▎ 摘　　要

We investigate the potential merits of using nanometer-sized graphene flakes as building blocks for two-dimensional (2D) quantum metamaterials. The choice of the building blocks of metamaterials is crucial to our ability to design quantum metamaterials with desired properties. In this context, graphene nanostructures are promising candidates to fulfill this role as they can be easily grown either by bottom-up chemical synthesis or top-down electron beam patterning in various shapes, topologies, and sizes, down to the molecular scale. This provides a broad range of parameters to tune the optical properties of graphene-based 2D quantum metamaterials. By using time-dependent density functional theory and quantum chemistry computations, we demonstrate that the graphene-based nanostructures accommodate collective charge oscillations, called quantum plasmons, which are qualitatively different in key aspects from their classical counterparts. In particular, our analysis reveals that the exponents characterizing the power-law scaling of plasmon energy with the size of the graphene flakes are markedly different in the classical and quantum regimes, proving that the quantum plasmons cannot be viewed as a trivial extension of the classical ones to the small-flake limit. In addition, the physical properties of quantum plasmons in graphene nanostructures exhibit significant dependence on their shape and size, and external control can be readily achieved with excess charge. Finally, we find that the energy of the fundamental quantum plasmon mode of triangular nanoflakes is larger than that of hexagonal nanoflakes, whereas in the classical case, the plasmon energy ordering is reversed.