Faraday Rotation in Graphene Quantum Dots: Interplay of Size, Perimeter Type, and Functionalization

Nanometer-sized graphene systems have optical properties that can be tuned in the visible range to enable new optoelectronic device applications. For such purposes it is of critical importance to fundamentally understand the behavior that is specific for the size, shape, and composition of the system. Recently, graphene has gained attention due to its capability to rotate the plane of polarization of linearly polarized light up to 6 degrees at 7 T magnetic field, which is a massive rotation for a single sheet of atoms. We present a computational study that contributes to understanding of this Faraday optical rotation (FOR) for graphene quantum dots (GQDs) of different size, perimeter structure, and composition. Based on first-principles calculations we predict FOR characterized by the Verdet constant, for a systematically growing series of hexagonal GQDs in the visible frequency range. We show evidence for the independence of FOR of the type of the perimeter, zigzag or armchair, in these hexagonal GQDs. In addition, we show how FOR is drastically changed at different levels of hydrogenation, leading to complete or partial sp(3) hybridization of the GQD. While FOR is a global property for a particular molecular system, the recently proposed technique based on optical rotation by polarized nuclear spins (nuclear spin optical rotation, NSOR) characterizes the system with atomic resolution. Here we demonstrate the capability of NSOR to distinguish between GQDs of specific size and edge structure.