Letter pubs.acs.org/NanoLett
Nanomesh-Type Graphene Superlattice on Au(111) Substrate Péter Süle,† Márton Szendrő,† Gábor Zsolt Magda,‡ Chanyong Hwang,§ and Levente Tapasztó*,‡ †
Institute for Technical Physics and Materials Science, Centre for Energy Research Konkoly Thege u. 29-33, Budapest, Hungary 2D Nanoelectronics “Lendület” Research Group, Institute for Technical Physics and Materials Science, Centre for Energy Research, Konkoly Thege u. 29-33, Budapest, Hungary § Center for Nanometrology, Korea Research Institute of Standards and Science, Daejeon 305-340, Republic of Korea ‡
S Supporting Information *
ABSTRACT: The adherence of graphene to various crystalline substrates often leads to a periodic out-of-plane modulation of its atomic structure due to the lattice mismatch. While, in principle, convex (protrusion) and concave (depression) superlattice geometries are nearly equivalent, convex superlattices have predominantly been observed for graphene on various metal surfaces. Here we report the STM observation of a graphene superlattice with concave (nanomesh) morphology on Au(111). DFT and molecular dynamics simulations confirm the nanomesh nature of the graphene superlattice on Au(111) and also reveal its potential origin as a surface reconstruction, consisting of the imprinting of the nanomesh morphology into the Au(111) surface. This unusual surface reconstruction can be attributed to the particularly large mobility of the Au atoms on Au(111) surfaces and most probably plays an important role in stabilizing the concave graphene superlattice. We report the simultaneous observation of both convex and concave graphene superlattices on herringbone reconstructed Au(111) excluding the contrast inversion as the origin of the observed concave morphology. The observed graphene nanomesh superlattice can provide an intriguing nanoscale template for self-assembled structures and nanoparticles that cannot be stabilized on other surfaces. KEYWORDS: Graphene, superlattice, nanomesh, scanning tunneling microscopy, surface reconstruction
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This is somewhat counterintuitive as the two morphologies are nearly the mirror symmetry of each other (Figure 1). Yet, on all the metallic substrates investigated so far, the convex (protrusion) pattern is the generally observed graphene superlattice morphology. Even when an apparently concave morphology has been observed in the STM images such as on Ir and Ru substrates, more detailed investigations revealed that the observed concave morphology is only due to the contrast inverted image of the common convex superlattice geometry.2,22,23 The nanomesh morphology has so far been observed in hexagonal boron nitride (h-BN) on Rh(111) 24 and Ru(0001).25 A nanomesh-like morphology has also been reported in the h-BN/graphene system4,5,10,26−28 by STM measurements. However, since both STM and AFM measurements are subject to contrast inversion2,29 due to various effects, there is a high controversy in the field concerning the unambiguous identification of the atomic topography of the emerging superlattices.
he adherence of 2D crystals to various crystalline substrates leads to moiré superlattices of nanoscale periodicity, displaying various types of patterns depending on the binding registry of the overlayer/substrate systems.1−4 In fact, it takes special substrates and orientation angles to maintain the flatness of graphene on a crystalline substrate.5,6 Graphene superlattices comprise a rich physics as the periodic modulation of the atomic structure substantially influences the electronic properties. One can exploit this correlation to tune the charge density distribution on the graphene sheet as well as alter the band structure through the periodic structural modulation. It has been experimentally demonstrated that the nanoscale periodic modulation of graphene can give rise to electronic superlattices,7 and new Dirac cones appear in the band structure at higher energies.8−11 Furthermore, the nanoscale landscape emerging from the interaction of the graphene and substrate can be employed as a template for molecular self-assembly enabling the formation of novel selfassembled structures not stable on planar surfaces.12 Graphene superlattices have predominantly investigated on various metallic surfaces4,12−15 as well as hexagonal boron nitride substrates.16,17 One of the observations is that on metallic surfaces the convex geometry is dominantly observed18−21 over the concave (nanomesh) morphology. © XXXX American Chemical Society
Received: September 24, 2015 Revised: November 3, 2015
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DOI: 10.1021/acs.nanolett.5b03886 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
graphene lattice, but a moiré superlattice of much larger (1.1 nm) periodicity. While most of the surface is covered by a hexagonal lattice of depressions, upon closer examination in several triangular areas of the herringbone junctions (marked by white circles) the graphene superlattice morphology changes to protrusions of the same periodicity, as evidenced in Figure 2b (for 2D STM images see Figure S3 of SI). High resolution STM images displaying both the atomic lattice of graphene and the superlattice have also been acquired as shown in Figure 3a. A honeycomb lattice is apparent for both
Figure 3. (a) Atomic resolution STM image (200 mV, 5 nA) of the graphene nanomesh on Au(111), displaying a hexagonal superlattice of 1.1 nm (± 0.1 nm) periodicity, 0.3 Å (± 0.1 Å) corrugation, and 12° (± 1°) rotation angle. The STM image had been Fourier filtered to remove high frequency noise. (b) Simulated STM image of the graphene superlattice emerging on Au(111) obtained by DFT calculations confirming the experimentally observed nanomesh morphology with a remarkable quantitative agreement (1.1 nm, 0.3 Å, 12°).
Figure 1. Two basic superlattice morphologies of the graphene/ Au(111) system. (a) The concave nanomesh graphene superlattice characterized by a periodic lattice of depressions, obtained using a Lennard-Jones interface potential. (b) The convex lattice of protrusions as generated by an Abell−Tersoff interface potential.
Here, we provide unambiguous evidence for the graphene nanomesh formation on Au (111) substrate, through the sideby-side observation of the convex and concave superlattice morphologies, excluding the contrast inversion origin of the concave superlattice. Results and Discussion. We have performed scanning tunneling microscopy (STM) measurements under ambient conditions on graphene samples grown by the CVD method on Cu foil and transferred onto gold substrates with large atomically flat Au(111) plateaus by using the standard PMMA assisted transfer technique. The parallel stripes with bending angles of about 120 degrees running across the STM image in Figure 2a are due to the well-known herringbone reconstruction of the Au (111) surface. The periodic honeycomb lattice visible in the STM image is not the atomic
the atomic lattice and superlattice. From both direct and Fourier transformed atomic resolution images one can experimentally determine the rotation angle between the graphene atomic lattice and the moiré superlattice, which was measured to be 12 ± 1 degrees. However, it is also known that in some cases what appears to be a nanomesh superlattice morphology in the STM images is only due to the contrast inversion of the far more common protrusion superlattice, depending on the imaging conditions, as revealed by combined STM/AFM measurements.2,22 Such cases have been reported for graphene on iridium2 and ruthenium.23 However, here we report the simultaneous observation of both nanomesh and protrusion lattice geo-
Figure 2. (a) Three-dimensional STM image of graphene on herringbone reconstructed Au(111) surface revealing a nanomesh-type hexagonal graphene superlattice of 1.1 nm periodicity as well as the more common protrusion superlattice in the areas marked by white circles. (b) Higher magnification STM image evidencing the coexistence of the convex (periodic protrusions within the triangular herringbone junction) and concave (periodic depressions) superlattice morphologies. B
DOI: 10.1021/acs.nanolett.5b03886 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 4. Large scale time lapsed classical molecular dynamics simulations of graphene/Au(111) system on herringbone reconstructed Au(111) surface. (a) The nanomesh morphology of graphene on herringbone reconstructed Au(111) is clearly reproduced. The inset shows a zoomed region. (b) The atomic structure of Au(111) surface underneath the graphene nanomesh reveals the surface reconstruction of the Au(111) induced by the formation of the graphene nanomesh. The inset shows the same reconstruction as obtained from DFT calculations.
reconstructed Au (111) surfaces30 and most probably plays an important role in stabilizing the elusive graphene nanomesh morphology, absent on other investigated substrates. DFT calculations show that both concave and convex graphene morphologies are stable, but the nanomesh geometry was found to be of lower energy on Au(111) in agreement with our experimental results. The energy of the concave (nanomesh) and convex configurations (per carbon atom) as calculated by the AIREBO potential after geometry optimization with the substrate are −7.3400 and −7.3320 eV/C, respectively. Therefore, we find that the concave (nanomesh) topography is slightly more stable than the convex one, although the energy difference is within the range of thermal energy at room temperature. The coexistence of the two morphologies can be explained due to this small energy difference, while the dominant nature of the nanomesh morphology might be related to the reconstruction of the Au(111) substrate further stabilizing the nanomesh (concave) morphology. Convex protrusion superlattice observed in the small triangular areas defined by the herringbone junctions of the Au(111) substrate most probably emerge due to the subtle changes in the local gold surface structure, altering the delicate overlayer/substrate interaction (e.g., the lack of nanoscale Au surface reconstruction stabilizing the graphene nanomesh morphology). However, the DFT method is not suitable for calculations including the larger scale reconstructions of the Au(111) surface, such as the experimentally observed herringbone structures due to a prohibitive increase of the computation time. To account for the effect of such large scale reconstructions we have also performed classical molecular dynamics simulations on the graphene/Au(111) system using a Lennard-Jones interface potential. The results of large-scale CMD simulations of graphene/Au(111) superlattice (timelapsed for 10000 time steps at 300 K) using the herringbone reconstructed Au(111) surface are shown in Figure 4a. The simulated herringbone reconstruction corresponds to the Au(111) − 22 × p(2) reconstruction in agreement with the literature.35 The morphology of graphene on hr-Au(111) reveals a homogeneous nanomesh geometry all over the hrAu(111) substrate (Figure 4a). The present CMD calculations could not reproduce the experimentally observed coexistence of the two superlattices, as no herringbone junctions emerged from the simulations of the Au(111) surface reconstruction. To capture these subtle features probably a much more detailed
metries within the same STM image, as clearly evidenced in Figure 2 (and Figure S3 in the SI). This excludes the possibility that the observed nanomesh pattern is merely due to contrast inversion, as both geometries reproducibly occur within the same scan lines. Nevertheless, this does not fully exclude the possibility of a contrast inversion all over the STM image raising the question of the dominant superlattice morphology. We did not observe any contrast inversion of the superlattice under various imaging conditions (5−500 mV, 0.5−5 nA). Even upon the contrast inversion of the atomic graphene lattice (from honeycomb to triangular lattice) attributed to a tip change, the contrast of the superlattice remained unaltered indicating that the observed nanomesh pattern is indeed of topographic origin and the dominant graphene morphology on Au(111). This is further supported by the nanomesh-type STM images found in the literature for the graphene/Au(111) system;30−32 though, the authors did not mention or analyze the nanomesh nature of the observed patterns. However, to further strengthen our interpretation and understanding of the experimentally observed graphene superlattices we have performed density functional theory calculations on the graphene/Au(111) system. The DFT-LDA method has been applied for a small periodic rotated gr/ Au(111) system with 9 Au layer (186 atoms) and 42 carbon atoms, which is sufficient to account for the experimentally found misorientation angle shown in Figure 3a. The structure preoptimized by classical molecular dynamics (CMD) simulations has been relaxed by cg-DFT minimizer until the residual forces dropped below 0.0005 eV/A. The SIESTA periodic DFT code has been used for this purpose.33,34 We have used Troullier−Martin, norm conserving, relativistic pseudopotentials in fully separable Kleinman and Bylander form for both carbon and Au. A double-ξ polarization (DZP) basis set was used. In particular, 22 valence electrons for Au atoms and four for C atoms have been considered. The DFT calculations for the graphene/Au(111) system predict a stable nanomesh (concave) superlattice morphology with a periodicity of 1.1 nm, a corrugation of 0.3 Å, and a rotation angle of 12 degrees (Figure 3b) in excellent quantitative agreement with our experimental findings. Besides the graphene nanomesh formation the DFT calculations also reveal a reconstruction of the Au(111) surface with a periodicity corresponding to that of the graphene nanomesh. This unexpected finding is nevertheless in agreement with the high mobility of the Au atoms on the herringbone C
DOI: 10.1021/acs.nanolett.5b03886 Nano Lett. XXXX, XXX, XXX−XXX
Nano Letters understanding of the fine structure of the Au (111) atomic surface is required. In Figure 4b the herringbone reconstructed Au(111) surface is shown. In the main image the graphene layer is removed after the simulation. It is apparent that the a pattern with the periodicity of the graphene nanomesh also appears “imprinted” on the Au(111) surface. Naturally, for CMD simulations of the herringbone reconstructed Au(111) surface without the presence of graphene no such pattern appears. The graphene induced reconstruction of the Au(111) surface also revealed by DFT calculations (Figure 4b inset) can be attributed to the particularly high mobility of the Au atoms on the Au (111) surface and most probably plays an important role in stabilizing the nanomesh morphology absent on other substrates. In summary, we have provided experimental evidence for the formation of a graphene nanomesh, emerging in graphene sheets deposited on Au(111) substrates. The coexistence of nanomesh and protrusion superlattices has been observed experimentally excluding the possibility of the contrast inversion as the origin of the observed graphene nanomesh. Based on the excellent quantitative agreement between the STM measurements and both DFT and classical molecular dynamics simulations we conclude that on herringbone reconstructed Au(111) surface graphene predominantly acquires a nanomesh morphology. The observation and stability of the nanomesh morphology so far elusive in graphene on metallic substrates can be related to the reconstruction of the Au(111) surface in the presence of graphene revealed by both DFT and molecular dynamics simulations. The fact that both convex and concave graphene superlattice morphologies are stable under similar conditions can open the way toward reversibly tuning their geometry between the two complementary landscapes that in turn can be exploited to control the adsorption and guide the assembly of various molecules on graphene.
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ACKNOWLEDGMENTS
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REFERENCES
The experimental work was conducted within the framework of the Korea Hungary Joint Laboratory for Nanosciences supported through the National Research Council of Science and Technology and the “‘Lendület”’ programme of the Hungarian Academy of Sciences. The DFT and CMD calculations (simulations) have been performed on the supercomputers of the NIIF center (Hungary). C.H. acknowledges funding by Nano-Material Technology Development Program (2012M3A7B4049888) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT,, and Future Planning. L.T. acknowledges financial support from OTKA grant K108753
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b03886. Details of DFT and classical molecular dynamics simulations, two-dimensional STM images on the coexistence of concave and convex superlattice morphologies, and simulation results on the herringbone reconstruction of the Au (111) surface (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
L.T. conceived and designed the experiments. G.Z.M. performed the STM experiments. C.H. supervised the sample preparation. P.S. and M S. performed the DFT and CMD calculations. L.T. and P.S. analyzed the data and wrote the paper. All the authors discussed the results and commented on the manuscript. Notes
The authors declare no competing financial interest. D
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DOI: 10.1021/acs.nanolett.5b03886 Nano Lett. XXXX, XXX, XXX−XXX
