Stacking of Colloidal CdSe Nanoplatelets into Twisted Ribbon Superstructures: Origin of Twisting and Its Implication in Optical Properties

Relatively large faces of colloidal CdSe nanoplatelets (NPLs) drive the anisotropic nanomaterials into one-dimensional superstructures through stacking of NPLs when solvent evaporates. We observe that the assembly could result in the formation of twisted ribbon superstructures with varying pitch length depending on lateral dimensions of CdSe NPLs. Transmission electron microscopy images and simulated projection reveal that stacked NPLs are distorted. The estimation of the contact area between distorted NPLs suggests that this distortion leads to lower energy of overall nanoribbon superstructures. The average pitch length of superstructures on the lateral dimension of NPLs depends on the dimension of NPLs as it alters the distortion angle of NPLs and thus the rotation angle between NPLs. We investigate the energy transfer between NPLs in the context of the lateral dimension of NPLs and the geometric structure of their superstructures via transient photoluminescence decay measurements. Our analysis on the energy-transfer rate indicates that extinction coefficients, which are determined by lateral dimension of NPLs, are more responsible for the energy-transfer change than the rotation angle between CdSe NPLs within twisted ribbon superstructures. The dependence of the energy-transfer rate on the lateral dimension of NPLs highlights the importance of geometry of individual NPLs in the context of optical properties of NPLs in ensemble.