continuity of graphene on polycrystalline copper -...

Continuity of Graphene on PolycrystallineCopperHaider I. Rasool,*,†,‡,# Emil B. Song,*,§,|,# Matthew J. Allen,† Jonathan K. Wassei,†Richard B. Kaner,†,⊥ Kang L. Wang,§ Bruce H. Weiller,| and James K. Gimzewski†,‡

†Deparment of Chemistry and Biochemistry and California NanoSystems Institute, University of California at LosAngeles, Los Angeles, California 90095, United States, ‡ International Center for Materials Nanoarchitectonics(MANA), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan, §Department of Electrical Engineering, University ofCalifornia at Los Angeles, Los Angeles, California 90095, United States, |Micro/Nano Technology Department, SpaceMaterials Laboratory, The Aerospace Corporation, Los Angeles, California 90009, United States, and ⊥Department ofMaterials Science and Engineering, University of California at Los Angeles, Los Angeles, California 90095, United States

ABSTRACT The atomic structure of graphene on polycrystalline copper substrates has been studied using scanning tunnelingmicroscopy. The graphene overlayer maintains a continuous pristine atomic structure over atomically flat planes, monatomic steps,edges, and vertices of the copper surface. We find that facets of different identities are overgrown with graphene’s perfect carbonhoneycomb lattice. Our observations suggest that growth models including a stagnant catalytic surface do not apply to graphenegrowth on copper. Contrary to current expectations, these results reveal that the growth of macroscopic pristine graphene is notlimited by the underlying copper structure.

KEYWORDS Graphene, STM, epitaxy, materials, growth

Knowledge of material growth mechanisms on solidsupports allows scientists to grow materials withhighly desired properties. In heteroepitaxy, lattice

matching between deposited materials and single crystalsubstrates dramatically affects the properties of grownmaterials. For instance, small atomic lattice mismatchesbetween substrates and growth materials creates significantstrain in single crystal films1 which can result in highlydesired electronic properties or harmful defects.2 In addition,when growing ionic films on metallic single crystals, theinitial facet identity and mobility of the underlying substrategreatly influence resulting surface structures. In these sys-tems, a strong interfacial binding between surface chargeson vicinal metal surfaces and ionic overlayers3,4 drivesmassive surface restructuring and selective growth on spe-cific metal facets.5,6 These exemplary studies illustrate thesignificant role of substrate-overlayer interactions in thefield of film growth.

Recently, growth of the two-dimensional covalentlybonded network of carbon atoms known as graphene7-9 hasbeen accomplished on the surface of several metal cata-lysts.10-15 Of particular interest is the work by Li andcolleagues,16 in which large scale graphene films were grownon copper foils and shown to be applicable for the produc-tion of practical transparent electrodes.17 In the original

work the authors suggested that the process occurs via asurface-catalyzed reaction, which was corroborated by amechanistic study using isotope-labeled carbon.18 For futureelectronic device applications and rational materials design,a fundamental understanding of the atomic structure of theas-grown graphene overlayer on copper substrates is of greatimportance.19,20 Understanding the interplay between thecopper surface and the growing honeycomb lattice ofgraphene will provide valuable information for the produc-tion of high-quality graphene.

In this work, we show scanning tunneling microscopy(STM) topographs of as-grown graphene produced by thethermal decomposition of methane on high-purity polycrys-talline copper disks (see Supporting Information). The STMallows us to observe the morphology of samples over severalhundred square nanometers and obtain atomic resolutionof graphene’s carbon lattice at specific surface features ofinterest in a single experiment. The pristine graphene latticeis shown to exist over the copper substrate despite underly-ing features that are generally thought to create defects in agrowing overlayer. Defect-free graphene is observed overedges and vertices of the copper substrate where line andpoint defects are expected under conventional wisdom.Furthermore, the perfect lattice is observed over crystallinefacets of a different symmetry than graphene and overhighly stepped surfaces, both of which should inhibit ef-ficient sheet extension.

Samples were also characterized by Raman spectroscopyand powder X-ray diffraction (XRD). Figure 1a shows atypical Raman spectrum of the as-grown graphene on acopper substrate. The 2D band at ∼2700 cm-1 is symmetric

* To whom correspondences should be addressed, [email protected] [email protected].# These authors contributed equally to this work.Received for review: 10/17/2010Published on Web: 11/30/2010

pubs.acs.org/NanoLett

© 2011 American Chemical Society 251 DOI: 10.1021/nl1036403 | Nano Lett. 2011, 11, 251–256

with a full width at half-maximum (fwhm) of 32 cm-1 andis more than twice as intense as the G band at ∼1600 cm-1.These results are consistent with the synthesis of high-qualitymonolayer graphene.21,22 Polycrystallinity of the copper diskafter graphene growth was established through XRD (Figure1b). Three low index facets of copper, (100), (110), and(111), are clearly oriented in the vertical direction, i.e.,accessible to the imaging plane of the microscope. The onlyhigh index facet clearly observed in the diffraction patternis the (311) plane.

While the substrate polycrystallinity is expected, it isremarkable that monolayer graphene can grow over asubstrate with multiple facets of different identities andwhose surface atomic configurations vary dramatically. Inaddition, it is interesting that the graphene layer has beenshown, through scanning electron microscopy (SEM) andoptical studies, to grow continuously across the surface ofdifferent crystal grains present in the copper substrate.16 The(111) and (100) facets of the face centered cubic (fcc) coppermetal have a “flat” hexagonal and cubic atomic arrangementat the surface of the bare metal with each atom having sixand four in-plane nearest neighbors, respectively. Con-versely, the (110) and (311) facets have complex corrugatedatomic arrangements at the surface with significant anisot-ropy. These fundamental geometric differences of the facetsare expected to have distinct surface energy potentials,affecting the carbon atom extension differently.

Large area scans of graphene-covered copper substratestaken at different locations show varied surface topography,but with common fundamental features (Figure 1c). Someareas have large atomically flat regions that span hundreds ofnanometers. Other areas have a highly corrugated surface,which is the result of small atomically flat planes oriented in

different directions and with surface areas of a few tens ofnanometers.

Continuity of graphene was established through atomicresolution imaging (Figure 1d) at features of interest. An STMtopograph of a corrugated area is shown in Figure 2a. Thesurface structure is characterized by different sets of parallelflat planes intersecting to form edges or vertices. An edge isdefined as the intersection of two distinct planes and a vertexas the intersection of three or more planes. Atomic resolutionimages taken in certain flat areas (Figure 2b) show a perfectgraphene lattice with a superimposed aperiodic modulation.The height variations proceed for a few nanometers withlocalized depressions of a half nanometer in width and up to30 pm in depth. These modulations are attributed to defectswithin the copper substrate since freely suspended grapheneshows larger intrinsic modulation amplitudes of 1 nm andlonger coherence lengths of approximately 10 nm.23 Thecopper may create these variations in the STM topographwhere atomic vacancies exist near or at the interface betweenthe metal and graphene overlayer.3 This is expected sincecopper mobility is extremely high at the growth temperatureof 1000 °C and vacancies can become pinned between thegraphene sheet and the copper during cooling.

Within the flat regions, single and double atomic coppersteps draped with a continuous layer of pristine graphene areobserved. Figure 2c shows an atomic resolution STM topographof a graphene sheet crossing over a monatomic copper step.While the continuity of graphene over monatomic steps ofsingle crystal metal surfaces such as Ru(0001) and Ir(111) isknown,10,11 the mechanism of growth at these features re-mains an active area of theoretical and experimental re-search.24,25 The growth mechanism associated with the exten-sion of a carbon front (the foremost atoms of a sheet) “up” or“down” a monatomic metal step is thought to be fundamen-tally different (Figure 2d). Recent findings suggest that thecarbon front terminates at metal step edges through bondingof the dangling carbon sigma bonds to metal step atoms,inhibiting upward growth.10 However, a different study sug-gests that the carbon front may displace metal atoms beneaththe sheet and onto nearby terraces in order to accomplishupward growth through an etching of the catalytic surface.25

In the downward direction, growth seems uninhibited and mayoccur through a carbon dimer extension to a lower terrace.24

Each of these formulations treats the graphene sheet as ex-tending over or affecting a stagnant catalytic surface. In ourexperiments graphene continuity over step edges is alwaysobserved with no signs of termination at copper steps and mayproceed via a more dynamic growth mechanism.

Remarkably, it is also possible to observe pristine grapheneconforming over the edges and vertices joining differentcopper facets. These two copper features can be furtherdefined as being either negative or positive based on theirinteraction with graphene. In the case of negative edges andvertices, the graphene bends toward the STM tip producingtrough and valley structures in the observed topography.

FIGURE 1. Characterization of as-grown graphene on polycrystallinecopper. (a) Typical Raman spectrum of graphene on a polycrystallinecopper substrate (after subtraction of copper luminescent back-ground). (b) Powder X-ray diffraction pattern of a copper disk aftergrowth. (c) Typical large area STM topograph of as-grown grapheneon a copper substrate: Itunneling ) It ) 0.9 nA, Vsample ) Vs )-75 mV.(c) Atomic resolution STM image of the pristine carbon honeycomblattice: It ) 5.0 nA, Vs ) -75 mV.

© 2011 American Chemical Society 252 DOI: 10.1021/nl1036403 | Nano Lett. 2011, 11, 251-–256

This creates a tension in the portion of the carbon pz orbitalsinteracting with the copper substrate and a compression inthe portion of the carbon pz orbitals interacting with the tip.In the case of positive edges and vertices, the sheet bendsaway from the STM tip producing ridge and hill structuresin the observed topography and an opposite strain affect.Parts a-d of Figure 3 exemplify the continuity of thegraphene atomic lattice over these four fundamentally dif-ferent copper surface features.

Under a stagnant growth model, the graphene front isexpected to extend differently over positive and negativecopper features. In the case of positive edges and vertices,the dangling bonds of the extending sheets are uninhibitedby the copper atoms and are expected to grow continuouslyover these copper features in a manner that is comparableto extension down a monatomic step. In the case of negativeedges and vertices, the copper atoms should terminate thesheet growth by bonding to the dangling bonds of the carbonlattice. This is analogous to what has been suggested forgraphene growth termination on Ru(0001) at single mona-tomic steps,10 since a change in facet orientation is es-sentially a stack of atomic steps. As an example, a schematicillustration is provided (Figure 3e) where Cu(100) facets areoriented along the horizon and stacked vertically to makeCu(111) facets that should terminate graphene growth.However, a number of atomic resolution images of grapheneover negative edges and vertices of the copper substratehave been acquired without sign of termination or creation

of defects. Furthermore, the observation of all four surfacefeatures within a single area necessitates an upward andthen downward extension across multiple facets, precludingonly downward extension (i.e., repeated positive features).

The picture including displacement of monatomic stepedges by the extending carbon front can adequately explainmicrometer size graphene monolayers on single crystalmetallic surfaces, which are known to have submicrometerstep densities. However, in the case of the polycrystallinesurface, the extending carbon front would need to displacea tremendous number of copper atoms beneath the growingsheet or to exposed copper atom aggregations to obtain theobserved features beneath graphene. An alternative expla-nation is that mobile copper atoms or clusters act as carboncarriers, allowing for sheet extension over the metal surface.

A natural question that arises when analyzing the STMtopographs is whether the underlying copper planes belong tocrystallographic facets of different identities. Using higher orderperiodic modulations to the graphene sheet, we find thatunique sets of modulations to graphene’s atomic structureappear along sets of parallel planes of the sample (Figure 4a).The modulations span the entirety of an atomically flat regionand fade when the edge of one plane meets another planeoriented in a different direction. The two-dimensional spatialmodulation frequency and apparent height of a unique patterndo not depend on the size or shape of the flat region. Thisindicates that the patterns are not a result of electron standingwaves created from geometrical confinement as has been

FIGURE 2. (a) STM topograph of a highly corrugated region of the sample. The magenta rectangle highlights an atomically flat plane, the bluecircle highlights where three positive edges meet to form a positive vertex, and the orange circle highlights where three negative edges meetto form a negative vertex. (b) Atomic resolution STM topograph of a graphene sheet above an amorphous copper plane. (c) Atomic resolutionSTM topograph illustrating continuous growth over a copper monatomic step. (d) Schematic illustration of graphene growth “down” (whitearrow) and “up” (green arrow) a monatomic Cu(100) step. Carbon atoms are represented by black spheres, foremost carbon atoms of thegraphene sheet expected to interact with the monatomic step are highlighted with red spheres, light purple spheres represent copper atomsof the upper terrace, and darker purple spheres represent copper atoms of the lower terrace. Imaging parameters are as follows: (a) It ) 0.5nA, Vs ) -500 mV, (b) It ) 0.9 nA, Vs ) -65 mV, and (c) It ) 30 nA, Vs ) -75 mV.


observed in graphene nanoribbons.26 Additionally, it is possibleto image these modulations with “moire defects”, where por-tions of the pattern are absent and replaced by slight depres-

sions of approximately 10 pm with the graphene layer main-taining a pristine structure (see Supporting Information). Theorigin of the periodic modulations is, hence, attributed to the

FIGURE 3. STM images illustrating defect-free growth of graphene over (a) a negative edge, (b) a negative vertex, (c) a positive edge, and (d)a positive vertex. (e) Schematic illustration of graphene growth “down” (white arrow) positive edges and vertices and “up” negative edgesand vertices. Carbon atoms are represented by black spheres, foremost carbon atoms of the growing sheet expected to interact with edgesand vertices are highlighted with red spheres, the blue circle illustrates where three positive edges unite at a positive vertex, and the orangecircle illustrates where three negative edges unite at a negative vertex. Imaging parameters are as follows: (a) It ) 5.0 nA, Vs ) -75 mV, (b)It ) 5.0 nA, Vs ) -75 mV, (c) It ) 0.9 nA, Vs ) -75 mV, and (d) It ) 1.0 nA, Vs ) -500 mV.

FIGURE 4. (a) STM image over multiple facets. White asterisks mark parallel copper planes where identical moire patterns are observed. (b)Atomic resolution STM image of graphene growth over a Cu(100) facet. The graphene [1120] direction makes a 17° angle with the copper [011]direction. (c) Moire pattern analysis of the STM image shown in (b). The green spheres and axis belong to copper and the blue lattice and axisbelong to graphene. (d) Atomic resolution STM image of graphene growth over a Cu(311) facet. The graphene [1120] direction makes a 6° anglewith the copper [011] direction. (e) Moire pattern analysis of the STM image shown in (d). The red and green spheres belong to planes 1 and2 of the Cu(311) facet (see Supporting Information). The green axis belongs to coppe,r and the blue axis belongs to graphene. Imaging parametersare as follows: (a) It ) 0.99 nA, Vs ) -75 mV, (b) It ) 0.99 nA, Vs ) -85 mV, and (d) It ) 5.1 nA, Vs ) -75 mV.


presence of different crystalline copper facets interacting withthe graphene sheet to produce unique moire superstructuresin an analogous manner to what has been observed forgraphene on metal single crystals.11,13,14

In order to identify the atomic arrangement of the copperfacets beneath the graphene, a moire pattern analysis27,28

was performed on each unique angle of a graphene over-layer on the four facets appearing in the X-ray diffractiondata. These images were then checked against the multitudeof patterns appearing in STM topographs. Of particularinterest is graphene growth observed over a Cu(100) facet.This is shown in Figure 4b where the graphene [1120]direction makes a 17° angle with the copper [011] direction(Figure 4c). The existence of the graphene layer over theCu(100) facet is interesting because the symmetry of theunderlying substrate is fundamentally different than that ofgraphene. The copper(100) square lattice (Figure S2a,b inthe Supporting Information) belongs to the p4m planecrystallographic group whereas the hexagonal carbon latticeof graphene belongs to the p6m group.

Another interesting aspect of the overgrowth is thecontinuity of the graphene lattice over the regularly steppedCu(311) surface. This facet is comprised of alternatingmonatomic (111) and (100) nanofacets with a dilated hex-agonal arrangement of the topmost atoms (Figure S2c,d inthe Supporting Information). An STM topograph of a graphenesheet on a Cu(311) surface is shown in Figure 4d. Thegraphene [1120] direction makes a 6° angle with the [011]direction of the Cu(311) surface (Figure 4e), which generatesa moire pattern with alternating bright and dark bands.

The structure of the (311) facet is of particular interestbecause the stepped surface is expected to produce a periodicstrain that is strictly mechanical in nature as well as a chargemodulation beneath the graphene. The modulation is a resultof charge smoothing and formation of electrostatic dipoles atstep edges due to the Smoluchowski effect.29 It has recentlybeen shown that surface adsorption and postmodification ofgraphene layers atop low index metal single crystal surfacesare sensitive to the registry of the overlayer with the underlyingmetal atoms.30,31 These affects have been exploited to inducemeasurable band gaps in graphene’s electronic structure withthe hope of making practical carbon-based logic devices. Byusing graphene growth over stepped surfaces such as Cu(311)it may be possible to also exploit the charge modulation of theunderlying metal surface for applications requiring postmodi-fication.

Graphene continuity over the various surface featuresmay be the result of mobile copper atoms or islands actingas carbon carriers which extend the graphene front over thecopper surface with minimal inhibition in a type of “tiling”growth. This is analogous to the way a regularly arrangedstone path may be laid over a complex terrain with hills andvalleys. At the growth temperatures of 1000 °C, only 83 °Cless than the metals melting point, the copper atoms have ahigh degree of mobility at the surface. This is corroborated

by X-ray diffraction data where narrowing and splitting ofthe diffraction peaks of the graphene-copper samples isobserved as compared to the bare copper substrates, sug-gesting a substantial amount of mass transport within thecopper catalyst during growth (Figure S3 in the SupportingInformation). While the trajectory of carbon carrying copperatoms or islands would primarily follow the copper sub-strate’s symmetry during graphene growth, the high tem-peratures could provide sufficient kinetic energy for thesurface diffusion required to generate trajectories that allowextension of the hexagonal carbon lattice over the squareatomic lattice of the Cu (100) surface or over the highlycorrugated and anisotropic Cu(311) facet.

In summary, we have established the continuity of theperfect graphene atomic structure over a multitude ofsurface features on the copper substrate which are expectedto inhibit efficient growth. Edges and vertices of the coppersurface reveal perfect graphene growth where line or pointdefects are expected to occur through substrate-inducedsheet termination. The two exemplary cases of Cu(100) andCu(311) reveal that overgrowth occurs on facets of differentfundamental symmetries than graphene as well as on highlystepped vicinal surfaces. With the possibility to grow grapheneon high index regularly stepped surfaces, it may be possibleto make use of periodic charge modulation as well as atomiclattice registry for postmodification studies. To date, this hasbeen limited to growth on low index hexagonal singlecrystals for band gap studies. Our results reveal that graphenegrowth on copper is a unique system where the underlyingsubstrate morphology and atomic arrangement do not affectthe atomic arrangement of the grown material. Finally, ourresults suggest that the growth of continuous macroscopicpristine graphene on copper is not limited by the substrate.

Acknowledgment. H.I.R. and J.K.G. thank the MEXT WPIProgram: International Center for Materials Nanoarchitec-tonics (MANA) of Japan for financial support. E.B.S., K.L.G.,and R.B.K. thank the Functional Engineered NanoArchitec-tonics (FENA) center at UCLA for financial support. E.B.S.acknowledges financial support from the NSF IGERT Materi-als Creation Training Program, Grant DGE-11443. B.H.W.acknowledges support from The Aerospace Corporation’sIndependent Research and Development program. E.B.S.would like to thank J. D. Fowler (The Aerospace Corporation).H.I.R. would like to thank Harry Lockart and the PhysicsMachine Shop at UCLA, J. B. Levine, and H. D. Tran. H.I.R.would also like to thank K. A. Lopata for invaluable help inmoire analysis of STM images and helpful discussions.

Supporting Information Available. Materials and Meth-ods, STM images of a “moire defect,” hard sphere illustra-tions of the Cu(100) and -(311) facets, and XRD dataillustrating the change in sample crystallinity during growth.This material is available free of charge via the Internet athttp://pubs.acs.org.


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