The atomic structure of epitaxial single and bilayer graphene on Ru(0001) was studied by scanning tunneling microscopy (STM). High-resolution imaging of the surface of single layer graphene shows a moiré with pronounced buckling and broken A/B carbon sublayer symmetry due to a strong interaction with the metal substrate. The top sheet of bilayer graphene is largely unperturbed by residual interactions with the substrate. Screened from the metal substrate, it shows the hallmarks of freestanding monolayer graphene: a honeycomb structure with equivalent carbon sublattices imaged in STM and a linear dispersion of bands near the Dirac point

Acharya, D.P.

Sadowski, J.T.

Sutter, Eli A.

Sutter, P.

English

DigitalCommons@University of Nebraska

University of Nebraska - LincolnDigitalCommons@University of Nebraska - LincolnFaculty Publications from the Department ofElectrical and Computer Engineering Electrical & Computer Engineering, Department of3-30-2009Scanning tunneling microscopy on epitaxial bilayergraphene on ruthenium (0001)Eli A. SutterUniversity of Nebraska–Lincoln, esutter@unl.eduD.P. AcharyaBrookhaven National LaboratoryJ.T. SadowskiBrookhaven National LaboratoryP. SutterBrookhaven National Laboratory, psutter@unl.eduFollow this and additional works at: http://digitalcommons.unl.edu/electricalengineeringfacpubPart of the Computer Engineering Commons, and the Electrical and Computer EngineeringCommonsThis Article is brought to you for free and open access by the Electrical & Computer Engineering, Department of at DigitalCommons@University ofNebraska - Lincoln. It has been accepted for inclusion in Faculty Publications from the Department of Electrical and Computer Engineering by anauthorized administrator of DigitalCommons@University of Nebraska - Lincoln.Sutter, Eli A.; Acharya, D.P.; Sadowski, J.T.; and Sutter, P., "Scanning tunneling microscopy on epitaxial bilayer graphene on ruthenium(0001)" (2009). Faculty Publications from the Department of Electrical and Computer Engineering. 458.http://digitalcommons.unl.edu/electricalengineeringfacpub/458Scanning tunneling microscopy on epitaxial bilayer grapheneon ruthenium „0001…E. Sutter, D. P. Acharya, J. T. Sadowski, and P. SutteraCenter for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, USAReceived 5 January 2009; accepted 4 February 2009; published online 30 March 2009The atomic structure of epitaxial single and bilayer graphene on Ru0001 was studied by scanningtunneling microscopy STM. High-resolution imaging of the surface of single layer grapheneshows a moiré with pronounced buckling and broken A /B carbon sublayer symmetry due to a stronginteraction with the metal substrate. The top sheet of bilayer graphene is largely unperturbed byresidual interactions with the substrate. Screened from the metal substrate, it shows the hallmarks offreestanding monolayer graphene: a honeycomb structure with equivalent carbon sublattices imagedin STM and a linear dispersion of -bands near the Dirac point. © 2009 American Institute ofPhysics. DOI: 10.1063/1.3106057Graphene, a two-dimensional honeycomb lattice of sp2bonded carbon atoms,1 has shown a number of remarkableproperties that make it a promising material for large-scaleapplications in microelectronics2,3 and sensing.4 To realizethis potential, reliable methods for fabricating large-areasingle crystalline graphene domains are required. Graphenesynthesis by epitaxy on noncarbide-forming transitionmetals5,6 has been considered recently as a promising alter-native to micromechanical cleavage for producing macro-scopic graphene domains. On the 0001 surface of ruthe-nium Ru, in particular, it has been shown that one canproduce large close to 500 m single or bilayer epitaxialgraphene nuclei in a controlled layer-by-layer fashion.5Graphene interacts strongly with Ru. Thus a single graphenelayer has a significantly altered electronic structure comparedto freestanding graphene and an atomic structure reflectingthe strong substrate coupling.5,7 While much effort has beeninvested in determining the properties of monolayergraphene MLG on Ru0001,7–9 the atomic structure of bi-layer epitaxial graphene has not been investigated yet.The possibility of controlled growth of bilayer grapheneBLG domains on Ru0001 raises several important issues,which can be addressed by high-resolution microscopy.While Raman spectroscopy suggests that the top sheet inBLG on Ru0001 recovers the electronic structure of free-standing MLG,5 it is unclear if the sheet is homogeneouslydecoupled from the first layer and substrate. Further, many ofthe exotic electronic properties of graphene derive from thechiral nature of its Dirac fermions,2,10 originating in the two-atom basis i.e., A and B sublattices of the honeycomb struc-ture of graphene. Thus, it needs to be verified if the A and Bcarbon sublattices of the top sheet of the bilayer are equiva-lent i.e., the charge carriers chiral or if the underlyingbuffer breaks the sublattice symmetry leading to nonchiralcarriers. Local spectroscopy, finally, can be used to establishthe residual charge-transfer doping of the bilayer by deter-mining the energy of the Dirac point, i.e., the crossing of thelinear -bands at the K-point of the two-dimensional Bril-louin zone.Here, we address these issues by scanning tunneling mi-croscopy STM imaging and spectroscopy of single and bi-layer epitaxial graphene on Ru0001. The study is based onthe ability to grow macroscopic epitaxial graphene sheetswith predictable thickness in a controlled layer-by-layermode on Ru. The graphene growth was carried out in apreparation chamber attached to an ultrahigh vacuum UHVSTM following the procedure described in Ref. 5. Briefly, aclean Ru0001 single crystal was loaded with interstitial car-bon by exposure to a hydrocarbon precursor ethylene athigh temperature 1000 °C. Graphene was grown inUHV by slowly lowering the sample temperature, driving thecontrolled segregation of C to the Ru surface. The graphenegrowth proceeds via the initial formation of monolayer thickgraphene islands that expand to a size corresponding to thespacing of the initial nuclei and coalesce to a completegraphene monolayer covering the entire Ru0001 substrate.Upon completion of the first layer, a second graphene layernucleates and can again grow to domain sizes of over100 m if the slow temperature ramp is continued. STMimaging and spectroscopy on samples with full monolayercoverage and with additional small second layer nuclei wasperformed in situ using a Createc low-temperature STMcooled to 77 K. The STM measurements were complementedby selected-area angle-resolved photoemission spectroscopymicro-ARPES, performed in situ at 300 K on individuallarger BLG domains in a Elmitec LEEM III low-energy elec-tron microscope with energy analyzer energy resolution200 meV, using 42 eV synchrotron radiation.On Ru0001, single layer graphene adopts a moiréstructure5,7,8 in which the 101¯0 directions of layer and sub-strate align, with moiré repeat vectors am=2.930.08 nm,equivalent to 10.80.3 times the nearest-neighbor distanceon Ru0001.5 Figure 1a is an overview STM image of thismoiré structure. The graphene monolayer shows large varia-tions in apparent height about 1 Å within a highly orderedmoiré structure that has no visible defects over very largesample areas. High-resolution STM images Fig. 1b showno atomic-scale defects, indicating a high crystalline qualityof the graphene layer. Superimposed over the large long-range height variations is a mostly triangular lattice structureat the atomic scale, similar to the one observed earlier7 andaAuthor to whom correspondence should be addressed. Electronic mail:psutter@bnl.gov.APPLIED PHYSICS LETTERS 94, 133101 20090003-6951/2009/9413/133101/3/$25.00 © 2009 American Institute of Physics94, 133101-1assigned to a moiré pattern whose atomic contrast variesacross the unit cell depending on the local registry with thesurface Ru atoms. The local registry determines which of thegraphene A or B sublattices is imaged. In most parts of themoiré unit cell the sublattice symmetry is broken and onlyone of the two carbon sublattices is imaged due to the strong,covalent interaction with the substrate. The full graphenehoneycomb lattice is only visible in areas directly adjacent tothe moiré maxima Fig. 1c. In these regions the graphenesheet is lifted higher above the Ru lattice and couples weaklyto it.9 As a result, the density of states DOS of the twosublattices is identical in these areas and the full honeycombof both A and B carbon atoms is imaged by STM.While the atomic structure of the graphene monolayershown in Fig. 1 is dominated by the strong covalent interac-tion with the metal substrate, it is important to establish thesurface structure of the graphene bilayer, whose topmostsheet is expected to be almost completely decoupled andonly interacting with the support via weak van der Waalsforces.5 In order to ensure the unambiguous identification ofthe bilayer, the growth was modified to yield only small nu-clei of the second graphene layer. This was achieved bygrowing the monolayer to saturation coverage, and then rap-idly lowering the sample temperature.Figure 2a shows a typical BLG island and the sur-rounding completed monolayer. The island is bounded onone side by a Ru surface step and on the other by a kinkedbut atomically sharp edge whose segments align with themajor crystallographic directions of the underlying moiréstructure of the monolayer. The surface of the bilayer islandshows distinct inhomogeneous STM contrast that appearsless ordered than the surrounding monolayer areas. While amajority of the sheet is flat, it also carries surface ripples ofvarying height, the highest of which protrude about 1 Å. Inthe sample plane, the positioning of these ripples shows nolong-range order. In some areas the ripples are in a localhexagonal arrangement with the periodicity and orientationof the superstructure of the adjacent monolayer but they gen-erally do not align with the maxima of the monolayer moiré.Figure 2b, a profile of apparent height in STM alongthe line traced in Fig. 2a, shows a height difference of2.2 Å between two terraces entirely covered by MLG, cor-responding to the interlayer spacing of the 0001-orientedRu 2.14 Å. These areas covered by MLG are thus grown onadjacent substrate terraces and separated by an atomic Rustep. Although the contrast in STM invariably reflects a com-bination of topographic and electronic structure and can bedifficult to interpret on inhomogeneous surfaces, the mea-sured height difference between the two sheets of the bilayer3.2 Å, relative to the monolayer minima is close to thec-axis spacing of graphite 3.35 Å, as expected if the topsheet of the bilayer is coupled to the underlying support byweak van der Waals forces only.The equivalence of A and B carbon sublattices ingraphene requires a description of electronic Bloch states interms of two component wave functions and gives rise toexciting phenomena such as chiral quantum Hall effects2,10and anisotropic group velocity renormalization11 and elec-tron supercollimation12 in lateral superlattices. It is thereforeimportant to establish if the decoupling of top sheet of epi-taxial BLG on Ru is sufficient to recover the A /B sublatticesymmetry. This question can be addressed by high-resolutionSTM. Atomically resolved images Figs. 2c and 2d ob-tained on the bilayer show symmetrical honeycomb contrastthroughout, i.e., in both planar and rippled areas. The honey-comb structure is characteristic of a single graphene layer forwhich all carbon atoms are equivalent and has been observedby STM on cleaved MLG on SiO2.13,14 The atomic structureobserved here shows that the top sheet in bilayer epitaxialgraphene on Ru0001 is weakly perturbed by residual sup-port interactions and has the characteristics of freestandingMLG. In particular its carbon sublattices are equivalent sothat its charge carriers should behave as massless and chiralDirac fermions, showing the rich behavior observed or pre-dicted for such charge carriers in isolated MLG.To further explore the electronic decoupling of the topsheet of epitaxial BLG, we have performed scanning tunnel-FIG. 1. Color Morphology of first-layer epitaxial graphene on Ru0001.a UHV STM image, obtained at 77 K, of a large area of monolayer epi-taxial graphene on Ru0001. b High-resolution STM image of the moiréstructure of the graphene layer V= +0.2 V, I=0.2 nA. c Magnified viewof the highest region of the moiré, showing a local honeycomb structure. FIG. 2. Color Morphology of the top sheet of bilayer epitaxial grapheneon Ru0001. a UHV STM image T=77 K of the surface consisting oftwo different phases: a BLG island on a completed MLG layer. b Heightprofile along the line displayed in a. c High-resolution STM image show-ing honeycomb contrast, i.e., sublattice equivalency, in rippled and flat re-gions of the bilayer. d Closeup view of the graphene honeycomb latticeV= +0.7 V, I=0.2 nA.133101-2 Sutter et al. Appl. Phys. Lett. 94, 133101 2009ing spectroscopy on both monolayer and bilayer domains.Representative local dI /dV spectra are shown in Fig. 3a.The spectrum of the monolayer shows pronounced modula-tions of the tunneling conductance near the Fermi energyEF, reflecting the DOS of a graphene sheet strongly inter-acting with the metal substrate. The state at about 0.2 eV,for instance, is also observed in micro-ARPES spectra atsmall in-plane momentum on the monolayer.15 From a mini-mum at EF, dI /dV increases sharply with energy for bothfilled and empty states, consistent with a finite DOS at EFnear the zone center. The situation is different for the bilayer.Here, the tunneling conductance remains close to zero withinan extended energy window around EF. A similar gaplikefeature observed in tunneling spectra on exfoliatedgraphene/SiO2 Ref. 14 was interpreted as due to a suppres-sion in elastic tunneling close to EF, followed by a sharp risein the tunneling conductance with the onset of inelastic tun-neling involving the emission of a K6 acoustic phonon withenergy 067 meV.16 For BLG on Ru0001, however,the gap is significantly wider. If the tunneling indeed is dueto the opening of an inelastic channel at the energy of aK-point acoustic phonon, our data suggest that the excitationof the lowest-energy out-of-plane K6 phonon mode is sup-pressed, and that the gap is instead defined by the in-plane K2phonon with 0125 meV.16 At higher negative bias, thetunneling spectrum of BLG on Ru0001 is similar toelectron-doped MLG SiO2, which showed a pronounced dipin dI /dV at the Dirac energy, i.e., the energy at which thelinear -bands intersect.14 Adopting the same interpre-tation for our spectra, we would place the Dirac energy atED−500 meV, consistent with a substantial electron dop-ing of the decoupled top sheet of the bilayer by the underly-ing graphene buffer and metal substrate. Micro-ARPES spec-tra on larger BLG domains on Ru0001 indeed show a linearband dispersion near K, with Dirac point about 500 meVbelow EF Fig. 3b, confirming that the top sheet of thebilayer behaves like freestanding MLG except for a down-shift of the Dirac point. Our STM and initial micro-ARPESresults suggest that the decoupling of the top sheet of thebilayer is due to a suppression of -states of the bottom layerby hybridization with Ru d-states. Such a process wouldleave only the -states of an isolated top layer near EF at theK-point.In conclusion, we have established the atomic and elec-tronic structure of bilayer epitaxial graphene on Ru0001.The interfacial graphene sheet in the bilayer plays the role ofa buffer, coupling strongly to the metal and screening the topsheet from the metal d-states. As a result, the top sheet of thebilayer recovers the structure a symmetric honeycomb lat-tice and electronic properties linear dispersion in Diraccones of freestanding MLG.Work performed under the auspices of the U.S. Depart-ment of Energy under Grant No. DE-AC02-98CH1-886.1A. K. Geim and K. S. Novoselov, Nature Mater. 6, 183 2007.2K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, M. I. Katsnelson,I. V. Grigorieva, S. V. Dubonos, and A. A. Firsov, Nature London 438,197 2005.3S. V. Morozov, K. S. Novoselov, M. I. Katsnelson, F. Schedin, D. C. Elias,J. A. Jaszczak, and A. K. Geim, Phys. Rev. Lett. 100, 016602 2008.4F. Schedin, A. K. Geim, S. V. Morozov, E. W. Hill, P. Blake, M. I.Katsnelson, and K. S. Novoselov, Nature Mater. 6, 652 2007.5P. W. Sutter, J.-I. Flege, and E. A. Sutter, Nature Mater. 7, 406 2008.6A. T. N’Diaye, S. Bleikamp, P. J. Feibelman, and T. Michely, Phys. Rev.Lett. 97, 215501 2006.7S. Marchini, S. Gunther, and J. Wintterlin, Phys. Rev. B 76, 0754292007.8A. L. Vazquez de Parga, F. Calleja, B. Borca, M. C. G. Passeggi, Jr., J. J.Hinarejo, F. Guinea, and R. Miranda, Phys. Rev. Lett. 100, 0568072008.9B. Wang, M.-L. Bocquet, S. Marchini, S. Günther, and J. Wintterlin, Phys.Chem. Chem. Phys. 10, 3530 2008.10Y. Zhang, J. W. Tan, H. L. Stormer, and P. Kim, Nature London 438,201 2005.11C.-H. Park, L. Yang, Y.-W. Son, M. L. Cohen, and S. G. Louie, Nat. Phys.4, 213 2008.12C.-H. Park, Y.-W. Son, L. Yang, M. L. Cohen, and S. G. Louie, Nano Lett.8, 2920 2008.13E. Stolyarova, K. T. Rim, S. Ryu, J. Maultzsch, P. Kim, L. E. Brus, T. F.Heinz, M. S. Hybertsen, and G. W. Flynn, Proc. Natl. Acad. Sci. U.S.A.104, 9209 2007.14Y. Zhang, V. W. Brar, F. Wang, C. Girit, Y. Yayon, M. Panlasigui, A. Zettl,and M. F. Crommie, Nat. Phys. 4, 627 2008.15P. Sutter, J. T. Sadowski, M. S. Hybertsen, and E. Sutter unpublished.16M. Mohr, J. Maultzsch, E. Dobardzic, S. Reich, I. Milosevic, M. Damn-janovic, A. Bosak, M. Krisch, and C. Thomsen, Phys. Rev. B 76, 0354392007.FIG. 3. Color online Electronic structure of the top sheet of bilayer epi-taxial graphene on Ru0001. a dI /dV tunneling spectra of MLG and BLGdomains. Horizontal lines mark dI /dV=0; vertical lines denote energies0 of the K2 acoustic phonon Ref. 16. b Micro-ARPES spectrum onBLG, obtained near the K-point of the Brillouin zone in a direction perpen-dicular to -	.133101-3 Sutter et al. Appl. Phys. Lett. 94, 133101 2009

Scanning tunneling microscopy on epitaxial bilayer graphene
on ruthenium (0001)

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Scanning tunneling microscopy on epitaxial bilayer graphene on ruthenium (0001)

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