Analysis of Magic-Size Clusters in Crude Reaction Mixtures Using Multiwavelength Analytical Ultracentrifugation

Magic-size clusters are ultrasmall, transient species that appear early during semiconductor nanocrystal growth and have been assigned to stoichiometrically and geometrically well-defined structures. Yet, a direct analysis has proved difficult due to their limited stability and challenging isolation. Analytical ultracentrifugation is a promising tool to analyze these compounds, because it circumvents the need to purify and, thereby, destabilize the clusters, while yielding information on their sedimentation and diffusion behavior, size, shape, and composition. We employ a multiwavelength detector that allows the detection of whole UV/vis absorption spectra during centrifugation to separate and identify individual magic-size clusters in crude reaction mixtures sampled directly from the colloidal syntheses. Analytical ultracentrifugation thus constitutes an elegant, independent, high-resolution, and statistically significant method to show the stepwise growth of magic-size clusters, confirming recent growth models.