How does a plasmon, the quintessential collective excitation of charge carriers, emerge from the response of individual carriers? I will discuss results of experiments on nanocrystals toward answering this question. Starting from a serendipitous finding of localized surface plasmon oscillations in nanocrystals of copper sulfide—the same material responsible for the recent LK-99 red herring—we were led to plasmon resonances in ultrasmall nanocrystals of zinc oxide. These plasmon oscillations are sustained by a handful of charge carriers much smaller in number than previously thought to be possible. Such few-electron collective oscillations expand the conventional view of plasmon resonances and potentially allow tests of electron–electron correlation models.
