The Promise of Graphene: A Survey of Microwave Devices Based on Graphene
Girish Chandra Ghivela, Joydeep Sengupta
Abstract
“Graphene is the smallest possible thinness layer of graphite, with a thickness of 0.34 nm” [1]. It is made up of sp <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> hybridized carbon atoms in such a way that each carbon atom bonded is attached to three others in a covalent bond, as shown in Figure 1(a) [2]. One s orbital and two p orbitals (p <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> , p <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> ) together result in sp <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> hybridization with 120° angles between the hybrid orbitals, forming a σ-bond. Figure 1(b) shows the crystal structure of graphite, which consists of parallel carbon layers. The perpendicular p <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">z</sub> orbital to the sp <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> hybrid orbital forms a π-bond. A weak Van der Waals force exists between the layers due to the π-bond. In the past, it was theoretically studied as the building block of graphitic materials with other dimensions [3], [4]. Because of the weak Van der Waals forces between the stacked layers of graphite in graphene, isolated graphene was considered to be thermodynamically unstable and, thus, was thought not to exist in reality [5]-[7].