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We present the synthesis of a noveltype of chalcogenide nanoplatelets.Starting from CdS core or CdSe/CdS core/shell nanoplatelets, we usesequential cation exchange to copper and then to either zinc or leadto obtain ZnS and PbS core or ZnSe/ZnS and PbSe/PbS core/shell structures.The procedure preserves well the 2D geometry of the nanoplatelets,provided that they are more than 6 monolayers (∼1.8 nm) thick.The core/shell structure is also well conserved during the cationexchange as verified by TEM images. The nanoplatelets exchanged withZn crystallize in a zinc blende structure, like the initial Cd-basedmaterial, whereas when Pb is used, the final nanoplatelets have arock-salt crystal structure. We explored the copper cation exchangeprocess using energy dispersive X-ray spectrometry with 1 nm resolutionon nanoplatelets standing on their edges, and we show that copperions diffuse uniformly from the outside of the nanoplatelet to theinside during the exchange. [ABSTRACT FROM AUTHOR]

We report the continuous lateral extension of cadmiumselenidenanoplatelets into nanosheets using continuous injection of precursorsat high temperature. We show that we can obtain CdSe nanosheets withlateral dimensions up to 700 nm and a well-defined thickness thatcan be tuned with atomic precision. When the nanosheets’ lateralsize increases, they roll on themselves to form nanoscrolls that canunroll upon surface modification. The final geometry of the nanosheetscan be tuned to different morphologies using precursors that favorthe growth of specific crystal facets. We provide a detailed studyof the CdSe nanosheets growth and its optimization for three differentthicknesses. [ABSTRACT FROM AUTHOR]

Colloidal CdSe nanoplatelets with monolayer control over their thickness can now be synthesized in solution and display interesting optical properties. From a fundamental point of view, the self-assembly of CdSe nanoplatelets can impact their optical properties through short-range interactions, and achieving control over their dispersion state in solution is of major relevance. The related issue of colloidal stability is important from an applicative standpoint in the perspective of the processing of these materials. Using UV-vis spectroscopy, we assess the colloidal stability of dispersions of CdSe nanoplatelets at different nanoparticle and ligand (oleic acid) concentrations. We unravel an optimum in oleic acid concentration for colloidal stability and show that even moderately concentrated dispersions flocculate on a time scale ranging from minutes to hours. Small-angle X-ray scattering shows that the precipitation proceeds through a face-to-face stacking of the nanoplatelets due to long-ranged van der Waals attraction. To address this issue, we coated the platelets with a carboxylic acid-terminated polystyrene, thus achieving colloidal stability while retaining the optical properties of the platelets.

Understanding the formation mechanism of colloidal nanocrystals is of paramount importance in order to design new nanostructures and synthesize them in a predictive fashion. However, reliable data on the pathways leading from molecular precursors to nanocrystals are not available yet. We used synchrotron-based time-resolved in situ small and wide-angle X-ray scattering to experimentally monitor the formation of CdSe quantum dots synthesized in solution through the heating up of precursors in octadecene at 240 °C. Our experiment yields a complete movie of the structure of the solution from the self-assembly of the precursors to the formation of the quantum dots. We show that the initial cadmium precursor lamellar structure melts into small micelles at 100 °C and that the first CdSe nuclei appear at 218.7 °C. The size distributions and concentration in nanocrystals are measured in a quantitative fashion as a function of time. We show that a short nucleation burst lasting 30 s is followed by a slow decrease of nanoparticle concentration. The rate-limiting process of the quantum dot formation is found to be the thermal activation of selenium.

We show that colloidal nanoplatelets can self-assemble to form a 1D superlattice. When self-assembled, an additional emission line appears in the photoluminescence spectrum at low temperatures. This emission line is a collective effect, greatly enhanced when the NPLs are self-assembled. It is attributed to the longitudinal optical (LO) phonon replica of the band-edge exciton, and its presence in self-assembled nanoplatelets is explained using a model based on an efficient photons reabsorption between neighboring nanoplatelets. The presence of phonon replica at low temperatures in ensemble measurements suggests the possibility to design a laser, based on self-assembled nanoplatelets.