Quantifying Order during Field-driven Alignment of Colloidal Semiconductor Nanorods

29 September 2021, Version 1
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

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 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 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 18.7°—indicating that mesoscale assembly helps facilitate field-driven alignment of colloidal NRs.

Keywords

colloidal nanorod
AC field alignment
nanorod assembly
optical anisotropy
electro-optic

Supplementary materials

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Description
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Title
Quantifying Order during Field-driven Alignment of Colloidal Semiconductor Nanorods — Supporting Information
Description
TEM images and XRD pattern; schematics describing experimental setups for measuring time-dependent absorbance and ensemble fluorescence anisotropy; additional optical response and ensemble anisotropy data measured using an alternate batch of CdSe/CdS nanorods; overhead images of UV-illuminated sample cell with and without an external AC field; plot of average deviation angle vs focus factor (κ); derivation of theoretical expression for ensemble fluorescence anisotropy
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