**4. Large-area single-crystal graphene**

Using a CVD system, large-area SLG can be synthesized with high yields; however, its crystallinity can represent an important disadvantage for its application. The polycrystalline nature of the Cu substrate and the presence of impurities within its surface lead to a high density of nucleation sites and adlayers, resulting in a polycrystalline material that will present diminished mechanical and electrical properties than the ideal product.

For this reason, in recent years, different alternatives have been proposed to solve this important factor. Li *et al.* and Yang *et al.* presented a methodology to reduce the nucleation sites during the growth, based on the use of Cu foil enclosures (folded as little pockets) that have the purpose to reduce the partial pressure of the precursor and undesired species inside the cavity of the enclosure, such as SiO2 particles proceeding from the decomposition of the quartz reaction tube, commonly found as contaminants in the graphene surface [48, 49], achieving single crystals in the size range of several hundreds of microns. On the other hand, other research groups have used other alternatives, such as Weatherup *et al.* and Huang *et al*. employed metal catalyst alloys (e.g., Au, Cu/Ni(III)) to grow high-quality single-crystal graphene, also by reducing the nucleation density [50, 51]. In addition, the action of oxygen (O2) as a reducer of nucleation has been also studied. Hao and coworkers found in 2013 that O2 can act as a passivation agent for the nucleation sites, reducing in consequence the density of the adlayers produced. They also concluded that O2 can be provided by an external source or even by segregation within the Cu substrate, such as a Cu oxide [52]. Now, millimeter-sized single crystals have been achieved [53], this holds promise for the scalable and affordable production of graphene for electronic devices.
