On the role of surface migration in the growth and structure of graphene layers
Introduction
There are a variety of carbonaceous materials whose growth is envisioned through extension of aromatic edges; familiar examples may include pyrolytic graphite, carbon black, combustion soot, interstellar “dust”, fullerenes, and nanotubes. Experimental studies with laminar premixed flames concluded that surface growth rate of soot particles is proportional to the acetylene concentration in the gas phase [1], [2]. These observations have been interpreted in terms of the hydrogen-abstraction-C2H2-addition (HACA) mechanism, a repetitive reaction sequence of two principal steps: abstraction of a hydrogen atom from the aromatic-edge C–H bond by a gaseous hydrogen atom, followed by addition of a gaseous acetylene molecule to the formed surface radical site [3]. That early analysis [4], leading to identification of the HACA mechanism, was based on the hypothesis of chemical similarity [5], which postulated that chemical reactions taking place on soot particle surface are analogous to those of large polycyclic aromatic hydrocarbons (PAHs). The postulate of chemical similarity provided a natural extension of the gas-phase chemistry of aromatics, enabling a description of surface processes in terms of elementary chemical steps.
The postulate of chemical similarity, however, is only an assumption. In reality, there could be substantial differences between gaseous and surface reactions, even in cases of seemingly analogous molecular interactions. The primary cause of the possible dissimilarity is the difference in steric confinements of reactive sites. In other words, a reaction of a gaseous species with a surface radical may have a “sticking probability” and equilibrium constant varying with the nature of neighboring sites and their occupancy. Furthermore, while the localized steric factors may affect the surface kinetics in it own right, sometimes it leads to substantially different global kinetic patterns, like in the case of surface migration. Extensive theoretical analysis of diamond surface chemistry has led to a conclusion that surface migration of CH2 bridges on diamond (1 0 0)–(2 × 1) surfaces must play the governing role in the growth of diamond films [6]. The newly identified migration of the five-member rings [7] opens a similar possibility to the growth of graphene layers. Here we explore further this phenomenon.
Section snippets
Graphene edge migration
Recent theoretical investigations [7], [8] revealed new reaction pathways for aromatic ring growth, such as enhanced formation of five-member aromatic ringsconversion of five- to six-member ringsand migration of the cyclopenta ring along zigzag aromatic edgesAll of these pathways have one critical mechanistic feature in common: the reaction pathway is induced or assisted by hydrogen atom migration.
The theoretical analysis [7], [8] indicated that these reactions should be sufficiently fast to
Surface model
The initial graphene edge was modeled by a one-dimensional zigzag substrate,The simulations were performed without imposing periodic boundary conditions, thus allowing reaction (2) to occur at the substrate corners.
Gas-phase composition
The gas contacting the substrate was assumed to have the same temperature as the surface and to be composed of H, H2, and C2H2––the principal gaseous species of the HACA mechanism [3]. The gas-phase species concentrations and temperature were maintained unchanged during an individual
Numerical results
A series of Monte Carlo simulations were performed at conditions described in Section 3.2. The simulations produce either smooth, completely filled surfaces or those interrupted with ring-size holes; a typical snapshot of the latter is shown in Fig. 1.
Conclusions
The sterically resolved Monte Carlo simulations provide further support to the critical role of five-ring migration in the growth of graphene layers. An important implication of this phenomenon is that while five-member rings are being constantly formed on the growing edge, they do not accumulate but rather converted to six-member rings.
Acknowledgements
The research was supported by the Director, Office of Science, Office of Basic Energy Sciences, Chemical Sciences Division of the U.S. Department of Energy, under the contract number DE-AC03-76SF00098.
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