MD

Article pubs.acs.org/JPCC

Graphene Nucleation from Amorphous Nickel Carbides: QM/MD Studies on the Role of Subsurface Carbon Density Menggai Jiao,†,‡ Hujun Qian,§ Alister Page,*,∥ Kai Li,† Ying Wang,*,† Zhijian Wu,*,† Stephan Irle,⊥ and Keiji Morokuma# †

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 130049, People’s Republic of China § State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun 130023, People’s Republic of China ∥ Discipline of Chemistry, School of Environmental and Life Sciences, The University of Newcastle, Callaghan 2308, Australia ⊥ Institute of Transformative Bio-Molecules (WPI-ITbM) and Department of Chemistry, Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan # Fukui Institute for Fundamental Chemistry, Kyoto University, Kyoto, 606-8103, Japan S Supporting Information *

ABSTRACT: The mechanism and kinetics of graphene formation from amorphous nickel carbides have been investigated employing quantum chemical molecular dynamics (QM/MD) simulations. Amorphous Ni3C, Ni2C, and NiC were employed to elucidate the role of the subsurface carbon density (ρC) on graphene formation. In each case, the nickel carbide phase underwent rapid carbon precipitation, resulting in a segregated nickel−carbon structure. The kinetics of graphene formation was most favorable for high carbon densities. At low ρC, i.e., Ni3C and Ni2C, there was a tendency for the formation of a number of small carbon fragments that failed to coalesce due to their inability to diffuse over the nickel surface. Graphene formation was only observed in the presence of high carbon densities that were relatively localized. These simulations, therefore, suggest that graphene nucleation is not immediately related to the presence of catalyst carbide phases.

1. INTRODUCTION Graphene is a two-dimensional allotrope of carbon consisting of a layer of carbon atoms arranged in a hexagonal, “chickenwire” pattern. It has been the focus of intense fundamental and applied research since its remarkable electronic properties were demonstrated in 2004.1 Graphene’s outstanding mechanical, thermal, and optical properties have also been demonstrated on a number of occasions.2−6 Owing to these properties, graphene potentially lays the foundation for future technologies. For instance, it is a promising candidate for replacing silicon in electronics7 and may also potentially enable efficient fuel cells through novel nanocomposite materials.8−10 However, development of a commercially viable production route for largescale, high-quality graphene with precise dimensions is of key importance if these applications are to be realized. Chemical vapor deposition (CVD) is currently the most favored method for the synthesis of graphene.11−16 It provides excellent process control and the ability to produce large-area graphene foils that can be easily transferred and incorporated into devices using existing microprocessing technology.14,15 CVD-graphene synthesis has been demonstrated on a number © 2014 American Chemical Society

of late transition metals, and transition-metal alloys, as detailed in recent reviews.17−19 However, Ni12,20−25 and Cu26−34 have been studied most extensively. Copper’s low carbon solubility prevents carbon dissolution and a bulk reservoir effect, and so it is widely used to synthesize monolayer graphene. However, the weak interaction between graphene and copper and the large number of graphene nucleation sites usually lead to a lack of process control, resulting in twisted domain boundaries.30 On the other hand, Ni was originally considered ideal for the synthesis of epitaxial graphene, due to the fact that the lattice constant closely matches that of graphite,22,35 and it provides fast growth rates.14 Weatherup et al. have demonstrated a threestep mechanism for graphene growth on Ni-based catalysts: (1) gaseous hydrocarbons decompose at a reactive site on the Ni surface, (2) carbon diffuses into the subsurface or bulk and (3) precipitates onto the Ni surface at a step-edge defect.25 The mechanism of graphene growth on Ni catalysts has also been Received: December 17, 2013 Revised: March 5, 2014 Published: March 11, 2014 11078

dx.doi.org/10.1021/jp4123612 | J. Phys. Chem. C 2014, 118, 11078−11084

The Journal of Physical Chemistry C

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nucleation and growth. It is noted that Te = 3000 K is the highest electronic temperature at which the pristine Ni(111) surface structure maintained its crystalline structure. Molecular dynamics calculations were performed by integrating the Newtonian equations with the Velocity-Verlet algorithm,61 using a time step of 1 fs. The nuclear temperature was maintained at 1180 K with a Nosé−Hoover chain thermostat62 (chain length three) throughout all simulations. 2.2. Model Systems. We have employed three different model systems depicted in Figure 1b−d, consisting of Ni108C36, Ni90C54, and

studied from a theoretical standpoint on a number of occasions.36−41 These studies have also highlighted the importance of the catalyst step-edge defect in graphene growth. Traditionally, these theoretical models assume the Ni catalyst topography to be static. Our own work, however, has shown that this is not the case at elevated temperatures relevant to CVD.36,37 Furthermore, we have recently shown38 that, as graphene forms and grows, it creates and destroys catalyst step edges as it needs to, in order to stabilize itself on the catalyst surface. Catalyst carbon solubility is a crucial parameter for controlling the number of graphene layers formed during graphene growth.29 Since Ni exhibits a relatively high carbon solubility, about ∼0.9 at. % at 900 °C,42 it has a tendency to form carbide phases during the CVD process.20,43,44 This generally results in the formation of less-desired multilayer graphene, as opposed to single-layer graphene. A great deal of effort has been directed at overcoming this problem. The typical strategy to counteract this problem is to use thin-metal catalyst films instead of bulk metal foils.17 Pertinent environmental factors, such as feedstock and hydrogen gas pressure, reactor temperature, and substrate cooling rate, were also found to be of great importance in this respect.31,45−47 Alternative approaches, such as ion implantation techniques by which carbon is introduced manually into nickel catalysts, also show promise.48,49 Trace levels of carbon in Ni have also been shown to yield graphene via segregation.50 However, there is currently no consensus over whether the formation of nickel carbide phases is related to graphene growth on nickel catalysts, with experiments providing evidence both for and against this possibility.20,25,43,51 A similar debate is currently ongoing in the carbon nanotube growth community.52,53 Despite intense experimental interest, and the wealth of theoretical studies of graphene growth on Ni catalysts, no theoretical study of the potential role of nickel carbide phases on graphene growth has been reported in the literature. In this work, we aim to address this shortcoming by presenting quantum chemical molecular dynamics (QM/MD) simulations of graphene formation from amorphous NixCy nickel carbide phases at 1180 K. We focus on the role that subsurface carbon density (ρC) plays regarding graphene nucleation and show that a highly localized concentration of subsurface carbon is required for graphene formation to take place. Thus, graphene formation from amorphous Ni2C and particularly Ni3C carbide phases becomes less likely with decreasing local carbon density. These simulations provide the first fundamental insight highlighting how subsurface carbon density may be exploited toward controlling the number of graphene layers formed during experimental CVD synthesis on nickel catalysts.

Figure 1. Optimized geometries of the (a) Ni144, (b) Ni108C36, (c) Ni90C54, and (d) Ni72C72 model systems. Brown and cyan spheres represent Ni and C atoms, respectively. Structures (b)−(d) correspond to the initial geometries for the Ni3C, Ni2C, and NiC trajectories, respectively. Ni72C72, respectively. These represent Ni/C ratios of 3:1, 2:1, and 1:1, respectively, and, therefore, correspond stoichiometrically to Ni3C, Ni2C, and NiC. Each model system is based on a four-layer Ni(111) (Figure 1a) consisting of 144 Ni atoms. Four layers will suffice here, since graphene nucleation on Ni catalysts is known to be a surface/ subsurface mediated process.25 Furthermore, the majority of relevant simulations use comparable systems to those employed here.63 The bottom layer in the surface was frozen in all simulations and approximates the underlying bulk region. Amorphous nickel carbide structures were produced by randomly replacing nickel atoms with carbon atoms in the first three layers, in a similar manner to our previous simulations of carbon nanotube nucleation.59 These model nickel carbide phases created here are presumably less stable compared with crystalline Ni2C and Ni3C phases, despite having the same stoichiometric ratios. However, the deformation of Ni surfaces in the presence of carbon, to the point of a loss of crystallinity, has been established experimentally.64 Periodic boundary conditions were enforced using the Γ point approximation, and a vacuum region of 10 nm was applied to avoid interactions between adjacent surfaces. These starting structures were initially optimized at 0 K (Figure 1). Ten independent trajectories were calculated for each nickel carbide system, yielding a total of 30 trajectories. These trajectories are labeled as Ni3C@N, Ni2C@N, and NiC@N, where N denotes the roman numerals (1−10) of the trajectories. The initial velocity of each atom in an individual trajectory was randomly chosen according to a Maxwell−Boltzmann distribution at 1180 K. Each trajectory was performed at 1180 K for 160 ps to investigate the graphene nucleation and growth from amorphous nickel carbide precursors.

2. COMPUTATIONAL METHODOLOGY 2.1. Quantum Chemical Molecular Dynamics Simulations. Graphene nucleation has been investigated with nonequilibrium QM/ MD simulations, which consist of the integration of classical equations of motion in conjunction with a QM potential. This potential was computed “on the fly” at each MD iteration using the SCC-DFTB method54 as implemented in the DFTB+ program,55 in combination with a finite electronic temperature (Te)56,57 of 3000 K. The standard trans3d-0-158 and mio-0-154 parameter sets were employed. The occupancy of each molecular orbital was described by a Fermi−Dirac distribution function of its energy, varying continuously between zero and two near the Fermi level. This method has been previously applied extensively in simulations of carbon nanotube59,60 and graphene36,39

3. RESULTS AND DISCUSSION 3.1. Graphene Nucleation Mechanism. Figure 2 shows snapshots of representative trajectories at the three carbon concentrations. Final snapshots of all trajectories following 160 ps are provided as Supporting Information (Figures S1−S3). Subsurface carbon rapidly precipitated onto the catalyst surface, 11079



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Figure 2. Evolution of SCC-DFTB/MD simulations of graphene nucleation at 1180 K from NixCy precursors: (a) trajectory Ni3C@8, (b) trajectory Ni2C@5, and (c) trajectory NiC@2. Color conventions as those in Figure 1. Snapshot times given with respect to the beginning of the simulation. The location of the periodic boundary is indicated by the red line. The Y-junction and rings formed are highlighted in purple, and the purple circle in the third side view for each model displays the formation of the ring in the subsurface.

graphene nucleation from the model NiC, Ni2C, and Ni3C bulk phases. In each case, the energy decreased, indicating a thermodynamically favorable process. As may be expected, the exothermicity was proportional to the carbon density in the bulk phase, i.e., E(NiC) < E(Ni2C) < E(Ni3C), and thus the extent of structural disruption to the bulk Ni-carbide phase, which was larger at higher ρC. As shown in Figure 2a, the majority of subsurface carbon atoms precipitated directly as single carbon atoms, although aggregation into carbon dimers or trimmers in the subsurface was occasionally observed. A single Y-junction was generated at 41.98 ps, followed by the instantaneous formation of a fivenumbered ring in the subsurface at 42.72 ps. Interestingly, this pentagon was repeatedly broken and reformed as it precipitated to the surface from the subsurface. Thereafter, polyyne chains on the surface coalesced further into polygonal rings during the subsequent 120 ps, when only small sp2 carbon clusters were found on the surface at this low carbon density. We have previously established that the rapid segregation of nickel carbide nanoparticles at high temperature is a consequence of thermodynamicsthe stronger C−C bond provides greater thermodynamic stability compared with the weaker Ni−C bondand so C−C bonds are naturally formed at the expense of Ni−C bonds.59 Mulliken charge analyses also show that the

in a similar manner to that observed during carbon nanotube nucleation from nickel carbide nanoparticles.59 The presence of a subsurface carbide phase is, therefore, presumably not related to graphene growth under these conditions, since such a crystalline phase is thermodynamically metastable. Figure 3 depicts the evolution of the total Mermin free energy during

Figure 3. Total Mermin free energy per carbon atom for Ni3C, Ni2C, and NiC model systems. The decrease in Mermin free energy is indicative of an exothermic process, and this decrease is proportional to the amount of carbon present in the bulk Ni structure. All data averaged over 10 trajectories. 11080



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Figure 4. Dependence of graphene nucleation from NixCy models on carbon concentration at 1180 K: populations of polygonal carbon rings from (a) Ni3C, (b) Ni2C, and (c) NiC trajectories. All data were averaged over 10 trajectories. Individual ring populations of all trajectories are shown in Figures S4−S6 (Supporting Information).

controlled, and driven by the large-amplitude vibration of the polyyne chain “arms” at the Y-junction.37 The positive curvature in the nascent sp2 carbon structure, induced by pentagon formation, underlies the propensity for the graphene nucleation process to disturb the pristine Ni structure, to one that is highly distorted through the formation of Ni−C σbonds.40,70 The Ni structure itself impacts on this reaction pathway by imposing physical constraints on the motion of the two arms, preventing their large amplitude vibrational motion. Taking Ni3C again as the example, this model system had the lowest ρC, and equivalently the densest surrounding Ni environment, and pentagons were often not the first polygonal carbon ring formed in the graphene precursor. Hexagons and defect heptagons were instead formed initially (see Figure S4, Supporting Information). Moreover, most of these nascent carbon clusters in the subsurface were not permanent in the subsurface. Instead, as entropic and enthalpic effects competed, these thermodynamically stable structures were repeatedly fractured and reformed as precipitation took place. The formation of pentaheptites (two-dimensional, flat carbon sheets consisting of pentagons and heptagons, a member of the “Haeckelite” family) was never observed here, while restricting comparable carbon densities to a Ni(111) surface does result in the formation of this graphene “sister-structure”.39 3.2. Graphene Nucleation Kinetics. While the atomistic mechanism of graphene nucleation is largely independent of ρC, this is not the case with respect to the kinetics of graphene nucleation observed in these simulations. This is evident from a comparison of carbon ring formation statistics in these three cases, which is shown in Figure 4. The extent of carbon polygonal ring production, which largely consists of hexagons and pentagons, is proportional to ρC as expected. However, as ρC decreases, so too does hexagon formation. For instance, the pentagon/hexagon ratios for the NiC, Ni2C, and Ni3C systems are 1:0.88, 1:0.42 and 1:0.36, respectively. This indicates that a carbide phase with high carbon density is more likely to produce graphene with fewer defects, at least on these time scales. Furthermore, Ni carbide phases with relatively low ρC, such as Ni2C or Ni3C, only produce small isolated graphene fragments and so may not be actively involved in the production of large-domain graphene sheets. Considering that experiments show graphene growth with as little as 0.15 atom %,25 this implies that a highly localized ρC is required in the catalyst subsurface in order for graphene nucleation to take place. Interestingly, this trend is the opposite of that observed for carbon nanotube nucleation on transition-metal carbide

strength of transition-metal−transition-metal bonds themselves are weakened in the presence of carbon, due to the effect of metal → carbon π-back-bonding, which produces electron depletion in the metal−metal bond.65,66 This ultimately leads to the disrupted initial NixC structures observed in Figure 1. This effect becomes tempered over time; as carbon precipitates and combines on the catalyst surface, the extent of Ni → C π-backbonding decreases. These factors underpin what is observed here in the case of bulk carbide phases. As shown in Figure 2b, for Ni2C@5, carbon atoms readily combined with each other to form dimers, longer polyyne chains, or even carbon polygonal rings in the subsurface region. Simultaneously, these fragments rapidly precipitated from the subsurface. A Y-junction was formed at 4 ps, followed by the formation of a subsurface pentagon at 4.88 ps. After ca. 89 ps, the number of polygonal carbon rings began to increase, as did the number of associated polyyne chains. Hexagons ultimately dominated this structure, and it is noted that hexagons rarely, if ever, were converted to either pentagons or heptagon defects. This is consistent with the relative thermodynamic stabilities of these carbon structures. Hexagons were actively stabilized on the catalyst surface via terminal Ni−C σ-bonds, and this demonstrates the essence of the step-edge defect’s primary role during graphene nucleation: stabilization of the nucleating precursors. It is this role that enabled the extension of this initial hexagon to a large sp2 carbon network structure via continual ring condensation within 160 ps. Figure 2c shows that, in the case of stoichiometric equivalence, the coalescence of polyyne chains and ring condensations proceeded far quicker compared to the previous two cases. In trajectory NiC@2, a Y-junction was formed instantaneously at 0.22 ps, and this was followed immediately by the formation of a subsurface pentagon at 0.44 ps. At this higher carbon density, the structure of the subsurface carbon cluster was essentially amorphous in nature. Following its precipitation to the catalyst surface, this carbon network structure isomerized from a planar to a dome-like structure at 160 ps, consistent with previous experimental and theoretical observations.12,40,67,68 From these observations, it is concluded that the atomistic mechanism of graphene nucleation starting from amorphous Ni3C, Ni2C, and NiC phases is largely independent of ρC. More generally, this mechanism parallels closely the “pentagon-first” mechanism observed during graphene and carbon nanotube nucleation on transition-metal catalysts,36,37,39,60 and even fullerene formation.69 This “pentagon-first” mechanism is attributed to the fact that pentagon formation is kinetically 11081



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Figure 5. Carbon cluster size as a function of simulation time in (a) Ni3C, (b) Ni2C, and (c) NiC simulations. Each color indicates a unique carbon cluster observed during the simulation; the way in which the sizes of these colored areas change indicates how these cluster sizes change. In each case, the largest cluster grows by consuming smaller fragments. All data were averaged over 10 trajectories.

Figure 6. Ratios of subsurface-C/total carbon atoms and surface-C/total carbon atoms as a function of time for (a) Ni3C, (b) Ni2C, and (c) NiC trajectories. All data were averaged over 10 trajectories.

nanoparticles59 and can be attributed to the relative curvatures of the underlying catalyst structure (the catalyst here is flat, i.e., 0 curvature, while nanoparticle catalysts for carbon nanotube growth have a positive, convex curvature). This fact could potentially be exploited toward controlling graphene/SWCNT synthesis through the modulation of catalyst curvature and subsurface ρC. As one may expect, the nucleation rate of graphene is also dependent on ρC. Figure 5 shows the evolution of each individual carbon cluster formed in the Ni3C, Ni2C, and NiC model systems. Variation of the ratios of subsurface and surface carbon to total carbon as a function of time is also shown in order to better understand and analyze the mechanism and kinetics of the entire graphene nucleation process (Figure 6). The nucleation time, which is defined here as the amount of time required before the first polygonal ring is formed (averaged over 10 trajectories), decreased from 17.4, 3.3, to 0.4 ps, respectively (Figure 4). Figure 5 suggests that graphene nucleation proceeds via a route reminiscent of Ostwald ripening: the largest carbon fragment grew at the expense of other, smaller fragments. This process is largely independent of ρC and is most noticeable at lower ρC. For example, in the Ni2C trajectories, the fragment sizes reached equilibrium at ca. 130 ps. This same equilibrium was reached after only ca. 40 ps for NiC. The largest clusters of C18, C29, and C59 (50, 54, and 82% over a total number of 36, 54, and 72 C) were observed in Ni3C, Ni2C, and NiC trajectories after the 160 ps, respectively. The growth of the largest cluster starts to increase dramatically when internal carbon precipitates from NixCy to the upper surface, but slows down after the majority of carbon atoms moved to the nickel surface, as seen in Figures 5 and 6. Figure 6

shows that, for all cases, the ratio of subsurface carbon reduces sharply in the initial stage of the simulation, which coincides with the rapid increase in the ratio of surface carbon atoms. The time required for 90% of all carbon to precipitate to the surface also decreased dramatically with increasing ρC, 58.6, 22.6, to 6.4 ps for Ni3C, Ni2C, and NiC, respectively. Together, these results indicate that these carbon fragments are dispersive at low ρC and that their coalescence is likely to be limited by their almost negligible surface diffusion, brought about by strong terminating Ni−C σ-bonding. On this basis, we propose that the participation of many small carbon fragments in the formation of large-domain graphene is unlikely. Instead, since higher ρC benefits the rapid precipitation of subsurface carbon, it will also benefit the formation of a single, dominant carbon structure, as opposed to many competing, smaller fragments (the sizes of which have a tendency to equilibrate with each other). A high carbon density that is also highly localized will thus enhance the formation of large-domain high-quality graphene on the nickel surface.

4. CONCLUSION QM/MD simulations of graphene nucleation from amorphous nickel carbide precursors demonstrated that, on subnanosecond time scales, the atomistic graphene nucleation mechanism is independent of the subsurface carbon density ρC. The observed mechanism followed three steps: the rapid precipitation of the internal carbon from the subsurface to the outer surface, the formation of dimer or longer polyyne chains, and the coalescence of them to the nickel-supported sp2-hybidrized carbon structures. Our results indicate that a high, localized 11082



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(9) Lim, D.-H.; Wilcox, J. Mechanisms of the Oxygen Reduction Reaction on Defective Graphene-Supported Pt Nanoparticles from First-Principles. J. Phys. Chem. C 2012, 116, 3653. (10) Lim, D.-H.; Negreira, A. S.; Wilcox, J. DFT Studies on the Interaction of Defective Graphene-Supported Fe and Al Nanoparticles. J. Phys. Chem. C 2011, 115, 8961. (11) Obraztsov, A. N. Chemical Vapour Deposition: Making Graphene on a Large Scale. Nat. Nanotechnol. 2009, 4, 212. (12) Gao, J.; Yip, J.; Zhao, J.; Yakobson, B. I.; Ding, F. Graphene Nucleation on Transition Metal Surface: Structure Transformation and Role of the Metal Step Edge. J. Am. Chem. Soc. 2011, 133, 5009. (13) Wei, D.; Liu, Y. Controllable Synthesis of Graphene and Its Applications. Adv. Mater. 2010, 22, 3225. (14) Reina, A.; Jia, X.; Ho, J.; Nezich, D.; Son, H.; Bulovic, V.; Dresselhaus, M. S.; Kong, J. Large Area, Few-Layer Graphene Films on Arbitrary Substrates by Chemical Vapor Deposition. Nano Lett. 2009, 9, 30. (15) Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S. Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils. Science 2009, 324, 1312. (16) Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Kim, K. S.; Ahn, J.-H.; Kim, P.; Choi, J.-Y.; Hong, B. H. Large-Scale Pattern Growth of Graphene Films for Stretchable Transparent Electrodes. Nature 2009, 457, 706. (17) Batzill, M. The Surface Science of Graphene: Metal Interfaces, CVD Synthesis, Nanoribbons, Chemical Modifications, and Defects. Surf. Sci. Rep. 2012, 67, 83. (18) Bartelt, N. C.; McCarty, K. F. Graphene Growth on Metal Surfaces. MRS Bull. 2012, 37, 1158. (19) Mattevi, C.; Kim, H.; Chhowalla, M. A Review of Chemical Vapour Deposition of Graphene on Copper. J. Mater. Chem. 2011, 21, 3324. (20) Lahiri, J.; Miller, T.; Adamska, L.; Oleynik, I. I.; Batzill, M. Graphene Growth on Ni(111) by Transformation of a Surface Carbide. Nano Lett. 2010, 11, 518. (21) Grüneis, A.; Kummer, K.; Vyalikh, D. V. Dynamics of Graphene Growth on a Metal Surface: A Time-Dependent Photoemission Study. New J. Phys. 2009, 11, 073050. (22) Cheng, D.; Barcaro, G.; Charlier, J.-C.; Hou, M.; Fortunelli, A. Homogeneous Nucleation of Graphitic Nanostructures from Carbon Chains on Ni(111). J. Phys. Chem. C 2011, 115, 10537. (23) Xu, M.; Fujita, D.; Sagisaka, K.; Watanabe, E.; Hanagata, N. Production of Extended Single-Layer Graphene. ACS Nano 2011, 5, 1522. (24) Hofmann, S.; Sharma, R.; Ducati, C.; Du, G.; Mattevi, C.; Cepek, C.; Cantoro, M.; Pisana, S.; Parvez, A.; Cervantes-Sodi, F.; Ferrari, A. C.; Dunin-Borkowski, R.; Lizzit, S.; Petaccia, L.; Goldoni, A.; Robertson, J. In Situ Observations of Catalyst Dynamics during Surface-Bound Carbon Nanotube Nucleation. Nano Lett. 2007, 7, 602. (25) Weatherup, R. S.; Bayer, B. C.; Blume, R.; Ducati, C.; Baehtz, C.; Schlögl, R.; Hofmann, S. In Situ Characterization of Alloy Catalysts for Low-Temperature Graphene Growth. Nano Lett. 2011, 11, 4154. (26) Yao, Y.; Li, Z.; Lin, Z.; Moon, K.-S.; Agar, J.; Wong, C. Controlled Growth of Multilayer, Few-Layer, and Single-Layer Graphene on Metal Substrates. J. Phys. Chem. C 2011, 115, 5232. (27) Wood, J. D.; Schmucker, S. W.; Lyons, A. S.; Pop, E.; Lyding, J. W. Effects of Polycrystalline Cu Substrate on Graphene Growth by Chemical Vapor Deposition. Nano Lett. 2011, 11, 4547. (28) Nie, S.; Wofford, J. M.; Bartelt, N. C.; Dubon, O. D.; McCarty, K. F. Origin of the Mosaicity in Graphene Grown on Cu(111). Phys. Rev. B 2011, 84, 155425. (29) Liu, W.; Li, H.; Xu, C.; Khatami, Y.; Banerjee, K. Synthesis of High-Quality Monolayer and Bilayer Graphene on Copper Using Chemical Vapor Deposition. Carbon 2011, 49, 4122. (30) Gao, L.; Guest, J. R.; Guisinger, N. P. Epitaxial Graphene on Cu(111). Nano Lett. 2010, 10, 3512. (31) Reina, A.; Thiele, S.; Jia, X.; Bhaviripudi, S.; Dresselhaus, M. S.; Schaefer, J. A.; Kong, J. Growth of Large-Area Single- and Bi-Layer

subsurface carbon density is required for the formation of largedomain graphene on nickel. The kinetics of graphene nucleation exhibits distinct dependence on ρC, with higher ρC values leading to faster nucleation and the formation of a single, dominant sp2 carbon graphene precursor. Interestingly, our simulations show that, at lower ρC, there is a tendency for the carbon from the carbide phase to form a number of smaller carbon fragments. Over time, it was observed that the size of these fragments equilibrates, and since these fragments are unable to diffuse over the nickel surface, we anticipate that they play little further role in graphene formation. These simulations, therefore, suggest that graphene nucleation on subnanosecond time scales is not directly related to the presence of catalyst carbide phases.

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ASSOCIATED CONTENT

S Supporting Information *

Extended discussion of graphene nucleation mechanism, structures of NixCy following 160 ps simulation at 1180 K, populations of polygonal rings and average spn-hybridized carbon atoms observed during graphene nucleation from NixCy precursors at 1180 K, and QuickTime movie depicting graphene precursor nucleation from NiC@2. 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] (Y.W.). *E-mail: [email protected] (Z.W.). *E-mail: [email protected] (A.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China for financial support (Grant Nos: 21203174, 21221061, 21273219) and the Natural Science Foundation of Jilin Province (Nos. 20130522141JH, 20130101179JC-07). The computational resource is partly supported by the Performance Computing Center of Jilin University, China. We are grateful to the Computing Center of Jilin Province for essential support.

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REFERENCES

(1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666. (2) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183. (3) Lee, C.; Wei, X.; Kysar, J. W.; Hone, J. Measurement of the Elastic Properties and Intrinsic Strength of Monolayer Graphene. Science 2008, 321, 385. (4) Balandin, A. A.; Ghosh, S.; Bao, W.; Calizo, I.; Teweldebrhan, D.; Miao, F.; Lau, C. N. Superior Thermal Conductivity of Single-Layer Graphene. Nano Lett. 2008, 8, 902. (5) Xia, F. N.; Mueller, T.; Lin, Y. M.; Valdes-Garcia, A.; Avouris, P. Ultrafast Graphene Photodetector. Nat. Nanotechnol. 2009, 4, 839. (6) Chen, D.; Tang, L.; Li, J. Graphene-Based Materials in Electrochemistry. Chem. Soc. Rev. 2010, 39, 3157. (7) Chen, J.-H.; Jang, C.; Xiao, S.; Ishigami, M.; Fuhrer, M. S. Intrinsic and Extrinsic Performance Limits of Graphene Devices on SiO2. Nat. Nanotechnol. 2008, 3, 206. (8) Lim, D.-H.; Wilcox, J. DFT-Based Study on Oxygen Adsorption on Defective Graphene-Supported Pt Nanoparticles. J. Phys. Chem. C 2011, 115, 22742. 11083



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Graphene by Controlled Carbon Precipitation on Polycrystalline Ni Surfaces. Nano Res. 2009, 2, 509. (32) Kumar, A.; Voevodin, A. A.; Zemlyanov, D.; Zakharov, D. N.; Fisher, T. S. Rapid Synthesis of Few-Layer Graphene over Cu Foil. Carbon 2012, 50, 1546. (33) Hu, B.; Ago, H.; Ito, Y.; Kawahara, K.; Tsuji, M.; Magome, E.; Sumitani, K.; Mizuta, N.; Ikeda, K.-i.; Mizuno, S. Epitaxial Growth of Large-Area Single-Layer Graphene over Cu(111)/Sapphire by Atmospheric Pressure CVD. Carbon 2012, 50, 57. (34) Guermoune, A.; Chari, T.; Popescu, F.; Sabri, S. S.; Guillemette, J.; Skulason, H. S.; Szkopek, T.; Siaj, M. Chemical Vapor Deposition Synthesis of Graphene on Copper with Methanol, Ethanol, and Propanol Precursors. Carbon 2011, 49, 4204. (35) Shelton, J. C.; Patil, H. R.; Blakely, J. M. Equllibrium Segregation of Carbon to a Nickel (111) Surface: A Surface Phase Transition. Surf. Sci. 1974, 43, 493. (36) Li, H.-B.; Page, A. J.; Wang, Y.; Irle, S.; Morokuma, K. Subsurface Nucleation of Graphene Precursors near a Ni(111) Step-Edge. Chem. Commun. 2012, 48, 7937. (37) Page, A. J.; Wang, Y.; Li, H.-B.; Irle, S.; Morokuma, K. Nucleation of Graphene Precursors on Transition Metal Surfaces: Insights from Theoretical Simulations. J. Phys. Chem. C 2013, 117, 14858. (38) Wang, Y.; Page, A. J.; Li, H.-B.; Qian, H.-J.; Jiao, M.-G.; Wu, Z.J.; Morokuma, K.; Irle, S. Step-Edge Self-Assembly During Graphene Nucleation on a Nickel Surface: QM/MD Simulations. Nanoscale 2014, 6, 140−144. (39) Wang, Y.; Page, A. J.; Nishimoto, Y.; Qian, H.-J.; Morokuma, K.; Irle, S. Template Effect in the Competition between Haeckelite and Graphene Growth on Ni(111): Quantum Chemical Molecular Dynamics Simulations. J. Am. Chem. Soc. 2011, 133, 18837. (40) Yuan, Q.; Gao, J.; Shu, H.; Zhao, J.; Chen, X.; Ding, F. Magic Carbon Clusters in the Chemical Vapor Deposition Growth of Graphene. J. Am. Chem. Soc. 2012, 134, 2970. (41) Artyukhov, V. I.; Liu, Y.; Yakobson, B. I. Equilibrium at the Edge and Atomistic Mechanisms of Graphene Growth. Proc. Nat. Acad. Sci. U.S.A. 2012, DOI: 10.1073/pnas.1207519109. (42) Li, X.; Cai, W.; Colombo, L.; Ruoff, R. S. Evolution of Graphene Growth on Ni and Cu by Carbon Isotope Labeling. Nano Lett. 2009, 9, 4268. (43) Jacobson, P.; Stöger, B.; Garhofer, A.; Parkinson, G. S.; Schmid, M.; Caudillo, R.; Mittendorfer, F.; Redinger, J.; Diebold, U. Nickel Carbide as a Source of Grain Rotation in Epitaxial Graphene. ACS Nano 2012, 6, 3564. (44) Benayad, A.; Li, X.-S. Carbon Free Nickel Subsurface Layer Tessellating Graphene on Ni(111) Surface. J. Phys. Chem. C 2013, 117, 4727. (45) Loginova, E.; Bartelt, N. C.; Feibelman, P. J.; McCarty, K. F. Evidence for Graphene Growth by C Cluster Attachment. New J. Phys. 2008, 10, 093026. (46) Addou, R.; Dahal, A.; Sutter, P.; Batzill, M. Monolayer Graphene Growth on Ni(111) by Low Temperature Chemical Vapor Deposition. Appl. Phys. Lett. 2012, 100, 021601. (47) Weatherup, R. S.; Dlubak, B.; Hofmann, S. Kinetic Control of Catalytic CVD for High-Quality Graphene at Low Temperatures. ACS Nano 2012, 6, 9996. (48) Baraton, L.; He, Z. B.; Lee, C. S.; Cojocaru, C. S.; Châtelet, M.; Maurice, J. L.; Lee, Y. H.; Pribat, D. On the Mechanisms of Precipitation of Graphene on Nickel Thin Films. Europhys. Lett. 2011, 96, 46003. (49) Garaj, S.; Hubbard, W.; Golovchenko, J. A. Graphene Synthesis by Ion Implantation. Appl. Phys. Lett. 2010, 97, 183103. (50) Liu, N.; Fu, L.; Dai, B.; Yan, K.; Liu, X.; Zhao, R.; Zhang, Y.; Liu, Z. Universal Segregation Growth Approach to Wafer-Size Graphene from Non-Noble Metals. Nano Lett. 2010, 11, 297. (51) Cao, Y.; Su, Q.; Che, R.; Du, G.; Xu, B. One-Step Chemical Vapor Synthesis of Ni/graphene Nanocomposites with Excellent Electromagnetic and Electrocatalytic Properties. Synth. Met. 2012, 162, 968.

(52) Yoshida, H.; Takeda, S.; Uchiyama, T.; Kohno, H.; Homma, Y. Atomic-Scale In-situ Observation of Carbon Nanotube Growth from Solid State Iron Carbide Nanoparticles. Nano Lett. 2008, 8, 2082. (53) Rao, R.; Pierce, N.; Liptak, D.; Hooper, D.; Sargent, G.; Semiatin, S. L.; Curtarolo, S.; Harutyunyan, A. R.; Maruyama, B. Revealing the Impact of Catalyst Phase Transition on Carbon Nanotube Growth by in Situ Raman Spectroscopy. ACS Nano 2013, 7, 1100. (54) Elstner, M.; Porezag, D.; Jungnickel, G.; Elsner, J.; Haugk, M.; Frauenheim, T.; Suhai, S.; Seifert, G. Self-Consistent-Charge DensityFunctional Tight-Binding Method for Simulations of Complex Materials Properties. Phys. Rev. B 1998, 58, 7260. (55) Aradi, B.; Hourahine, B.; Frauenheim, T. DFTB+, a Sparse Matrix-Based Implementation of the DFTB Method. J. Phys. Chem. A 2007, 111, 5678. (56) Weinert, M.; Davenport, J. W. Fractional Occupations and Density-Functional Energies and Forces. Phys. Rev. B 1992, 45, 13709. (57) Wentzcovitch, R. M.; Martins, J. L.; Allen, P. B. Energy Versus Free-Energy Conservation in First-Principles Molecular Dynamics. Phys. Rev. B 1992, 45, 11372. (58) Zheng, G.; Witek, H. A.; Bobadova-Parvanova, P.; Irle, S.; Musaev, D. G.; Prabhakar, R.; Morokuma, K.; Lundberg, M.; Elstner, M.; Köhler, C.; Frauenheim, T. Parameter Calibration of TransitionMetal Elements for the Spin-Polarized Self-Consistent-Charge Density-Functional Tight-Binding (DFTB) Method: Sc, Ti, Fe, Co, and Ni. J. Chem. Theory Comput. 2007, 3, 1349. (59) Page, A. J.; Yamane, H.; Ohta, Y.; Irle, S.; Morokuma, K. QM/ MD Simulation of SWNT Nucleation on Transition-Metal Carbide Nanoparticles. J. Am. Chem. Soc. 2010, 132, 15699. (60) Page, A. J.; Ohta, Y.; Irle, S.; Morokuma, K. Mechanisms of Single-Walled Carbon Nanotube Nucleation, Growth, and Healing Determined Using QM/MD Methods. Acc. Chem. Res. 2010, 43, 1375. (61) Swope, W. C.; Andersen, H. C.; Berens, P. H.; Wilson, K. R. A Computer Simulation Method for the Calculation of Equilibrium Constants for the Formation of Physical Clusters of Molecules: Application to Small Water Clusters. J. Chem. Phys. 1982, 76, 637. (62) Martyna, G. J.; Klein, M. L.; Tuckerman, M. Nosé-Hoover Chains: The Canonical Ensemble via Continuous Dynamics. J. Chem. Phys. 1992, 97, 2635. (63) Haghighatpanah, S.; Börjesson, A.; Amara, H.; Bichara, C.; Bolton, K. Computational Studies of Graphene Growth Mechanisms. Phys. Rev. B 2012, 85, 205448. (64) Rinaldi, A.; Tessonnier, J. P.; Schuster, M. E.; Blume, R.; Girgsdies, F.; Zhang, Q.; Jacob, T.; Abd Hamid, S. B.; Su, D. S.; Schlogl, R. Dissolved Carbon Controls the Initial Stages of Nanocarbon Growth. Angew. Chem., Int. Ed. 2011, 50, 3313. (65) Ohta, Y.; Okamoto, Y.; Page, A. J.; Irle, S.; Morokuma, K. Quantum Chemical Molecular Dynamics Simulation of Single-Walled Carbon Nanotube Cap Nucleation on an Iron Particle. ACS Nano 2009, 3, 3413. (66) Larsson, P.; Larsson, J. A.; Ahuja, R.; Ding, F.; Yakobson, B. I.; Duan, H. M.; Rosen, A.; Bolton, K. Calculating Carbon Nanotube− Catalyst Adhesion Strengths. Phys. Rev. B 2007, 75, 115419. (67) Lacovig, P.; Pozzo, M.; Alfè, D.; Vilmercati, P.; Baraldi, A.; Lizzit, S. Growth of Dome-Shaped Carbon Nanoislands on Ir(111): The Intermediate between Carbidic Clusters and Quasi-Free-Standing Graphene. Phys. Rev. Lett. 2009, 103, 166101. (68) Wang, B.; Ma, X.; Caffio, M.; Schaub, R.; Li, W.-X. SizeSelective Carbon Nanoclusters as Precursors to the Growth of Epitaxial Graphene. Nano Lett. 2011, 11, 424. (69) Irle, S.; Zheng, G.; Wang, Z.; Morokuma, K. The C60 Formation Puzzle “Solved”: QM/MD Simulations Reveal the Shrinking Hot Giant Road of the Dynamic Fullerene Self-Assembly Mechanism. J. Phys. Chem. B 2006, 110, 14531. (70) Yuan, Q.; Hu, H.; Gao, J.; Ding, F.; Liu, Z.; Yakobson, B. I. Upright Standing Graphene Formation on Substrates. J. Am. Chem. Soc. 2011, 133, 16072.

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