4. 2D Materials

Two-dimensional (2D) materials can be defined as a crystalline structure that consists of a single- or few-layer of atoms. In fact, those materials can be described as a “hybrid” system of a solid (in the periodic plane) and a molecule (nonperiodic perpendicular direction). The atoms along the in-plane directions are strongly bonded by covalent or ionic interactions to form the atomic layer and the electrons are free to move throughout the plane. On the other hand, along the out-of-plane direction, these atomic layers are weakly held by van der Waals force, in that case, the movement of the electrons is restricted and governed by quantum mechanics [BHC13].