Defect Healing of Chemical Vapor Deposition Graphene Growth by

Article pubs.acs.org/JPCC

Defect Healing of Chemical Vapor Deposition Graphene Growth by Metal Substrate Step Lijuan Meng, Zilu Wang, Jian Jiang, Yonghong Yang, and Jinlan Wang* Department of Physics, Southeast University, Nanjing, 211189, China S Supporting Information *

ABSTRACT: The evolution of carbon structures and the kinetics of graphene nucleation on nickel step surfaces are investigated by classical molecular dynamics simulations and density functional theory calculations. It is found that the evolution mechanism of C structures on the step surface is the same as that on the flat terrace when no substrate Ni atom is pulled out of the surface. But the defects involved with the pulled-out Ni atoms can be efficiently healed with the assistance of the step atoms on the step surface, while they are rather difficult to be healed on the terrace. Compared with the terrace, the step significantly lowers the healing barrier of the defect involved with the pulled-out Ni atom and therefore results in a very fast healing of the defect. These results demonstrate that the presence of the step is beneficial to synthesize better graphene for chemical vapor deposition growth on Ni substrate.

I. INTRODUCTION Graphene exhibits numerous excellent properties,1−5 making it an ideal material for future applications in field-effect transistors,6,7 sensors,8,9 electrodes,10,11 supercapacitors,12,13 etc. Presently, the most promising way of producing highquality large-size graphene is chemical vapor deposition (CVD) on transition metal (TM) surfaces.14−16 Beyond growth temperature,17−19 partial pressure,17,20 and carbon precursors,21,22 the effect of TM substrate on the CVD growth of graphene has recently been investigated.23−28 Karoui et al. compared the healing of graphene with and without a nickel substrate via Monte Carlo simulations and found that the metal substrate is beneficial to the healing of graphene defects.23 Jacobson et al. calculated the healing barriers of a Stone−Wales (SW) defect of freestanding graphene and graphene on Ni(111) surface and showed that the metallic substrate helps the healing of SW defects.24 Gao et al. explored the reconstruction of graphene edge on TM surface and revealed that the presence of TM surface stabilizes the zigzag edge while it reconstructs the armchair edge of the graphene.25 These studies are all focused on the TM terrace. However, the real morphology of TM surface is complicated and crucially influences the graphene growth.26,27 Surface defects such as steps readily form in experiment and may affect the quality of graphene significantly during the CVD growth.29,30 Plenty of studies have been devoted to CVD growth of graphene on metal step surface. They primarily focus on the preferred binding sites of C monomers,31 dimers,32 or chains.33 To date, the study on growth kinetics and microscopic growth mechanism of graphene on metal step surfaces is still rather lacking. Furthermore, many experiments34,35 have demonstrated that carbon adsorption would first induce the reconstruc© 2013 American Chemical Society

tion of metal surface and the occurrence of monatomic steps, thereafter the growth of C nanostructures. This probably suggests that the surface steps play a key role in C nanostructure growth except for acting as nucleation centers.30−32 Additionally, we have recently investigated the growth kinetics of graphene on Ni(111) terrace18 and found that the dissolution of C into the nickel lattice leads to the deformation of the crystalline surface via the migration of Ni atoms out of the surface, which has already been observed in experiment.36 Theoretical investigations37,38 also show that surface metal atoms like Cu and Ni are readily pulled upward from the surface. However, in our previous work18 we found that in some cases the emergence of Ni atoms from the terrace hinders the healing of graphene defects. Such phenomenon is highly undesirable in the CVD growth of graphene. Can the steps heal these defects? The answer to this question would provide valuable insight into the experimental design of CVD growth of graphene. In this work, we investigate the evolution of carbon structures and the kinetics of graphene nucleation on the step surfaces by employing classical molecular dynamics (MD) methods and density functional theory (DFT) calculations. Our calculations show that the evolution mechanism of C structures on the step surface is the same as that on the flat terrace if no substrate Ni atom is pulled out of surface plane by C atoms. Interestingly, the defects involved with the pulled-out Ni atoms are hard to be healed on the terrace, but they can be successfully healed with the assistance of the step atoms on the Received: December 28, 2012 Revised: June 30, 2013 Published: July 3, 2013 15260

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1200, 1400 K) are considered, and the best temperature for graphene growth is about 1000 K (Figure S1 in Supporting Information). In our previous work, the C structure evolution on the (111) terrace at different temperatures has been studied and the optimal temperature for graphene growth is around 1000 K as well.18 Therefore, we focus on the evolution of C structures and the kinetics of graphene nucleation at a temperature of 1000 K in the following discussion. We first look into the C structure evolution on the (100)step surface. The initial and final C structures obtained in 50 ps MD simulation at 1000 K are displayed in Figure 1. Similar to

step surface. Compared with the terrace, the step significantly lowers the healing barrier of the defect involved with the pulled-out Ni atom and therefore results in a very fast healing of the defect.

II. MODELS AND METHODS The C/Ni interactions are described by the ReaxFF potential of Mueller et al.,39 same as in our previous work.18 Although ReaxFF is less accurate than quantum mechanics methods, it provides a fairly reliable description of energies, transition states, reaction paths, and reactive events.39−41 Thus, we expect that the fundamental nature obtained based on ReaxFF is reasonably accurate. In the MD simulation, we integrate the equations of motion using the velocity Verlet42 algorithm with a time step of 0.25 fs and employ a Berendsen43 thermostat to control the system temperature with a damping constant of 100 fs. The DFT calculations are performed using the Vienna ab initio simulation package (VASP).44,45 The ion−electron interactions are treated with the projected augmented wave (PAW) method,46 and the general gradient approximation (GGA) parametrized by Perdew, Burke, and Ernzerhof (PBE)47 is used as the exchange-correlation functional. The climbingimage nudged elastic band (cNEB) method48 incorporated with spin-polarized DFT is employed to locate the minimum energy path and possible transition states. A five-layer slab model of Ni(111) surface is built, of which every layer contains 96 Ni atoms. Periodic boundary conditions are applied along the two horizontal directions, that is, ⟨112⟩ and ⟨110⟩. We adopt a 20 Å thick vacuum along the vertical direction and fix atoms in the bottom layer of the slab to mimic an infinitely large bulk under the surface. Eight atomic rows in the top layer of the slab are removed to form a monatomic step. Two types of steps are formed on the flat (111) surface, namely, ⟨110⟩/(100)- and ⟨110⟩/(111)-microfaceted steps. For simplicity, these two microfaceted steps are referred as (100)- and (111)-steps. Then 48 C atoms are deposited randomly around these two kinds of steps. On the basis of our previous simulation results18 that C monomers readily enter into the subsurface and the C dimers and trimers are hard to diffuse to participate in graphene nucleation, here we just choose the initial C structures that all atoms are bonded together (see Figure 1a and Figure 2a). It is noted that the observed phenomena presented here are based on many MD trajectories.

Figure 1. Initial and final structures obtained in 50 ps MD simulation at 1000 K for (100)-step.

the case of the terrace,18 the C atoms on the (100)-step surface spontaneously form C chains or C rings due to the C−C interaction. During the period of MD annealing, the long C chains interact with the adjacent C chains, resulting in the formation of the six-membered rings (6-MRs). The short C chains with five or six C atoms turn into a 5- or 6-MR directly. The small C rings such as the 3-MRs or 4-MRs are very unstable and soon break into C chains. The large C rings such as the 9-MRs or 10-MRs can easily evolve into the 5-MRs or 6MRs. Ultimately, a graphene island is formed mainly containing 5-, 6-, and 7-MRs and continuously extends over the stepped surface (Figure 1b). The C structure evolution on the (111)step surface at 1000 K is also investigated. As clearly seen from Figure 2, the C structure also evolves into a defective graphene island and the evolution details are very similar to the case of the (100)-step. Therefore, we conclude that the evolution mechanism of the C structures on the step surfaces has not much difference from that on the flat terrace when no substrate Ni atom is pulled out of the surface plane. Nevertheless, in our earlier work18 of graphene on the terrace, we notice that some Ni atoms in the first layer of the substrate were pulled out of the surface plane by C atoms during the evolution of the C structures, which is also observed on the step surfaces. We call the long C chains or rings with pulled-out Ni atoms as a new kind of defect and name them as the D-Ni defects for simplicity. The D-Ni defects on the flat terrace are very rigid and never healed even at temperatures up to 1400 K (Figure S2).18 Very interestingly, these D-Ni defects on the step surfaces can be successfully healed with the assistance of the step Ni atoms. Trajectories show that the step Ni atoms are quite active and helpful to the D-Ni defect

III. RESULTS AND DISCUSSION 1. C Structure Evolution on Different Step Surfaces. In the initial stage of graphene growth, the hydrocarbon molecules decompose at the Ni surface, and as-produced C atoms diffuse into the substrate. After adequate C saturation, graphene nucleation occurs in which C atoms should come from the subsurface and the ongoing dissociated carbon. Evidently, the subsurface C atoms are crucial for graphene nucleation and growth, as clearly revealed by Weatherup et al.49 and Li et al.37 However, in this work we do not consider the precipitation of the subsurface carbons and only concern the surface catalysis process of graphene nucleation. To investigate the effect of substrate morphology on the nucleation of graphene, we place 48 C atoms on the (100)- and (111)-step surfaces, respectively, and monitor their diffusion and nucleation at different temperatures. Four annealing temperatures (T = 800, 1000, 15261

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Figure 4. Time dependence of the z coordination of the lilac Ni atom. Figure 2. Initial and final structures obtained in 50 ps MD simulation at 1000 K for (111)-step. Color definitions are the same as that in Figure 1.

direction after it is pulled out of the surface. This is different from the case on the terrace that the pulled-out Ni atoms move slightly in all directions and nearly have no chance to return to the original positions. At 14.4 ps, the lilac Ni atom is pulled upward from the surface again by 0.90 Å. Subsequently, the lilac Ni atom begins to migrate down, resulting in the approach of two of the three C atoms bonded with the lilac Ni atom (yellow ellipse of Figure 3c). The two approaching C atoms are bonded, and a 5-MR is simultaneously formed after 0.6 ps (yellow ellipse of Figure 3d). The lilac Ni atom continues to diffuse down, leading to the Ni−C bonds breaking, and it is never pulled out of the surface by the C chain atoms afterward. The further healing of the C chain can be achieved readily without the pulled-out Ni atoms. Finally, the C chain in the initial structure (yellow ellipse of Figure 3a) is completely healed at 19.95 ps (yellow ellipse of Figure 3e). We also investigate the healing of the D-Ni defects on the (111)-step at 1000 K. First, the step Ni atom with magenta circle (Figure 5b) is pulled out of surface after 13 ps MD annealing. Then it quickly goes back to the surface layer and is never drawn out. So the 5-MR (yellow ellipse of Figure 5c) is formed with no D-Ni defect. Afterward, the step Ni atom with red circle is gradually pulled out of the surface by the left C atoms of the ring, leading to the formation of a D-Ni defect. At

healing. An explicit discussion about the healing of the D-Ni defects on the step is presented in the next section. 2. Defect Healing with the Assistance of the Step. The graphene has various defects,50 and some of them can be healed via experiment conditions such as using prepatterned growth seed51 or controlling the crystal orientation of the substrate52 during the CVD growth of graphene. Considering that the DNi defects on the terrace are very difficult to be healed by raising the temperature, we investigated the possibility of the healing of the D-Ni defects through changing of the substrate morphology. Figure 3 presents a few snapshots of the trajectories of a D-Ni defect healing with the assistance of (100)-step atoms at 1000 K. It was soon observed that a substrate Ni atom (lilac atom) is pulled out of the surface by three atoms of the C chain (yellow ellipse of Figure 3b). This is evidently manifested by the z coordination variance of the lilac Ni atom within the first 20 ps of the annealing presented in Figure 4; the lilac Ni atom is 0.96 Å higher than the original position at 1.8 ps. The structure in the yellow ellipse of Figure 3b is a typical D-Ni defect aforementioned. Figure 4 shows that the lilac Ni atom is still movable within a large range along the z

Figure 3. Snapshots of trajectories of a D-Ni defect healing with the assistance of (100)-step at 1000 K. Color definitions are the same as that in Figure 1. The lilac sphere represents the pulled-out Ni atom. 15262

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Figure 5. Snapshots of trajectories of a D-Ni defect healing with the assistance of (111)-step at 1000 K. Color definitions are the same as that in Figure 1.

Figure 6. Healing process of C10-1 (a), C10-2 (b), and C10-3 (c). Color definitions are the same as that in Figure 1. The lilac sphere represents the pulled-out Ni atom.

40.4 ps, the red-circled Ni atom stably bonds with three C atoms (Figure 5d), and then it begins to go down. After 0.7 ps, two of the three C atoms bonded with the red-circled Ni atom are bonded and at the same time, the C chain is healed to be a 6-MR and a 7-MR (yellow ellipse of Figure 5e). The large C ring (yellow ellipse of Figure 5a) eventually turns into a graphene island with a 5-MR, a 6-MR, and a 7-MR (yellow ellipse of Figure 5f). In fact, no matter whether the D-Ni defects are on the terrace or the step surfaces, the pulled-out Ni atoms generally bond with three C atoms. On the terrace, the pulled-out Ni atoms bond not only with three C atoms but also with the hexagonal close packed substrate Ni atoms. The strong Ni−C bonds and Ni−Ni bonds make the D-Ni defects very stable and hard to be healed. Hence, the pulled-out Ni atoms hinder the growth of C structures and the D-Ni defects are not healed even up to 1400 K on the terrace (Figure S2).18 With the presence of the step, partial Ni atoms such as the step edge atoms or the nearby atoms become quite active because they have less coordination numbers and more space to diffuse than the other substrate atoms. This can greatly enhance the activity of the pulled-out Ni atoms, allowing them to diffuse down, and thereby the two C atoms bonded with the pulled-out Ni atom have the chance to approach each other. The two approaching C atoms form a

new bond rapidly, accompanying the healing of the D-Ni defects. The pulled-out Ni atom keeps going down and at last breaks the bonds with all the C atoms and returns to the substrate. 3. Healing Barrier on Terrace and Step Surface. In order to supplement the dynamic simulation results, we further calculate the energy barriers for the D-Ni defect healing on the terrace and step surfaces by using the DFT calculations via cNEB method. We take a 10-MR as an example and assume that it is healed to be two 6-MRs. We consider three different positions for the 10-MR on the substrate as shown in Figure 6. C10-1 and C10-2 represent the structures that the 10-MR is on the terrace without and with Ni atom pulled out from the first layer of the substrate surface, respectively, and C10-3 refers to the 10-MR being on the step edge with the pulled-out Ni atom as well. It is noted that C10-2 and C10-3 are typical D-Ni defects aforementioned. For the case of C10-1, the energy barrier is only about 0.02 eV (see Figure 6a), indicating that the defect without the pulledout Ni atom can be healed very easily on the terrace. In contrast to C10-1, the energy barrier for the healing of C10-2 increases to 0.56 eV because of the presence of the pulled-out Ni atom. Although this barrier is not too high to overcome, such healing process is energetically unfavorable; the product is 0.30 eV 15263

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higher in energy than the reactant (Figure 6b). Fortunately, when the healing occurs on the step edge (Figure 6c), the energy barrier reduces to only 0.15 eV with a remarkable energy drop of 0.69 eV even though the pulled-out Ni atom is also present as in the case of the C10-2. This result indicates that the defect with the pulled-out Ni atom can be healed very readily on the step edge. Kinetically, the healing rate of C10-2 (labeled as R2) or C10-3 (labeled as R3) can be considered as a function of the energy barrier, ΔE:

where k is the Boltzmann constant and T is the temperature of graphene growth. Choosing T = 1000 K or kT ≈ 0.08 eV as an example, we have R2/R3 ≈ 1/150. Obviously, the step can make the healing of the D-Ni defect on the step edge surface more easily than that on the terrace. Therefore, we conclude that the presence of the step is actually beneficial to the healing of the D-Ni defects.

IV. CONCLUSIONS In summary, using molecular dynamics and density functional theory approaches, we have studied the evolution of carbon structures and the kinetics of graphene nucleation on the step surfaces. Combined with our previous work of graphene growth on the terrace, we found that the evolution mechanism of the C structures on the step surfaces is the same as that on the flat terrace if no substrate Ni atom is pulled out of the surface plane by C atoms. However, the defects involved with the pulled-out Ni atoms called D-Ni defects are rather difficult to be healed on the terrace, but they can be efficiently healed with the assistance of the step atoms on the step surface. We have also calculated the healing barriers of a D-Ni defect on the terrace and step surface. It is found that the step significantly lowers the energy barrier of the D-Ni defect healing, resulting in a very fast healing of the D-Ni defect. Our present findings of a fundamental nature reveal that it may be a new way to synthesize high-quality graphene by use of a substrate step. ASSOCIATED CONTENT

S Supporting Information *

Figure S1 showing the initial structure and final configurations obtained in a 50 ps MD simulation at four different temperatures (800, 1000, 1200, and 1400 K) on the (100)step surface and Figure S2 showing the initial structure and final configurations obtained in a 100 ps MD simulation at four different temperatures (800, 1000, 1200, and 1400 K) on the flat terrace. This material is available free of charge via the Internet at http://pubs.acs.org.

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R ∝ exp[−ΔE /(kT )]

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-25-52090600-8304. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by NBRP (Grants 2011CB302004 and 2010CB923401), the NSF (Grants 21173040 and 11074035), and Peiyu Foundation of SEU. The authors thank the computational resource at Department of Physics, SEU, and National Supercomputing Center in Tianjin, China. 15264

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dx.doi.org/10.1021/jp312802e | J. Phys. Chem. C 2013, 117, 15260−15265