Molecular Dynamics Simulation of Chemical Vapor Deposition

Article pubs.acs.org/JPCC

Molecular Dynamics Simulation of Chemical Vapor Deposition Graphene Growth on Ni (111) Surface Lijuan Meng,† Qing Sun,† Jinlan Wang,*,† and Feng Ding*,‡ †

Department of Physics & School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China Institute of Textiles and Clothing, Hong Kong Polytechnic University, Kowloon, Hong Kong, China

‡

S Supporting Information *

ABSTRACT: Grasping the fundamentals of graphene growth is vital for graphene synthesis. By employing classical molecular dynamics with the ReaxFF potential, we have investigated the evolution of carbon structures and the growth kinetics of graphene on Ni(111) surface at different temperatures. Our results showed that low C concentration leads to the dissolution of C atoms into Ni only, whereas high C concentration leads to the formation of graphene island. By efficient defect annealing at the optimal temperature of ∼1000 K, the quality of graphene island can be significantly improved. Furthermore, a graphene island can grow larger by capturing the deposited C atoms and form more hexagons on the edge with its self-healing capability during the growth. These underlying observations and understandings are instructive for the control of the CVD growth of graphene. favorable than those with only hexagons.36 Cheng et al. have studied the structure, energetics, and mobility of C aggregates up to 10 atoms on the Ni (111) surface and proposed a homogeneous nucleation mechanism.37 Using tight-binding Monte Carlo simulation, Amara et al. have examined the nucleation of graphene on a Ni(111) surface38 and revealed that the substrate metal is beneficial to the defect healing during graphene growth.39 These theoretical studies have mainly employed static models. Nevertheless, the graphene growth is typical a kinetic and nonequilibrium process. Molecular dynamics (MD) is a powerful tool to explore such process at the atomic level. Up to date, there is only very limited theoretical work concentrating on the dynamics of graphene growth.40 In CVD experiments, the evolution of graphene structures is driven by C atom addition. What are the preferred configurations of C aggregates on TM surface at different C concentrations and temperatures? How do the C atoms nucleate on catalysts? Can defects formed during C addition be healed efficiently? In this work, we studied the impact of C concentration and temperature on graphene growth. Our results showed that (i) high C concentration may induce the formation of graphene island directly, (ii) the quality of graphene island can be significantly improved by efficient defect healing at 1000 K, and (iii) a graphene island can grow larger by capturing the deposited C atoms around to form more hexagons.

I. INTRODUCTION Graphene has surged a new knowledge of science and technology owing to its excellent mechanical,1−3 thermal,4−6 optical,7−9 and electronic properties.10−13 These intriguing properties make graphene the most promising candidate for various applications in many fields,14−18 such as material science and microelectronics. To realize these potential applications, the synthesis of high-quality graphene in large quantity is a prerequisite. Experimentally, there are many methods of synthesizing graphene, including mechanical peeling,19,20 high-temperature sublimation of SiC,21−23 chemically functionalized graphene reduction,24,25 and transition metal (TM)-catalyzed chemical vapor deposition (CVD).26−30 Presently, CVD is considered the most promising, low-cost approach to synthesize high-quality large-area graphene.31,32 Controlling the growth of graphene on TM surface requires a primary understanding of its mechanism at the atomic level. Current experimental techniques are unable to provide the atomic details of graphene growth. To achieve the high quality and large scale graphene, extensive theoretical efforts have been made to understand the details of graphene nucleation and growth. Loginova et al. have found that graphene grows by adding C clusters to the edge rather than the insertion of C monomers.33 Chen et al. have studied the formation of C dimers on different TM terraces or near a step edge and predicted that the nucleation behavior of graphene depends on the type of the metal substrate.34 Gao et al. have systematically explored graphene nucleation on TM terrace and near the step edge.35 They further investigated C clusters supported on Ni (111) surfaces and found that graphene islands with one to three 5-membered rings (MRs) are energetically more © 2012 American Chemical Society

Received: December 16, 2011 Revised: February 10, 2012 Published: February 15, 2012 6097

dx.doi.org/10.1021/jp212149c | J. Phys. Chem. C 2012, 116, 6097−6102

The Journal of Physical Chemistry C

Article

Figure 1. Initial and final structures obtained in 100 ps MD simulations at 1000 K for C16 (a, b), C32 (c, d), and C64 (e, f). Cyan and orange spheres represent Ni and C atoms, respectively.

II. MODELS AND METHODS Our MD simulations were based on the reactive force-field (ReaxFF)41 potential energy surface, which parameters are recently developed for the C/Ni system.42 It was found that energies, transition states, reaction paths and reactive events calculated by ReaxFF are consistent with those obtained by quantum mechanical calculations.42−44 Certainly, despite the ReaxFF was validated reasonable, it is less accurate than the quantum methods. In our MD simulation, the velocity Verlet algorithm45 with a time step of 0.25 fs was used. The relatively small time step ensures the energy conservation at the temperatures of T ∼ 1000 K. The system temperature was maintained by a Berendsen thermostat46 with a damping constant of 100 fs. A four-layer slab model of Ni (111) surface, which contains 256 Ni atoms in total or 64 per layer, was built to represent the catalyst surface. The axis x is along the [110] direction, and axis y is along the [1̅12] direction. We adopted a 20 Å thick vacuum along the z axis and applied periodic boundary conditions along the other directions. Atoms in the bottom layer of the slab were fixed to mimic the semi-infinite large surface. 16, 32, and 64 C atoms (i.e., 1/8, 1/4, and 1/2 of pristine monolayer graphene laying over the metal surface) were randomly deposited on the Ni (111) surface, respectively. Four different annealing temperatures, T = 800, 1000, 1200, and 1400 K, were considered in the simulations. Each combination of C concentration and temperature is named as Cn@T, where n denotes the number of C atoms and T is the simulation temperature in K.

and 1/2 monolayer. The initial and final structures obtained in 100 ps MD trajectories at 1000 K are presented in Figure 1. In the case of 16 C atoms, C monomers are readily to enter into the subsurface. This is due to the low barrier from surface adatom to subsurface one, only 0.6 eV.37 Such a small barrier can be overcome easily at 1000 K. As C monomer preferentially adsorbs on the subsurface, it is difficult to observe the formation of dimer on the Ni (111) surface. In contrast, C dimers and trimers on the surface are energetically more stable at 1000 K. Trajectories show that the surface Ni atoms between two C dimers or trimers will first be lift out of the substrate by the strong C−Ni interaction, forming stable bridging-metal structures. Similar results were observed in previous DFT calculation,47 which provides a strong support of the validity of the force field employed in this work. Thus, the diffusion barrier of these dimers or trimers must be quite large owing to the strong C−Ni interaction. This dramatically hampers the diffusion and coalescence of C atoms on Ni (111) surface. As the annealing proceeds, more C atoms penetrate gradually into the subsurface and some Ni atoms are kicked out of the surface to form adatom defects. As a result, the substrate surface becomes very disordered. For the case of 32 C atoms, the initial structure was built with a little coincidence that all atoms are bonded together (Figure 1c). Because of the increase of the C concentration, the C atoms tend to form long C chains or large polygonal rings in which many C atoms are sp1-hybridized. A long chain (red ellipse of Figure 1c) has two “arms”, and the two “arms” may easily form a C−C bond between them, which eventually leads to the formation of a 6-MR. Similarly, the polygonal ring containing 11 C atoms (black ellipse of Figure 1c) ultimately turns into a 6-MR and a 7-MR. In addition, short chains with five or six C atoms can evolve directly into a 5-MR or a 6-MR (yellow ellipse of Figure 1d). After a 100 ps annealing, nearly all these chains and rings eventually form a 2D sp2 network of pentagons, hexagons, and heptagons (Figure 1d). It can be seen that the dome-like graphene island is very defective, which

III. RESULTS AND DISCUSSION 1. C Structure Evolution at Different Concentrations. In a CVD experiment of graphene growth, the C concentration is raised gradually with the dissociation of the C precursor on the TM surface. To investigate the effect of C concentration on the nucleation of graphene, we considered three different C concentrations with 16, 32, and 64 C atoms, namely 1/8, 1/4, 6098



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Figure 2. Equilibrium configurations at (a) 0, (b) 800, (c) 1000, (d) 1200, and (f) 1400 K. Color captions are the same as that in Figure 1.

contains a number of 5- and 7-MRs, in good agreement with the results reported in the literature.36,40 As the C concentration further increases, taking 64 C atoms as an example, the initial C structure contains a monomer, a dimer, a trimer, many chains, and rings. Similar to the case of C16, the monomer preferentially occupies the subsurface site after a short period of annealing. Both the C dimer and trimer favor to locate on the surface, but they do not coalesce with the large framework because of the lack of diffusion. For chains or rings, they are apt to interact with each other to form 5-, 6-, or 7-MR, as the case of C32. It is observed that majority of 5-, 6-, or 7-MRs forms within the first 10 ps in the MD simulation. On the other hand, once a 2D sp2 C network forms, it is very stable because of the enhanced C−C interaction.38 From above description, we conclude that low C concentration does not allow the formation of large sp2 network, and high concentration is required to induce the formation of graphene island. Now, let us turn to the C structure evolution at different temperatures. The initial and final structures obtained by a 100 ps MD simulation at 1200 and 1400 K are depicted in Figures S1 and S2 in the Supporting Information, respectively. It is noticeable that the quality of final C structures highly depends on the temperature. Taking C32 as an example, at 1000 K, the final C structure contains four 5-MRs, three 6-MRs, and two 7MRs, whereas the numbers are 3, 2, and 2, respectively, at 1200 K. An explicit discussion about the effect of temperature on the evolution of C structures is shown in the next section. 2. Defect Healing at Different Temperatures. Experimentally, the quality of graphene strongly depends on the growth temperature. Here, taking the high C concentration with 64 C atoms as an example, we studied the effect of temperature variations on the defect healing in graphene growth. The starting structure and the final configurations after 100 ps MD trajectories at four different temperatures (800, 1000, 1200, and 1400 K) are presented in Figure 2 (top and side views). Table 1 summarizes the numbers of i-MRs (i = 3, 4, ..., 9) in the C structures formed at different temperatures. As clearly shown in Table 1, the numbers of 3- and 4-MRs are greatly reduced at high temperature, indicating that they are very unstable. While 5-MRs are very stable and the only 5-MR in the initial structure has never been healed at the temperature up to 1400 K. Furthermore, due to the thermal activation, some new pentagons are formed via the interaction of the C chains; the number of the 5-MRs increases at high temperature. This is

Table 1. Numbers of Various Polygons in the Initial and Final Configurations Annealed at Various Temperatures T (K)

3-MR

4- MR

5- MR

6- MR

7- MR

8- MR

9- MR

0 800 1000 1200 1400

6 1 1 1 0

3 0 1 0 0

1 2 2 2 4

2 3 8 5 3

1 2 1 1 1

0 1 1 0 1

1 1 0 0 1

in agreement with the previous result revealed by ab initio calculations that a graphene island should be stabilized by the incorporation of a few pentagons.35,36,48 Similar to the 5-MR, the 7-MR in the initial structure is also very stable and not be healed. The 9-MR remains present at 800 K but is transformed into a 5-MR and a 6-MR at 1000 and 1200 K. Different from the results obtained by Monte Carlo simulation39 that the quality of graphene became better at higher temperature, the number of 6-MR reaches the maximum at 1000 K and decreases as the temperature further increases in our MD simulations. At the temperature higher than 1000 K, the surface Ni atoms between C chains become so active that some diffuse out of the surface and the substrate becomes very disordered, which heavily hinders the defect healing of the C structures. A trajectory of 64 C atoms at 1000 K is shown in Movie S1 of the Supporting Information. It clearly shows that two 3-MRs and one 4-MR form a new 6-MR. A 10-MR turns into two 6-MRs. Two adjacent chains move closely and form a 5-MR and a 6MR. Therefore, we can conclude that the quality of graphene growth depends on the temperature and the optimal growth temperature of graphene should be around 1000 K, as most CVD experiments of graphene growth were applied.49,50 3. Growth of Graphene Island by Adding C Atoms. With the deposition of C atoms, a graphene island acts as a nucleation center and will grow large by the continuous addition of C atoms. Here we take the annealed configuration of 32 C atoms at 1000 K (Figure 1d) as an initial seed and randomly place 8 C atoms near it. Figure 3 presents a few snapshots of the trajectories of the island growth by adsorbing those added C atoms. Clearly, the graphene island grows larger with the appearance of new polygons, most of which are hexagons (Figure 3f). Here, we focus on the formation of new hexagons. When a deposited C atom (atom 1) is captured by a 5-MR of the island edge, atom 1 tends to form two bonds with atoms 2 and 3 and, simultaneously, the bond between atoms 2 and 3 is broken, which leads to a 6-MR formation (red ellipse 6099



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Figure 3. Snapshots of trajectories of an island growing larger by adding C monomers and a dimer at 1000 K. Color captions are the same as those in Figure 1. Green spheres represent newly added carbon atoms.

Figure 4. Snapshots of trajectories of an island growing larger by adding C monomers and a dimer at 1200 K. Color captions are the same as in Figure 3.

adjacent 7-MR is opened (red ellipse of Figure 4d). This is expected and probably provides a pathway to the healing of heptagon defect via the continuous addition of C atoms. The newly added atom 11 moves slowly toward atom 3, forming a new 6-MR (black ellipse of Figure 4f). This is significantly different from the case of 1000 K, where the atom 11 interacts with the island edged atoms and forms a 4-MR (purple ellipse of Figure 3d). Furthermore, a new 5-MR is formed and ascribed to the presence of atom 9 at 1200 K (yellow ellipse of Figure 4f), which is readily transformed into a 6-MR in the subsequent growth process, which is similar to the formation of the 6-MR involved atom 1 and is not shown here. For the case of T = 1400 K, it is also evident from Figure 5 that atoms 1 and 4 are incorporated into the formation of two 6-MRs at ca. 0.09 ps. Different from the cases at 1000 and 1200 K, however, atom 9 bonded with atom 5 diffuses closely to atom 4 to form a triangle at 0.32 ps. The triangle is unstable

of Figure 3c). Atom 4 falls in the vicinity of a polygonal C chain and is captured by atoms 5 and 6, resulting in the formation of another 6-MR (black ellipse of Figure 3c). A dimer that is composed of atoms 7 and 8 moves gradually and ultimately incorporates into a new 6-MR with the edge atoms of the island (yellow ellipse of Figure 3e). As discussed above, the 5-MR is very stable and difficult to be healed. However, it can be easily healed to be a 6-MR once it captures another C atom. It is noted that, as the newly added C atoms mainly interact with the edge atoms of an island, the addition of C atoms only affects the edge structure of the island. The growth of graphene island by adding C monomers and a dimer at 1200 and 1400 K are further explored. The snapshots of trajectories are shown in Figures 4 and 5, respectively. From Figure 4, one can see that atoms 1 and 4 contribute to the formation of two new 6-MRs, the same as that at 1000 K. At 1200 K, the vibration motion of the dimer is promoted and the 6100



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Figure 5. Snapshots of trajectories of an island growing larger by adding C monomers and a dimer at 1400 K. Color captions are the same as in Figure 3.

(111) surface should be around 1000 K. Furthermore, we have also explored the growth of graphene island by adding C atoms around. By capturing the deposited C atoms, graphene island grows larger and larger and defect formed by the addition of C atoms can be efficiently healed. The deeply insightful understanding on the kinetic process of graphene growth is expected to guide the synthesis of high-quality graphene in CVD experiments.

and the bond between atom 4 and 5 is broken after 0.19 ps, thereby leading to the conversion of a 6-MR into a 7-MR (red ellipse of Figure 5e). At 1400 K, the incoming atom 11 breaks a 7-MR. Contrary to our expectation, the two moving chains eventually form a new 7-MR by the reformation of C−C bonds (black ellipse of Figure 5d). Owing to the relatively high temperature (T = 1400 K), the activity of Ni atoms is enhanced. The two Ni atoms nearby the dimer (yellow ellipse of Figure 5f) move out of the surface and become adatoms, which actually stonewalls the motion of the dimer. Thus, the dimer neither joins in the formation of a 6-MR nor breaks the connected 7-MR. On the basis of the discussion above, we can conclude that the graphene island growth possesses the capacity of defect healing. The graphene island is a sp2 C structure with substantive defects which are uniformly called nonhexagonal rings here. We find among these defects 5-MRs or 7-MRs are hard to be healed by raising temperature. Surprisingly, the defect healing of graphene island can be achieved during the growth by the continuous breaking and reformation of C−C bonds. The efficiency of defect healing of the growing graphene island depends on the temperature. The medium high temperature (1200 K) is most optimal for the rearrangement of the growing sp2 C network, leading to more hexagons formed by defect healing.

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ASSOCIATED CONTENT

S Supporting Information *

Initial and final structures obtained following 100 ps MD simulation at 1200 and 1400 K for C16, C32, and C64 (Figures S1 and S2); the defect healing process of C64 at 1000 K (Movie S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.W.); [email protected] (F.D.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by NBRP (2011CB302004 and 2010CB923401), the NSF (21173040 and 11074035), SRFDP (20090092110025), and Peiyu Foundation of SEU. The authors thank the computational resource at Department of Physics, SEU, and National Supercomputing Center in Tianjin.

IV. CONCLUSIONS In summary, we have investigated the influence of C concentration and temperature on the kinetics of graphene growth on Ni (111) surface using classical molecular dynamics approach. Lower C concentration leads to the dissolution of C atoms into Ni and part of surface Ni atoms are kicked out of the surface. Consequently, the substrate surface becomes disordered, and thus it is unfavorable for the formation of sp2 graphene island. In the opposite, high C concentration induces the formation of graphene island whose C atoms mostly are three coordinated. The study on the defect healing of C structures at four temperatures (800, 1000, 1200, and 1400 K) shows that the quality of C structures at 1000 K is the highest, and thus the optimal growth temperature of graphene on Ni

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Molecular Dynamics Simulation of Chemical Vapor Deposition

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