Quantifying Order during Field-Driven Alignment of Colloidal Semiconductor Nanorods

Aligning large populations of colloidal nanorods (NRs) into ordered assemblies provides a strategy for engineering macroscopic functional materials with strong optical anisotropy. The bulk optical properties of such systems depend not only on the individual NR building blocks but also on their meso- and macroscale ordering, in addition to more complex interparticle coupling effects. Here, we investigate the dynamic alignment of colloidal CdSe/CdS NRs in the presence of AC electric fields by measuring concurrent changes in optical transmission. Our work identifies two distinct scales of interaction that give rise to the field-driven optical response: (1) the spontaneous mesoscale self-assembly of colloidal NRs into structures with increased optical anisotropy and (2) the macroscopic ordering of NR assemblies along the direction of the applied AC field. By modeling the alignment of NR ensembles using directional statistics, we experimentally quantify the maximum degree of order in terms of the average deviation angle relative to the field axis. Results show a consistent improvement in alignment as a function of NR concentration-with a minimum average deviation of 36.2 degrees-indicating that mesoscale assembly helps facilitate field-driven alignment of colloidal NRs.

Automated summary

This study investigates the dynamic alignment of colloidal CdSe/CdS nanorods in the presence of AC electric fields. The results identify two scales of interaction that lead to the field-driven optical response: the spontaneous self-assembly of the nanorods into structures with increased optical anisotropy at the mesoscale, and the macroscopic ordering of the nanorod assemblies along the direction of the applied AC field. The alignment of the nanorod ensembles improves with increasing nanorod concentration, indicating that mesoscale assembly facilitates the field-driven alignment.