Letter pubs.acs.org/NanoLett
Atomic Level Spatial Variations of Energy States along Graphene Edges Jamie H. Warner,*,† Yung-Chang Lin,‡ Kuang He,† Masanori Koshino,‡ and Kazu Suenaga*,‡ †
Department of Materials, University of Oxford, Parks Road, Oxford, OX1 3PH, United Kingdom Nanotube Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
‡
S Supporting Information *
ABSTRACT: The local atomic bonding of carbon atoms around the edge of graphene is examined by aberration-corrected scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy (EELS). High-resolution 2D maps of the EELS combined with atomic resolution annular dark field STEM images enables correlations between the carbon K-edge EELS and the atomic structure. We show that energy states of graphene edges vary across individual atoms along the edge according to their specific C−C bonding, as well as perpendicular to the edge. Unique spectroscopic peaks from the EELS are assigned to specific C atoms, which enables unambiguous spectroscopic fingerprint identification for the atomic structure of graphene edges with unprecedented detail. KEYWORDS: Graphene, EELS, STEM, Edge states, TEM, Carbon
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overcome this problem, a high-temperature holder is used to perform imaging and spectroscopy at 500 °C. At these elevated temperatures, the edges of graphene remain in stable configurations for sufficient time to obtain high quality 2D STEM images and EELS maps with single atom resolution. Our results reveal significant variations in the local energy states of individual carbon atoms at the edges of graphene that relate to their bonding coordination. Monolayer graphene was synthesized by chemical vapor deposition on Cu using a previously reported method and transferred onto SiN TEM grids, which enable their extensive cleaning at high temperatures.22 Experiments were performed at elevated temperatures of 500 °C, where edges were primarily terminated with arm-chair or reconstructed zigzag (rec5−7) edges, (Supporting Information Supporting Information Figure S1a). EELS line scans were taken across both edge types to acquire high signal-to-noise EELS spectra from the edge atoms and the graphene bulk to accurately calibrate the new peaks observed for edge states. The π* peak in the EELS is set as Peak 0 = 285 eV, corresponding to bulk graphene. The heptagon from the rec5−7 edge shows a single EELS peak at 284.2 eV, whereas the arm-chair edge has two EELS peaks at ∼283 eV and ∼285.0 eV (Supporting Information Figure S2). To correlate EELS peak positions with specific atoms requires 2D EELS maps with simultaneous ADF-STEM
he edges of graphene have unique energy states that differ from the bulk and influence the properties of nanoribbons.1 To date, specific novel energy states of graphene edges have been linked to the three different edge terminations, armchair, zigzag or reconstructed zigzag.2−7 However, our understanding can be further refined at single atom level by exploring the spatial variation in energy states along the edge as well as perpendicular. Atomic resolution imaging of graphene by either electron microscopy or scanning probe microscopy provides structural information that can be used to predict the properties.1−13 However, to experimentally probe the properties of graphene with single atom sensitivity requires the addition of some form of spectroscopy to the imaging methods, such as scanning tunnelling spectroscopy, and for scanning transmission electron microscopy (STEM), it can be energy dispersive X-ray spectroscopy (EDX) or electron energy loss spectroscopy (EELS).14−18 Using a STEM with angstrom-sized probe combined with single atom sensitive EELS enables information about the local density of states to be obtained. Current work has been limited to obtaining atomic resolution 2D maps of specific elements, such as distinguishing a single N or Fe atom within hexagonal C atoms of graphene and nanotubes.19−22 Detecting changes of C−C bonding in graphene using EELS has been limited to line-scans with spatial information restricted to 1D, perpendicular to the edge.2,7 Accurately correlating EELS information to a specific atom requires the simultaneous acquisition of an ADF-STEM image with the 2D EELS map, and to date, this has been challenging for graphene edges due to their dynamic reconstruction.5 To © XXXX American Chemical Society
Received: June 19, 2014 Revised: September 26, 2014
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imaging. Figure 1 shows ADF-STEM images of the rec5−7 edge in high magnification and the positions of atoms resolved.
Figure 2. STEM:EELS mapping of an armchair graphene edge. (a) ADF-STEM image of armchair edge. Scale bar: 2 Å. (b) EELS from the numbered regions in (a). (c) Density functional theory atomic model of armchair edge. (d) Simulated EELS from the numbered regions in (c).
Figure 1. STEM:EELS mapping of reconstructed 5−7 graphene edge. (a) ADF-STEM image of rec 5−7 edge. Scale bar: 2 Å. (b) EELS from the numbered regions in (a). (c) Density functional theory atomic model of rec5−7 edge. (d) Simulated EELS from the numbered regions in (c).
The region of the π* peak (∼280−287 eV) changes atom-byatom around the heptagon and pentagon edge with one peak at 284.2 eV and another at 286.2 eV. The peak at 284.2 eV is strongest for position 1 within the heptagon and the 286.2 eV peak is associated with the atoms from the pentagon structure, see also Supporting Information Figure S4. Density functional theory simulations of the EELS, Figure 1c and 1d support the experimental results with position 1 in the heptagon in the DFT model (Figure 1c) having a strong peak at low energy of 283.9 eV and position 2 with the higher energy peak at 286.1 eV. Probing an armchair edge reveals differences compared to the rec5−7 edge, Figure 2 and Supporting Information Supporting Information Figure S5. The outermost atom in position 1 has a low energy peak at 282.8 eV and the bulk graphene peak at 285 eV. The low energy peak at 282.8 eV disappears when moving inward within the graphene. The DFT simulations show a similar trend, with a peak at 282.6 eV for the outermost atom. Even though both the heptagon in the rec5−7 edge and the armchair edge both have two outermost carbon atoms that are only bonded to two nearest neighbors, changes in their local atomic structure leads to a difference of 1.4 eV between their low energy EELS peak. The third periodic edge termination of graphene is the zigzag edge and Figure 3 explores its 2D EELS mapping. A new low energy peak of 281.0 eV is seen in the outermost atom, position 1 in Figure 3, along with the 285 eV peak from bulk graphene, and Supporting Information Figures S5. This new peak is the lowest energy peak found out of the three edge states and quickly fades away by position 2 in Figure 3 and Supporting
Figure 3. STEM:EELS mapping of a zigzag graphene edge. (a) ADFSTEM image of zigzag edge. Scale bar: 2 Å. (b) EELS from the numbered regions in (a). (c) Density functional theory atomic model of zigzag edge. (d) Simulated EELS from the numbered regions in (c).
Information Figure S5. The DFT simulations, Figure 3c and d, replicate this behavior and show a strong low energy peak at 282.7 eV for the outermost atom. We have also mapped the B
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Table 1. Summary of EELS Peak Positions for Different Graphene Edges peak (eV)
zig-zag (Pz)
Klein (Pk)
armchair (Pa)
heptagon (Ph)
bulk (Pb)
pentagon (Pp)
experiment DFT shift (exp.) shift (DFT)
281.0 282.7 −4.0 −2.3
282.8 283.5 −2.2 −1.5
282.8 282.6 −2.2 −2.4
284.2 283.9 −0.8 −1.1
285.0 285.0 0 0
286.2 286.1 +1.2 +1.1
Figure 4. Fingerprinting graphene edges. (a) ADF-STEM image of an edge interface between armchair and rec5−7 terminations. (b) ADF-STEM image acquired simultaneously as 2D EELS map of the armchair−rec5−7 interface. (c) Spectrum images extracted from the 2D EELS overlaid onto ADF-STEM image. Red corresponds to rec5−7 edge region (peak Ph, Table 1) and green corresponds to arm-chair edge region (peak Pa Table 1). (d) ADF-STEM image of another edge boundary between armchair and rec5−7 acquired simultaneously with 2D EELS. (e) Spectrum images extracted from the 2D EELS overlaid onto (d), where red corresponds to rec5−7 edge region (peak Ph Table 1) and green corresponds to arm-chair edge region (peak Pa Table 1). (f) ADF-STEM image of a long rec5−7 edge acquired simultaneously with 2D EELS and (g) Spectrum image (peak Ph Table 1) overlaid onto (f). (h) and (i) Same edge as in (f) and (g), but after 20 s and reconstruction (orange arrow).
within one specific rec5−7 edge the fingerprint changes after edge reconstruction through spatially relocation of the heptagons. Figures 4f−i demonstrate how the 2D spectroscopic maps produced from the spatial location of peak Ph match the periodicity of the repeating 5−7 units, and the appearance of two sequential heptagons, orange arrow in Figure 4i. In Figure 5, line profiles are used to measure the magnitude of the peak Ph signal as a function of distance, with a phase shift in the periodic signal oscillations observed in the line profile in Figure 5c, orange line. The two line profiles in Figure 5c show minima at the location of pentagons and maxima at the location of heptagons. The blue curve from the periodic rec5−7 edge reads a sequence 757575757 and the orange curve from the nonperiodic rec5−7 edge reads a sequence 7575757757. We have demonstrated the ability to generate 2D spectroscopic images based on EELS from a single element, carbon, at atomic resolution. We have shown how the energy states of individual C atoms vary at the edge of graphene and the strong dependence on the atomic bonding coordination. Even though the arm-chair and the heptagon in the rec5−7 edge both have two outermost atoms that are only bonded to two nearest neighbors, their EELS signal are distinctly different, indicating variation in the energy states. We have successfully correlated several peaks of different energy (around the π*
EELS for the zigzag:armchair stair edge structure and also an edge that contains a single Klein edge atom in Supporting Information Figures S5−S8. Both edge types also show unique peaks in their EELS maps associated with atoms at the very edge. The carbon K-edge EELS signal varies atom by atom along the edge as well as perpendicular to the edge toward the bulk, Figures 1−3 and Supporting Information Figures S2−S8, with unique peaks for each edge structure. This spectral region corresponds to an excitation transition of a core s-electron to an unoccupied π* or σ* orbital and changes to the profile of the EELS is related to variations in the local density of states. We summarize all peak values and their edge state association in Table 1. Color coding is assigned to each unique edge peak for further identification processes. When the edge is terminated with a sequence of different reconstructions, it can also be read out as a spectroscopic fingerprint, demonstrated in Figures 4 and 5. An armchair edge shows a sequence of high armchair peak Pa signals, green spots in Figure 4c, before switching to rec5−7 with high heptagon peak Ph signal, red spot in Figure 4c. A higher magnification ADF-STEM and spectroscopic image of the junction between armchair and rec5−7, Figure 4d and e, shows a unique fingerprint of alternating red to green spectral regions. Even C
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Research Acceleration programme. Financial support by JSPS KAKENHI 23681026 and 26390004 is acknowledged by M.K.
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Figure 5. (a) 2D spectroscopic map of peak Ph (greyscale) acquired at the same time as the ADF-STEM image in (b), which is the same image as in Figure 4f. Yellow box indicates region used for line profile in (c) (blue line). (c) Line profiles of peak Ph signals for the two corresponding images in (a) and (e). (d) ADF-STEM image of the same edge as in (b), but after reconstruction, which is the same image as in Figure 4h. (e) 2D spectroscopic map of peak Ph (greyscale) acquired at the same time as the ADF-STEM image in (d). Yellow box indicates region used for line profile in (c) (orange line).
region in the carbon k-edge EELS) to specific atoms within defined edge terminations. Each of the three edge configurations (arm-chair, zigzag and rec5−7) displayed unique peaks in the EELS profiles, indicated in color in Table 1, which can be used to “fingerprint” the atomic structure of graphene edges.
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ASSOCIATED CONTENT
S Supporting Information *
Graphene synthesis and transfer to TEM grids, transmission electron microscopy, carbon K-edge ELNES simulations for edge structures of graphene by pseudopotential method, and supporting figures. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS J.H.W. thanks the Royal Society and the Sasakawa Fund for support. Y.C.L., M.K., and K.S. acknowledge support from JST D
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