the mipolr trnsfer chrcteristics nd the shift of neutrlity point to positive gte voltge, tht is, wek p-type ehviour 1,12,21, in mient conditions re oserved, ll expected electricl properties for grphene-sed FETs. The estimted crrier (hole) moility of this prticulr device in mient conditions is ~667 cm 2 V 1 s 1 t room temperture, suggesting tht the s-synthesized grphene films re of resonle qulity. Intensity (.u.) c Intensity (.u.) 1,200 1,200 1,600 2,000 Rmn shift (cm 1 ) 1,600 2,000 2,400 2,400 Rmn shift (cm 1 ) 2,800 2,800 G FWHM (cm 1 ) d Using our pproch, we hve synthesized grphene lyers over rnge of tempertures etween 25 nd 260 C. OM imges nd Rmn spectroscopic mesurements of G-to-2D intensity rtio (I G /I 2D ) nd FWHM of G nds 9 (Fig. 2, nd Supplementry Fig. S4) indicte tht few-lyer grphene films with similr qulity cn e otined t ll T, even t room temperture (Fig. 2). However, the surfce coverge of grphene on SiO 2 shows strong dependence on T, incresing linerly from ~60 to ~98% with incresing T from 25 to 260 C s shown in Figure 2. (The corresponding OM imges of grphene on SiO 2 grown t different T re shown in Supplementry Fig. S4.) We note tht continuous grphene lyers over lrge res cn only e otined t T 160 C. The sheet resistnce of grphene lyers grown t T = 160 C, mesured using trnsmission line model method 22,23, is found to e ~1,000 Ω per squre (Supplementry Fig. S3), which mkes the s-synthesized grphene promising s trnsprent electrode mteril. We hve lso explored the possiility of using our pproch to grow grphene in ir insted of inert Ar tmospheres. Surprisingly, we find tht the surfce morphology, rel coverge nd Rmn structure (Fig. 2c) of the grphene films grown in Ar s well s in ir re similr, demonstrting the potentil of our pproch to grow grphene in ir t low tempertures Growth temperture ( C) Figure 2 Structurl nd opticl chrcteriztion of DAS-grown grphene films versus growth temperture. () Rmn spectr showing the presence of one (red), two (lue) nd three (green) lyers (from ottom to top) of grphene film grown t room temperture for 10 min on SiO 2 (300 nm)/si sustrte. () FWHM of G nds in Rmn spectr (lck squres) nd the surfce coverge (red circles) of grphene films grown for 10 min on SiO 2 s function of growth temperture T. For comprison, the FWHM of G nd for highly oriented pyrolytic grphite sustrte is 15 cm 1. The insets show the opticl microscopy imges of few-lyer grphene films grown t room temperture (left; scle r, 20 µm) nd t 205 C (right) (scle r, 100 µm) on SiO 2 (300 nm)/si sustrtes. (c) Comprison of the Rmn spectr of 1 ML-re grphene films grown t T = 25 C (lck), 60 C (red) nd 160 C (green) in rgon nd t T = 25 C (lue), 160 C (purple) in ir (from ottom to top). (d) Representtive lowmgnifiction pln-view TEM imge of grphene film grown t T = 160 C on SiO 2 then trnsferred onto TEM support grid (scle r, 100 nm). Inset is selected re diffrction pttern otined from wrinkle-free region highlighted y the dotted circle. Scle r in inset is 5 1/nm. Coverge (%)

4 nture communictions DOI: /ncomms1650 C Ni GB SiO 2 /Si templte C Ni GB SiO 2 /Si templte Figure 3 Schemtic digrms of grphene growth mechnisms in DAS process. The digrms represent (from left to right) the elementry steps of the process including dissocition of C C onds t C/Ni interfce, diffusion of cron toms, followed y nucletion nd growth of grphene. Grphene growth mechnism in cse () 460 C T 600 C; ulk diffusion of C toms through Ni crystllites, leding to homogeneous nucletion t multiple sites resulting in the formtion of nnocrystlline grphene film y precipittion nd () T 260 C; preferentil diffusion of C toms vi GBs in Ni, followed y heterogeneous nucletion t the defect sites nd growth vi lterl diffusion of C toms long Ni/sustrte interfce. Growth mechnism of grphene films in DAS process. So, how does grphene form t such low tempertures? We propose mechnism for the growth of grphene lyers in DAS s follows: to form grphene t the Ni SiO 2 interfce, C from the solid source will hve to diffuse through the Ni film nd crystllize s grphene t the interfce. Given tht our tempertures re well elow 260 C, we rule out the possiility of grphene growth vi C precipittion from the ulk tht requires tempertures 460 C or higher 24. (In cse of 460 C T 600 C, we oserved the formtion of nnocrystlline grphene lyers nd the lyers contin no grphene ridges t ll, s shown in Supplementry Fig. S5.) Insted, we suggest the following: C toms from solid C source hve to diffuse through the Ni film. Previous experimentl 17,24 nd theoreticl 17,18 studies indicte tht Ni surfce cn ctlyze the dissocition of C-C onds nd promote diffusion even t room temperture. The resulting C toms re trnsported cross the Ni film primrily long the GBs to the Ni SiO 2 interfce 19,20,25. Following Fisher 19 nd Blluffi nd Blkely 20, we estimte the rtio of two trnsport fluxes of C toms through the Ni lttice (ň L ) nd long the GBs (ň GB ) in 111- textured Ni films nd it is evident tht lower tempertures gretly fvour GB diffusion reltive to lttice diffusion, tht is, ň GB ň L (detils of the clcultions re presented in Methods). It is noted tht the C tom trnsport long GBs in Ni films is highly susceptile to residul impurity concentrtions such s hydrogen (Supplementry Fig. S6). Upon reching the Ni SiO 2 interfce, C toms precipitte out s grphene t the GBs. This is consistent with the fct tht for reltively short growth times (t 1 3 min), independent of T ( C), we otined grphene-free surfces with trces of grphene ridges presumly long the GBs in Ni films. We verified the ridge composition to e grphene nd not morphous C using Rmn spectroscopy nd cross-sectionl trnsmission electron microscopy (TEM) (Supplementry Fig. S7). Excess C toms reching the grphene ridges, diffuse lterlly long the grphene-ni(111) interfce nd led to the growth of grphene over lrge res, driven y the strong ffinity of C toms to self-ssemle nd expnd the sp 2 lttice 26. To understnd the origin of the interfcil processes, we hve performed density functionl theory clcultions on the trnsport of C toms long the grphene-ni(111) interfce (detils of the clcultions re presented in Methods). We otin n energy rrier of ~0.51 ev for diffusion of C toms t the grphene Ni(111) interfce in greement with erlier reports 27,28, which indictes tht the trcer diffusion coefficient (D t ) of C toms is ~200 nm 2 s 1 t room temperture, ccording to the Arrhenius eqution. The suggested grphene growth mechnism in DAS process is schemticlly illustrted in Fig. 3. Eventully, continuous ut polycrystlline grphene film forms t the Ni SiO 2 interfce. Using comintion of TEM techniques including selected re diffrction pttern nd drk-field TEM, we find tht the grin sizes in grphene lyers vry from few hundred nnometres to few micrometres (Fig. 2d nd Supplementry Fig. S8), nd re comprle to those of Ni grins. Our results imply tht we hve more room for improvement in grphene qulity y Ni GB engineering, tht is, micrometre-scle or lrger grin sizes in grphene lyers cn e otined y creting mcrocrystlline Ni films with GBs locted fr enough from ech other nd/or y wet chemicl doping 8,29 of the films with high controllility. Lrge-scle synthesis of grphene films on plstic nd glss sustrtes. We demonstrte the pplicility of our pproch to prepre lrge-re grphene on non-conducting, plstic nd glss sustrtes. To this purpose, we use T 160 C nd do not nnel the Ni thin films so s to minimize therml degrdtion of the sustrtes. Figure 4 shows typicl surfce morphology of grphene lyers grown t T = 60 C nd t = 10 min on PMMA then trnsferred to SiO 2 (300 nm)/si sustrte (detils of the trnsfer process re descried in the Supplementry Methods). In contrst to grphene grown on SiO 2, the grphene films on plstic nd glss sustrtes re continuous over lrge res t ll T (even t room temperture), possily due to the decrese in distnce etween GBs. The s-grown lyers re nnocrystlline grphene, whose Rmn structure hs the following chrcteristics (Fig. 4): two peks centred t 1,359 ± 4 cm 1 (the D nd) nd 1,594 ± 2 cm 1 (the G nd) with reltively lrge FWHM nd I D /I G rtio of ~0.7 ± 0.1, ll expected for nnocrystlline grphene 30. We hve mesured the thicknesses of grphene films trnsferred on to SiO 2 sustrtes y tomic force microscopy. Figure 4c shows film thickness plotted s function of T for smples grown t t = 10, 30 nd 60 min nd ll of our smples grown for t = 10 min yield film thicknesses of 1.3 ± 0.3 nm. In cse of grphene lyers grown on glss sustrtes t T 60 C for t = 10 min, we otin ~97.4% trnsmittnce (Fig. 4d). Considering 2.3% sorption of incident white light in n individul grphene lyer 31, it cn e inferred tht these films re ML thick, in good greement with tomic force microscopy dt of s-grown grphene lyers.

5 nture communictions DOI: /ncomms1650 ARTICLE c 6 Thickness (nm) Growth temperture ( C) Discussion In our DAS process, the GBs in poly-ni(111) films fford the C toms the chnce to preferentilly diffuse to other surfce t low tempertures. Upon reching the Ni sustrte interfce, C toms precipitte out s grphene t the GBs (Supplementry Fig. S7) nd growth occurs vi lterl diffusion long the interfce. This process gives rise to continuous ut polycrystlline grphene film. This finding indictes tht we cn control the verge grin size of resulting grphene films y Ni GB engineering, tht is, control the positions where grphene grins strt to grow. For grphene on Ni/SiO 2, using therml nneling process efore DAS process, we enlrged the grin size of poly-ni films y up to 5 20 µm (tht is, crete GBs locted fr enough from ech other); therey, the mcrocrystlline grphene grins up to severl micrometres in size re otined, s confirmed y pln-view TEM nlysis. However, for Ni on plstic nd glss sustrtes, the s-deposited poly-ni films were not nneled further, therey the grin sizes were ~40 50 nm. As result, resulting grphene films consist of grins of the order of nnometres. Similr results re lso otined in cse we grow grphene films on SiO 2 using s-deposited, unnneled poly-ni(111) films. Finlly, we hve lso used polycrystlline Ni foils nd otined lrge-re grphene lyers vi DAS process t low tempertures. The detils of synthesis nd structurl nd optoelectronic chrcteriztion results will e presented elsewhere. We found tht precise control over the foil thickness, surfce roughness nd crystlline qulity Intensity (.u.) d 100 Trnsmittnce (%) ,200 1,600 2,000 2,400 2,800 Rmn shift (cm 1 ) Wvelength (nm) Figure 4 Lrge-scle growth of grphene films on plstic nd glss sustrtes. Representtive () opticl microscopy imge of grphene film grown t T = 60 C for 10 min on PMMA then trnsferred to SiO 2 (300 nm)/ Si sustrte (scle r, 100 µm). () Typicl Rmn spectr, with ech of the colours corresponding to the coloured spots on the smple. Folded regions such s those highlighted y the green open circle re occsionlly oserved nd re formed during trnsfer process. (c) The thicknesses of grphene films trnsferred onto SiO 2 (300 nm)/si sustrtes fter growth t T = C for 10 min (lck squres), 30 min (red circles) nd 60 min (lue tringle) on PMMA. Error rs re the stndrd devition of the thickness mesured in 30 different edge regions in trnsferred grphene films. (d) Trnsmittnces of grphene films grown t T = 25 C (lck), 45 C (red), 60 C (lue) nd 110 C (green) for 10 min on glss sustrtes. The inset shows photogrph of grphene film grown t temperture T = 60 C. of the foils is criticl to otining grphene films over lrge res. We note tht similr experiments crried out using single-crystl Ni foils do not yield grphene on either side of the foils, indicting tht the GBs re necessry for low-temperture synthesis of grphene. In summry, we hve demonstrted trnsfer-free, lrge-re growth of grphene films vi DAS method t close to room temperture. Our pproch cn, in principle, e used to grow deviceredy grphene lyers on ny ritrry sustrte even in ir nd the resulting grphene lyers exhiit controllle structurl nd optoelectronic properties y Ni GB engineering. This reltively simple method of synthesizing grphene films is potentilly sclle nd opens up new possiilities for vriety of electronic nd optoelectronic pplictions. Methods Predeposition of Ni films on nonconducting sustrtes. All of our experiments were crried out using SiO 2 /Si, PMMA-coted SiO 2 /Si, poly-dimethylsiloxne nd glss sustrtes (1 2 cm 2 size). Polycrystlline Ni (poly-ni) films with thickness of 100 nm re deposited using n electron-em evportor (operting pressure: ~ Torr) with solid Ni (99.99% purity) s the source. The sustrte temperture is set to 400 C for SiO 2 /Si nd room temperture (~25 C) for plstics (PMMA nd poly-dimethylsiloxne) nd glss. Microstructure of the Ni thin films is determined using tomic force microscopy (Veeco Multimode V), scnning electron microscopy (SEM; FEI Qunt 200) nd X-ry diffrction (Bruker D8 Advnce) techniques. As-deposited Ni thin films on SiO 2 /Si sustrte hve n verge Ni grin size of ~100 nm (Supplementry Fig. S1). To further control the grin size, crystllinity nd morphology of poly-ni films, the s-deposited smples were trnsferred to n ultrhigh vcuum chmer ( < 10 9 Torr) nd nneled t tempertures s high s ~1,000 C for times etween 1 nd 10 min in vcuum (~10 9 Torr) or in H 2 mient ( % purity) t pressures of Torr. After nneling, we otin Ni films with predominntly 111-texture, with lrge crystlline grins of sizes round 5 20 µm nd tomiclly flt terrces s shown in Supplementry Figures S1 nd S1c. We find tht H 2 mient is preferred to vcuum to otin lrger grins (Supplementry Fig. S1d). Using Rmn spectroscopy (WiTec lph 300R M-Rmn), we rule out the presence of ny crystlline C on the Ni films due to residul C segregtion fter nneling in H 2 mient. In ddition, we do not oserve ny crystlline C on SiO 2 surfces fter etching wy the metl films in n queous solution of FeCl 3. In cse of the Ni thin films deposited t room temperture on plstic nd glss sustrtes, the verge grin size is round nm (Supplementry Fig. S2). Owing to the limited therml stility of these mterils, we did not nnel these smples. Synthesis of grphene films y DAS process. As C sources, we used pste composed of grphite powder (Aldrich, product ) dispersed in ethnol. The verge size of grphite powder is ~40 µm. The pste ws plstered to the Ni surfce to mke C Ni/sustrte diffusion couples nd dried y heting the smples on hot plte ( < 50 C). Pressure ( < 1 MP) is uniformly pplied y mechniclly clmping the C Ni/sustrte diffusion couple using molydenum holding stge. Then, the smples re seled in qurtz tue nd heted for 1 60 min while flowing inert Ar gs (1 stndrd l min 1 ) or ir t tempertures etween 25 nd 600 C. Following grphene growth, the smples re clened vi soniction in deionized wter, nd the Ni films re removed y etching in n queous solution of FeCl 3, leving ehind grphene film on desired sustrtes. The etching time ws found to e function of the etchnt concentrtion nd type of the sustrtes. Typiclly, 1 cm 2 y 100-nm thick Ni film on SiO 2 /Si sustrte cn e dissolved y 1 M FeCl 3 solution within 30 min. Fisher model. A highly idelized poly-ni film mtrix with squre-shped grins of side l, GB sls of width δ nd film thickness d is considered. Among ll competing diffusion mechnisms, we only consider the lttice nd GB diffusion quntities nd ignore the disloction quntity s insignificnt disloctions threding norml to surfce exist in fce-centered cuic (FCC) Ni(111) film. Under these conditions, the numer of toms (ň) tht flow per unit time is essentilly equl to the product of the pproprite diffusivity (D i ), concentrtion grdient (dc/dx) nd trnsport re involved nd the rtio of two fluxes cn e estimted for FCC metls s ň GB / ň L = δ D GB /l D L = [( )/l (cm)]exp(8.1t M /T) 20. Assuming l = 10 µm = 10 3 cm nd using T M of Ni = 1726 K, we hve ň GB /ň L = t 473 K, t 773 K. Even if we consider the fcts tht rel polycrystlline films contin vrious types nd orienttions of GBs nd the lttice diffusion of C toms in Ni film would occur y interstitil diffusion rther thn sustitutionl diffusion used in the model, it is evident tht lower tempertures gretly fvour GB diffusion reltive to lttice diffusion. Density functionl theory clcultion. All clcultions were performed using the VASP pckge sed on the spin-polrized density functionl theory 32,33. We used projector ugmented wve potentil 34, nd the generlized grdient pproximtion

6 nture communictions DOI: /ncomms1650 of Perdew nd Wng 35. All configurtions were fully relxed until the mximum residul forces were less thn 0.01 ev/å. The kinetic energy cutoff ws 400 ev nd the Monkhorst Pck 36 k-point grid ws We clculted the stle dsorption site of single C tom on free-stnding grphene sheet. The grphene model consisted of 32 C toms with periodic oundry conditions long the in-plne directions. We found tht ridge-like site, where the dded C tom sits on the centre of the ond etween two neighouring C toms of grphene, is most fvourle. To otin the prefctor for the Arrhenius eqution, the virtionl frequency of the dded C tom on the dsorption site ws clculted 37. The dsorption energy nd the virtionl frequency of the dded C tom on the site were found to e 1.65 ev nd 22.8 THz, respectively. In ddition, the minimum energy pthwy for the dtom diffusion ws exmined y the nudged elstic nd method 38. The dditionl C toms in oth initil nd finl configurtions were locted t the most energeticlly fvourle sites, which were neighours. Nine intermedite imges, initilly constructed y liner interpoltion etween the initil nd finl configurtions, were optimized long the diffusion pthwy, which enles the determintion of the minimum energy rrier. This minimum energy rrier ws found to e 0.24 ev. We then performed the sme clcultion of single C tom diffusion on free Ni(111) surfce. The Ni thin film model consisted of 6 Ni(111) lyers with 16 Ni toms in ech lyer for totl of 96 Ni toms. Periodic oundry conditions were pplied long the in-plne directions, wheres the three ottom lyers of the Ni film were held fixed during the diffusion process. We found tht the C tom is most stle on the hexgonl close-pcked (HCP) site, for which the dsorption energy is 7.01 ev, just 0.06 ev higher thn on FCC site. In ddition, the clculted energy rrier for the C tom diffusion on the Ni(111) surfce is 0.49 ev. To exmine the diffusion of single C tom long the grphene Ni(111) interfce, we otined the stle configurtion of grphene sheet on free Ni(111) surfce. For this, eight C toms for grphene sheet nd 24 Ni toms for the Ni(111) surfce were used. The grphene sheet ws ixilly stretched using 1.22% tensile strin to remove the lttice mismtch etween grphene nd the Ni(111) surfce. The equilirium distnce etween the grphene sheet nd the Ni(111) surfce is Å, nd the interction energy is 0.02 ev per ech C tom. This is in good greement with the erlier clcultions 27. When we introduce n dditionl C tom t the interfce etween grphene nd the Ni(111) surfce, the energeticlly preferred position for the C tom is in the HCP site on the Ni(111) surfce. The distnce etween the C tom nd the Ni surfce is 1.03 Å nd the distnce etween the C tom nd the grphene sheet is 2.62 Å, which indictes tht the diffusion of C tom long the interfce will e more strongly influenced y the Ni surfce thn the grphene ML. The clculted diffusion rrier from the HCP site to the FCC site of the Ni(111) surfce long the interfce is found to e ~0.51 ev, which is slightly higher thn tht on free Ni(111) surfce. During the diffusion, there is no significnt chnge in the distnce etween the C tom nd the Ni surfce. The hopping rte of C tom cn e found y the Arrhenius eqution Γ = Γ 0 exp( E /kt), which results in room-temperture hopping rte of out 20,000 s 1 with n ttempt frequency of Γ 0 = Hz. In ddition, the trcer diffusion coefficient cn e expressed in terms of the hopping rte Γ nd the men squre jump length < l 2 > : D t = Γ < l 2 > / (2d), nd is found to e ~200 nm 2 s 1 using the lttice constnt of Ni 3.52 Å. If the chemicl diffusion or the presence of grphene edges were considered, the diffusion coefficient would e higher thn the clculted vlue of D t. References 1. Novoselov, K. S. et l. Electric field effect in tomiclly thin cron films. Science 306, (2004). 2. Novoselov, K. S. et l. Two-dimensionl gs of mssless Dirc fermions in grphene. Nture 438, (2005). 3. Zhng, Y., Tn, Y.- W., Stormer, H. L. & Kim, P. Experimentl oservtion of the quntum Hll effect nd Berry s phse in grphene. Nture 438, (2005). 4. Zhou, S. Y. et l. Sustrte-induced ndgp opening in epitxil grphene. Nture Mterils 6, (2007). 5. Kwon, S.- Y. et l. Growth of semiconducting grphene on plldium. Nno Lett. 9, (2009). 6. Blndin, A. A. et l. Superior therml conductivity of single-lyer grphene. Nno Lett. 8, (2008). 7. Lee, C., Wei, X. D., Kysr, J. W. & Hone, J. Mesurement of the elstic properties nd intrinsic strength of monolyer grphene. Science 321, (2008). 8. Be, S. et l. Roll-to-roll production of 30-inch grphene films for trnsprent electrodes. Nture Nnotechnol. 5, (2010). 9. Rein, A. et l. Lrge re, few-lyer grphene films on ritrry sustrtes y chemicl vpor deposition. Nno Lett. 9, (2009). 10. Kim, K. S. et l. Lrge-scle pttern growth of grphene films for stretchle trnsprent electrodes. Nture 457, (2009). 11. Li, X. et l. Lrge-re synthesis of high-qulity nd uniform grphene films on copper foils. Science 324, (2009). 12. Sun, Z., Yn, Z., Beitler, E., Zhu, Y. & Tour, J. M. Growth of grphene from solid cron sources. Nture 468, (2010). 13. Yn, Z. et l. Growth of ilyer grphene on insulting sustrtes. ACS Nno 5, (2011). 14. Peng, Z., Yn, Z., Sun, Z. & Tour, J. M. Direct growth of ilyer grphene on SiO 2 sustrtes y cron diffusion through nickel. ACS Nno 5, (2011). 15. Shelton, J. C., Ptil, H. R. & Blkely, J. M. Equilirium segregtion of cron to nickel(111) surfce: surfce phse trnsition. Surf. Sci. 43, (1974). 16. Isett, L. C. & Blkely, J. M. Segregtion isosteres for cron t the (100) surfce of nickel. Surf. Sci. 58, (1976). 17. Vng, R. T. et l. Controlling the ctlytic ond-reking selectivity of Ni surfces y step locking. Nture Mterils 4, (2005). 18. Tn, X. & Yng, G. W. Ctlytic ond-reking selectivity in the ethylene decomposition on Ni surfces: kinetic Monte Crlo simultions. J. Phys. Chem. C 112, (2008). 19. Fisher, J. C. Clcultion of diffusion penetrtion curves for surfce nd grin oundry diffusion. J. Appl. Phys. 22, (1951). 20. Blluffi, R. W. & Blkely, J. M. Specil spects of diffusion in thin films. Thin Solid Films 25, (1975). 21. Schedin, F. et l. Detection of individul gs molecules sored on grphene. Nture Mterils 6, (2007). 22. Reeves, G. K. & Hrrison, H. B. Otining the specific contct resistnce from trnsmission line model mesurements. IEEE Electron Dev. Lett. 3, (1982). 23. Xi, F., Pereeinos, V., Lin, Y., Wu, Y. & Avouris, P. The origin nd limits of metl-grphene junction resistnce. Nture Nnotechnol. 6, (2011). 24. Lmer, R., Jeger, N. & Schulz-Ekloff, G. Electron microscopy study of the interction of Ni, Pd, nd Pt with cron: I. Nickel ctlyzed grphitiztion of morphous cron. Surf. Sci. 197, (1988). 25. Berry, B. S. Diffusion of cron in nickel. J. Appl. Phys. 44, (1973). 26. Ajyn, P. M. & Ykoson, B. I. Grphene: pushing the oundries. 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From ultrsoft pseudopotentils to the projector ugmented-wve method. Phys. Rev. B 59, (1999). 35. Perdew, J. P. et l. Atoms, molecules, solids, nd surfces: pplictions of the generlized grdient pproximtion for exchnge nd correltion. Phys. Rev. B 46, (1992). 36. Monkhorst, H. J. & Pck, J. D. Specil points for Brillouin-zone integrtions. Phys. Rev. B 13, (1976). 37. Al-Nissil, T., Frerrndo, R. & Ying, S. C. Collective nd single prticle diffusion on surfces. Adv. in Phys. 51, (2002). 38. Mills, G., Jonsson, H. & Schenter, G. K. Reversile work trnsition stte theory: ppliction to dissocitive dsorption of hydrogen. Surf. Sci. 324, (1995). Acknowledgements We grtefully cknowledge support from Reserch Fund of the Ulsn Ntionl Institute of Science nd Technology (Grnts No , ) nd from the Bsic Science Reserch Progrm (Grnts No , , , ) nd from Ntionl Nucler R&D Progrm (Grnt No ) nd from World Clss University Progrm (Grnt No. R ) through the Ntionl Reserch Foundtion (NRF) of Kore funded y the Ministry of Eduction, Science nd Technology. SK grtefully cknowledges support from UCLA COR-FRG. This work hs enefited from the use of the fcilities t UNIST Centrl Reserch Fcilities. Author contriutions S.-Y.K. plnned nd supervised the project; E.Y. nd S.K. dvised on the project; J.K., J.H.C., J.-K.C. nd S.-Y.K. designed nd performed experiments; S.-D.K. nd Y.-W.K. mde TEM smples nd the structurl mesurements; J.K, H.G. nd K.P. performed the electricl mesurements; nd S.-D.P. nd S.Y.K. performed density functionl theory clcultions nd the nlyses; S.-Y.K., S.K., S.Y.K., K.P. nd J.K. nlysed dt nd wrote the mnuscript; All uthors discussed nd commented on the mnuscript.

7 nture communictions DOI: /ncomms1650 Additionl informtion Supplementry Informtion ccompnies this pper t nturecommunictions Competing finncil interests: The uthors declre no competing finncil interests. ARTICLE Reprints nd permission informtion is ville online t reprintsndpermissions/ How to cite this rticle: Kwk, J. et l. Ner room-temperture synthesis of trnsfer-free grphene films. Nt. Commun. 3:645 doi: /ncomms1650 (2012).