Quantification of meso- and macro-scale ordering of colloidal semiconductor nanorods in the presence of AC electric fields

Colloidal semiconductor nanorods (NRs) have attracted significant interest as potential building blocks for optoelectronic devices due to their tunable and highly anisotropic absorption and band-edge emission. Aligning large populations of NRs into ordered assemblies provides a basis to the engineering of macro-scale functional materials with strong optical anisotropy. The bulk optical properties of NR assemblies are determined not only by the individual NR building blocks, but also by their meso- and macro-scale ordering, as well as more complex inter-particle 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 highlights the spontaneous meso-scale self-assembly of colloidal NRs and its manifestation in the transient optical response. Using a statistical model based on the von Mises-Fisher distribution in conjunction with ensemble anisotropy measurements, we calculate the limiting degree of order at different NR concentrations in terms of the mean deviation angle relative to the field axis. Our results indicate a decrease in average deviation with increasing NR concentration, suggesting that meso-scale self-assembly is likely to promote field-induced ordering of colloidal NRs. The fundamental insights from this study will add fresh perspective to the evolving state-of-the-art understanding of the electro-kinetic behavior of colloidal semiconductor NRs.