Gold-semiconductor hYbrid nanoparticles as Novel light sources – GYN

A first part of the project involves the synthesis of hybrid nanostructures. The LPEM developed the synthesis of golden QDs (for quantum dots) with a geometry that will optimize the enhancement of the spontaneous emission rate. The LPEM will seek to extend this synthesis methods to nanoplaquettes (NPs). These new synthesis methods will lead the LCF to adapt its models to take into account the specificities of the new nanoparticles. GEMaC will characterize in detail their fluorescence properties by time resolved photoluminescence experiments.
The second phase deals with the effects associated with the emission of an ensemble of QDs or NPs coupled to various types of plasmonic resonators. The LPEM will synthesize hybrid superparticles consisting of an ensemble of colloidal emitters surrounded by a gold shell. In addition to theoretical modeling, the LCF will also realize gratings integrating NPs and patch antennas in which QDs are incorporated. In the collective emission regime, the GEMaC will examine the influence of the number of emitters and of the temperature on the radiative lifetime or the emission and extinction spectra. It will allow the characterization of the collective effects.
The next step will consist in transposing the structures already realized in the infrared by MPQ to the visible domain. In particular, it will be necessary to adapt the electrical injection structures of the holes and the electrons to QDs of greater bandgap. In parallel, the LCF will develop a model for understanding experimental observations. Finally, the GEMaC and the MPQ will conduct a detailed study of the photoluminescence and especially the electroluminescence properties of structures by time-resolved experiments, Fourier microscopy, or spectroscopy to demonstrate superradiance in ultrafast light-emitting diodes.

1) Synthesis of nanocrystals (NCs) aggregates
Since the beginning of the GYN project, the LPEM team has been able to synthesize aggregates (or superparticles, SPs) made up of NCs. In particular, the optical losses of fluorescence induced by the formation of aggregates and the synthesis of the silica shell have been greatly reduced. The size of the SPs now reaches 130 nm in a controlled way.

2) Characterization of the optical properties of SPs at 300K
Experiments conducted at the LPEM and GEMaC on ensemble or individual SPs have highlighted a red shift and an acceleration of the emission of NCs when they are integrated in SPs. Measurements resolved spectrally and in time have shown that these observations are due to a very efficient non-resonant energy transfer (FRET for Förster resonance energy transfer).

3) Strong coupling between a gold film and a film of nanoplatelets
The LCF team has demonstrated a significant strong coupling effect (the Rabi splitting reaches 80 meV). These studies have also highlighted the relevance of modeling these systems in terms of effective index.

4) Emitting properties of the LEDs
Still in this theoretical framework, the teams of the LCF and MPQ have shown that the emission of LEDs associating metal nano-antennas and nanocrystals of PbS did not come from the enhancement of the emission of individual NCs by the nano-antennas. Only a model based on a statistical description of all NCs coupled to nano-antennas makes it possible to model the experimental results.

GYN is a project with challenges that are first and foremost fundamental. In this highly interdisciplinary project, the complementarity of the consortium teams, with recognized expertise in the fields of synthesis, modeling and optical characterization, will lead to major breakthroughs. The work on the emitter ensembles coupled to plasmonic resonators will thus make it possible to explore original light-matter interaction regimes.

Beyond the scientific benefits, the nanostructures and devices developed in this project are part of the emerging field of nanophotonics and more specifically the use of colloidal nanostructures. The latter can be used in a wide range of applications ranging from the labeling of biological molecules to optoelectronic devices. Recently, many groups have begun marketing TVs in which the emitting structures are QDs. LEDs based on QDs and pumped electrically can also reach external quantum efficiencies of the order of 20%.

Individual nanoemitters confined in a small volume, in interactions, and coupled with a plasmon resonance will give rise to original collective emission properties such as superradiance or strong coupling. Working at room temperature, this new class of hybrid nanostructures finally opens the way to the realization of bright, ultra-fast light sources, innovative design that can be pumped electrically.

1. H. Wang, A. Aassime, X. Le Roux, N. J. Schilder, J.-J. Greffet, and A. Degiron, « Revisiting the Role of Metallic Antennas to Control Light Emission by Lead Salt Nanocrystal Assemblies », Physical Review Applied 10, 034042 (2018)
2. L. Wojszvzyk, H. Monin, J.-J. Greffet, « Light Emission by a Thermalized Ensemble of Emitters Coupled to a Resonant Structure », Adv. Opt. Mat. 7, 1801697 (2019)
3. I. Shlesinger, H. Monin, J. Moreau, J.-P. Hugonin, M. Dufour, S. Ithurria, B. Vest, and J.-J. Greffet, « Strong coupling of nanoplatelets and surface plasmons on a gold surface », ACS Photonics (accepted)

GYN is a multidisciplinary project centered on the coupling between colloidal semiconductor nanocrystals and plasmonic resonators. Taking advantage of collective and antenna effects, it aims at designing a new generation of hybrid light sources and electrically driven devices that emit non-conventional forms of light.

The first goal is to synthesize and to model fast (sub ns) emitters with stable emission and excellent resistance to photobleaching at the single particle level. Individual CdSe/CdS core-shell colloidal nanocrystals with different shapes, spherical (QDs) and quasi 2D nanoplatelets (NPs), will be encapsulated in a silica or polymer layer and coated with a gold nanoshell by solution chemistry methods. The structure of these hybrid materials will be optimized by theoretical modelling.

Beyond this single particle study, we will look into the coupling of an ensemble of QDs or NPs to plasmonic cavities. Super-particles (SPs) consisting in a set of colloidal QDs or NPs surrounded by a gold shell will be designed. NPs coupled with the plasmon resonance of a gold grating and a patch antenna encapsulating a dense ensemble of QDs will be also studied. We will seek emission enhancement through collective effects (superradiance, strong coupling) at room temperature. These systems pave the way for the emission of light on extremely short time scales (sub-picosecond) and offer model objects to study the influence of the number of emitters in these original regimes of cavity quantum electrodynamics.

The last goal of this project is devoted to the electrical pumping of such advanced systems, a step that is a critical milestone for applications. Based on superradiance emission, these devices will represent a new class of sources beside lasers. They will open tantalizing prospects for directional and ultrafast LEDs that can be modulated at rate similar to lasers (tens of GB/s) for a fraction of the energy cost.