Collective Fluorescence of Graphene Quantum Dots on a surface

TL;DR

This work investigates how graphene quantum dots (GQDs) arrange and interact on the surface of a MAPbBr3 perovskite substrate. Using spin-coated GQDs and confocal, time-resolved measurements, the study reveals that GQDs aggregate into clusters on MAPbBr3, displaying an excimer-like emission at higher densities and dynamic coupling–decoupling behavior at lower densities. Under continuous illumination, some clusters transition to a redshifted, brighter state with a shorter lifetime, consistent with a partially collective emission arising from reorganized GQDs on the surface. These findings establish MAPbBr3 as a platform for surface-induced collective photophysics and point to potential routes for assembling coherently coupled emitter arrays, though the underlying mechanisms require further exploration, including temperature and cluster-size dependent studies.

Abstract

This study explores the organization of graphene quantum dots on the surface of monocrystalline halide perovskite. We show that graphene quantum dots tends to aggregate on the surface of perovskite unlike in solution or on other substrates, even at very low concentration of the initial solution that should yield single-molecule samples. Spectral analysis on small clusters shows a back-and-forth dynamical transition between an uncoupled, monomer-like state, and an excimer state. Following this "dance" between states, a drastic one-way increase in fluorescence intensity combined with a shortening of the excited state lifetime has been observed on some clusters. This behavior is related to the emission of a collective state that may be a consequence of the dynamical organization of graphene quantum dots under illumination on the surface of the perovksite.

Figures (5)

• Figure 1: \ref{['fig:molecule gqd']} Schematic representation of C96tBu8, with the bulky tert-butyl groups (blue). \ref{['fig:pl poly gqd']} Typical optical properties of the GQDs and MAPbBr3 perovskite: Emission spectrum of a single C96tBu8 measured in a polystyrene matrix (blue). Absorption (dashed green line) and emission (solid green line) spectra of the MAPbBr3 substrate. The single molecule emission spectrum and the perovskite absorption spectrum have been smoothed to represent their typical shape.
• Figure 2: \ref{['fig:PL vs concentration map']} PL raster scan of C96tBu8 films on MAPbBr3 crystal, the brighter points indicate higher integrated PL and thus higher concentration of emitters. \ref{['fig:PL vs concentration spectra']} PL spectra taken on points indicated in \ref{['fig:PL vs concentration map']}. A typical spectrum in a polystyrene matrix is plotted in light cyan for reference. The inset shows the ratio $r$ between the peak at high energy and the peak at lower energy. The yellow line is a guide to the eye and corresponds to a power law fit of the ratio against intensity, with characteristic exponent $-0.62$.
• Figure 3: \ref{['fig:confocal raster scan']} Confocal raster scan of C96tBu8s drop-casted onto an MAPbBr3 millimetric crystal. \ref{['fig:pl spectrum map']}PL spectra taken repeatidly over 5s exposure time. \ref{['fig:PL spectra downsampled']} Highlight some of the spectra from \ref{['fig:pl spectrum map']}, showing the 'dancing' dynamics of the emission spectrum. The excitation was set to $2.18~\text{eV}$, with an intensity of $15.47~\text{kW}/\text{cm}^2$.
• Figure 4: PL spectra taken repeatedly over a ten-second exposure. The sample was excited at 2.18eV, with an intensity of 3kW^2. The solid blue line shows the reference PL spectrum of a single C96tBu8 in polystyrene. The intensity of the reference spectrum is arbitrary and should not be compared to the other spectra. The inset shows the evolution of the integrated intensity over time.
• Figure 5: \ref{['fig:TRPL decay filter']} Comparison of the TRPL decay curves for photons integrated over the high-energy range of the spectrum (in green), the low-energy range (in orange), and over the whole spectrum (in black). The specific energy ranges are explicit in the upper part of the figure. The baseline of experimental data has been subtracted, see \ref{['sec:methods']} and Supp. Fig. \ref{['fig:explanation fitting procedure']} for details of the fitting procedure. The IRF is plotted in dashed grey. The fitted mono-(for experiments with filters) and bi-(for the whole spectrum) exponential decays are shown with solid lines. \ref{['fig:TRPL of time']} Evolution of the intensity of the fast (in orange) and of the slow component (in green) of the decay of the integrated PL over time. The components are fitted over $1~\text{s}$ integrated decays. The inset highlights the un-normalised decays at $t=15~\text{s}$ (in purple) and $t=50~\text{s}$ (in light blue).