Plasmonic evolution of atomically size-selected Au clusters by electron energy loss spectrum.

The plasmonic response of gold clusters with atom number ( N ) = 100-70 000 was investigated using scanning transmission electron microscopy-electron energy loss spectroscopy. For decreasing N , the bulk plasmon remains unchanged above N  = 887 but then disappears, while the surface plasmon firstly redshifts from 2.4 to 2.3 eV above N  = 887 before blueshifting towards 2.6 eV down to N  = 300, and finally splitting into three fine features. The surface plasmon's excitation ratio is found to follow N 0.669 , which is essentially R 2 . An atomically precise evolution picture of plasmon physics is thus demonstrated according to three regimes: classical plasmon ( N  = 887-70 000), quantum confinement corrected plasmon ( N  = 300-887) and molecule related plasmon ( N  < 300).