Shape-Dependent Linear Dichroism Spectra of Colloidal Semiconductor Nanocrystals

Semiconductor nanocrystals are normally dispersed in the solvent for property studies as well as practical applications. However, rare attention has been paid to their orientation status in the colloidal solution. Herein, with the help of linear dichroism (LD) spectroscopy, we demonstrate that isotropic NCs of high symmetry (i.e., quantum dots, QDs) and anisotropic NCs (e.g., quantum rods, QRs and nanoplates, NPLs) but under diluted concentration are randomly dispersed without any preferential orientation. Meanwhile, anisotropic NCs under a high concentration can behave with some net orientation along a certain direction. For example, CdSe quantum rods (QRs) and nanoplatelets (NPLs) both show an obviously preferred orientation along the vertical direction in solution when their solution absorbances increase to certain values. An in-depth analysis of QRs' LD spectrum shows that the first excitonic transition of QRs is strongly quantumly confined while its higher-energy excitonic transitions are weakly quantumly confined. In contrast, the NPLs' LD spectrum indicates that their excitonic transitions are isotropic in the spatial space. This work provides a new viewpoint of the real status of anisotropic semiconductor NCs in solution.