During the first 30 months of the project, 4 students and I have dedicated our research to the 3 work-packages (WPs). A Chemical review on II-VI semiconductor nanoplatelets (NPLs) has been written.
WP 1: We have demonstrated that native carboxylate ligands of NPLs can be replaced by halides followed by a partial dissolution of cadmium chalcogenide NPLs at the edges. The released monomers then recrystallize on the wide top and bottom facets, increasing NPL thickness. This dissolution/recrystallization method is used to increase NPL thickness to 9 MLs while using 3 ML NPLs as the starting material. This method is not limited to CdSe and is 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 homostructures are comparable to type I heterostructures, leading to the exciton recombination in the thicker area of 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. The main objective was also to show dual emission coming from one single nanoparticle. We have shown that CdSe/CdTe/CdSe core/crown/crown heterostructures enable to localize the exciton either in the external crown, leading to a green emission or at the interface between CdSe/CdTe leading to a red emission. The ratio between these two emission can be tuned through the lateral dimensions of NPLs or the excitation power. These NPLs can be inserted in LEDs. ANd, using thinner NPLs, the dual emission is extended to the blue and the yellow.
WP 2: We have worked on the comparison between HgTe NPLs 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 and under simultaneous pressure, up to 4 GPa. While the pressure dependence of NPLs follows the one observed for bulk and nanocrystals, the temperature dependence dramatically differs for NPLs. 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. In the objective to get narrow NIR emitters, we have shown that cation exchange with mercury could be performed on alloy CdSeTe NPLs. The choices of solvent, temperature, precursors and ligands have their own importance. So, we have demonstrated the synthesis of HgSeTe NPLs with tunable optical properties depending on the composition of the alloy and that emit in the NIR range. We are now working and the control of cation exchange on 2D particles with the surface chemistry. When ligands bring a stress in the direction of the targeted material lattice parameter, the 2D shapes of the NPLs is preserved. We are showing that the kinetic of cation exchange is a key parameter to preserve the 2D shape of particles. This way atomically precise Cu2-xSe NPLs capped with thiolate have been synthesized with various thicknesses. We also synthesized thicker than 3 ML HgSe NPLs using a cation exchange from Cd2+ to Hg2+ co-catalyzed with silver cation. The obtained NPLs present optical properties tunable with their thicknesses and can now reach 0.9eV in photoluminescence.
WP 3: NPLs are extremely thin and present a subatomic roughness all along the particles with lateral dimensions extended up to few 100's 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, 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. This work represents one of the objectives of the third WP. We are now studying the effect of NPLs shape and thickness with various chiral ligands on the circular dichroism properties of these particles.
These results lead to 10 main publications.
