Efficient measurement of length distribution of 1D nanoparticles in solution via electric-field alignment and optical polarimetry

The efficient measurement of the length distribution of nanotubes, nanowires, and other 1D nanoparticles in solution is important to enable their incorporation into materials and devices, and to optimize their solution-based processing. Existing length-measurements techniques like atomic-force microscopy or electron microscopy require a deposition and drying step and are time-consuming and tedious. Other techniques like analytical ultracentrifugation are best suited for measuring shorter nanoparticles in the sub-micron range but are less useful for several-micron-long particles. Here, we report an electric-field- (E-field-) assisted optical-polarimetry technique for efficiently measuring the length distribution of ensembles of longer high-aspect-ratio particles in dilute suspension. The degree of alignment of polarizable 1D particles suspended in a fluid under Brownian motion explicitly depends on the E-field strength and the particle length. We show that it is possible to extract the length distribution of the 1D nanoparticles suspended in an insulating fluid by applying a range of E-fields and using optical polarimetry to measure the corresponding alignment-order parameter. Notably, the method is relatively insensitive to the diameter of the 1D particles, which can be poorly known or vary within a sample. The technique is validated with silver nanowires and carbon nanotubes grown to known lengths, as well as with polymer-depletion-length-separated single-wall-carbon-nanotube samples with length distributions independently measured with analytical ultracentrifugation. Finally, we demonstrate the ability of the technique to quantify changes in length distribution of ultra-narrow, sub-nanometer-diameter SWCNT samples with different durations and types of ultrasonication. Within its range of applicability (polarizable 1D nanoparticles in the 0.5 – 15 micron length range), the E-field-assisted optical-polarimetry method is a particularly efficient and accurate method to measure the length distribution of nanowires and nanotubes in suspension.