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Creating the new generation of 2D light emitters

Periodic Reporting for period 1 - Ne2DeM (Creating the new generation of 2D light emitters)

Reporting period: 2020-02-01 to 2021-07-31

Semiconductor nanocrystals (NCs) have size tuneable optical features. Among them, nanoplatelets (NPLs) are 2D NCs with especially well controlled optical features. As a result, these are the semiconductor nanoparticles with the narrowest luminescence signal, which make them very appealing for light emission application and in particular large gamut displays. In the Ne2DeM project, the NPLs will be used as a platform for the design of a new generation of light emitters which applications span from display lightning, to infrared sensing or 3D imaging displays.
The development of a new tool box based on NPLs will be the main ambition of the Ne2DeM project. More specifically I intend to develop three new functionalities to NPLs dedicated to light emission. First, I aim to synthesize particles with dual green and red narrow emissions. This goal relies on the design of a new highly complex 2D heterostructure that will be used as light down converter for white light production with an easier integration than the current technology or materials. The second objective will relate to expanding the wavelength range in the infrared where light can be emitted from NPLs while preserving narrow and fast luminescence. Such short-wave infrared source will be suited in applications such as biolabeling or active imaging. Finally, I intend to develop NPL with circularly polarized emission. To do so, I will develop enantioselective synthesis of 2D NPLs and control the folding of these lasts. This provides high potential for 3D imaging display.
The targeted materials are highly promising to bring back Europe into the market of NC for light emission, while it remains currently uninvolved at the industrial scale. Beyond the development of a new generation of NCs, Ne2DeM also aims to develop new original method for in-situ investigation of the NC growth and to investigate the development of materials electronic structure to fasten their integration into devices.
During the first 18 months of the project, Students and I have mostly dedicated our research to the first and third work-packages (WPs).
Workpackage 1: We have demonstrated that native carboxylate ligands of semicondcutor nanoplatelets (NPLs) can be replaced by halides1 followed by a partial dissolution of cadmium chalcogenide NPLs at the edges.2 The released monomers then recrystallize on the wide top and bottom facets, leading to an increase in NPL thickness. This dissolution/recrystallization method is used to increase NPL thickness to 9 MLs while using 3 ML NPLs as the starting material. We also demonstrate that this method is not limited to CdSe and can be extended to CdS and CdTe to grow thick NPLs. When a metal halide precursor is introduced with a chalcogenide precursor on the NPLs, CdSe/CdSe, CdTe/CdTe, CdSe/CdTe core/shell homo- and heterostructures are achieved. Finally, when an incomplete layer is grown, NPLs with steps are synthesized. These stress-free homostructures are comparable to type I heterostructures, leading to recombination of the exciton in the thicker area of the NPLs. Following the growth of core/crown and core/shell NPLs, it affords a new degree of freedom for the growth of structured NPLs with designed band engineering. This work represents one of the objectives of the first WP. We are now working on relative band alignments (draft in preparation) between alloyed semiconductors to then design dual emitters NPLs.
Workpackage 2: We have worked on the comparison between HgTe nanoplatelets and low-energy-absorption HgTe nanocrystals and probed the optical transmission of HgTe nanoparticles over the 0.26–1.8 eV range, from 0 to 300 K temperatures and under simultaneous pressure, up to 4 GPa. While the pressure dependence of nanoplatelets follows the one observed for bulk and nanocrystals, the temperature dependence dramatically differs for nanoplatelets. The modeling of the electronic energy dispersion using up to 14-band k.p formalism suggests that the second conduction band and higher bands of HgTe play a vital role in describing and explaining the HgTe nanoparticle spectroscopies.3
Worpackage 3: NPLs are not only extremely thin but their thickness present a subatomic roughness all along the particles with lateral dimensions extended up to few hundreds nm. The crystal structure of the NPLs in ad-equation with the wide facets orientation induces the assembly of organic ligands over thousands of nm2. And, in order to release the surface stress brought by these ligands, NPLs fold as helices. We are working on the control of the NPLs helices radii through the ligands described as an anchoring group and an aliphatic chain of a given length. A mechanical model accounting for the misfit strain between the inorganic core and the surface ligands enables to predict the helices radii (draft in preparation). This work represents one of the objectives of the third WP. We are now dedicated our work to the optical properties of these folding NPLs.
1. Dufour, M., Qu, J., Greboval, C., Méthivier, C. & Ithurria, S. Halide Ligands to Release Strain in Cadmium Chalcogenide Nanoplatelets and Achieve High Brightness. ACS Nano 13, 5326–5334 (2019).
2. Moghaddam, N. et al. Surface Modification of CdE (E: S, Se, and Te) Nanoplatelets to Reach Thicker Nanoplatelets and Homostructures with Confinement-Induced Intraparticle Type I Energy Level Alignment. J. Am. Chem. Soc. 143, 1863–1872 (2021).
3. Moghaddam, N. et al. The Strong Confinement Regime in HgTe Two-Dimensional Nanoplatelets. J. Phys. Chem. C 124, 23460–23468 (2020).
My teams and I are working on the three different workpackages in order to synthesis NPLs with dual emission, NPLs with narrow optical properties in the near IR range and NPLs with circularly polarized optical properties.
We have recently shown that even in homostructured NPLs, we could specifically localized exciton in thicker areas of the NPLs. Joining these results with the right choice of semicondcutors, we will be able to synthesis NPLs with dual narrow emission. Our second objective is to expand optical properties of NPLs to the infrared while preserving narrow and fast luminescence. Different strategies will be explored but after having shown that CdSe NPLs could be grown in the thickness, we plan to expand this strategy with mercury chalcogenides NPLs. Such short-wave infrared source will be suited for active imaging. Finally, We will synthesize NPLs with circularly polarized optical properties while carefully choosing precursors to induce enantioselective growth of nanoparticles.
Control of nanoplatelets thickness through halides ligands