Morphological changes of Au nanoparticles in Au–polystyrene nanocomposites: A combined spectroscopy, atomic force microscopy and X-ray scattering study

Au nanoparticles sputter deposited on polystyrene-coated Si and fused quartz substrates have been studied using optical spectroscopy, atomic force microscopy and X-ray reflectivity. Under the same deposition conditions, both spectroscopy and atomic force microscopy indicate clearly that the nanoparticles undergo a shape transition from near-spherical to progressively ellipsoidal as the polystyrene film becomes thinner than 4Rg, Rg being the radius of gyration of the polymer. There is a gradual increase in the in-plane ellipticity a/b, a(b) = semi-major (semi-minor) axis, with decrease in polystyrene film thickness from 230 to 20 nm, where b remains almost invariant for a particular deposition time while the semi-major axis increases in dimension. Electron density profiles along the depth of the films, extracted from X-ray reflectivity data, show that the Au nanoparticles sit on the top of the polystyrene film with c, the third ellipsoid axis having a dimension of about 3.0 nm irrespective of film thickness or deposition times used (7, 10, 12 and 15 s).