Manipulating the Phonon Bottleneck in Graphene Quantum Dots: Phonon-Induced Carrier Relaxation within the Linear Response Theory

Slow phonon-induced relaxation of excited carriers in quantum dots is crucial for next-generation photovoltaics; however, the phenomenon has been difficult to realize experimentally due to defects and ligands that introduce strongly coupled states. Graphene quantum dots are an intriguing system to avoid these problems due to chemical synthesis methods that minimize defects. Modeling electron-phonon interactions using a state-of-the-art method combining the reduced density matrix formalism with linear response theory, we find that 100 Ps lifetimes are possible for electron-hole pairs in graphene quantum dots due to large transition energies and weak coupling to excited states near the band edge. By calculating carrier relaxation rates in dots with and without ligands, with armchair or zigzag edges, and of increasing size, we show how excited state lifetimes are sensitive to structural changes. In contrast to other types of quantum dots in which ligands can increase phonon-induced relaxation, carbon ligands on graphene quantum dots extend lifetimes by nonadiabatically decoupling excited states. Changing carbon edge termination type between armchair and zigzag patterns increases and decreases the electron-phonon coupling, respectively, and geometrically symmetric dots significantly increase nonadiabatic electronic coupling. These results provide guidance for experimental routes to control excited state lifetimes.