APL102-023112-日本索尼公司制备的100米长的石墨烯透明导电薄膜 - 图文-

1、 Production of a 100-m-long high-quality graphene transparent conductive film by roll-to-roll chemical vapor deposition and transfer processToshiyuki Kobayashi, Masashi Bando, Nozomi Kimura, Keisuke Shimizu, Koji Kadono et al.Published by the American Institute of Physics.Related ArticlesTuning grap

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3、 graphene based nanofluidsJ. Appl. Phys. 113, 084307 (2013Graphene hydrogenation by molecular hydrogen in the process of graphene oxide thermal reductionAppl. Phys. Lett. 102, 071910 (2013Atomistic simulation study of brittle failure in nanocrystalline graphene under uniaxial tensionAppl. Phys. Lett

4、. 102, 071902 (2013Additional information on Appl. Phys. Lett. Production of a 100-m-long high-quality graphene transparent conductive film by roll-to-roll chemical vapor deposition and transfer processToshiyuki Kobayashi,aMasashi Bando,Nozomi Kimura,Keisuke Shimizu,Koji Kadono,Nobuhiko Umezu,Kazuhi

5、ko Miyahara,Shinji Hayazaki,Sae Nagai,Yukiko Mizuguchi,Y osuke Murakami,and Daisuke HobaraAdvanced Materials Laboratories,Sony Corporation,Atsugi-Shi,Kanagawa 243-0014,Japan(Received 25July 2012;accepted 3January 2013;published online 17January 2013A high-quality graphene transparent conductive lm w

6、as fabricated by roll-to-roll chemical vapor deposition (CVDsynthesis on a suspended copper foil and subsequent transfer.While the high temperature required for the CVD synthesis of high-quality graphene has prevented efcient roll-to-roll production thus far,we used selective Joule heating of the co

7、pper foil to achieve this.Low pressure thermal CVD synthesis and a direct roll-to-roll transfer process using photocurable epoxy resin allowed us to fabricate a 100-m-long graphene transparent conductive lm with a sheet resistance as low as 150X /sq,which is comparable to that of state-of-the-art CV

8、D-grown grapheneC 2013American Institute of Physics ./10.1063/1.4776707Graphene is an atomically thin carbon sheet with extra-ordinary electronic properties that are attractive for severalFor efcient production of graphene,roll-to-roll CVD synthesis using a copper foil has been a maj

9、or goal.In the roll-to-roll CVD process,graphene is continuously synthe-sized on the metal foil as it passes through a high tempera-ture reactor using several guide rollers.Hesjedal carried out atmospheric pressure thermal CVD by passing a copper foil through a 25-mm-diameter quartz tube furnace,whi

10、ch was heated to a temperature of 1000 C.7Although graphene was observed to form as a continuous lm over the entire copper foil,it was found to be multilayered and defective,probably because it was grown under atmospheric pressure at a high methane concentration.Yamada et al.have reported large scal

11、e and high throughput roll-to-roll CVD synthesis of gra-phene on a 297-mm-wide copper foil at a relatively low tem-perature of 400 C by assisting the dissociation of methane using microwave plasma.8However,the sheet resistance of the obtained graphene lm was higher than 10k X /sq,which is insufcient

12、 for use in electronic applications.Hence,pro-ducing high-quality graphene lms by an efcient roll-to-roll process remains a challenge.From previous studies,it is known that the quality of graphene improves with increasing growth temperature.9The growth rate of graphene is also an important factor to

13、 be considered for efcient graphene lm production.If the growth rates of graphene lms of similar quality are com-pared,graphene can be grown at a low pressure by more than one order of magnitude faster 10than at atmospheric pressure.11,12Based on these studies,we carried out a roll-to-roll CVD synth

14、esis of graphene at high temperature and low pressure by developing a reactor capable of not only selectively heating a large copper foil,but also capable of roll-to-roll handling of the copper foil under vacuum.In this paper,we describe a scalable technique to handle a high-temperature metal foil f

15、or use in obtaining signicant improvement in the quality of roll-to-roll processed graphene transparent conductive lms.aAuthor to whom correspondence should be addressed.Electronic mail:Toshiyuki.Kobayashi.C 2013American Institute of Physics 102,023112-1APPLIED PHYSICS LETTERS 102,023112 (2013roll-t

16、o-roll system,the CVD process was stable throughout the experiment,which lasted more than 16h.The rst 52m of graphene lm was grown at temperature T g %950 C by applying a constant direct current J ¼82A/mm 2to the sus-pended copper foil,and the remaining 48m of graphene lm was grown at J ¼8

17、3A/mm 2(T g %980 C.The temperature was estimated from the visible spectrum of emission from the copper foil.For the structural characterization of the synthesized graphene lm,we transferred the graphene lm from the copper foil to a SiO 2/Si substrate using a polymethyl meth-acrylate(PMMAsupport lm.1

18、4Raman spectroscopy Fig.2(drevealed that the graphene lm consisted of predomi-nantly single-layer graphene.A scanning electron micros-copy (SEMimage analysis Fig.2(bshowed that each graphene grain is nearly hexagonal 15,16and in many cases,a thicker multi-layer grain existed at the center.We observe

19、d more than 800grains and found that multi-layer grains with a diameter larger than 100nm covered $5%of the lm.Since Joule heating of a copper foil may result in a non-uniform temperature distribution,the spatial uniformity of the temperature within the copper foil was investigated by growing graphe

20、ne lms that only partially covered the cop-per foil surface,and then measured the coverage distribution Fig.2(a.After baking the copper foil at 180 C for 5min,the graphene-covered regions can be observed with an opti-cal microscope because of oxidation of the bare copper sur-face.The graphene covera

21、ges on (001copper surfaces were determined at different constant direct currents to eliminate the inuence of a facet-dependent growth rate.17Near the center of the foil,the graphene coverages were 89%and98%for J ¼82A/mm 2and 83A/mm 2,respectively.The lower coverage near the edge,as shown in Fig

22、.2(c,suggests that there is a non-negligible temperature drop due to a larger heat loss at the edges.However,uniform graphene coverage was observed near the center of the copper foil,indicating that the temperature variation across the majority of the sur-face area of the copper foil is negligibly s

23、mall.These results demonstrate that selective Joule heating is a reasonable heat-ing technique for graphene CVD synthesis.To evaluate the electrical and optical properties of the grown graphene lms,we fabricated a 100-m-long transpar-ent conductive graphene lm on a 125-l m-thick,230-mm-wide polyethy

24、lene terephthalate (PETlm Figs.1(dand 2(e.In this case,we transferred the graphene lm 18by bonding the graphene/copper foil to the PET lm with a 5-l m-thick photocurable epoxy resin 19Fig.1(b.The cop-per foil was then etched away by spraying the foil with a copper chloride solution Fig.1(c.All trans

25、fer processes were performed using the roll-to-roll process.The direct transfer of the graphene lm using the epoxy resin signicantly improved the yield of the transfer process compared with the conventional two-step transfer process,which involves transferring the graphene lm to a support lm prior t

26、o moving it to the target substrate.3,14However,in the direct transfer process,the epoxy resin conforms to the copper surface and transfers the rough surface morphologyFIG.2.(aOptical microscopy image of the graphene lm (J ¼82A/mm 2grown on copper foil after baking at 180 C.The image is capture

27、d 20mm from the edge of the copper foil.The light red areas in the image represent the copper surface covered with graphene and the darker areas represent the oxidized copper surface.(bSEM image of the graphene lm (J ¼82A/mm 2on an SiO 2/Si substrate transferred from near the center of the copp

28、er foil.(cCoverages of graphene at different positions from the edge of the copper foil having a (001copper surface.(dRaman spectrum of graphene on an SiO 2/Si substrate measured at an excitation wavelength of 437nm and a spot size of 1l m.Raman G-and 2D-band peaks can be tted with a single Lorentzi

29、an curve,indicating that the lm consists of predominantly single-layer graphene.(ePhotograph of the graphene/epoxy/PET roll before dop-ing.The widths of the graphene/epoxy and base PET lm are 210mm and 230mm,respectively.(fOptical transmittance of the base PET lm (blue,AuCl 3/graphene/expoxy/PET (re

30、d,and PET/adhesive/AuCl 3/graphene/epoxy/PET (purple.The transmission loss includes the reection at the lm surface.FIG.1.(aContinuous roll-to-roll CVD system using selective Joule heating to heat a copper foil suspended between two current-feeding electrode roll-ers to $1000 C to grow graphene (Gra.

31、(bReverse gravure coating of a photocurable epoxy resin onto a PET lm and bonding to the graphene/cop-per foil,followed by curing of the epoxy resin.(cSpray etching of the cop-per foil with a CuCl 2solution.(dStructure of the fabricated graphene/epoxy/PET lm.to the PETlm together with the graphene.A

32、lthough the transmission loss of the as-transferredlm was slightly higher because of the increased diffusion,the rough surface can be buried in a stack oflms in many applications,such as capacitive-type touch screens.After planarizing the rough surface by bonding another125-l m-thick PETlm with a 25

33、-l m-thick double-coated transparent adhesive tape,the transmission loss in the PET/adhesive tape/AuCl3/graphene/ epoxy was2.9%at a wavelength of550nmFig.2(f,indi-cating that thelm is a single-layer graphenelm consisting of partially covered multi-layer regions.The sheet resistance,R s,of the graphe

34、nelm was meas-ured with a4-probe resistivity meter,whose electrode sepa-rations were1.5mm.Without intentional doping,R s of the graphenelm grown at J¼82A/mm2was$500X/sq,show-ing a marked improvement from those of previously reported roll-to-roll synthesized graphenelms.8The sheet carrier densit

35、y,n¼1.4Â1013cmÀ2,measured by the Hall effect in the van der Pauw geometry,yielded a carrier mobility of l %900cm2VÀ1sÀ1using the Drude formula l¼(eR s nÀ1, where e is the elementary charge.This carrier mobility is comparable with that obtained using conventional th

36、ermal CVD synthesis6considering the high carrier density,which results in a lower mobility.20Since the mobility did not show a notable degradation as a result of the roll-to-roll transfer process,the defects in the graphene sheet are mainly attrib-uted to the CVD process.The conductivity of the grap

37、henelm was improved by wet chemical doping of the surface of the graphenelm using an AuCl3solution(20mM in nitromethaneplaced in direct contact with the graphene surface for30s.The AuCl3solution was then removed using a stream of dry air.3,4,21,22After dop-ing with AuCl3,R s initially decreased to$1

38、50X/sq and then gradually increased with time,t,until it eventually stabilized at$250X/sq.The increase in R s is due to a decrease in the car-rier density,which decays in two stepsFig.3(a.Fitting the curve with the double exponential function n(t¼n0þA1eÀk1t þA2eÀk2t yielded

39、a stable carrier density,n0¼4.4Â1013 cmÀ2,and two half-lives,t1/2,of0.33days and10days,respec-tively,where t1/2¼(ln2/k.The carrier mobility obtained one day after doping was$600cm2VÀ1sÀ1.Figs.3(b3(dshow spatial distributions of R s.In the R s maps,such as the one shown

40、in Fig.3(b,we did not observe large variations in either of the width or length directions. Long-range uniformity was also conrmed by measuring R s every1m over a length of100mFig.3(e.The representa-tive R s of the graphenelm was$500X/sq before doping and$200X/sq after doping.In both cases,R s is su

41、fciently low for touch screen applications.However,these values are still higher than those of state-of-the-art graphene grown by a conventional CVD process in a quartz tube furnace whose R s is111X/sq after doping with AuCl3.22The standard devi-ation of10%determined in this study is more than2times

42、 larger than that obtained in previous reports.3These observa-tions suggest some approaches to further improve our roll-to-roll CVD process.The relatively high R s is partly due to a high defect den-sity.The Raman D-band intensity,which represents the defect density,was8%and11%of the G-band intensit

43、y for J¼82A/mm2and83A/mm2,respectively.The higher D-peak intensity at J¼83A/mm2is consistent with the observed higher R s and suggests that the high defect density was probably caused by a relatively high growth rate at a high methane partial pressure.To lower the defect density,it is cons

44、idered that decreasing the methane partial pressure,as well as decreasing the total pressure,would be effective measures.While a high defect density degrades the overall conduc-tivity of the transparent graphenelm,the high deviation of the sheet resistance values is probably caused by micro-cracks a

45、ssociated with the plastic deformation of the copper foil.When a suspended copper foil is locally heated under a line tension,the copper foil buckles because of thermal stress.23The local radius of curvature of the corrugation can be less than10mm and the associated bending stress becomes sufciently

46、 high for triggering plastic deformation. If such plastic deformation occurs after the graphene has covered the copper surface,the graphene might form cracks, as shown in Figs.4(aand4(b.Such microcracks were observed only on the spatially distributed plastically deformed areas,which can be identied

47、by measuring the local crystal orientation of the copper foilFigs.4(cand 4(d.The density of the microcracks as well as the plasti-cally deformed area obviously increases with increasing tem-perature or decreasing line velocity.This trend can explain the smaller deviation of sheet resistance in the g

48、raphene FIG.3.(aDecay of carrier density measured by the Hall effect in the van der Pauw geometry at a magneticeld of0.7T and electrode separations of 10mm.Each data point corresponds to an average of8measurements taken at different points on the surface.The error bars correspond to the standard dev

49、iation.The purple curve represents the curve of bestt with a double-exponential function.The horizontal broken line represents the stable carrier density obtained by thet.(bSpatial distribution of sheet resistance R s taken at a position of30m along the graphene transparent conductivelm, and measure

50、d1day after doping with AuCl3.Histograms of R s at positions of30m(cand92m(d.Measurements were carried out within the 210mmÂ300mm area at a15-mm interval(n¼280before(blueand 1day after(reddoping with AuCl3.The R s value stated above each peak shows the mean6standard deviation of each histo

51、gram obtained by a Gaus-siant(solid lines.(eR s of the graphene/epoxy/PETlm along the longi-tudinal direction measured every1m(red line.Each point on the red line is an average of5measurements taken at different positions in the transverse direction.The5-m averages(total of25measurementsare also sho

52、wn by the blue circle symbols.The horizontal broken lines represent the averages of all of the measured data in the82A/mm2(052mand83A/mm2(52100mregions.lms grown at a lower temperature due to the lower micro-crack density.Therefore,microcracks could be the major ori-gin of the spatial variation in s

53、heet resistance.The increased microcrack density at high temperatures also suggests that the temperature must be lowered to minimize plastic defor-mation of the catalytic substrate during the growth of the graphene lm,as long as a copper foil is used as the catalytic substrate.It is important to note that a prolonged annealing of the copper foil prior to the growth might also reduce the microcrack density in the resulting graphene lm.Thus,the reduction of the microcrack density is pr