www.sciencemag.org/cgi/content/full/323/5922/1701/dc1 Supporting Online Material for Controlled Formation of Sharp Zigzag and Armchair Edges in Graphitic Nanoribbons Xiaoting Jia, Mario Hofmann, Vincent Meunier, Bobby G. Sumpter, Jessica Campos- Delgado, José Manuel Romo-Herrera, Hyungbin Son, Ya-Ping Hsieh, Alfonso Reina, Jing Kong, Mauricio Terrones, Mildred S. Dresselhaus* This PDF file includes: *To whom correspondence should be addressed. E-mail: millie@mgm.mit.edu Materials and Methods SOM Text Figs. S1 to S6 References Published 27 March 2009, Science 323, 1701 (2009) DOI: 10.1126/science.1166862
Supporting Online material Methods: The experiments were conducted inside a JEOL 2010F HRTEM equipped with a Nanofactory STM holder, which is further attached to a piezoelectric stage. This TEM- STM system enables us to manipulate the nanoribbons as well as to make two point electrical measurements across the ribbons, while simultaneously observing the structural behavior under the HRTEM (S1). The graphite nanoribbon samples were produced by a single-step chemical vapor deposition (CVD) process (S2). In short, an aerosol was produced from a solution containing ethanol, ferrocene, and a very small concentration of thiophene. This aerosol was pyrolyzed at 950ºC for 30 minutes, and after that time, the system was allowed to cool down to room temperature as previously described (S2). As stated in Ref. (S2), the presence of S is crucial for synthesizing the graphitic nanoribbons. We could not detect S on the flat areas of the ribbons using EDX or XPS because the detection limits of these instruments were higher than 1 at%. However, it is quite possible that lower concentrations of S are present in highly curved areas (e.g. along the ribbon ripples or in regions containing heptagons or pentagons, such as 5-7 Stone- Wales defects), a result which is consistent with previous experimental findings as well as theoretical calculations (S3). Regarding the Fe atoms coming from ferrocene, we never found them on the ribbon sites. Nevertheless we believe that individual atoms of S, Fe and O are somehow bonded to the graphitic sheets (e.g., as adatoms or atoms within the hexagonal lattice) so that under Joule heating and electron irradiation, these atoms are likely the first to move towards the ribbon edges and to detach from the carbon network at low voltages. However at high voltages (1.6V), these atoms no longer play such an important role in the reconstruction process because of the high temperature that is reached in the Joule heating process, in contrast to the catalytically driven edge cutting process reported by Ci, et al. (S4). When we applied a bias voltage (up to 1.6 V) across the length (315 nm) of a 66 nm wide ribbon, the nanoribbon was reconstructed into a more crystalline material, as can be observed in figure S1 A) and B), in which A) shows the nanoribbon before the annealing treatment and B) is after 20 min of annealing. We should also note that after 20 minutes of irradiation (acceleration voltage of 200 KeV and electron beam density of ca. 100 A/cm 2 ), the ribbon thickness is reduced (fewer stacked graphene layers are seen, especially near the central region when comparing Figs. S1A and S1B) because carbon atoms are knocked out from the graphitic lattice and from the edges of the ribbons (S5). However, these edges and graphene sheets can be reconstructed through Joule annealing. 2
We commonly observed defective regions which are transformed by Joule heating to a highly crystalline region showing many zigzag edges. The fast Fourier transform (FFT) image of the sample before significant annealing (Fig. S2A) shows a hexagonal diffraction pattern with a small cloudy circular region near the center. This shows evidence for some crystalline ordering, but the zigzag and armchair edges are either not yet well formed or are very defective, and the average spacing between well formed edges (inversely proportional to the radius of the cloudy circular region near the center) is much larger than the lattice spacing. After 20 minutes of Joule annealing, as shown in the FFT image of Fig. S2B, the hexagonal diffraction pattern becomes much sharper, and clear diffraction lines 30º away from each other are developed in the center of the FFT, corresponding to the formation of sharp zigzag and armchair edges. The length of the diffraction lines gets much longer than before, indicating that the average spacing between edges is getting closer and the edges are forming edge arrays. For our experiments on the highly crystalline few-layered graphene samples, no ripple was observed for spacial regions up to 18 nm by 18 nm, as is observed in monolayer or few layer graphene (S6). Figure S3 shows the evaporation of carbon atoms and the movement of a zigzag edge in a single graphene layer. The inset of Figure S3A is an image of the sample before significant annealing. An irregular curved opening edge is indicated in the circled region. After applying a constant high bias, the opening edge of the graphene layer started to move towards the inner part of the nanoribbon, probably due to the high resistance and thus high temperature at the edge. However, the angle of the opening edge remained constant after a 60º angle was formed, indicating zigzag edge boundaries. Further evaporation of carbon atoms resulted in a one dimensional movement of the edge, as shown by the arrow in Figs. S3A-D. This is due to the fact that the activation energy of atoms forming zigzag/armchair edges is lower than for other configurations at elevated temperatures. As indicated in Fig. S3A, the current flow and the heat flow are both along the same direction in this case, considering the metal electrode to be the heat sink. Observe that the edge movement follows the same direction as the current flow and the heat flow. In order to estimate the temperature of the graphitic nanoribbons during the Joule heating experiment, Pt nanoparticles were deposited chemically on the as-prepared nanoribbon surface (Fig. S4), and the structural changes in the Pt nanoparticles were monitored in-situ (S7). The Pt anchoring process consisted of sonicating for 15 minutes a mixture of graphitic nanoribbons (10 mg), plus 10 ml of N,N-dimethylformamide (Sigma-Aldrich, 99% ), (1,5-Cyclooctadiene)dimethylplatinum(II) (Aldrich, 97%) as a platinum source, and polyvinylpyrrolidone (Sigma-Aldrich, average mol wt 10,000) as a passivating agent. After sonication, the suspension was maintained under an 3
Ar-H 2 (5% H 2 ) atmosphere to increase the reduction rate (see Fig. S4), and the suspension was subsequently placed in a glycerin bath at 110 ºC for 40 minutes. Next, the suspension was allowed to cool down to room temperature and the composite material (graphitic nanoribbons with Pt particles) was recovered by filtration. These graphitic nanoribbons exhibited platinum nanoparticles (with an average size of 6 nm) anchored to their surface. Finally a thermal treatment was carried out at 350 ºC under an Ar atmosphere for 15 minutes in order to eliminate any residues of organic material that could remain on the surface of the composite material. We confirmed the presence of Pt nanoparticles via EDX (energy dispersive X-ray) studies and XRD (X-ray diffraction) measurements. The modified nanoribbon material was then mounted on the Joule heating set-up (Fig. S5A). As we increased the applied voltage across the nanoribbons, the Pt nanoparticles near the central region of the ribbon started to melt and merge with small neighboring Pt nanoparticles (some particles finally reached a diameter of 13 nm). Subsequently, and starting from the central region, the Pt nanoparticles evaporated, resulting in a clean surface (devoid of Pt nanoparticles) near the center of the ribbon sample (Fig. S5(B)). When a higher voltage is applied, additional Pt nanoparticles evaporate and eventually almost the entire ribbon is free of Pt nanoparticles (Fig. S5(C)). From these experiments, we confirmed that good thermal contacts are made near the electrodes, and that the two electrodes serve as heat sinks. The central region of the ribbon (Fig. S5(C)) exhibits the highest temperature at a given applied voltage. Given the bulk Pt boiling point of 3827 ºC, and that the boiling point of the Pt nanoparticles will have a lower boiling point than their bulk counterpart material due to size effects (S8), we estimate the temperature of the suspended ribbon sample under Joule heating to be ca. 2800 ºC based on the loop formation morphology for furnace annealed samples (S9). Quantum molecular dynamics calculations were performed using the DFT program Vienna ab initio simulation package (VASP), version 4.6.6. (S10-S13) The Kohn-Sham equations were solved using the projector augmented wave (PAW) approach (S14, S15) and a plane-wave basis with a 400 ev energy cutoff. The Local Density Approximation (S16) was utilized to define the exchange-correlation. The graphene ribbons were placed in a cell that ensured at least 10 Å of vacuum in each Cartesian direction between the edges and its reflection. k-point sampling was restricted to a single point, the Γ point, a choice that is relevant for the finite cluster calculations performed here. Quantum molecular dynamics simulations (1 fs dynamical time step) with a Nosé-Hoover thermostat (S17) to regulate the ion temperature to ~2500 K over trajectories of up to 1 picosecond were performed. These preliminary studies address the dynamic behavior of the graphene edges and their reconstruction at temperatures relevant to that obtained during Joule heating. We also note that recently, similar calculations on Ni assisted 4
cutting of graphene also indicated that the dissociation of C atoms from armchair edges is more facile than from a zigzag edge. (S4) In Fig. S6, structural snapshots were taken from the initial phase of the edge reconstruction of a graphene ribbon with a zigzag-armchair-zigzag junction towards one with a zigzag-zigzag-zigzag edge. In this case a C-C bond located at the armchair edge dissociates preferentially, providing some evidence that armchair edges are easier to evaporate, which is in good agreement with the experimental results discussed in this paper. 5
Figure S1. HRTEM images of the nanoribbon sample (A) before and (B) after Joule annealing for 20 min at 1.6 V. (scale bar = 10 nm) Figure S2. The same region of the ribbon sample (A) before and (B) after annealing, and their Fast Fourier Transform (FFT) images (on the right of each image) show clear development of the crystallinity and edge quality after annealing. (Scale bars are 2 nm.) 6
Figure S3. (A)-(D) Successive TEM images show a zigzag edge of a single graphene layer (indicated by the solid arrow) moving into the interior region of the graphene along both the current and heat flow direction, while keeping the zigzag edge configuration unchanged. (The scale bar is 4 nm.) Figure S4. Diagram of the set up used in the process of anchoring Pt nanoparticles to the graphitic nanoribbon material (S7). 7
Figure S5. A sequence of TEM images showing Pt nanoparticles on the ribbon surface (A) before Joule heating, (B) after Joule heating for 11 minutes under a constant bias of ~2V, and (C) after Joule heating for another 4 minutes under a constant bias of ~2V. Here we see that the Pt particles melt and merge into bigger clusters (B), and start to evaporate from the central region of the ribbon (B), and eventually evaporate across almost the entire ribbon sample (C). (Scale bar is 100 nm) (S7). Quantum MD Results Bond dissociation Time = 0 Time = 0.37 Figure S6. Snapshots taken from the quantum molecular dynamics of a zigzagarmchair-zigzag junction showing the dissociation of the bond at the zigzag-armchair junction. (The unit of time is picoseconds). 8
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