JOURNAL OF APPLIED PHYSICS 115, 024311 (2014)
Molecular dynamics simulation of graphene growth on Ni(100) facet by chemical vapor deposition R. Rasuli,a) Kh. Mostafavi, and J. Davoodi Department of Physics, Faculty of Science, University of Zanjan, Zanjan, Iran
(Received 4 October 2013; accepted 31 December 2013; published online 14 January 2014) We present a molecular dynamics simulation of chemical vapor deposition of graphene. Single layer graphene growth on a Ni (100) facet was studied at different substrate temperatures, C flow rates, and C flow energies. Results show that a single layer graphene film grows through a combined deposition mechanism on a Ni substrate, rather than by surface segregation. These simulations suggest that high quality graphene deposition is theoretically possible on Ni (100) facet C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4862164] under high flux energy. V
I. INTRODUCTION
Unique properties of graphene introduce it as a most promising candidate for applications in nanotechnology.1,2 It has potential application in nano-electronics and nanodevices.3–5 To realize these potential applications, one needs to produce high-quality graphene in large quantity. So far, several methods have been reported to synthesize graphene. Mechanical exfoliation,6 chemical methods,7 and transition metal-catalyzed chemical vapor deposition (CVD)8,9 are commonly use in graphene production. Presently, CVD is the most promising and low-cost approach to synthesize graphene in large-area as well as high-quality.5 Understanding of CVD growth mechanism at the atomic level is useful for controlling and improving quality of produced graphene. However, experimental techniques are presently unable to give details of graphene growth at the atomic scale. Graphene growth in the large-scale has been subject of theoretical researches to understand the details of nucleation and growth of graphene. Loginova et al. have reported that adhesion of C clusters to the graphene edge is main process in CVD growth of graphene.10 Chen et al. have predicted that nucleation of graphene is dependent on the type of transition metal substrate.11 Gao et al. have studied graphene nucleation on terrace of metal substrate and near the step edge.8 Cheng et al. have reported a homogeneous nucleation process for graphene growth.12 Amara et al.13 revealed that defect healing occurs when metal substrate is applied for graphene growth.14 Theses studies have explored graphene growth in the static condition. According to the fact that graphene growth is a kinetic and nonequilibrium process, it is instructive to study the process dynamically on the Ni (100) facet. Molecular dynamics (MD) is a powerful method to study dynamic process at the atomic scale. So far, there is some theoretical work focused on the dynamics of graphene growth on the Ni (111) facet.15 Chae et al. reported that after graphene synthesis on poly-crystalline Ni substrates, the most abundant (110) direction, as well as (100) and (111) a)
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directions, is transformed into (100) direction.9 However, these studies focus on the thermodynamic stability of structures on the (111) facet and have not presented dynamics of the growth process on Ni (100) facet. In this study, we investigate the effect of C flow energy, substrate temperature, and C flow rate as three important parameters in CVD graphene growth. According to the transformation of (110) and (111) directions to (100) direction [9], we have selected Ni (100) facet to study the graphene growth instead of the more stable Ni (111) facet. Results show that high C mobility significantly improves graphene quality. In addition, formation of a single layer graphene film occurs through a combined deposition mechanism on a Ni substrate, rather than by surface segregation. II. METHODS
MD simulations were performed using LAMMPS code by the Verlet algorithm16 with a time step of 0.20 fs. Temperature of system was controlled by a Berendsen thermostat with a damping constant of 100 fs.17 A four-layer slab model of Ni (100) surface including 256 Ni atoms was ˚ applied to represent the catalyst surface. We take on a 20 A thick vacuum along the z-axis and periodic boundary conditions along the other directions. To mimic the semi-infinite surface, we fix atoms in the bottom layer of the slab. C atoms were randomly deposited on the Ni (100) surface at different substrate temperatures, C flow rates, and C flow energies. We have applied Airbao, Embedded atom model, and LenardJones potentials between C-C, Ni-Ni, and Ni-C atoms, respectively. Parameter values of Lenard-Jones potential were set to ˚ for epsilon and sigma, respectively.18 0.0486 eV and 3.0665 A III. RESULTS AND DISCUSSION
In CVD method, metal catalyst is placed in a mixed flow of hydrocarbons usually methane and other gases, e.g., hydrogen. In this method, graphene growth occurs by C atom addition on the previous structure. During graphene growth, C precursor on the catalyst surface dissociates to C atom and its concentration is raised gradually. To explore the effect of substrate temperature on the nucleation of graphene, we considered the concentrations with 72 and 144 C
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FIG. 1. Final configurations of the graphene layers grown on a Ni (100) surface for the various substrate temperatures with concentrations of 72 (a) and 144 C atoms (b).
FIG. 2. (a) Dissolved C atoms (red sphere) into Ni substrate, (b) equilibrium configuration at 250 K from both top and side view, (c) surface segregation of dissolved C in the Ni (100) surface at 1600 K from both top and side view.
atoms. To maintain the crystalline state of substrate during the graphene growth sufficiently, low temperatures are essential. The initial and final structures simulated in 100 ps at 400–1400 K are presented in Figure 1. Results show that increase in substrate temperature cause to formation of some C rings on the substrate due to increase in C atoms diffusion on the catalyst surface. At the temperature of 1200 K, for both concentration 72 and 144 C atoms, some hexagonal is formed. In the case of 144 C atoms, graphene nucleation is improved while the obtained graphene has defect and dislocation. This is in agreement with the previous reports.19 In a typical CVD graphene growth on Ni substrate, C monomers readily dissolve into the subsurface. In this part, we present simulation results of C surface segregation as a final step in CVD growth of graphene. The barrier for an adatom to diffuse from the surface to the subsurface is 0.6 eV,12 which is small enough to be overcome easily at 1000 K. C dimmers and trimmers are more stable on the surface at 1000 K than subsurface. According to snapshots in
FIG. 3. Flow rate (R) effect versus various temperatures on the number of hexagons at temperature ranged 500–6500 K.
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Figure 2, the surface Ni atoms between two C dimers or trimers lift out of the substrate due to strong C Ni interaction. This was observed in previous ab initio calculation that justifies validity of the employed potential in this work.20 In CVD growth, flow rate of precursor also affect the obtained graphene quality. The first step in the CVD growth of graphene is the decomposition of gas-phase C precursors on the catalyst surface, which can be controlled by the thermochemistry. Here, we name the rate of C production as C flow rate. The C ions diffuse on the catalyst surface or across its interior. Afterward, nucleation of graphene is followed by the incorporation of C into the growing process. At the initial stage of graphene growth, a dimmer acts as a nucleation center and grows large by adding C atoms. C
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flow rate is corresponded to the required time for diffusion and incorporation in graphene growth process. Figure 3 shows the number of 6-membered rings at various flow rate and temperatures. The process have been simulated in the flow rate range 0.05–0.5 ps 1 and temperature range 1000–6500 K. Results show that at the flow rate of 0.1 ps 1 and at the flow temperature range 4000–5000 K, the number of 6-membered rings is maximized. This means that under such condition, C adatom have enough energy and time to diffuse on the surface and find a stable location, which leads to formation of 6-membered C rings. Figure 4 shows a snapshot of final structure of obtained graphene under different C flux energy. It is apparent that the energy of C flux has a main role in high quality graphene production.
FIG. 4. Snapshot of final structure of obtained graphene under different C flux energy. Substrate temperature and flow rate are 1200 K and 0.06 ps 1, respectively.
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FIG. 5. Snapshots of graphene growth on the Ni (100) surface after 25, 50, 75, 100, 125, 150 C atom impacts on the surface.
Surface diffusion on the substrate is dependent on the C ion energy. The C ion energy in CVD experiment is corresponded to the C flux energy in MD simulation. A C atom deposited on the catalyst surface with specific energy. After collision to the substrate surface, it diffuses on the substrate until a stable location is found. To study of graphene growth in detail, we take typical snapshots of formed hexagons after 25, 50, 75, 100, 125, 150 C atom impact on the surface (Figure 5). As shown in Figure 5, graphene grows with the appearance of C chain and then convert to hexagons by addition of more C atoms. A deposited C atom on the C chain form two bonds with two other atoms; simultaneously, the bond between these atoms is broken, which finally forms a 6-membered ring. IV. CONCLUSIONS
Single layer graphene growth on a Ni (100) surface was studied at different substrate temperatures, C flow rate, and C flow temperature. Results show the formation of a single layer graphene film occurs through a combined deposition mechanism on a Ni substrate, rather than by surface segregation. These simulations suggest that high quality single layer graphene deposition is theoretically possible on Ni (100) under high flux energy. 1
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