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 μm. 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.
