2, Korea Institute of Science and Technology, Seoul, , Korea (the Republic of)
Colloidal CdSe nanoplatelets (NPLs), which have atomically flat geometry with well-defined thickness of several monolayers, exhibit unique optical properties, such as narrow emission band width, giant oscillator strength transition and linearly polarized emission. Considering the isotropic zincblende crystal structure of CdSe NPLs, the linearly polarized emission is particularly intriguing. In general, geometric anisotropy is responsible for the degree of polarization from anisotropic nanocrystals. Since NPLs have three-dimensionally anisotropic morphology, the lateral aspect ratio between two lateral edges should also play an important role.
In this study, we report the shape-dependence of fluorescence polarization from colloidal CdSe NPLs. We controlled morphology of CdSe NPLs with lateral aspect ratios ranging from 1.1 to 4.5 by varying the ratio of precursor, acetate hydrate salt, which triggers lateral growth of NPLs. From measurement of fluorescence polarization of CdSe NPLs with controlled morphology, we revealed that elongated NPLs with higher lateral aspect ratio exhibit higher optical anisotropy at the same thickness. In order to identify the origin of the shape dependence of optical anisotropy, we measured emission and absorption polarization separately with NPLs aligned by electric field. As a result, only absorption polarization depends on the lateral aspect ratio, while the emission polarization stays nearly unchanged regardless of shape anisotropy of NPLs. 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.