Computational Studies of Metal−Carbon Nanotube Interfaces for

NANO LETTERS

Computational Studies of Metal-Carbon Nanotube Interfaces for Regrowth and Electronic Transport

2009 Vol. 9, No. 3 1117-1120

Anders Bo¨rjesson,*,†,| Wuming Zhu,† Hakim Amara,‡ Christophe Bichara,§ and Kim Bolton†,| UniVersity of Gothenburg, SE-412 96, Gothenburg, Sweden, Laboratoire Francis Perrin, CEA Saclay, 91191 Saclay, France, CINaM, CNRS, 13288 Marseille, France, and UniVersity of Borås, SE-501 90, Borås, Sweden Received November 30, 2008; Revised Manuscript Received January 20, 2009

ABSTRACT First principles and tight binding Monte Carlo simulations show that junctions between single-walled carbon nanotubes (SWNTs) and nickel clusters are on the cluster surface, and not at subsurface sites, irrespective of the nanotube chirality, temperature, and whether the docking is gentle or forced. Gentle docking helps to preserve the pristine structure of the SWNT at the metal interface, whereas forced docking may partially dissolve the SWNT in the cluster. This is important for SWNT-based electronics and SWNT-seeded regrowth.

The interest in carbon nanotechnology since the discovery of multiwalled carbon nanotubes in 19911 and single-walled carbon nanotubes (SWNTs) in 19932,3 is partially due to their potential applications in electronics.4,5 However, the transport properties of SWNT-based devices depends on the SWNT chirality and the structure and bonding at the electrode-SWNT contact.6 It is therefore imperative that one can obtain SWNTs with the desired chirality and that one can include them in electronic devices in a controlled and reproducible manner. For example, sputtering of electrodes onto SWNTs is expected to yield very different contacts, and hence electrical transport, than positioning of SWNTs over existing electrodes. The structure of the metal catalyst-SWNT interface may also affect the chirality of the grown SWNT. Although significant advances have been made in chirality controlled growth7-9 and in separating SWNTs with a desired chirality from other SWNTs,10-12 there are still no efficient ways for chirality controlled nanotube growth or separation. SWNT regrowth,13 where existing SWNTs are used as seeds to grow longer SWNTs, offers a potential method for chirality controlled growth. In this method SWNT growth is interrupted and the SWNTs are purified and cut to obtain clean SWNT ends before the growth is continued after docking nanometer sized metal particles on the seed SWNTs.13 Analysis of the bulk material shows that the regrown * Corresponding author, [email protected]. † University of Gothenburg. ‡ Laboratoire Francis Perrin. § CINaM, CNRS. | University of Borås. 10.1021/nl8036245 CCC: $40.75 Published on Web 02/09/2009

 2009 American Chemical Society

nanotubes inherit the diameters and chiralities of the seed SWNTs, although it is not known if there is a 1:1 correspondence of the regrown and seed SWNT chiralities at the individual SWNT level. Computational studies complement experiment by deepening our understanding of SWNT properties and growth at the atomic level, since they allow for detailed monitoring of atomic positions during chemical reactions. For example, first principle methods have been used to study carbon diffusion relevant to root-growth mechanisms of nanotube growth,14 carbon surface and subsurface diffusion on nickel,15,16 and carbon-catalyst interactions.17-20 Many of these studies assume that the metal-SWNT interface is on the surface of the catalyst particle, and not at subsurface sites, even though other studies15,16,21 indicate that carbon atoms, which are often assumed to be important precursors for SWNT growth, are more stable at subsurface sites. A disadvantage of computational methods when studying complex chemical processes is that they usually cannot include all chemical species and/or reaction processes, and one needs to limit the studies to key reactions. In addition, one needs to use models that are sufficiently accurate and, at the same time, computationally affordable to give chemically meaningful (statistically converged) results. This can be done by combining methods that have different levels of accuracy but also different computational expense. For example, first principles methods are usually sufficiently accurate to yield correct energetic and structural trends but are often restricted to single-point calculations, geometry optimizations at 0 K, or extremely short molecular dynamics trajectories at finite temperatures.22 In

Figure 2. Typical final configurations of Ni55-SWNT systems, for (6,5) (left) and (9,2) (right) nanotubes. The initial Ni55 clusters were cut from the bulk fcc structure, and were initially located slightly inside the SWNTs. The final configurations are obtained from TB MC simulations at 1000 K. Only the carbon atoms near the SWNT-metal interface are shown for the sake of clarity.

Figure 1. Initial (top), optimized (middle), and typical finite temperature (bottom) structures with Ni55 clusters and (10,0) (left) and (5,5) (right) SWNTs. The initial Ni55 geometry is fcc and the optimizations (middle) are done with DFT forces at 0 K. The typical structures at finite temperature are obtained from TB MC simulations at 1000 K.

contrast, molecular dynamics (MD) or Monte Carlo (MC) methods based on empirical force fields23-25 allow for statistically converged results but are often limited by the accuracy of the force field. In this contribution we study the effect of nanoparticle docking on the metal-SWNT interfacial structure, by varying the metal-SWNT structures to represent gentle or forced docking. This work complements previous investigations of metal deposition on carbon nanotubes26 and the effect of the metal-carbon nanotube interface on electronic transport properties.27,28 We combine density functional theory (DFT) geometry optimizations using the Vienna ab initio simulation package29 with finite temperature MC simulations based on a semiempirical tight binding (TB) Hamiltonian that has been developed specifically for nickel-carbon systems.30 Details of these methods are given in the Supporting Information. Two types of initial configurations, representing gentle and forced docking, have been investigated. The first type, representative of gentle docking and illustrated in the top panels of Figure 1, was obtained by placing nickel clusters, cut from the bulk nickel face centered cubic (fcc) structure, at low energy configurations relative to the SWNT end. Due to the computational expense, the DFT calculations focused only on capped (10,0) and (5,5) SWNTs. Except for the calculations that were done to directly compare with these DFT optimizations, long (≈30 Å) open-ended SWNTs were used in the TB calculations, and a wide variety of SWNT chiralities were investigated. The SWNTs used in the TB calculations are longer than the TB cutoff distance, and the similarity between the TB and DFT results supports the fact that the use of long or capped SWNTs does not affect the data presented here. In contrast to the 0 K DFT geometry 1118

optimizations, the TB MC calculations were performed at 1000 K, which is more relevant to chemical vapor deposition growth and regrowth of SWNTs. The second type of initial structure was used in the TB simulations only and was used to simulate forced docking of clusters. The same structures as those described above were used, except that the SWNT end was submersed ≈1.5 Å into the cluster. The structure of the metal cluster was relaxed before starting the MC simulations at finite temperatures. TB simulations were also used to study the equilibration of Ni atoms that are initially randomly dispersed over the inside and outside of a (9,0) SWNT (as discussed later with reference to Figure 4). These simulations mimic SWNT regrowth where the new metal catalyst particles are aggregated from metal atoms that are vaporized over the SWNT walls.13 For these systems the positions of nickel atoms were chosen so that the Ni-C atomic separations were between 1.75 and 2.25 Å, and the Ni-Ni separations were at least 2 Å to ensure a relatively low energy initial structure. The nanotube was kept rigid during the simulations presented here. As discussed below, similar results were obtained using flexible nanotubes. Simulations were performed at temperatures between 1000 and 2000 K. Figure 1 shows the initial (top panels) and final Ni55-SWNT structures from both DFT optimizations (middle panels) and TB MC simulations (bottom panels). The fcc geometry of the initial Ni55 structure resulted in the fact that four Ni atoms were initially inside the nanotube. After DFT geometry optimization these nickel atoms retract from the nanotube and the cluster became more spherical. For the (10,0) SWNT all four of these atoms are below the lowest carbon ring in the optimized structure, and for the (5,5) SWNT three of these atoms are below the lowest carbon ring while the top atom is at approximately the same height as the ring. Hence, for Ni clusters of this size, it is clear that the cluster adapts to the shape of the nanotube and not vice versa. It is also evident that the metal-SWNT interface is on the cluster surface and not at subsurface sites. In order to compare TB MC with the DFT results, the structures shown in the top panels of Figure 1 were also used to initialize TB MC simulations at 1000 K. Similarly to the DFT studies, and as shown in the bottom panels of Figure 1, the Ni55 cluster retracts from the SWNT. Hence, in Nano Lett., Vol. 9, No. 3, 2009

Figure 3. Local energy distributions of interfacial C atoms (top) and interfacial Ni atoms (bottom) for a Ni55 cluster attached to a (10,0) SWNT by forced docking. The lowest SWNT ring is initially inserted into the Ni cluster. The local energy distributions for the initial configuration (before C dissolution) are shown as dashed lines and energy distributions for the final configurations (after carbon dissolution) as dotted lines. The energy distribution for the dissolved C atoms is shown by the solid line in the top panel.

agreement with the DFT studies, the TB MC calculations show that the metal-SWNT interface is on the surface of the cluster, even at finite temperatures relevant to SWNT growth. In addition, the structure of the SWNT end is not changed during the MC simulation when one has a gently docked cluster (i.e., from the initial structures shown in top panels of Figure 1). The TB MC simulations were extended to numerous nickel cluster-SWNT systems, two of which are shown in Figure 2. The results discussed above are valid for all of the SWNT chiralities that were investigated. That is, low-energy docked initial cluster-SWNT structures result in the cluster retracting from the nanotube and adhering at the nanotube end. The structure at the nanotube end does not change during equilibration (e.g., the nanotube end does not dissolve into the cluster during the time scale of the simulation). The retraction of the Ni cluster can be explained by analyzing the local energies of the Ni atoms in the cluster, where three types of Ni atoms are identified in a similar way as done in ref 31. Interfacial Ni atoms are defined as Ni atoms that have C neighbors within the distance dCNi < 2.3 Å. Bulk Ni is defined as Ni with at least six Ni neighbors within the distance dNiNi < 2.5 Å while surface Ni has less than six Ni neighbors (and no C neighbors). The average local energy is -4.0 eV for bulk Ni, -3.6 eV for interfacial Ni, and -3.4 eV for surface Ni. In contrast, the average local energy for Ni atoms situated in the interior of the SWNT (at the beginning of the MC simulation) is -3.6 eV, and hence there is a reduction in energy when these Ni atoms retract from the SWNT into the bulk of the cluster. It is also clear that Ni atoms on the cluster surface have a higher energy (-3.4 eV) than the atoms that were initially inside the SWNT. Hence, retraction is not favored if it leads to a significant increase in the cluster surface area. Retraction is therefore more likely to occur for larger clusters, which is in agreement with recent reports that SWNTs only absorb nanoparticles that are below a critical size.32 Nano Lett., Vol. 9, No. 3, 2009

Figure 4. Capped (9,0) SWNT with nickel atoms randomly distributed on the inside and outside of the nanotube wall (left) and the 1500 K equilibrated structure (right).

TB MC simulations that began with the SWNT end inserted ≈1.5 Å into the cluster (to mimic forced docking of the metal on the SWNT) showed that SWNT atoms that were initially in the cluster typically dissolved into the cluster (simulations using rigid SWNTs yield similar structures to those discussed above with reference to Figure 2). The driving force for dissolution was, once again, revealed by analyzing the atomic local energies. This is exemplified in Figure 3 for a (10,0) SWNT where the lowest C ring is initially inserted into a Ni55 cluster. The top panel is for the 20 interfacial C atoms (i.e., the two (10,0) rings that are closest to the cluster). As shown by the dashed curve, the initial distribution is bimodal, with the 10 C atoms that have two C neighbors and that are inserted into the cluster having a higher energy (≈-5.9 eV) than those with three C neighbors and that are approximately at the cluster interface (≈-7.2 eV). These peaks shift to ≈-5.8 eV and ≈-7.3 eV, respectively, at equilibrium. However, of interest here is that the energies of the dissolved C atoms, which were initially -5.9 eV are ≈-6.5 eV and ≈-6.9 eV after dissolution (shown by the solid line in the figure). That is, dissolution is favored by changes in the dissolved atom energies. As seen in the bottom panel of Figure 3, dissolution and retraction of the cluster from the SWNT are also favored by the changes in the interfacial Ni atom energies (from ≈-3.5 to -4.1 eV). The bimodal shape of the final energy distribution (dotted line) is primarily due to the difference in energies of Ni atoms neighboring dissolved C (-4.1 eV) and Ni atoms neighboring undissolved C (-3.5 eV). The lower energy of Ni atoms neighboring dissolved C atoms compared to undissolved C atoms indicates that the energy gain for the entire Ni system is a combined effect due to dissolution of C atoms and the retraction of the Ni cluster 1119

(which also lowers the energies of the bulk Ni atoms and is not shown in this figure). Hence, the simulations that mimic forced docking showed that the structure at the end of the SWNT changed, as did the composition of the cluster. This yields different metal-carbon bonding at the cluster-SWNT interface compared to the interface obtained from gentle docking, which effects the electron transport across the junction.6 However, even though the structure of the SWNT end is changed, its chirality remains intact. It should be noted that the forced docking procedure used here differs considerably from what is expected under experimental seeded SWNT regrowth conditions, e.g., where the SWNT end may be capped, oxidized, or hydrogenated. In addition, there may be large activation barriers (that are overcome by our forced docking) that hinder dissolution of the SWNT into the cluster. The simulations where the Ni atoms were randomly distributed on the inside and outside of SWNTs, as exemplified in the left panel of Figure 4, show that nickel prefers to cluster at the SWNT end instead of distributing over the entire SWNT. The difference in average energies between initial and final configurations is approximately 20 eV. This also supports the possibility of SWNT regrowth where the catalytic clusters, that must be at the SWNT end for regrowth, are produced by depositing metal vapor atoms over the surface of the SWNT seeds.13 The gain in energy from retraction and clustering of the metal atoms is due to the increase in the number of bulk nickel atoms. These results were obtained for equilibration temperatures of 1000, 1500, and 2000 K. In conclusion, both first principles DFT 0 K geometry optimizations and TB MC simulations at finite temperatures show that the cluster-SWNT junction is on the surface of the metal nanoparticle and not in subsurface sites. This is important since some calculations often assume this, despite the fact that carbon atoms in low concentrations often favor subsurface sites in the metal cluster.31 Also, both methods show that there is significant restructuring of the metal cluster to fit the SWNT end, and not vice versa. This agrees with experimental results, e.g. ref 33, and must be taken into account in calculations of metal-SWNT interfaces, which are sometimes based on large metal surfaces that do not allow for large reconstruction. In addition, gentle docking of clusters at the simulated conditions allows the SWNT to retain its structure at the nanotube end, even at 1000 K, whereas forced docking can lead to dissolution of some SWNT end atoms. Hence, the docking procedure is important for reproducible production of metal-SWNT contacts for electron transport. Even when the SWNT end atoms dissolve in the cluster, the SWNT retains its chirality. This indicates that the detailed docking procedure may not be as important for SWNT regrowth. Acknowledgment. The calculations were performed on C3SE computing resources and the Swedish National Su-

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percomputing facilities. Financial support was obtained from the Swedish Research Council, the Swedish Foundation for Strategic Research (CARAMEL consortium), and University of Gothenburg Nanoparticle Platform. We thank Franc¸ois Ducastelle for fruitful discussions. Supporting Information Available: A more detailed description of the computational methods used. This material is available free of charge via the Internet at http:// pubs.acs.org. References (1) Iijima, S. Nature 1991, 354, 56. (2) Iijima, S.; Ichihashi, T. Nature 1993, 363, 603. (3) Bethune, D. S.; Klang, C. H.; de Vries, M. S.; Gorman, G.; Savoy, R.; Vazquez, J.; Beyers, R. Nature 1993, 363, 605. (4) Anantram, M. P.; Leonard, F. Rep. Prog. Phys. 2006, 69, 507. (5) Avouris, P.; Chen, Z.; Perebeinos, V. Nat. Nanotechnol. 2007, 2, 605. (6) Vitale, V.; Curioni, A.; Andreoni, W. J. Am. Chem. Soc. 2008, 130, 5848. (7) Bachilo, S. M.; Balzano, L.; Herrera, J. E.; Pompeo, F.; Resasco, D. E.; Weisman, R. B. J. Am. Chem. Soc. 2003, 125, 11186. (8) Lolli, G.; Zhang, L.; Balzano, L.; Sakulchaicharoen, N.; Tan, Y.; Resasco, D. E. J. Phys. Chem. B 2006, 110, 2108. (9) Li, X.; Tu, X.; Zaric, S.; Welsher, K.; Seo, W. S.; Zhao, W.; Dai, H. J. Am. Chem. Soc. 2007, 129, 15770–15771. (10) Krupke, R.; Hennrich, F.; Lohneysen, H. V.; Kappes, M. M. Science 2003, 301, 344. (11) Zheng, M.; Jagota, A.; Semke, E. D.; Diner, B. A.; Mclean, R. S.; Lustig, S. R.; Richardson, R. E.; Tassi, N. G. Nat. Mater. 2003, 2, 338. (12) Chen, Z.; Du, X.; Du, M.-H.; Rancken, C. D.; Cheng, H.-P.; Rinzler, A. G. Nano Lett. 2003, 3, 1245. (13) Wang, Y. H.; Kim, M. J.; Shan, H.; Kittrell, C.; Fan, H.; Ericson, L.; Hwang, W.-F.; Arepalli, S.; Hauge, R. H.; Smalley, R. E. Nano Lett. 2005, 5, 997. (14) Gavillet, J.; Loiseau, A.; Journet, C.; Willaime, F.; Ducastelle, F.; Charlier, J.-C. Phys. ReV. Lett. 2001, 87, 275504. (15) Abild-Pedersen, F.; Nørskov, J. K.; Rostrup-Nielsen, J. R.; Sehested, J.; Helveg, S. Phys. ReV. B 2006, 73, 115419. (16) Hofmann, S.; Csa´nyi, G.; Ferrari, A. C.; Payne, M. C.; Robertson, J. Phys. ReV. Lett. 2005, 95, 036101. (17) Fan, X.; Buczko, R.; Puretzky, A. A.; Geohegan, D. B.; Howe, J. Y.; Pantelides, S. T.; Pennycook, S. J. Phys. ReV. Lett. 2003, 90, 145501. (18) Zhang, Q.-M.; Wells, J. C.; Gong, X. G.; Zhang, Z. Y. Phys. ReV. B 2004, 69, 205413. (19) Reich, S.; Li, L.; Robertson, J. Chem. Phys. Lett. 2006, 421, 469. (20) Larsson, P.; Larsson, J. A.; Ahuja, R.; Ding, F.; Yakobson, B. I.; Duan, H.; Rose´n, A.; Bolton, K. Phys. ReV. B 2007, 75, 115419. (21) Yazyev, O. V.; Pasquarello, A. Phys. ReV. Lett. 2008, 100, 156102. (22) Raty, J.-Y.; Gygi, F.; Galli, G. Phys. ReV. Lett. 2005, 95, 096103. (23) Shibuta, Y.; Maruyama, S. Chem. Phys. Lett. 2003, 382, 381. (24) Zhao, J.; Martinez-Limia, A.; Balbuena, P. B. Nanotechnology 2005, 16, 575. (25) Ding, F.; Bolton, K. Nanotechnology 2006, 17, 543. (26) Henley, S. J.; Watts, P. C. P.; Mureau, N.; Silva, S. R. P. Appl. Phys. A: Mater. Sci. Process. 2008, 93, 875–879. (27) Mann, D.; Javey, A.; Kong, J.; Wang, Q.; Dai, H. Nano Lett. 2003, 3, 1541–1544. (28) Nemec, N.; Toma´nek, D.; Cuniberti, G. Phys. ReV. Lett. 2006, 96, 076802. (29) Kresse, G.; Furthmu¨ller, J. Phys. ReV. B 1996, 54, 11169. (30) Amara, H.; Bichara, C.; Ducastelle, F. Phys. ReV. B 2006, 73, 113404. (31) Amara, H.; Bichara, C.; Ducastelle, F. Phys. ReV. Lett. 2008, 100, 056105. (32) Schebarchov, D.; Hendy, S. C. Nano Lett. 2008, 8, 2253. (33) Helveg, S.; Lo´pez-Cartes, C.; Sehested, J.; Hansen, P. L.; Clausen, B. S.; Rostrup-Nielsen, J. R.; Abild-Pedersen, F.; Nørskov, J. K. Nature 2004, 427, 426.

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Nano Lett., Vol. 9, No. 3, 2009