## 1 Graphene and Graphene Nanoplatelets  
Graphene is a single freestanding monolayer of graphite [1]. It is the first 2D-material ever manufactured by mankind, having a thickness of one atom (0.34 nm), and lateral size orders of magnitudes larger [2; 3; 4]. Graphene combines diverse and unique physical properties (see Figure 1), and as a result, is an ideal building block for miniaturized next-generation devices, with applications in fields like photonics, opto-electronics, protection coatings, gas barrier films, and advanced nanocomposites [5; 6; 7]. In recent years, many studies have focused on solutions to conveniently mass-produce defect-free graphene. More than twelve different fabrication techniques were proposed [6; 8; 9; 10; 11; 12; 13]. Two noteworthy processes are chemical vapor deposition on copper or metals [14] and liquid phase exfoliation of graphite [15]. The first method is a bottom-up approach: it makes wide graphene films grow on top of metallic foils, starting from volatile carbon based precursors. In contrast, liquid phase exfoliation is a top-down method which singles out the graphene monolayer by sonicating graphite immersed into solvents with low surface tension or water with surfactants [15; 16]. Single layer graphene flakes are then isolated only after additional ultracentrifugation steps. Pure graphene is not yet mass-produced though. There is still a lack of a large scale manufacturing techniques that isolate these 2D crystals with the same outstanding performance as that required to produce the samples fabricated in research laboratories [10]. The main limitations are the low fabrication rates and high sales costs. On the other hand, graphene nanoplatelets (also known as graphite nanoplatelets, GnPs, or GPs) combine large-scale production and low costs with remarkable physical properties. This nanoflakes powder is normally obtained following the liquid phase exfoliation procedure without further centrifugation steps. Other widespread GnP manufacture methods are ball-milling [17], the exposure of acid-intercalated graphite to microwave radiation [17], shear-exfoliation, and the more recent wet-jet milling [18]. These manufacturing techniques produce a large variety of powders in terms of thickness, lateral size of the flakes, aspect ratio, and defect concentrations [18]. GnPs are composed of single and few layer graphene mixed with thicker graphite (see Figure 2); hence, structurally they are in between graphene and graphite. In literature, graphene based materials are classified according to their thickness, lateral size, and carbon to oxygen atomic ratio [19]. Considering the morphological characteristics, the graphene family can be classified as single layer graphene, few layer graphene (2-10 layers), and graphite nano- and micro-platelets. Commercially available GnPs are a mixture of single layer, few layers, and nanostructured graphite. In other words, GnPs thickness can vary from 0.34 to 100 nm within the same production batch [20; 21]. Note that graphite is typically considered a 2D-like material (i.e., not bulky) when its number of layers is \(\leq\)10 [10]. GnPs exhibit exciting properties such as light weight, high aspect ratio, electrical and thermal conductivity, mechanical toughness, low cost, and planar structure. As such, they are attractive options to replace different nanostructured fillers in material science, such as other carbon allotropes (i.e., carbon black or carbon nanotubes), metallic nanoparticles, and clay [21; 22]. They are appealing for nanocomposites, since they can easily and successfully be included in polymeric matrices by solvent or melt compounding [23]. GnPs are cheaper than carbon nanofibers and nanotubes, and are comparable with such tube-like nanofillers in modifying the mechanical properties of polymers [21; 24]. Moreover, GnPs' electrical conductivity is orders of magnitude higher than those of graphene oxides [25]. Figure 1: Graphene hexagonal honeycomb chemical structure and its remarkable physical properties. The black dots are carbon atoms.