Graphene is a two-dimensional (2D) sheet of carbon atoms in which each carbon atom is bound to its three neighbors to form the honeycomb structure shown in Figure 19.1a. Graphene occurs naturally as the basic building sheet of graphite. In graphite, each graphene layer is arranged such that half of the atoms lie directly over the center of a hexagon in the lower graphene sheet, and half of the atoms lie directly over an atom in the lower layer, seen in Figure 19.1b. ™is stacking of the graphene layers is called the Bernal stacking. ™is particular arrangement of the graphene layers in graphite greatly a£ects the electronic properties of graphite; other types of stacking give di£erent electronic properties.