Optical Properties of Bilayer Graphene Nanoflakes

The optical properties of coupled graphene nanoflakes are investigated theoretically within the framework of HartreeFock based semiempirical methods, with the aim of unraveling the role of pi pi interactions. Two different types of pi-stacking are considered, obtained either by coupling two identical flakes with different relative displacement or by coupling flakes having different width or edge functionalization, i.e., with different electronic gap or ionization potential. Our results indicate that a systematic red shift and broadening of lowest excitations occur: an overall widening of the optical absorption range can therefore be expected in an ensemble of flakes. However, the coupling prevents a strong enhancement of the absorption intensity. In the case of a heterogeneous ensemble of flakes, the possibility of introducing low-energy excitations with considerable charge transfer character is also demonstrated by properly exploiting the chemical edge functionalization.