Investigation of the dissipative properties of Au-Ag nanostructures through Electron Energy Loss Spectroscopy

We describe the unusual dielectric properties of Au-Ag nanostructures at a single particle level using electron energy loss spectroscopy. It has been shown previously that these nanostructures deviate from their usual metallic characteristics of metallic Ag and Au. In particular certain variants of these materials appear to undergo a transition to an immeasurably low resistive state as well as a strong diamagnetic state that is not usually associated with Au and Ag themselves. More significantly these nanostructures exhibit absence of Mie like resonances associated with Au and Ag. Instead, these exhibit negligible optical absorption in the UV-VIS-NIR range. Here this unconventional electromagnetic response was probed at a single particle level. We find that the absence of dissipation is also reflected in the electron loss spectra. The lack of distinct electron energy loss features in particles with a known composition enables us to unambiguously assign these observations to actual particle properties. We tentatively interpret our observations as arising due to a reduction of free electron density in Au and Ag nanostructures. Further it was also shown that the conventional plasmonic modes can be recovered by altering material composition.