**2. Graphene**

 Remarkable technical developments, based on carbon's ability to bind to itself in a variety of hybridized states, have seen the emergence of synthetic carbons with exotic and exceptional properties, e.g., fullerenes, nanotubes, graphene, graphyne, nanorings, etc. Graphene, the ultimate thin material, is composed of a single sheet of graphite, and is currently one of the most investigated 2D carbon allotrope [15]. Its existence was considered to be purely theoretical for a long time. Single graphene layers were prepared in 2004 by a simple mechanical exfoliation of graphite using a Scotch tape. Graphene is finding extensive applications in nanotechnology, optoelectronics, water desalination, dispersion in polymers, etc. Several of these applications, such as capacitive energy storage and heat transfer coefficient, depend strongly on graphene's wettability and its surface-based interactions with various liquids [16].

 With all its carbon atoms exposed to high levels of surface activity, graphene is a material of choice for interfacial carbon materials, especially for carbon-aqueous liquid interactions. Poor wettability of graphene with water and its hydrophobic nature can limit device contamination during fabrication; however, there are concerns regarding the hydrophilic or hydrophobic nature of graphene and its dependence on operating parameters. The contact angle of graphene with water and aqueous solutions has been found to vary depending upon various conditions, e.g., surface wrinkle morphology, extent of tensile strain, vibrational strain, etc. Other variables include the influence of doping, presence of defects, temperature, and electric field, among others. Graphene monolayer also finds application as an ideal coating material due its wetting transparency [17].

Vertical graphene sheets for applications, such as high-performance super capacitors, are presently being produced using plasma-enhanced chemical vapor deposition technique. During sputtering and deposition process, a number of defects get invariably incorporated in graphene sheets. The density of defects can be controlled through sputtering time. Graphene with low density of defects showed a hydrophobic behavior and poor wettability; significant improvements in the wetting were, however, observed for high defect concentrations [18]. Laser-induced

*Introductory Chapter: Factors Influencing the Wettability of Nanomaterials DOI: http://dx.doi.org/10.5772/intechopen.86451* 

 graphene is produced by irradiating polyimide film with CO2 laser to burn away all elements except carbon. Operating conditions during their production can impact the chemical composition of the graphene surface affecting its wettability with water; the presence of H2 gas in the chamber along with limited O and CO contents led to hydrophobic graphene [19].

 Graphene, typically a zero-band-gap semiconductor, can be doped as n-type or p-type using sub-surface polyelectrolyte or metals, electrical voltage chemicals, etc., and its wetting properties can be modified with significant changes in contact angles [20]. Doping can modify and regulate the surface electron density of graphene and its interaction with external molecules in the contact region. It may also help to develop a feasible route for tuning the surface wettability of graphene, especially for coating applications. Studies have also been reported on the conversion of its surface wettability from hydrophobic to hydrophilic and vice versa with help of external stimuli [21]. Extensive research is being carried out in this field on several aspects of the wetting behavior of graphene with focus on controlled and reversible tuning.
