I will describe our group’s efforts to understand heterogeneity in the chemical reactivity of semiconductor nanocrystals by imaging reaction events one at a time. In one project, we have used the change in fluorescence intensity to image the transformation of individual lead bromide (PbBr2) nanocrystals to methylammonium lead bromide (CH3NH3PbBr3) via intercalation of CH3NH3Br. Analyzing this reaction one nanocrystal at a time reveals information that is masked when the fluorescence intensity is averaged over many particles. Sharp rises in the intensity of single nanocrystals indicate they transform much faster than the time it takes for the ensemble average to transform. Based on these observations, we propose a phase-transformation model in which the solid-state immiscibility between PbBr2 and CH3NH3PbBr3 initially create a high energy barrier for ion intercalation.
In related work, we have used chemically-triggered fluorogenic probes to study the spatial distribution of catalytically active regions in individual tungsten oxide nanowires using super-resolution fluorescence microscopy. Activation of the first probe molecule requires photoexcitation above the bandgap of the semiconductor to generate hydroxyl radicals. The second reaction does not require photoexcitation but instead relies on the presence of either oxygen vacancies or hydroxyl groups at the surface of the nanowires. Through quantitative, coordinate-based colocalization of probe molecules activated by the same nanowires, we demonstrate that the nanoscale regions most active for the photocatalytic generation of hydroxyl radicals also possess a greater concentration of oxygen vacancies.









Bryce Sadtler (Washington University in St. Louis)