**Growth Study and Characterization of Single-Layer Graphene Structures Deposited on Copper Substrate by Chemical Vapour Deposition**

Stefanos Chaitoglou, Enric Bertran and

Jose Luis Andujar

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Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/67439

#### **Abstract**

Currently, the graphene industry is moving forward to the import of graphene in a number of novel applications. To take full advantage of the excellent properties of the material, the standardization of the growth process is an emergency. The suitable growth technique should ensure the high yield, accompanied by high quality of single-layer graphene sheets. Chemical vapour deposition is the technology that fulfils the above requirements, promoting the growth of largescale graphene films through automatized processes. In the present chapter, we present the latest advances in this field, summarizing the most recent publication activity of the authors. The results outline how the control in the growth process over parameters like the gases flow, growth temperature and pressure can affect the nucleation density of graphene domains, the growth rate and percent coverage. Growth of graphene domains with different morphologies depends on the crystallographic orientation of the copper lattice. At the same time, the formation of ripples occurs in the graphene surface as a result of the copper foil compression during the cooling step. These ripples are responsible for the appearance of a compressive stress in the graphene sheets. We demonstrate the control over such stress through the variation in the hydrogen flow during the growth.

**Keywords:** graphene films, chemical vapour deposition, ripples formation, hydrogen effect

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### **1. Introduction**

Bottom-up synthesis methods like chemical vapour deposition (CVD) are those that better ensure the growth of continuous graphene films [1]. Evidence of the potential to move to industrial scale synthesis has been proven [2, 3]. When the various graphene nuclei reach the coalescence phase, they join to each other, forming a continuous layer. In this growth model, the grain boundaries are proven defects points affecting the quality of the graphene layer, both concerning mechanical and electrical properties. Thus, it is an emergency to proceed with the growth of wafer-scale single crystal graphene domains [4–10]. Efficient growth approaches should ensure a low nucleation density of graphene domains, followed by high growth rates. Parameters that affect the synthesis process are the gas mixture and growth pressure, temperature and time [11]. Copper foil is one of the preferred substrates for the CVD growth as it permits the production of large-scale, single-layer graphene domains [12]. Carbon atoms show a low solubility into the copper. More carbon species remain in the surface of the foil participating in the formation of the graphene sheet [1]. A conventional CVD growth method consists of a continuous flow of carbon gas precursor/H<sup>2</sup> /Ar mixture in order to generate the graphene growth [13]. In all experiments described in this chapter, methane is used as carbon precursor. As described in the work of Li et al. [14], the steps for the graphene CVD growth are the following:


The hydrogen presence has proved to play a critical role in the process, as it affects different mechanisms. It reduces the native copper oxide layer and activates the graphene growth. At the same time, it can apply an etching on the material. Thus, it is fundamental to fully understand the above mechanisms. It has been investigated before how hydrogen can affect thickness, shape, size, edge configuration and crystalline quality of graphene islands/domains as well the control of the nucleation density [15–21], which is very important, taking advantage of the low solubility of carbon into copper [21–27]. Polishing of the copper surface or even enclosure of the substrate in special 'pockets', enables the control of the gases mixture, has proven to help in the control of the nuclei density [12]. The use of high growth temperatures favours the copper recrystallization, permitting the growth of high-quality graphene [28]. Searching for means to further reduce the nucleation, it has been proposed the oxidation of the copper surface [29, 30] resulting in very low domain density with size up to 1 cm2 [31]. In order to reduce the copper surface, thermal annealing processes are usually applied, which require long periods of baking [32].

In this chapter, we present new experiments and results evidencing the effect of the partial pressure of H<sup>2</sup> at a given CH<sup>4</sup> /H<sup>2</sup> gas flow ratio on the growth of bi-dimensional crystals of graphene and on their morphology.

To provide a better understanding of the physicochemical mechanisms that govern film formation, we include them in the framework of atomic level phenomena such as adsorption, diffusion and nucleation, providing the possibility to have a full control on the growth process. We study the CVD growth of graphene in the range between 970 and 1070°C, examining the temperature effect in the graphene growth ratio and nucleation density. By fitting the graphene coverage ratio as a function of growth time, we estimate an activation energy of 3.01 eV.

Finally, we study the formation of ripples in the surface of the graphene sheet. Ripples in the graphene surface are proven to affect its electronic structure [33], carrier transport[34] and chemical properties.[35] Therefore, the capability to control the formation of this kind of corrugation is urgent.

In other recent works, Park et al. have demonstrated the capability to control the density and height of such ripples through control of the cooling ratio, which follows the growth step [36, 37]. Slow cooling permits the formation of molecular hydrogen, which can result in the suspension of the graphene film. Moreover, they have demonstrated the importance of such ripple morphology in the mobility properties of the graphene. Graphene with fewer and smaller ripples demonstrates better transport properties when it is embodied in electronic devices.
