Characterization of Core-Shell Nanoparticles
Noble metals such as Ag and Au has very interesting optical properties due to so-called surface plasmon resonance which is related resonance of collective oscillation of conduction electrons with alternating electric field of incident electromagnetic radiation. Hence, Au and Ag nanoparticles with ca. 15 nm have sharp characteristic surface plasmon resonance bans at 520 nm and 400 nm respectively. Any spectral changes due to size, shape composition and charging ability of the nanoparticles are extensively studied in our group.
The following figure display time evaluated formations of Au@Ag and Ag@Au nanoparticles by UV-Vis Spectroscopy. When Ag+ ions are reduced onto preformed Au nanoparticles Au(core)@Ag(shell) nanoparticles with ca. 25 nm are obtained. The formation of Ag nanoshell onto preformed Au nanocore can be followed by the large blue-shift of SPR band of Au nanoparticles (520 nm) towards 400 nm, which is the characteristic SPR band of Ag nanoparticles.
Incorporating metal nanoparticles within dielectric shells introduces extra stability, and new optical and dielectric properties. Au and Ag Metal(core)@Dielectric(shell) nanoparticles with ca. 5-15 nm core diameter and ca. 3-5 nm shell thickness are prepared using procedures reported in the literature. Characterization and dielectric properties of the corresponding particles are carried out using UV-Vis spectroscopy, and XPS, recorded while the samples are subjected to external voltage bias. Binding energy shifts as a function of the external bias and its polarity are monitored for the corresponding to the metal in the core, and the dielectric in the shell for assessing the extent of charging in each layer. Binding energy shifts in the XPS peaks as a function of the external bias and its polarity are monitored for the corresponding metal in the core, and the dielectric in the shell for assessing the extent of charging in each layer. Dependence of charging behavior on the chemical nature of the metal core (Au or Ag), and/or the dielectric shell (SiO2 or TiO2) is also studied.
Representative XPS spectra of Au(core)@SiO2(shell) on a Cu substrate, under ±10V external bias is depicted in the following figure. On top of the figure TEM image of the corresponding particles is also depicted.
