Letter pubs.acs.org/NanoLett
Topographic and Spectroscopic Characterization of Electronic Edge States in CVD Grown Graphene Nanoribbons Minghu Pan,† E. Costa Giraõ ,⊥,¶,# Xiaoting Jia,‡ Sreekar Bhaviripudi,‡ Qing Li,† Jing Kong,§ V. Meunier,⊥,# and Mildred S. Dresselhaus*,§,∥ †
Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States Department of Materials Science and Engineering, §Department of Electrical Engineering and Computer Science, and ∥Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States ⊥ Department of Physics, Astronomy, and Applied Physics, and #Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180, United States ¶ Departamento de Fisica, Universidade Federal do Ceará, Caixa Postal 6030, Fortaleza, Ceará, 60455-900, Brazil # Departamento í de Física, Universidade Federal do Piauí, Teresina, Piauí, 64049-550, Brazil ‡
ABSTRACT: We used scanning tunneling microscopy and spectroscopy (STM/S) techniques to analyze the relationships between the edge shapes and the electronic structures in asgrown chemical vapor deposition (CVD) graphene nanoribbons (GNRs). A rich variety of single-layered graphene nanoribbons exhibiting a width of several to 100 nm and up to 1 μm long were studied. High-resolution STM images highlight highly crystalline nanoribbon structures with welldefined and clean edges. Theoretical calculations indicate clear spin-split edge states induced by electron−electron Coulomb repulsion. The edge defects can significantly modify these edge states, and different edge structures for both sides of a single ribbon produce asymmetric electronic edge states, which reflect the more realistic features of CVD grown GNRs. Three structural models are proposed and analyzed to explain the observations. By comparing the models with an atomic resolution image at the edge, a pristine (2,1) structure was ruled out in favor of a reconstructed edge structure composed of 5−7 member rings, showing a better match with experimental results, and thereby suggesting the possibility of a defective morphology at the edge of CVD grown nanoribbons. KEYWORDS: Graphene, nanoribbon, chemical vapor deposition, edge states, scanning tunneling microscopy/spectroscopy
A
foil as a substrate and CH4 as the carbon source. Similar to the low-pressure CVD synthesis,15 a uniform layer of graphene is grown on the Cu surface, but additionally, graphene flakes of different shapes are found on the uniform layer background, including nanoribbons. For STM/STS measurements, the sample has been transferred to a Si/SiO2 substrate. A large area scan shows a rough surface with surface corrugations of around 0.6−1 nm. This surface corrugation can be attributed to the roughness of the Si/SiO2 substrate. A number of GNRs are found on top of the graphene layer. They have various widths, lengths, and geometries. Large area scans indicate a rich variety of ribbons on the surface: some are 100 nm wide, some are 40 nm wide with a folded edge, and some are less than 10 nm wide but more than 1 μm long (Figure 1a−c), respectively). In total, about 40 GNRs were studied in this work with the majority of the observed ribbons being about 20−60 nm wide and at least several hundred
rmchair-edged GNRs can be either quasi-metallic or semiconducting depending on their nanometer size widths,1−6 while zigzag-edged GNRs are quasi-metallic with one-dimensional edge states localized on both sides of the ribbon.7−10 Recent experimental work has demonstrated the existence of spin-split edge states on the edge of GNRs synthesized via the carbon nanotube unzipping method.11 In general, however, the edge structure in chemically prepared nanoribbons is usually irregular and complicated compared to the materials obtained by unzipping nanotubes. To investigate the conditions under which the edge state survives for more realistic edge terminations, we investigated GNRs prepared by the CVD method. As-synthesized ribbon samples show arbitrary edge shapes with very few long-range zigzag and armchair edges.12 Such irregular edges can be understood as a mixture of zigzag, armchair, chiral, and even defective morphologies. How the electronic edge states are affected by such complicated local edge structures remains largely an open question both experimentally and theoretically. The GNRs investigated here were produced by atmosphere pressure chemical vapor deposition (APCVD),13,14 using Cu © 2012 American Chemical Society
Received: December 13, 2011 Revised: February 13, 2012 Published: February 24, 2012 1928
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Figure 1. STM images of CVD grown GNRs on a Si/SiO2 substrate. (a−c) Large area scans show a variety of different GNRs: (a) size 400 nm × 400 nm, bias 0.8 V, set point 100 pA; (b) size 300 nm × 300 nm, bias 1.0 V, set point 20 pA; (c) size 300 nm × 800 nm, bias 0.8 V, set point 20 pA; The insert in the upper left of (c) shows the same nanoribbon after tip cutting and moving. (d) Topographic image of a section of GNR, pointed by the black box in panel c, shows two bright stripes running along the edges. Image size is 52 nm × 80 nm. The insert figure is a schematic drawing showing three possibilities of edge configurations, the folded edges, morphological curved opened edge (C) and flat opened edge with a high intensity of edge states (F). (e) The height profile measured from the topographic STM image, indicated by the dashed line in panel d. (f) The calculated LDOS profile across a scan perpendicular to the axis of a 6.5 nm wide ribbon. The left edge of the modeled ribbon is 5−7 reconstructed, and the right edge is a pristine (3,1) structure.
nanometers long. In addition to imaging the topography, the STM tip was also successfully used to manipulate the ribbons, including moving and cutting them, as shown in Figure 1c, where a cut ribbon is found at the location pointed by the black arrow. We emphasize that the possibility of cutting individual pieces confirms that these are indeed ribbons, thereby ruling out the possible misinterpretation of a localized ripple of the graphene basal plane.16 Figure 1d is a topographic STM image of a specific GNR chosen from those observed in a large-area scan. This particular GNR presents straight, smooth edges and “bright stripes” running along each edge. The stripe is located about 2−3 Å above the average height of the middle terrace of the ribbon, which is in agreement with results reported for unzipped carbon nanotube GNRs,11 where their stripe was interpreted as a manifestation of an edge-curved region near the edge of the GNR. Alternatively, the presence of “bright” stripes can be interpreted as the folding of the edge (see the schematic drawing insert in Figure 1d). However, a careful analysis of the height profile measured across the ribbon (Figure 1e) favors more the possibility of an opened edge with a curved region, given the decrease in the average height of the middle of the terrace of the ribbon. The height of the middle of the terrace is 4.6 Å, which is much smaller than 6.71 Å for double stacked graphene layers (the distance between adjacent stacked layers in graphite is 3.36 Å). The slightly larger height of this ribbon can be understood by a larger stacking distance and less coupling between the ribbon and the underneath graphene layer. Even with ruling out the possibility of a folded edge, the stripes observed here in our CVD-grown GNRs are still not solely interpretable as a curvature effect in light of our experimental and theoretical observations. Indeed, using a combination of spectroscopic mapping and electronic structure
calculations, the stripe characterized here seems to be the result of a combination of morphological curvature and a locally high density of electronic edge states that proceeds along the entire edge of the structure. The calculated local density of states (LDOS) for a 4 nm wide ribbon shown in Figure 1f indicates the strongly enhanced LDOS at the edge, which decays exponentially into the middle terrace of the ribbon. This LDOS enhancement originates from strong edge states and can account for an up to 2 Å height difference at the ribbon edges that is observed during STM topographic imaging. Differential conductance mapping that provides spatial information based on the purely electronic local density of states on the surface, recorded for a ribbon simultaneously with the topographic image, also supports the calculated results: the spectroscopic bright edge stripes coincide with the stripe features in the normal constant current topographic image. This result shows that the edge stripe feature observed in our CVD grown ribbons is rather a combined effect, through which a high LDOS at a ribbon edge and a real morphologic curvature both contribute to the “bright stripe” feature along the ribbon edges in the STM image. The theoretical calculations presented here were performed using a self-consistent tight-binding + U (TBU) approach based on the model of ref 17 with first-, second-, and thirdnearest neighbor hopping integrals given by t1 = 3.2 eV, t2 = 0 eV, and t3 = 0.3 eV, respectively. This parametrization has been shown to give an excellent description of graphene nanostructures compared to state-of-the-art density functional theory (DFT) calculations.18,19 The different chemical environment for the atoms located at the structure’s edges was modeled by including a 0.2 eV correction to the t1 parameter of the 2-fold coordinated frontier atoms.18 An accurate description of the magnetic interaction in GNRs is known to be possible by supplementing the tight-binding Hamiltonian with an explicit 1929
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mean-field Hubbard U term to account for spin−spin interactions.17,27 The U parameter quantifies the magnitude of the on-site electron−electron interaction. A value of the U = 0.92t1 for the interaction strength was obtained previously by fitting the TBU and DFT band structures for a large number of GNR systems in different magnetic configurations.19 The STM images were simulated using the Tersoff and Hamann formalism.20 Here we employed the density matrix elements computed using the TBU model and we assumed a simple sorbital-like density of states for the tip.21 The approach used here was successfully employed in a number of previous works by some of the present authors.22 High-resolution STM images reveal the fine details of the structure in both the interior GNR terrace and the edge region. These high-resolution images are the first to be reported for CVD-grown GNRs; they enable a detailed characterization of the atomic structure and of the chirality of the GNRs and allow for building structural models for the observed edge regions. Figure 2a shows a typical single layer graphene nanoribbon
structure. It has been reported that the reduced coupling between the graphene layers is responsible for the triangular/ honeycomb atomic lattice transition phenomenon.23 The observation in Figure 2b further confirms that the nanoribbons are only weakly coupled to the under-lying graphene layer, and therefore the properties measured here are more representative of the properties of the nanoribbon. The atomically resolved image of the edge region of this ribbon is shown in Figure 2c. We can here see a series of protrusions (with a spacing of ∼6.5 Å, corresponding to the width of nearly three graphene hexagons) at the edge of a GNR. The appearance of these protrusions in STM images is 1.5−2 Å higher than the adjacent graphene lattice. As a reminder that STM measurement involves not only the atomic structure but also the electronic density, the height difference here manifests the electronic contribution originating from the high LDOS at the ribbon edge, and further supports our previous conclusion that high electron density can contribute to the “bright stripe” feature along the edge. The protrusions have the spacing expected for adjacent graphene hexagons, and a careful analysis seems to indicate that the edge of this ribbon possesses a (2,1) structure. However, the atomically resolved STM image does not clearly show the details of the atomic structure alone. By coupling this result with theoretical simulations, we can rationalize the observed variations in STM intensity and infer the atomic structure of the GNR. To that end, we have employed the TBU model to simulate the STM images of a number of proposed structures. Figure 2d shows the simulated STM image based on a (2,1) edge model. The image clearly indicates three significantly brighter spots at the location of the edge atoms along a zigzag line. The image in Figure 2d corresponds to a perfectly planar structure and has features that are not fully compatible with the details of the experimental image. However, because edge atoms only have two carbon neighbors, these atoms can more easily undergo out-of-plane distortions when compared to atoms located deeper inside the ribbons. We therefore considered a wavy corrugation for the edge atoms in such a way that the C−C edge dimer is slightly dislocated by 0.2 Å above the plane (going out of the plane of the paper), while the other dicoordinated C is moved 0.2 Å below the plane (into the paper plane). While this does not modify the electronic structure (within our TBU model), the perpendicular displacements have a strong influence on the computed STM image. This makes the brightest atom appear dimmer in the simulated image at the same time that the other two bright carbons get brighter, resulting in an image (Figure 2e) that is in much closer in agreement with the experimental one (Figure 2c). While this model satisfactorily reproduces the experimental image, the ad hoc structural modifications are not very appealing as they do constitute a strong working hypothesis. For this reason, we explored the possibility of edge reconstructions to account for the fine details of the STM images along the edges. Here we consider a case where two carbon atoms from the (2,1) edge are removed and a pentagonal reconstruction takes place. This structure is shown in Figure 2f along with the corresponding simulated STM image. The simulation clearly highlights features that are very similar to the experimental image, thereby indicating that reconstruction is a likely possible structure that corresponds to the measured GNR. Note that such defects have been previously reported in a graphene layer, such as a line defect containing octagonal and pentagonal sp2-hybridized carbon
Figure 2. Atomic-resolution images on both an interior GNR terrace and an edge region. (a) A typical GNR observed in STM measurements with an image size of 100 nm ×100 nm. (b) An atomic resolution image taken on an interior GNR terrace showing a well-ordered hexagonal lattice. The inset figure shows an atomic resolution image taken on a HOPG surface. (c) An atomic resolution image of an edge region (pointed by arrows). Simulated STM images (d) based on the (2,1) edge structure, (e) wavy corrugated edge, and 5−7 reconstructed edge (f). Tunneling current is 50 pA and the sample bias voltage is 0.6 V for panels a, b, and c.
observed experimentally under lower resolution. Figure 2b,c presents the atomically resolved interior GNR terrace and edge regions of the monolayer GNR and indicate unambiguously the clean atomically ordered termination of the periodic GNR. No special heat treatment was used to obtain these atomically clean and sharp edges, except for a gentle degassing at 200 °C in ultrahigh vacuum to eliminate the polymer residuals. In Figure 2b, the atomic lattice is resolved on the ribbon, which indicates that CVD grown GNRs are highly crystalline, extremely clean and suitable for STM/S measurements. By comparing the CVD GNRs to the atomic resolution image on a highly orientated pyrolytic graphite (HOPG) sample (inset in Figure 2b), instead of the typical triangular lattice of HOPG, the atomically resolved STM image of CVD ribbons shows a hexagonal 1930
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rings embedded in a perfect graphene sheet.24 The appearance of 5-member ring defects at the ribbon edge could be due to the CVD sample preparation.25 The defects could be introduced into the graphene lattice during CVD growth. The Raman spectra suggested the defect density in the graphene samples synthesized at intermediate methane concentrations.14 The kinetics (cooling rate, pressure of the reaction chamber during synthesis) of the CVD process has important ramifications on the density of defects. As diffusion through the boundary layer is the rate limiting factor, a variation in the thickness of the boundary layer, due to the surface inhomogeneity of the wafer, leading to a variation in the amount of active species that are diffusing through the boundary layer, could result in the nonuniformity of as-grown graphene and the formation of local defects. Some defects such as vacancies or 5−7 defect lines could play an important role in forming the nanoribbons by preferring the growth along one dimension. The high quality of the atomically well-defined edge structures observed here allows us to quantitatively compare our experimental data to theoretical calculations of the electronic structure for chiral GNRs. We measured the local electronic structure of GNR edges using low temperature STS, which reflects the energy-resolved LDOS.26 Here, we chose a narrow ribbon (as shown in Figure 3a) with straight and clean
states. This leads to a spin-polarization of the edge states that splits the single low-energy peak into a series of van Hove singularities. The occurrence of spin-polarization at the edges (more specifically at the zigzag part of the edge) is intrinsically related to the pairlike behavior for the low-energy levels (Figure 3e and 4a) since it is known that nonpolarized zigzag edges
Figure 4. Two spin-split peaks around the Fermi energy taken perpendicularly to the axis of a graphene ribbon. (a) Spatial dependence of the tunneling spectra across the ribbon width, dI/dV spectra obtained at five different positions (as marked in Figure 3a) in the low-energy regime. Tunneling current 50 pA, sample bias 300 mV. The red curves are Gaussian fittings to the peaks. Although all spectra show two peaks around the Fermi energy, the energy separation between the peaks and the intensity of the peaks varies according to the position on the ribbon (1, 2, 3, 4, 5). (b) Color mapping for the calculated LDOS obtained by moving perpendicularly away from the pristine (3,1) edge (top) to the 5−7 reconstructed edge (bottom).
possess flat low-energy bands which are likely to become polarized and split in pairs into a more stable electronic configuration.27 For this reason, we chose a (3,1) ribbon instead of the previous (2,1) structure since the zigzag extent in the latter is too short to develop such a spin polarization. A (3,1) ribbon with a pristine edge and a 5−7-reconstructed edge having the same width as the actual GNR shown in Figure 3a is constructed in our calculations (Figure 3c). This splitting can be seen in Figure 3d, which shows the calculated LDOS at the ribbon edges. Our calculations (black curve) show two lowenergy peaks in the LDOS (at −67 and +48 meV) with an energy separation of about 115 meV with somewhat asymmetric lineshapes and different peak intensities relative to the reconstructed edge and two peaks (−166 and +147 meV) with similar intensities for the pristine edge (red trace). By comparing the theoretical results with our experimental STS measurements at the actual ribbon edge (Figure 3e), we can attribute a pair of low energy peaklike features (with black arrows) symmetrically located ±46 meV in the STS spectra to the spin-split van Hove singularities (VHSs). The asymmetry of the peak intensities reflects the broken electron−hole
Figure 3. Tunneling spectra measured at a ribbon edge. (a) A ribbon with a width of 6.4 nm and a height of 6 Å. (b) Line profile of height measurement across the ribbon showing a “bright edge”, which is about 2.4 Å higher than the plain terrace of the ribbon. (c) Ball−stick model of proposed ribbon structure, where the left edge is a pristine (3,1) edge. Right edge of the ribbon is a reconstructed structure composed of 5−7 member rings. (d) The calculated electronic structure for the (3,1) (red) and for the 5−7 reconstructed edge of a GNR (black) obtained by using a model with an identical width as the ribbon in panel a. (e) dI/dV spectrum obtained at the edge of the GNR in panel a. Tunneling current 50 pA, sample bias 300 mV. The measurement was taken at 25 K, and an a.c. bias modulation of 6 mVrms and a frequency of 746 Hz were used.
edges (the ribbon width is about 6.4 nm, shown in the line profile measurement in Figure 3b). From the STM image (Figure 3a), a “bright stripe” along the edges can be clearly observed. Figure 3e shows an experimental dI/dV spectrum vs bias voltage obtained at the position (as marked by a star) near the edge of the GNR pictured in Figure 3a. Within the description of the TBU model Hamiltonian, the electron−electron interactions lift the degeneracy of the edge 1931
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can potentially manipulate and control the electronic edge states of GNRs.
symmetry, which could be induced by a local structural defect such as 5- and 7-membered rings at each edge site. Now focusing on the pair of VHSs at low-energy in Figure 3e, we recorded the dI/dV spectra obtained at different positions 1, 2, 3, 4, 5 (as marked in Figure 3a) as a black trace in Figure 4a. On one edge of the ribbon, the two peaks are separated in energy by a splitting of 55 meV (positions 1, 2). These two peaks are asymmetric both in intensity and in energy positions. The middle point has a 6 meV offset from zero bias. The energy positions of the two peaks remain the same (positions 1, 2 in Figure 3a) and just the intensity decreases as we move toward the middle of the ribbon. When moving to the other side from the middle, the amplitude of the two peaks again grows higher when getting closer to the opposite terminal edge. The peaks become more symmetric and the energy separation jumps from 68 meV in the middle to 76 meV at the edge. Theoretical calculations predict a similar spatial dependence based on the modeled ribbon. Along the perpendicular direction away from one edge and into the ribbon interior, a series of calculated LDOS are shown as a color mapping as a function of transverse position in Figure 4b with a 2.5 Å interval. Obviously the energy separations of the peaks predicted in the theoretical calculations have much larger values (313 and 115 meV) than experimental results (76 and 55 meV). These discrepancies may be the result of the fact that the actual ribbon has a different chirality rather than the (3,1) chirality we have chosen for Figure 4b. But comparing the spatial dependence of the calculated edge state intensities with our experimental results, we find that both show an exponential decay behavior. According to Figure 4a, the predicted edge states decay similarly to the experiment with a decay length of 14 Å. Two different edge structures produce different spin-split edge states, though both show an exponential decay with distance away from the edges. These spin-split edge states vanish in the middle of the ribbon. Both the experimental results and our theoretical calculations suggest that this CVD grown ribbon has two parallel similar-looking edges but with different local edge structures. This finding shows that CVD grown nanoribbons can be significantly more complicated than GNRs synthesized via carbon nanotube unzipping methods. For the unzipped nanotube, the ribbon edge would tend to be rather regular. However, two parallel edges of a CVD grown ribbon in our case could have very different local edge structures, such as reconstructed and corrugated edges or a mixture of these two types. In summary, we have characterized highly crystalline CVD grown graphene nanoribbons using atomically resolved STM. The observations were rationalized using tight-binding + U electronic structure calculations to determine the atomic structure of the edge. The presence of nonhexagonal rings on the edge of the ribbons is expected to present a number of interesting chemical, magnetic and electronic properties, as pointed out by a number of studies.28 We observed different edge states for each side of a ribbon, which suggests that this CVD ribbon has very different edge structures and chiralities locally for both edges, even though the two edges overall look quite parallel. Not only the chirality but also the edge defects induce an enhanced local electronic density of states, as measured in dI/dV spectra. All these edge and defect states are expected to dramatically change the electronic structure and properties of graphene nanoribbons. By modifying the local edge structure (by using methods such as Joule heating), one
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Office of Basic Energy Sciences, U.S. Department of Energy. The work at MIT was done under ONR-N00014-09-1-1063 (X.J., M.S.D., and J.K.) while S.B. and J.K. acknowledge the support from National Science Foundation NSF DMR 0845358. E.C.G. acknowledges support from the Brazilian agencies CNPq and CAPES (process 032710-7).
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