Exciton Coherence Length and Dynamics in Graphene Quantum Dot Assemblies

Abstract

Exciton size and dynamics were studied in assemblies of two well-defined graphene quantum dots of varying size: hexabenzocoronene (HBC), where the aromatic core consists of 42 C atoms, and carbon quantum dot (CQD) with 78 C atoms. The synthesis of HBC and CQD were achieved using bottom-up chemical methods, while their assembly was studied using steady-state UV/vis spectroscopy, X-ray scattering, and electron microscopy. While HBC forms long ordered fibers, CQD was found not to assemble well. The exciton size and dynamics were studied using time-resolved laser spectroscopy. At early times (∼100 fs), the exciton was found to delocalize over ∼1−2 molecular units in both assemblies, which reflects the confined nature of excitons in carbon-based materials and is consistent with the calculated value of ∼2 molecular units. Exciton−exciton annihilation measurements provided the exciton diffusion lengths of 16 and 3 nm for HBC and CQD, respectively.

Long-lived Excited State in a Solubilized Graphene Nanoribbon

“…The calculated natural transition orbitals (NTOs) also indicate that the exciton is not delocalizing past the dimer (Figure 2b). We obtained similar results in calculations of dexc in one-dimensional -stacks of graphene flakes, 8 indicating that the excitons do not delocalize significantly in these carbon-based chromophores, irrespective of the direction of their growth (along the -stack in graphene flakes or via in-plane bonding in graphene ribbons). It may be noted that the particle-accepting NTO in the trimer appears to be localized over one monomer unit and therefore potentially smaller than the particle NTO of the dimer.…”

“…Photoluminescence of H-GQD and H-GNR in aqueous surfactant solutions is very weak, as evident from the spectra presented in Figures 3a and b. Since it is known that -stacking causes a significant increase in excited-state nonradiative decay rates in aromatic chromophores, 8,[48][49][50] we hypothesize that the use of aqueous surfactants results in solubilization of -stacked aggregates of H-GQD and H-GNR, rather than fully dissolved isolated molecular species. Therefore, we attribute the steady-state spectroscopic signatures of H-GQD and H-GNR in Figure 2 to aggregates.…”

“…Quite similar emission profiles consisting of intense and skinny bands in the higher energy range and broad emission features at lower energies were observed by us and others concentration-dependent emission spectra of analogous GQDs. 8,[51][52][53] Based on this similarity, we assign the 500 nm band to the emission from monomeric Cl-GQD, while the emission at 625 nm is assigned to the -stacked aggregate of Cl-GQD. The absorption profiles of monomeric and aggregated Cl-GQD can be derived from the excitation spectra collected at monomer emission (490 nm) and aggregate emission (625 nm) wavelengths.…”

“…6 These photocatalytic reactions have also been demonstrated for GQDs used in 0D/2D van der Waals heterojunctions with graphene. 7 GQDs can also self-assemble into π-stacked structures that efficiently harvest light along the stack [8][9][10] . Furthermore, the photocatalytic properties of GQDs are enhanced upon coordination of a metal catalyst, such as cobaloximes for hydrogen evolution chemistry.…”

“…34 However, the exciton lifetimes were found to be relatively short-lived due to the excimer formation and the presence of dark states. 32,33 As part of our ongoing interest in light-harvesting properties of nanographenes 8,11,35 , we investigate here the photophysical properties of hydrogenated and chlorinated GQDs and GNRs that contain bipyrimidine moieties in their edges (Scheme 1). Here, H-GQD and Cl-GQD can be thought of as the basic building blocks of H-GNR and Cl-GNR, respectively.…”