The discovery of Graphene, has sparked a renaissance in the study of two-dimensional materials and their potential technological applications. Graphene quantum dots (GQD’s) bring another new opportunity with potential applications in fields ranging from quantum computation to solar energy. However, in order to tailor the properties of a GQD to a specific purpose it is vital to understand the relationship between the size and shape of the dot and its physical properties.
On this work we explore the role that size, shape, edge-type and atomic vacancies play in the optical response of GQD’s. Using group theory, we reveal optical selection rules which follow from the symmetry of a regular shaped GQD, identifying the different roles played by linear and circularly-polarised light in different GQD’s with different shape. The study of the optical conductivity σ(ω) of GQD’s within a simple tight-binding model showed that for GQD’s of intermediate size (≈ 10nm), σ(ω) is sensitive to the edge-type of the dot, with the presence of zigzag edges heralded by a new peak in σ(ω) at the UV end of the visible spectrum (see Fig.). States causing this peak were identified to be 1-dimensional, providing a natural explanation for large binding energies of the associated excitons in bulk graphene. Shape-asymmetry and atomic vacancies in the optical response of GQD’s can be used to even enhance the number of these states. Since GQD’s with both regular and irregular shapes are now available we hope that this article will stimulate further experimental work on this topic.

This work was published as “Symmetry and optical selection rules in graphene quantum dots”, R. Pohle, E. G. Kavousanaki, K. M. Dani, and N. Shannon.  Phys. Rev. B 97, 115404 (2018)