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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - DECAGON (Doping Colloidal Nanoplatelets: Synthesis, Self-assembly and Spectroscopy)

Teaser

Summary

Copper doped CdSe nanoplatelets luminescence in the red to near infrared has been synthesized with remarkably large Stokes shift and quantum yields of about 40%. Doping of nanoplatelets of three different thicknesses is achieved through post-synthetic addition of copper oleate followed by annealing at high temperature. Quantity of copper and annealing time is the key factor of getting higher quantum efficiency. To prove the applicative relevance of these doped nanoplatelets, we formulate composites in which they are dispersed in a polymer matrix and show that these films act as efficient luminescent solar concentrator with strongly reduced self-absorption as compared with undoped nanoplatelets.

Work performed

The Fellow has developed a simple way to synthesize Cu doped CdSe NPLs by adding a Cu precursor to a dispersion of freshly prepared NPLs. These Cu doped NPLs readily show color tunable photoluminescence from red to NIR with remarkably large Stokes shift and quantum yields of about 40%. In the case of doped NPL, we observed the same absorbance spectra for doped and undoped NPLs but the PL exhibits a broad peak with a maximum at 690 nm corresponding to a large Stokes shift (~200 nm) independent of the dopant concentration and broad nature of PL spectrum (full width at half maximum~ 100 nm). The observed large Stokes shift can be attributed to the presence of Cu within the CdSe NPLs, creating additional lower energy transition states. The emission intensity of these doped NPLs is stable under ambient atmosphere (more than 6 months). Since the excitonic properties of the NPL only depend on their thicknesses, the fellow applied the same doping strategy to NPL with different thicknesses. This enabled the tuning of the dopant emission from 620 to 780 nm as the thickness of the NPL varies. The Fellow also made a transparent composites of doped NPLs and poly-(lauryl methacrylate), which displays high optical efficiency and could be used as solar light harvester or color conversion devices.
Another aspect of the work performed by the Fellow was the stacking of CdSe NPLs. We showed that ribbons of stacked board shaped CdSe NPLs twist upon addition of oleic acid ligand, leading to chiral ribbons with several micrometer in length and well defined pith of ~400 nm. In order to assemble the NPL, the fellow used simple drying technique with successive additions of oleic acid during drying. After drying, the precipitate was re-dispersed in hexane and shows long ribbon ranging from 1-4 Î¼m. Closer observation reveals that the ribbons are twisted. Instead of stacking face to face with edges parallel, consecutive NPL in the same thread are rotated by a small angle. We demonstrated that the chirality originates from surface strain caused by the ligand since the addition of various amounts of OA to a dispersion of NPL in hexane without drying, so as to prevent ribbon formation, lead to a twist of the individual NPLs. In this case, the NPL are not stacked and can therefore change their shape freely upon ligand addition hence the increase of OA surface concentration induces a transition of the individual NPL from a flat to a curved shape. This twist at the single-particle level propagates over the whole ribbon length through the release of stress when platelets are confined within stacked ribbons. This study has led to a publication in a prestigious journal (Science Advances) in Open Access for a better dissemination to the public. A press release has been issued by the CNRS about these results.
The fellow also continued his work on chirality of CdSe NPLs by adding a chiral ligand on surface of NPLs. This chiral NPLs results is in very preliminary stage and needs more experiment.

Final results

The easy synthesis process of doped NPLs implies the possibility of mass scale production by avoiding the complexities of colloidal reaction. Efficient dopant related red to NIR emission with no self-reabsorption, higher absorption coefficient, higher emission stability in air, are making them suitable candidate for fabricating luminescence solar concentrator though further research needs for device engineering. On the other hand for chiral assembly, the physical origin of the twist lies in the strain induced by the ligands at the surface of the NPLs. This approach should be easily generalized to other types of 2D NPLs and could be used to produce other original self-assembled nanostructures by tuning the internal strain transmitted by the ligands. Further understanding the self-assembly of these objects would be interesting and lead to important
breakthroughs on both fundamental and applied standpoints.