Proceedings of September Meeting 2016 (NFM16)
Publication date: 14th June 2016
In contrast with more conventional colloidal quantum dots (QDs) that are confined in all three dimensions, 2D nanoplatelets (NPLs) combine the benefits of strong confinement in one direction, with expanded lateral sizes that lead only to weak in-plane confinement of charge carriers. Their thickness can be controlled with monolayer precision, yielding narrow, homogeneously broadened band-edge absorption and emission peaks.
Colloid chemistry offers the advantage that the NPL dimensions can be precisely controlled, not being limited to lattice matching with an underlying growth substrate. Importantly, lateral sizes and size dispersion, while not strongly influencing spectral position of the band-edge absorption, still determine their self-assembly and associated interparticle charge and/or energy transfer in solid films. Additionally, almost independently from the confinement energy, they allow tuning of the emission lifetime and excited-state photoluminescence properties. Taking 4.5 monolayer thick CdSe NPLs as a starting point, we will discuss how different short-chained ligands lead to a tuning of the final particle dimensions, either using CdCl2 to increase the NPL thickness, or via a controlled addition of Cd(Ac2).2H2O to the reaction medium to determine the lateral size and aspect ratio.
Next, we developed procedures to coat the NPLs with CdxZn(1-x)S and ZnS shells with a focus on maintaining the fast PL lifetime that CdSe core NPLs exhibit. Interestingly, upon increasing the ZnS shell thickness we observed a continuous shift of the band-edge absorption and emission, in contrast with the stepwise increase of the PL emission peak position as one increases the number of monolayers within the CdSe NPL. The tailored synthesis of CdSe core and CdSe/CdxZn(1-x)S NPLs thus yields materials with precisely controlled opto-electronic properties across the visible spectral range. Furthermore, the high-band gap shell enhances optical stability, allowing us to transfer the NPLs to different media and environments without significant loss of fluorescence. This opens the way to practical photonic applications of 2D-confined colloidal nanocrystal on a solution-processed platform.
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