Cite this: RSC Advances, 2013, 3,13926

Fabrication of vertically aligned graphene sheets on SiCsubstrates

Received 19th February 2013,Accepted 28th May 2013

DOI: 10.1039/c3ra40840j

www.rsc.org/advances

Lianlian Chen,a Liwei Guo,*a Yue Wu,b Yuping Jia,a Zhilin Lia and Xiaolong Chena

Fabrication of unidirectional arrays of VAGSs were achieved on nonpolar (101̄0) and (112̄0) SiC substrates

by thermal decomposition of SiC. The morphology and structural characteristics of the VAGSs were studied

and compared with that of the VAGSs grown on the (0001̄) (C-face) SiC substrate. It is found that the basal

plane orientations of the VAGSs are predominantly influenced by the orientation of the SiC substrate. The

mechanisms behind the phenomena were studied and discussed. As an example, the anisotropic

magnetism of graphene was studied with applied magnetic field parallel and perpendicular to the

graphene plane based on fully carbonized VAGSs. A nonpolar SiC substrate is a new choice for the

reproducible and convenient fabrication of massive ordering graphene arrays for both fundamental

research and potential applications of graphene based functional materials.

1 Introduction

Graphene based nanostructures show exceptional electronic,photo-electric, magnetic, mechanical, and chemical proper-ties, and therefore have potential applications in numerousnanodevices.1–4 The performance and possible applications ofthese nanodevices are critically dependent on their structuralmorphology and arrangement ordering decided by thefabrication methods used for the nanostructures.1 So moretechniques for their fabrication are being developed.5–7

Vertically aligned graphene sheets (VAGSs), as one of theforms of graphene nanostructures, have brought additionalbenefits for advanced applications in large area field emissionsources,8 electrodes for lithium-ion batteries,9 supercapaci-tors10 and as templates for fabricating high performancephotocatalytic materials,11,12 due to their well-defined reactiveedges, large surface area and surface-to-volume ratios, andexcellent electrical conductivity. Overviewing the syntheticmethods for vertically aligned carbon nanowalls (CNWs) orVAGSs, plasma-enhanced chemical vapor deposition (PE-CVD)is a widely used technique due to its feasibility and potentialfor large-scale production with reasonable growth rates atrelatively low temperatures.12,13 However, almost all of thereported vertically aligned graphene nanosheets showentangled and disordered networks, while a few showed thatdirectional two-dimensional alignments could be realized byapplying an electrical field to the substrate during CVD

growth,14,15 where density and height of the vertically alignedgraphene nanosheets were limited by the method itself.

Thermal decomposition of SiC was found to be a simpleand useful method for fabricating carbon nanotubes(CNTs)16,17 or VAGSs.18 However, almost all of the work onCNWs or VAGSs derived from SiC were based on the (0001̄) (C-face) SiC substrate. These studies16–18 stressed that the CNTsor CNWs can be grown on the C-face of SiC, but not on the Si-face of SiC. Meanwhile, it was found that the final products(CNTs or CNWs) are critically dependant on the oxygencontent of the growth ambient.18 If CNWs were prepared,their basal plane orientations were random and disordered.These experimental results imply that the orientation of theSiC substrate is a crucial factor in governing morphology of thederived product from SiC. Up to now, the growth mechanismof CNWs on C-face SiC remains elusive and the study on SiCsubstrates with other orientations for VAGSs has not beenreported. Therefore, exploring experimental phenomena andgrowth mechanism of VAGSs on SiC substrates with otherorientations are necessary and imperative to pursue aneffective and convenient method for the controllable fabrica-tion of structurally ordered VAGSs. Furthermore, the fabri-cated VAGSs will act as highly pure massive graphenematerials for application in fundamental science researchand as templates for graphene based functional materials forenergy storage, biosensors and other applications.

Here, fabrication of VAGSs with a unidirectional basalplane orientation on nonpolar SiC substrates was achieved.The structural characteristics and growth mechanisms of theVAGSs were analyzed. Based on a nonpolar SiC substrate, suchas the (101̄0) SiC substrate which has seldom been used forgraphene growth, VAGSs with a unidirectional basal planeorientation parallel to the SiC {0001} plane were achieved. A

aResearch & Development Center for Functional Crystals, Beijing National

Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of

Sciences, P.O. Box 603, Beijing 100190, China. E-mail: lwguo@iphy.ac.cnbNational Lab for superconductivity, Beijing National Laboratory for Condensed

Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603,

Beijing 100190, China

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13926 | RSC Adv., 2013, 3, 13926–13933 This journal is � The Royal Society of Chemistry 2013

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similar regime was also found in the VAGSs grown on the(112̄0) SiC substrate. The unidirectional alignment of thegraphene basal planes was well maintained during the wholegrowth procedure due to an intrinsic constraint from the SiCsubstrate. Comparatively, the basal planes of the VAGSs grownon a C-face SiC substrate show a petaloid shape arrangementof the graphene basal planes, and manifested multi-orienta-tion alignments which were inferred to be nearly parallel tothe {101̄0} plane of the SiC substrate. The ordering alignmentsof the VAGSs on the nonpolar SiC substrate make it possible tostudy the anisotropic properties of graphene sheets andexplore potential applications of the VAGSs, such as fabricat-ing graphene based composition materials or intercalationmaterials based on the gram-scale graphene template.

2 Experimental

The VAGSs samples were grown on SiC substrates withphysical vapor transport (PVT) equipment. The nitrogen-doped6H-SiC crystal was sliced and lapped along the (101̄0) and(112̄0) planes and then cut into rectangular plates in lateralsizes of several centimeters to prepare the sample A andsample B, respectively. Simultaneously, a commercially avail-able nitrogen-doped on-axis C-face 6H-SiC substrate (Tanke-Blue, Beijing, with Si face polished and C face lapped) wasused to preparing sample C. The miscut angle of the SiCsubstrate orientation is less than 0.3u. Before loading the SiCsubstrates to the PVT system, they were degreased withacetone and ethanol, rinsed with deionized water, and blowndry with nitrogen gas. Then they were put into a graphitecrucible and loaded into the PVT equipment. The SiCsubstrates were heated to about 1800 uC and kept for 2 h in

an atmosphere of argon and hydrogen mixed gas (95 vol% Ar +5 vol% H2) with a pressure of 30 kPa. After that, the pressure ofthe growth atmosphere was reduced to 0.01 Pa and kept therefor 10 min. Then, the PVT system was refilled with the mixedgas to 10 kPa and kept there for 5 min. After repeating thepumping and filling procedures three times, the samples werecooled down to room temperature naturally. The as grown thinfilm on the SiC substrate has a grey–black appearance. The asgrown samples were cut into small pieces to performmorphology observation in top view and cross-section viewon the fresh cut faces. To analyze the morphology of theprepared VAGSs film samples, scanning electron microscopy(SEM) (HITACHI S-4300) was used, which worked at anaccelerating voltage of 10 kV. High-resolution TEM (HRTEM)analysis was performed on a Tecnai G2 F20 U-TWIN operatingat 200 kV. The structural quality of the VAGSs was analyzedusing a high-resolution Raman spectrometer HR800 with a 532nm laser excitation focused to a spot with a diameter of about1 mm.

3 Results and discussion

Fig. 1(a), (b) and (c) show the top view images of the as grownsamples of A, B and C, respectively. It can be seen that theirsurfaces are covered with a graphene layer with a thickness ofseveral tens of nanometers, hereinafter called the graphenecap. Because the SiC substrates used are lapped withoutperforming chemical mechanical polishing with a coarsesurface, the surfaces of the VAGSs are uneven and coveredwith pieces of graphene cap in sizes of micrometers separatedby irregular hollows. In detail, there are small linear hollows asmarked by the red rectangles shown in Fig. 1(a) and (b), and

Fig. 1 Top-view SEM images of the as grown samples with (a–c) and without (d–f) the top graphene cap. (a) and (d) sample A on (101̄0) SiC substrate, (b) and (e)sample B on (11̄20) SiC substrate, (c) and (f) sample C on (0001̄) SiC substrate. The red rectangles and the irregular curves mark the hollows in the graphene cap. Theinset in (f) is the magnified image of a selected region marked by the red square, where the short red lines mark the basal planes of the VAGSs.

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some irregular big hollows on the top of the sample C asselectively marked with the closed red curves in Fig. 1(c). Moreinterestingly, the linear hollows are all aligned along the samedirection especially in sample A [Fig. 1(a)], which is deter-mined to be perpendicular to the [0001] direction of the SiCsubstrates deduced from the relative orientations of the linearhollows and the edges of the rectangular SiC substrates whoseedges are correspondingly along the [0001] and [12̄10], [0001]and [11̄00] directions for the (101̄0) and (112̄0) SiC substrates,respectively. The different appearance of the hollows on thethree SiC substrates suggests the orientations of SiC substrateshave much influence on the structure of the VAGSs.

To shed light on the inner morphology of the VAGSs, thetop graphene cap on each surface of the three samples wasremoved by reactive ion etching (RIE) with oxygen plasmausing a power 100 W for 5 min. After the top graphene cap isremoved, a deep-black appearance can be seen by the nakedeye in contrast to the original grey–black ones, and their topview morphology images are shown in Fig. 1(d), (e) and (f). It isobserved that alignments of the graphene sheets on the threesubstrates are clearly different. The morphologies of sample Aon a (101̄0) SiC substrate and sample B on a (112̄0) SiCsubstrate are similar as shown in Fig. 1(d) and (e), and areboth very different from that of sample C on a C-face SiCsubstrate as shown in Fig. 1(f), although the VAGSs are allcomposed of the dense carbon nanosheets. In Fig. 1 (d) and(e), most of the basal planes of the VAGSs are parallel to theSiC {0001} plane showing a unidirectional alignment. To thebest of our knowledge, the so high-density ordering align-ments of VAGSs were seldom reported before. While inFig. 1(f), the crystalline planes of the VAGSs show a petaloidshape arrangement similar to previously reported results.9

Magnifying the petaloid shape arrangement of the graphenesheets, it is noted that the neighbouring graphene sheetsprefer to intersect into 60u or 120u angles as schematicallymarked in the inset of Fig. 1(f) with the red short lines. Thepreferred basal plane arrangements of the VAGSs on differentSiC substrates suggest there are two factors at least playing keyroles in forming the regular VAGS structures in addition to thegrowth procedure conditions. One is the intrinsic surfacestructure of SiC substrate and the other is the interactionbetween the SiC frontal surface and the released C atomsduring the thermal decomposition of SiC.

To reveal the growth mechanism of the VAGSs on SiC,cross-sectional images of the VAGSs were taken and the detailsof the VAGSs at the frontal surface of SiC substrate wereanalyzed. For sample A, its cross-sectional SEM images in lowand high magnifications are shown in Fig. 2 viewing along the[12̄10] and [0001] directions of the (101̄0) SiC substrate. In thefull scene images of Fig. 2(a) and (b), it is seen that the two-dimensional carbon sheets were grown vertically on thesubstrate and their total height was about 44 mm. Uponinspection in detail, the VAGSs are not composed of a perfectcontinuous graphene sheet along the growing direction. Theyare physically overlapped or partly bonding while touchingeach other as identified from their magnified images of

Fig. 2(c) and (d). From Fig. 2(c) and (d), a clear difference inlateral width of the graphene sheets is noted. It is a little widerin Fig. 2(d) than in Fig. 2(c), which is consistent with theobserved phenomena in Fig. 1(d), indicating that basal planesof the VAGSs are preferred parallel to the SiC {0001} plane.Observing the interface region between the frontal surface ofthe SiC and the fresh growth end of the graphene sheets, it isfound that the graphene base edges connecting or bonding tothe SiC frontal surface possess two typical features. One ispartial bonding to the SiC surface in punctate form at the endsof two opposite edges of a free standing graphene ribbon asmarked with arrows in the region with a symbol P, which likesto have two feet to support the free VAGSs steadily. The other isa full connecting or bonding of its base edge with the SiCfrontal surface as marked in the region with a symbol F, wherethe graphene base edge shows an arc line shape to enhancestability of the VAGSs. The two forms have been proved to bemechanically stable as is common knowledge. Accompanyingthe growth of VAGSs at the region of the symbol P, aninterruption or a breakage of the VAGSs may happen.However, a continuous VAGSs growth may appear at theregion with the symbol F. Both of these are reasonable toexplain the observed cross-sectional images of sample A.

Furthermore, the mechanism of forming unidirectionalalignment of the VAGSs on (101̄0) SiC substrate is explained.While growing graphene at a high temperature of about 1800uC under a vacuum, the remnant C atoms were arranged into aplanar graphene layer at the initial growth as observed in theFig. 1(a) due to a rapid evaporation of Si atoms. Accompanyingthe graphene layer covering the surface gradually, the top openchannel facilitated escape of the decomposed Si atoms wasblocked. The escape of the Si atoms from the lateral spacebecame the only route. Therefore, the remaining C atoms wereprone to arrange into vertical standing graphene sheets with a

Fig. 2 Cross-sectional SEM images of sample A. Viewing along (a) and (c)[12̄10], (b) and (d) [0001], (c) and (d) are magnified images of (a) and (b) nearthe interface region between the VAGSs and SiC substrate, respectively. Thearrows in Fig. 2(d) position the punctate contact of the VAGSs with the SiCfrontal surface.

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wider space between them to facilitate the further escape ofthe thermal decomposed Si atoms. Since the C atomsbelonging to the neighboring SiC (0001) plane are prone toaccumulate into a graphene sheet, the unidirectional align-ment of the graphene sheets is preferred. As is well known, theC atoms in about every three Si–C bilayers contributed andconstructed a monolayer graphene while fabrication ofgraphene occurred by thermal decomposition of polar (0001)SiC substrate.19 In our experiments, the height of the VAGSson the nonpolar SiC substrate is nearly the same as thethickness of the decomposed SiC substrate. Therefore, it isestimated that on average one monolayer of grapheneoccupies the same space as that of three Si–C bilayers. Thus,it is reasonable that space exists between the graphene sheetsas shown in the magnified cross-section images of Fig. 2, sinceeach piece of the VAGSs was composed of several to dozens ofgraphene monolayers as shown by a transmission electronmicroscopy (TEM) image in Fig. 4(b). Fig. 3 is a schematiccross-section structure of the VAGSs grown on (101̄0) SiCsubstrate, which corresponds to Fig. 2(a) revealing a clearcross-section structure.

Fig. 4(a) and (b) show low and high magnified TEM imagesof the VAGSs, which indicate that the VAGSs are sheets ofseveral micrometers in size with numbers of layers rangingfrom several to several tens. The inset in Fig. 4(b) is thediffraction pattern from a thick multilayer graphene sheet inthe VAGSs. The polycrystalline-like diffraction pattern suggeststhe multilayer graphene sheet may be in rotational stackingfault or existing polycrystalline domains in a single graphenesheet.20 The later Raman scattering analysis on the VAGSs

suggests the multilayer graphene sheets behave like monolayergraphene, due to a weak coupling of the adjacent graphenelayers, which is consistent with the inference of a rotationalstacking fault existing in the multilayer graphene.

For sample B, the cross-sectional SEM images are shown inFig. 5, where view directions were along the [1̄100] and [0001]directions of the (112̄0) SiC substrate at the same magnifica-tions as in Fig. 2. The total height of the VAGSs is about 41 mm,slightly shorter than that of sample A. Observing Fig. 5(a) and(c), there is no clear difference from Fig. 2(a) and (c). However,it is noted that a crossover arrangement of the graphene sheetsappeared in Fig. 5(b) and (d), that is obviously different fromthe observation on sample A as shown in Fig. 2(b) and (d).Inspecting the interface of the VAGSs and the SiC substrate inthe bottom region of Fig. 5(d), it is seen that the exposed frontsurface of the (112̄0) SiC substrate shows protrusions ofregular ridge shape. The vertex angles of the ridges are about120u as clearly shown in Fig. 5(d). Therefore, it is deduced thatthe facets composed of the ridges belong to the {101̄0} plane.Here, they are (011̄0) and (101̄0) facets. It is noted that thegraphene sheets were grown on the (011̄0) and (101̄0) facetsvertically, in other words, they are inclined relative to the[112̄0] direction of the SiC, that makes the total height of theVAGSs a little shorter than that grown on the (101̄0) SiCsubstrate. Meanwhile, it is noted that when the graphenesheets developed from two opposite facets (belonging todifferent ridges) touching each other the intersected morphol-ogy is observed. Similar to the VAGSs grown on the (101̄0) SiCas shown in Fig. 2, the spread area of a single graphene sheetviewed along the C axis of the SiC as shown in Fig. 5(d) is widerthan that viewed along the direction perpendicular to the Caxis as shown in Fig. 5(c). Combined with the observation ofthe graphene basal plane alignment in Fig. 1(e), it is inferredthat the basal planes of the graphene sheets of sample B areparallel to the SiC {0001} plane. The inference is same as thatof the VAGSs grown on the (101̄0) SiC substrate, because the

Fig. 4 The TEM images of the VAGSs in low (a) and high (b) magnifications. Theinset is the diffraction pattern from a multilayer graphene sheet.

Fig. 3 A schematic cross-sectional structure of the VAGSs grown on (101̄0) SiCsubstrate.

Fig. 5 Cross-sectional SEM images of sample B. Viewed along (a) and (c) [1̄100],(b) and (d) [0001], (c) and (d) are magnified images of (a) and (b) in the interfaceregion. In Fig. 5(b), the red short lines give a guide to the eyes to show theintersections of the VAGSs.

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graphene sheets grown on the (112̄0) SiC substrate expandedtheir growth from the exposed SiC {101̄0} plane, which iscomposed of the ridges formed on the (112̄0) SiC substrate. Asis well known, the exposed crystalline planes usually possess arelatively low specific surface energy.21 The surface energies ofnonpolar planes of a-SiC were estimated to be 3.88 and 3.36 Jm22 for {112̄0} and {101̄0}, respectively, considering thesublimation energy and the density of dangling bonds onthe surfaces.22 This means the SiC {101̄0} plane is more stablecompared with the SiC {112̄0} plane. Decomposition of {112̄0}facets into pairs of {101̄0} facets would thus decrease thesurface energy. So, the (011̄0) and (101̄0) facets exposed andbecame the frontal surfaces of the (112̄0) SiC substrate duringthermal decomposition of SiC at high temperature. In thiscase, the VAGSs grown on SiC (112̄0) substrate transforms intogrowth on the (101̄0) facets, which obeys the same growthmechanism as on the (101̄0) SiC substrate. The growth processcan be described as graphene growth proceeded downwardperpendicular to the {101̄0} facets accompanied by the escapeof the Si atoms on the surface of the SiC substrate. Withongoing SiC decomposition and VAGSs growth, the middleregion of the initial {101̄0} facets becomes the vertex later, sothe graphene sheets derived from them intersected at thevertex. Therefore, the crossover morphology of the graphenesheets is observed. Meanwhile, it is noted that the basal planesof the graphene sheets are kept parallel to the SiC {0001} planein the whole growth process. Therefore, the growth mechan-ism of unidirectional arrangement of the VAGSs on the (112̄0)SiC substrate is similar to that on the (101̄0) SiC substrate,although a slight different in cross-sectional morphology isfound on the two SiC substrates.

A comparative study on VAGSs of sample C grown on the(0001̄) (C-face) SiC substrate was performed. Cross-sectionalSEM images of the VAGSs are shown in Fig. 6, where viewdirections were parallel to the SiC [12̄10] direction as shown inFig. 6(a) and (c), and parallel to the SiC [101̄0] direction asshown in Fig. 6(b) and (d), in low and high magnifications

respectively. As can be seen, the two-dimensional carbonsheets have grown vertically on the substrate and their totalheight is about 35 mm as measured from Fig. 6(a) and (b),which is 9 mm shorter than that of sample A grown on the(101̄0) SiC substrate. The possible reasons for this result willbe analyzed in the next paragraph. Viewing the magnifiedimages as shown in Fig. 6(c) and (d), it is difficult to tell thedifference between the VAGSs in shape and size along the twodirections. This means that it is difficult to analysis basalplane orientation of the VAGSs from the cross-sectional SEMimages. From observation of Fig. 6(c) and (d) in detail, it isnoted that the VAGSs are not a continuous growth along thegrowing direction similar to the observation in the abovementioned two substrates. The VAGSs are bent or curved attheir base to keep stable with increasing height. The width ofthe VAGSs is less than 0.4 mm and the height of a near perfectsingle sheet is over 3 mm (about 5–10 mm inferred from otherSEM images not shown here). These graphene sheets are partlyoverlapped or connected at their contacting part similar to thecase of samples of A and B. The narrow graphene sheetscontribute a clear D peak signal in the Raman scatteringspectrum of the sample C, which is shown in Fig. 7.

From an overview of the VAGSs grown on the three SiCsubstrates, it is noted that the height of the VAGSs is about 44mm in sample A, 41 mm in sample B and 35 mm in sample C,respectively, although the three samples were grown in thesame run with the same thickness of about 45 mm of SiCdecomposed. We consider three possible reasons may governthe results. The first is the morphology of the top graphene capon the top surface of the VAGSs. The hollows in the top surfaceof the VAGSs facilitate escape of the evaporated Si atoms fromthe inner space of the VAGSs to the outer space. The bigger thehollows, the easier it is for the Si atoms to escape. The secondis the evaporation rate of the Si atoms on different SiCsurfaces, which is closely related to surface energy of the freecrystal surface under the same ambient pressure.23 The thirdis the density of the VAGSs. The higher the density of theVAGSs is, the shorter the VAGSs are, in the case of carbon atomnumber conserved. Since the decomposed SiC thickness in ourexperiment is similar in the three kinds of SiC substrates, theformer two aspects are excluded to play a key role in ourexperiments. The density of the VAGSs on the three SiCsubstrates is the main reason for the different heights of theVAGSs. In other words, a smaller total height of the VAGSs insample C is ascribed to its higher density than that of sample Aand sample B, as evidenced by Fig. 1(d)–(f). For sample B, itsslightly smaller height compared with that of sample A isascribed to the inclined growth of the VAGSs on the SiC (112̄0)face, resulting in a short effective height of the VAGSs alongthe [112̄0] direction.

In addition to the insight of the morphology characteristicsof the VAGSs grown on the three SiC substrates, the structuralqualities of the VAGSs from the three samples were analyzedby Raman scattering as shown in Fig. 7. Fig. 7(a) and (b) recordthe back scattering Raman spectra on the top surface of thesamples with and without the top graphene cap as shown in

Fig. 6 Cross-sectional SEM images of sample C. View along (a) and (c) [12̄10], (b)and (d) [101̄0], (c) and (d) are magnified images of (a) and (d).

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Fig. 1 All the spectra show three peaks located around 1350,1580 and 2700 cm21 attributed to D, G and 2D peaks, whichare typical fingerprints of a free graphene and agree well withthose of CNW films prepared by other methods.24 It is notedthere is very weak D peak in Fig. 7(a), but prominent D and D9

peaks in Fig. 7(b), besides the significant G and 2D peaks. As iswell known, the emergence of a D peak and a D9 peak indicatedefects or disorder exist in the graphene, and their intensity isproportional to the edge or defect density in the graphene. Theweak D peak in the as-grown samples means the VAGSs arehigh quality at least in the scale of the laser spot in the Ramanscattering measurement. This inference is supported by thenarrow full width at half maximum (FWHM) WG and W2D ofthe G and 2D peaks in Fig. 7(a). They are 22 cm21, 19 cm21

and 21 cm21 for the G peaks, and 46 cm21, 32 cm21 and 46cm21 for the 2D peaks. The FWHM values are comparable tothose of the reported high quality graphene.24 The D peakbecomes prominent after the top graphene cap was removedby RIE treatment as shown in Fig. 7(b), and an additional D9

peak appeared which is also related to defects or disorder asobserved in the hydrogenation graphene25 and narrowingwidth of graphene ribbons.26 The enhanced D and D9 peaks inthe Raman spectra of the RIE treated samples are consideredto be ascribed to the exposed top edges of the VAGSs after thesurface graphene cap was removed and the defects introducedin the VAGSs by plasma bombardment. This inference is

supported by the universal experimental observations thatboth the WG of the G peak and the intensity ratio ID/IG betweenthe D and G peaks increase with increasing disorder in thegraphite structure.26,27 Here WG increased by about 5 cm21,while the ID intensity enhanced prominently, after the topgraphene cap was removed. In addition, it is noted that the 2Dpeaks are all well fitted by a symmetry Lorenzian function, thatmeans interaction of the adjacent graphene layers in the samegraphene sheet is weak, although the graphene layer numbersin each piece of VAGSs are ranged from a few to dozens oflayers as shown by TEM analysis in Fig. 4.

Based on the fully carbonized unidirectional arrays of theVAGSs derived from the (101̄0) SiC substrate, one of theintrinsic properties of graphene: anisotropic magnetic prop-erty was studied. Fig. 8(a) shows magnetization as a functionof temperature with an applied magnetic field 1000 Oeperpendicular (black curve) and parallel (red curve) to thegraphene basal plane respectively. The observed diamagnet-ism is derived from the quantized orbital motions of thecarriers in a magnetic field similar to that observed ingraphite28,29 whose susceptibility has a large temperaturecoefficient and is about 221.5 6 1026 emu g21 at roomtemperature while the magnetic field is perpendicular to thegraphene basal plane. The anisotropy is ascribed to the greatdifference of free carrier density in and out of the graphenebasal plane. The room temperature susceptibility of our VAGSs

Fig. 7 Raman scattering spectra of the (a) as grown samples, (b) RIE treated samples. The Raman spectra from top to bottom are corresponding to samples A, B and Crespectively.

Fig. 8 (a) Susceptibilities of VAGSs versus temperature with applied field of 1000 Oe perpendicular (black curve) and parallel (red curve) to the graphene basal planes.(b) The anisotropic susceptibility difference xe = x) 2 x// as a function of reciprocal temperature.

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is a little smaller compared with that of graphite. Thetemperature dependence of the anisotropic susceptibility xe

= x) 2 x// at temperature ranges from 30 K to 300 K as shownin Fig. 8(b), where x) and x// present susceptibility withapplied magnetic field perpendicular and parallel to thegraphene basal plane, respectively. The variations of theanisotropic susceptibility xe of the VAGSs with temperatureare similar to that of graphite.30 Here, deduced xe at hightemperature tends to reach asymptotically the relation xe =20.002/T emu g21 as expressed by the asymptotical lineexpressed by the triangle symbols, which is about one-fifth ofthe anisotropic susceptibility 20.010/T emu g21 of graphite,suggesting a lower carrier concentration in our VAGSscompared with that of graphite. At low temperature, it tendsto reach the temperature independent value xe = 222.5 6 1026

emu g21, which is a little smaller compared to the value 2306 1026 emu g21 of graphite. The slightly smaller anisotropicsusceptibility of the VAGSs compared with that of graphite isascribed to the smaller size of the VAGSs. In other words, thesmall size of graphene renders more edges or defective statesin the VAGSs, which introduces local states31,32 and traps somefree carriers. It is the edges in the VAGSs that generate somenovel magnetism at low temperature, which is beyond thestudy scope here and will be investigated elsewhere.

4 Conclusions

VAGSs with unidirectional basal plane alignment have beenfabricated on nonpolar (101̄0) and (112̄0) SiC substrates, wherethe graphene basal planes are parallel to the SiC {0001} plane.However, most of the graphene basal planes are parallel to the{101̄0} plane of the SiC, while growing the VAGSs on a C-faceSiC substrate. The growth mechanism of the unidirectionalalignment of the VAGSs is discussed. The quality of the VAGSsis high and the adjacent graphene layer coupling is weak,which makes them behave like monolayer graphene suitablefor acting as a platform to study anisotropic intrinsic proper-ties of graphene where gram-scale graphene is a prerequisite.As an example, anisotropic magnetism of the VAGSs at 30–300K was studied and carrier concentration in the graphenesheets is lower than that of graphite due to the existing edgesor defects in the micrometer sized graphene sheets. Therefore,the study here provides a simple and reproducible promisingway to fabricate high quality graphene in gram-scale withunidirectional arrays of graphene sheets, which will help us tostudy intrinsic properties of graphene and explore novelfunctional materials based on the massive ordering graphene.

Acknowledgements

This work is partly supported by the ministry of science andtechnology of china under Grant No. 2011CB932700, theknowledge innovation project of Chinese academy of scienceunder Grant No. KJCX2-YW-W22, the national science founda-

tion of china under Grants No. 51272279, 51072223 and50972162.

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