On the role of surface migration in the growth and structure of graphene layers

doi:10.1016/j.carbon.2004.01.025

Carbon

Volume 42, Issue 7, 2004, Pages 1209-1212

https://doi.org/10.1016/j.carbon.2004.01.025 Get rights and content

Abstract

The principle of chemical similarity, postulating analogy of chemical reactions taking place on a graphene edge to those of aromatic species, has lead to the formulation of detailed reaction models for the growth of carbonaceous surfaces. While meeting a reasonable success, for instance in the areas of soot and carbon-black formation, such an analogy-based approach cannot be expected to provide a truly realistic description of surface processes. The primary cause of the possible dissimilarity is the difference in steric confinements of reactive sites. A recent quantum chemical study of possible reaction steps led to a proposed phenomenon of surface migration of five-member rings along an edge of a graphene layer. The migrating ring can either desorb or convert to a six-member ring upon an encounter with another six-member ring or a reaction with a gaseous species. All these processes––migration, desorption, and transformation reactions––are mediated by reactions with gaseous hydrogen atoms. The subject of the present communication is analysis of this coupled phenomenon. We will report the results of new Monte Carlo simulations that examine the interplay among surface migration, gas–surface reactions, desorption, and surface transformation reactions of the five-member rings, focusing on emergent morphologies 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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On the role of surface migration in the growth and structure of graphene layers

Abstract

Introduction

Section snippets

Graphene edge migration

Surface model

Gas-phase composition

Numerical results

Conclusions

Acknowledgements

Combust. Flame

Proc. Combust. Inst.

Proc. Combust. Inst.

Proc. Combust. Inst.

Chemical kinetics of soot particle growth

Annu. Rev. Phys. Chem.