(Invited) Variance Spectroscopy Studies of Single-Walled Carbon Nanotube Aggregation

Abstract

Understanding aggregation in samples of dispersed, well-individualized single-walled carbon nanotubes (SWCNTs) is important for many basic studies and applications. We will describe a study that uses the recently developed technique of variance spectroscopy to monitor aggregation kinetics of SWCNTs in aqueous surfactant dispersions. Unlike other techniques used to investigate colloidal dispersions, variance spectroscopy resolves individual subpopulations in mixed samples based on their distinct fluorescence signatures. Following the addition of NaCl coagulant to unsorted SWCNT dispersions, we monitored changes in the independent particle concentrations of six semiconducting (n,m) species. These data were supplemented by time-resolved measurements of fluorescence intensity, which showed pseudo-first order kinetics that was independent of SWCNT structure and was attributed to quenching by energy transfer to metallic SWCNTs. The decreases in particle concentration directly revealed aggregation. They were found to fit reasonably well to a second-order rate law with some dependence on structure at higher salt concentrations. Finally, covariance analysis was applied to the variance data to monitor correlated spectral fluctuations arising from specific heteroaggregates. Those results showed equivalent associations between different semiconducting SWCNT species during salt-induced aggregation.