Semi-conducting colloidal nanoparticles have been studied since the 1980s and 1990s, starting with the spherical nanocrystals and expanding in the 2000s to elongated nanorods and tetrapods and in the 2010s to planar nanoplatelets. An elaborate understanding of this class of emitters is now available and has led to various developments in opto-electronics, including the commercially-available QD-LED TV displays, as well as in other fields like bio-imaging, nano-medicine and quantum optics. These studies were usually performed on isolated emitters: either on ensembles of emitters in solution, or on single emitters under fluorescence microscopy. There is now an increasing interest of both the solid-states community and the nano-optics community for the collective behaviour of a large number of coupled nano-emitters. When a densely-packed stacking of emitters is considered, interactions between them are expected. However, experimental results are diverse and their interpretation is complex, especially because the level of disorder in stacked structures makes them difficult to model and reproduce. A fundamental understanding of the coupling between semi-conducting nanoparticles is crucial for opto-electronic applications such as quantum-dot-LEDs or quantum-dot-sensitized solar cells as these applications involve dense layers of semiconductor nanoparticles.
CdSe nanoplatelets, also coined colloidal quantum wells (fig. 1(a)), are outstandingly bright and well controlled fluorescent emitters and constitute an attractive system for studying such interactions between nanoparticles. Benjamin Abécassis has developed unparalleled experience on the self-assembly of chains of hundreds of platelets (up to 4 µm length), with constant platelet center-to-center distance of 6 nm and excellent linear order (fig. 1(b,c)). As part of a collaboration initiated in 2018, Laurent Coolen has imaged the fluorescence from individual chains and found a FRET (Förtser resonant energy transfer : dipole-dipole non-radiative exciton hopping) migration length of 500 nm (around 90 platelets). From this, a diffusion-equation model leads us to estimate the characteristic time of FRET transfer between neighbour platelets to 1-2 ps, much shorter than all decay mechanisms known to occur in fluorescent semiconductor nanoparticles. Densely-packed nanoparticles can thus be expected to present, because of FRET, totally new photophysical behaviour involving tens or hundreds of emitters collectively instead of each platelet emitting individually.
Figure: (a) Typical CdSe nanoplatelets, (b) spontaneously-stacked platelets, (c) µm-length chains of self-assembled platelets prepared at ENS de Lyon by B. Abécassis.
Ensembles of emitters are also studied in the nano-optics community with the perspective of using collective optical effects to enhance light emission. In particular, identical emitters in a small volume may reach the regime of superradiance, where constructive interferences lead to extraordinary emission intensity enhancements. Various solid-state nano-optics systems have recently demonstrated superradiance but its characterization can be ambiguous and superradiance in solid-state systems is still not fully understood.
Our self-assembled chains of nanoplatelets are a perfect platform for exploring these various collective effects. Their architecture can be adjusted by physico-chemical means and microscopic fluorescence analysis of single chains allows a combination of spatially-resolved study of energy transfer, spectroscopy, time-resolved measurements, polarisation and Fourier-plane analysis of the radiating in-plane dipole etc. The aim of the project is to optimize the properties of NPLs chain assemblies in order to enhance interactions between platelets and analyse FRET-mediated and optical collective photophysics.
