Ultrafast Exciton Dynamics in 2D van der Waals Nanostructures: Probing the Hot Exciton Relaxation of Size-Controlled & Well-Dispersed Graphene Nanoflakes

Graphene nanostructures, such as graphene quantum dots (G-QDs), graphene nanoribbons (G-NRs) and carbon nanotubes (C-NTs), combine the unique mechanical and electronical transport properties of sp2- hybridized carbon materials and the optical properties of direct semiconductors provided by the optical gap resulting from the reduction of dimensionally. Among them, the recent developments within the well-known synthesis of G-QDs though bottom-up approach [1] have led to exceptionally well-controlled nanostructures in terms of size, shape and dispersion [2]. The resulting graphene nanoflakes provide tunable emission in the red range, with fluorescence quantum yield close to 1. Furthermore, these nanostructures have revealed to be promising stable emitters of single photons, as shown in our laboratory [2-5]. Here we use transient absorption of 30 fs temporal resolution with polarization-controlled configuration to probe the hot exciton relaxation (internal conversion, Sn→S1) in rectangular G-QDs of various lateral lengths. The nanoflakes are composed of exactly 96, 114 and 132 conjugated carbons (respectively 2.30, 2.71 and 3.11 nm). While the ultrafast electronic dynamics in graphene nanostructures are often being blurred by large broadband photo-induced absorption signals [6-8] (in particular involving triplet states, T1→Tn), here the suppressed aggregation in the studied graphene nanoflakes allows a clear observation and identification of the discrete ground state bleaching and photo-induced emission signals. We selectively excite the different samples at the second optically active electronic transition and, thought the appearance of a photo-induced emission signal at the energy corresponding to the bandedge and red-shifted vibrational replica (i.e. at the position of the steady-state photoluminescence peaks), the dynamics of relaxation were unveiled. The resulting relaxation times range from 100 fs to 175 fs. These results allowed to discuss the mechanism of relaxation, with the effect of the length of the graphene nanoflakes and of the fluence excitation [Quistrebert et al., in preparation]. References: [1] A. Narita, X.-Y. Wang, X. Feng, K. Müllen. Chem. Soc. Rev. 44, 6616 (2015). [2] D. Medina-Lopez, T. Liu, S. Osella, H. Levy-Falk, N. Rolland, C. Elias, G. Huber, P. Ticku, L. Rondin, B. Jousselme, D. Beljonne, J.-S. Lauret, S. Campidelli. Nat. Commun. 14:4728 (2023). [3] T. Liu, B. Carles, C. Elias, C. Tonnelé, D. Medina-Lopez, A. Narita, Y. Chassagneux, C. Voisin, D. Beljonne, S. Campidelli, L. Rondin, J.-S. Lauret. J. Chem. Phys. 156, 104302 (2022) [4] T. Liu, C. Tonnelé, S. Zhao, L. Rondin, C. Elias, D. Medina-Lopez, H. Okuno, A. Narita, Y. Chassagneux, C. Voisin, S. Campidelli, D. Beljonne, J.-S. Lauret. Nanoscale 14, 3826 (2022). [5] S. Zhao, J. Lavie, L. Rondin, L. Orcin-Chaix, C. Diederichs, P. Roussignol, Y. Chassagneux, C. Voisin, K. Müllen, A. Narita, S. Campidelli, J.-S. Lauret. Nat. Commun. 9 :3470 (2018). [6] M.L. Mueller, X. Yan, B. Dragnea, L.s. Li. Nano Lett. 11, 56-60 (2011). [7] D. Sebastian, A. Pallikkara, H. Bhatt, H.N. Ghosh, K. Ramakrishnan. J. Phys. Chem. C 126, 11182-11192 (2022). [8] M. Reale, A. Sciortino, M. Cannas, E. Maçoas, A.H.G. David, C.M. Cruz, A.G. Campana, F. Messina. Materials 16, 835 (2023).