National Research Council of Canada. Measurement Science and Standards
We report on a particular type of CdSe nanocrystals (NCs) that exhibit a single optical absorption doublet. The two peaks in the doublet are relatively sharp with a full width half-maximum as narrow as 10 nm. The peak positions vary with passivation ligands (at ∼426 and ∼453 nm for amine ligand passivation and at ∼432 and ∼460 nm for carboxylate ligand passivation). To date, it has been generally concluded that these NCs have a two-dimension (2D) morphology with 1D quantum confinement. Here, we report that zero-dimension (0D) NCs with 3D quantum confinement can exhibit a very similar static optical feature consisting of a sharp absorption doublet. We show that our as-prepared CdSe NCs (without further purification) were mainly 0D NCs, as observed when they were deposited on transmission electron microscopy (TEM) grids directly from toluene or hexane dispersions. We further demonstrate that it was possible to alter this 0D morphology by using dispersion additives and/or purification solvents to result in the appearance of 2D NCs under TEM. Although the 0D and self-assembled 2D NCs displayed similar static optical features, the two morphologies behaved quite differently in polarized emission. The 2D NCs exhibited detection angle dependent polarized emission, whereas the 0D NCs do not. Our findings indicate that a well-like morphology can be induced by the presence of hexadecylamine (HDA) in the dispersion with sonication for amine-passivated 0D NCs or by the use of ethanol during purification with dispersion storage for carboxylate-passivated 0D NCs. In this way, it is possible to manipulate the NC morphology for a targeted application through the appropriate post-treatment. This study highlights that more sophisticated theoretical studies are required to account for the experimental observations in which both 0D NCs and their self-assembled 2D NC products display similar static optical features.