Insights into the Formation Mechanism of CdSe Nanoplatelets Using in situ X-ray Scattering

Two dimensional ultra thin CdSe nanoplatelets have attracted a large interest due to their optical properties but their formation mechanism is not yet well understood. Several different mechanisms and models have been proposed but quantitative in situ data that could validate or disprove them are lacking. We use synchrotron-based small-angle and wide-angle X-ray scattering to probe in situ the formation mechanism of CdSe nanoplatelets synthesized using a heating-up method. We prove the absence of a molecular mesophases in the reactive medium at the onset of nanoplatelet formation ruling out a templating effect. A q^-2 regime is observed from the start of the reaction which extends towards smaller wave vectors with time, consistent with the continuous lateral growth of nanoplatelets from a limited number of seeds fed by reactive monomers. A ripening mechanism where small cluster fuse to yield nanoplatelets can also be ruled out by our data. When the final temperature is lowered, larger nanoplatelets are obtained and the SAXS patterns exhibit marked oscillations due to their rolling into curved nanoscrolls. Our experiments thus show that nanoplatelet curvature appears during their synthesis.