Evolution of plasmonic response of individual size-selected Au clusters from nanoscale to atomic-scale

We have studied the evolution of plasmonic response of individual size-selected Au clusters by scanning transmission electron microscopy-electron energy loss spectrum (STEM-EELS) from nanoscale to atomic scale. The clusters are prepared and selected by megnetron sputtering time-of-flight cluster source to get an atomic number precision. To distinguish surface and bulk plasmon mode, we put the electron beam at the edge or center of clusters, where we can get a pure surface mode or a mixure of surface and bulk mode. Both surface and bulk mode exist when the gold clusters are large enough. However when the atom numbers decrease to 887, the bulk mode disappears abruptly while the surface mode still appears. The surface mode excitation probability of a single incident electron shows a square relationship versus atom numbers from 70000 to 300, but deviates from it when the atom number reaches 175. What’s more, a new peak higher than surface mode appears when the gold clusters are smaller than Au₅₀₀. After a slight redshift from Au₇₀₀₀₀ to Au₈₈₇, an obvious blue-shift happens in the range Au₈₈₇ to Au₃₀₀, showing a combined influence between quantum size effect and classical size effect of electron mean free path.