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Received Oct 18, 2017; Revised Jan 26, 2018; Accepted Jan 30, 2018
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1. Introduction
Graphene is a two-dimensional material with sp2 hybridization and carbon atoms in a honeycomb arrangement [1, 2]. Nevertheless, it was not until 2004, after its isolation by mechanical exfoliation, that electric properties of a monolayer were published [3], followed by many other studies that showed extremely interesting properties, such as electronic mobility in room temperature higher than 2.105 cm2 V−1S−1, Young’s Modulus of 1 TPa, thermal conductivity above 3000 WmK−1, 2.3% of optical absorption, and the capacity to be functionalized by a wide range of organic groups [4–6]. Hence, graphene is a material with a broad range of applications from mechanical, electrical, and optical applications to medical ones [5, 7]. Several methods have been used to prepare graphene, including graphite micromechanical exfoliation, epitaxial growth over SiC, graphite oxide reduction, and CVD [8] with the unique purpose to obtain large areas and more production [9, 10].
Therefore considering its high productivity, CVD is one of the most promising processes to grow high quality graphene in large areas [11–13]. Accordingly, in recent years a considerable effort has been done to optimize the process, improve the kind of substrate, and control graphene growth [12, 14].
In initial experiments to form graphene, CVD systems with methane as the carbon precursor and hydrogen as a balancing gas were used and are still commonly used today. Thereafter, atmospheric pressure CVD (AP-CVD) or other carbon feedstocks have been explored (other gaseous hydrocarbons or carbon containing liquid or solid precursors) [15, 16]. Hydrogen is almost always used to achieve a better control in the synthesis of graphene on Cu. In industry, due to safety and cost issues, the use of pure hydrogen may not be preferred. Although processes using diluted hydrogen and methane have been developed, it should be noted that, in practice, these gases invariably contain trace amounts of oxidative impurities, which may have a significant effect on graphene growth [17].
At the same time, a deeper understanding has been acquired for each...