Optics of quasi-particle phase transitions in nanostructured Ag thin films

We describe and discuss XRD, AFM and optical absorption behaviour of nanostructured Ag films 2, 5 and 10 nm thick obtained by thermal evaporation. While all films are quasi-amorphous, 2 nm thick films exhibit nanotriangles in the nature of Mie particles which exhibit surface plasmon resonance (SPR). Increase of thickness produces a shape change from nanotriangle to spherical triangle and a red shifted SPR. Most interestingly systematic iodization throws up a quasi-particle transition namely plasmon–exciton transition even as Ag metal film is converted into an insulating/semiconducting AgI film.