The Growth Mechanism of Semiconductor CdSe Nanoplatelets: Their Connection to Magic-Size Clusters and a Synthesis Pathway Enabling Future Applications

Abstract

Anisotropic semiconductor nanoplatelets possess exceptional optical properties. They result from the strong confinement of the exciton through the thickness of the nanoplatelets so that narrow light-hole and heavy-hole absorption peaks, as well as narrow emission peaks, emerge. Due to their optical properties, CdSe nanoplatelets would be more suitable candidates for ap-plications like LED screens to replace spherical quantum dots in the long term. However, it is relevant here that the synthesis of CdSe nanoplatelets can be scaled to produce large quantities on an industrial level. Currently there is no known synthesis method for CdSe nanoplatelets in a flow reactor. Ideally, further insights into the growth mechanism of the CdSe nanoplatelets should also be found, and additional modifications should be made to these nanoplatelets, de-pending on the application.

In this work, a synthesis of CdSe nanoplatelets was developed for a flow reactor. With this combined precursor method, mainly 3 monolayer thick CdSe nanoplatelets resulted with their typical absorption peaks at 435 nm and 462 nm and a narrow full width at half maximum of 13 nm for the light-hole and 9 nm for the heavy-hole. The emission of the 3 monolayer CdSe nanoplatelets at 463 nm is also narrow, with a full width at a maximum of 8 nm. In addition to 3 monolayer CdSe nanoplatelets, 4 monolayer CdSe nanoplatelets are also synthesized with absorption peaks at 481 nm and 513 nm, including the respective emission at 514 nm. As a result of combining the cadmium precursor and selenium precursor, these CdSe nanoplatelets produced from the flow reactor have large and non-uniform lateral areas of approximately 1000 nm2. Accordingly, their quantum yield is below 1 percent. In the laminar flow profile of the flow reactor, the nucleation temperature of the CdSe nanoplatelets was determined to be at 180 °C.

Thanks to the combined precursor solutions, the earliest synthesis stages of the CdSe nanoplate-lets could be reached and analyzed using UV/Vis spectroscopy, transmission electron micros-copy, and small-angle X-ray scattering. CdSe-360 and CdSe-405 magic-size clusters were ob-served and, thus, were discovered as important building blocks in the CdSe nanoplatelet syn-thesis as part of this work. These magic-size clusters agglomerate and undergo a size-focusing process in order to form intermediate structures. In particular, these intermediates were detected by the blue shift in the photoluminescence excitation spectra and the size focussing visible in the small-angle X-ray patterns. Based on these results, a growth model was developed that shows how the magic-size clusters can grow through the intermediate stage to the CdSe nano-platelets. Based on the X-ray diffraction, the CdSe nanoplatelets have a zinc blende structure. The mechanism for precisely these zinc blende CdSe nanoplatelets is not yet fully understood. Therefore, the obtained results provide important and new information to fully understand the growth mechanism of CdSe nanoplatelets in the future.

The CdSe nanoplatelets were successfully modified with a CdS-shell or a polystyrene-PI-b-PEO shell. A water-stability could be obtained with the addition of the polystyrene-encapsula-tion. The CdS-shell allowed to change the optical properties of the CdSe nanoplatelets. Thanks to the flow reactor synthesis and the modifications, a foundation has been created to use the CdSe nanoplatelets for various applications in the future.