Origin of Shape-Dependent Fluorescence Polarization from CdSe Nanoplatelets

In this study, we report the shape dependence of fluorescence polarization from colloidal CdSe nanoplatelets (NPLs). Despite the symmetry of their cubic unit cell structure, CdSe nanocrystals grow into two-dimensional platelets in the presence of acetate precursors, and the resulting NPLs exhibit polarized emission. The amount of acetate salts introduced during the synthesis plays a critical role in controlling the lateral aspect ratio of CdSe NPLs. Specifically, the more the acetate hydrate presents, the more squarelike face of NPLs emerges. As a result, we achieved CdSe NPLs with varying lateral aspect ratios ranging from 1.1 to 4.5 in our experimental conditions. At the same thickness, CdSe NPLs with higher lateral aspect ratios exhibit higher fluorescence polarization. We analyzed the shape dependence by preparing films of CdSe NPLs dropcast under an electric field and measuring emission and absorption polarizations from CdSe NPL films of known orientation parameters. The emission polarization stays nearly unchanged regardless of shape anisotropy of CdSe NPLs, while the absorption polarization is affected by the lateral aspect ratio. Now that these results allude to a likelihood that absorption polarization is responsible for the shape dependence of fluorescence polarization, we design a model to assess the correlation between the geometry of NPLs and the optical transition polarization by way of the local field effect. Theoretically estimated absorption polarization also shows shape dependence similar to experimental data, which suggests that the anisotropic local field effect is a primary denominator of shape-dependent fluorescence polarization in CdSe NPLs.