Optical Response of Graphene Quantum Dots in the Visible Spectrum: A Combined DFT-QED Approach

Abstract

We propose a model based on density functional theory (DFT) and quantum electrodynamics (QED) to study the dynamical characteristics of graphene quantum dots (GQDs). We assume the GQD edges are saturated with hydrogen atoms, effectively making it a polycyclic aromatic hydrocarbon (PAH) such as coronene. By combining the GQD spectrum calculated from a time-dependent DFT (TDDFT) with the dynamical behavior of a QD model derived from QED, we calculate the main optical characteristics of the GQD, such as its transition frequencies, the dipole moment associated to each of those transitions, life-time and the population dynamics of the molecular levels. Owing to the close match between the calculated spectrum and experimental results, our results represent a significant contribution to research on quantum treatments of light-matter interactions in realistic 2D nanomaterials.

Figures (2)

• Figure 1: (a) Absorption-emission spectra of the coronene molecule from TDDFT calculations for electric field kick perturbations along the $x,\,y,$ and $z$ directions. The $z$-axis spectrum (green curve) is scaled by a 100 factor in order to be visible. The experimental absorption-emission spectrum obtained by Hirayama et. al.hirayama2014 is shown in red. The inset provides a schematic of the coronene molecule with blue spheres representing carbon atoms and gray hydrogen. (b) Scheme for the three-level quantum system used to model coronene as a GQD.
• Figure 2: (a) TDDFT-calculated data and the corresponding fit from the QED model (Eq. \ref{['eq:espectro']}) for an initial perturbation along the $y$ axis. (b) Population dynamics of levels 1 and 2, with initial values $|a_1(0)|^2=0.0325$ and $|a_2(0)|^2=0.9675$ derived from the model fit. The inset shows an enlarged view of the level 1 population dynamics at short times.