Core-shell nanoparticles with ca. 15-nm gold core and 6-nm silica shell were prepared and characterized by XPS. The Au/Si atomic ratio determined by XPS is independent of the electron takeoff angle because of the concentric spherical shape of the nanoparticles. The formula given by Wertheim and DiCenzo (Phys. ReV. B 1988, 37, 844) for spherical nanoparticles and the modified one by Yang et. al. (J. Appl. Phys. 2005, 97, 024303) for core-shell nanoparticles are used to correlate the XPS-derived composition with the geometry of the nanoparticles only after significantly modifying either the bulk density of the silica shell or the attenuation length of the photoelectrons.

Label free imaging of the chemical environment of biological specimens would readily bridge the supramolecular and the cellular scales, if a chemical fingerprint technique such as Raman scattering can be coupled with super resolution imaging. We demonstrate the possibility of label-free super-resolution Raman imaging, by applying stochastic reconstruction to temporal fluctuations of the surface enhanced Raman scattering (SERS) signal which originate from biomolecular layers on large-area plasmonic surfaces with a high and uniform hot-spot density (>1011/cm2, 20 to 35 nm spacing). A resolution of 20 nm is demonstrated in reconstructed images of self-assembled peptide network and fibrilated lamellipodia of cardiomyocytes. Blink rate density is observed to be proportional to the excitation intensity and at high excitation densities (>10 kW/cm2) blinking is accompanied by molecular breakdown. However, at low powers, simultaneous Raman measurements show that SERS can provide sufficient blink rates required for image reconstruction without completely damaging the chemical structure.

The photocatalytic activity of TiO 2 under sunlight irradiation depends on the bandgap energy. Due to the relatively low solar intensity in the UV region (<10%) and the fact that the bandgap of TiO 2 is usually greater than 3 eV (below 400 nm), many attempts have been made to shift the bandgap towards lower energies. Here, we investigate the structure, chemical composition, bandgap shift and charge transfer processes of Ag@TiO 2 core-shell nanoparticle thin films by field emission scanning electron microscopy, atomic force microscopy, XPS, and UV-Vis spectroscopy. As a solid support, Au-coated Si wafers and Si surface covered with a native oxide were used and homogenously covered by Ag@TiO 2 core-shell nanoparticles with overall film thicknesses of 80-100 nm and size distributions between 8 and 15 nm. The shell thickness of the adsorbed Ag@TiO 2 particles was estimated to be 1.5-2.0 nm. The effect of the Ag core on the bandgap of TiO 2 and photoinduced charge separation of Ag@TiO 2 nanoparticle films was studied by UV-Vis reflectance spectroscopy using the Kubelka-Munk formalism. Films of Ag@TiO 2 core-shell nanoparticles revealed a substantially reduced bandgap of 2.75 eV (corresponding to 450 nm), and an electron charge transfer to the Ag core occurring upon UV irradiation on nonconductive surfaces. These features make Ag@TiO 2 particulate films a promising candidate for photocatalytic surfaces under sunlight irradiation.

a b s t r a c tAu and Au-Pt alloy nanoparticles are prepared and patterned at room temperature within the PMMA polymer matrix by the action of 254 nm UV light or X-rays. The polymer matrix enables us to entangle the kinetics of the photochemical reduction from the nucleation and growth processes, when monitored by UV-vis spectroscopy. Accordingly, increase of the temperature to 50 C of the reaction medium increases the nucleation and growth rates of the nanoparticle formation by more than one order of magnitude, due to enhanced diffusion and nucleation at the higher temperature, but has no effect on the photochemical reduction process. Presence of Pt ions also increases the same rate, but by a factor two only. Similar photochemical reduction and particle growth take also place within the PMMA matrix, when these metal ions are subjected to prolonged exposure to X-rays, as evidenced by XPS analysis. Both angle-resolved and charge-contrast measurements using XPS reveal that the resultant Au and Pt species are in close proximity to each other, indicating the Au-Pt alloy formation to be the most likely case.

Prolonged exposure to X-rays of HAuCl 4 , PtCl 4 and their mixtures, deposited from an aqueous solution onto a silicon substrate, causes chemical reduction of the metal ions to their metallic states. The corresponding oxidation reaction is the conversion of chloride ions to chlorine. The resultant metal atoms aggregate to form metallic/bimetallic nanoclusters as evidenced from their XPS chemical shifts. Hence, X-rays are usable for in-situ nanoparticle production or for direct-writing applications on silicon substrates.