Proceedings of nanoGe Fall Meeting 2018 (NFM18)
Publication date: 6th July 2018
Blue (460 nm) and green (510, 550 nm) emitting 2D colloidal CdSe nanoplatelets (NPLs) have well-established synthesis procedures,[1] yet thicker NPLs, needed to cover the remaining visible spectrum, have not been explored to the same extent. Here we present a novel reaction scheme, where we start from 4.5 monolayer (ML) NPLs and induce a 3D growth in a two-step reaction including CdCl2, resulting a series of nearly monodisperse CdSe NPLs with thicknesses between 5.5 ML and 8.5 ML. Investigation of the reaction kinetics and control experiments with purified NPL dispersions suggest that the thickness increases via Ostwald ripening.[2] Transmission electron microscopy and X-ray diffraction revealed that the NPLs maintain a zinc-blende crystal structure, despite growing them for 2-5 hours at temperatures of ca. 300 °C.
By varying the reaction temperature (280-320°C), we gained control over the final NPL thickness, and as a result we could synthesize NPLs with an emission peak from 554 nm up to 625 nm. They show narrow emission line widths with a full width at half maximum of 9-13 nm, and fast emission lifetimes of 5-11 ns. Due to the increased red shift of the NPL band edge, photoluminescence excitation spectra revealed several high-energy peaks, that we could identify using k.p calculations.
In conclusion, the introduction of chloride ions to the CdSe NPL synthesis has opened the possibility to tune the thickness in colloidal 2D materials, resulting in new insights into both the synthesis as well as their opto-electronic properties.
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