Optical properties of nanocomposites with iron core–iron oxide shell structure

Iron nanoparticles of diameter ∼5 nm were produced within a gel-derived silica glass by reducing a suitable gel composition. By heating these composites in the temperature range 573–973 K, Fe3O4 shells of a few nanometer thickness were grown around the iron nanoparticles. Three peaks were observed in the optical absorption spectra of the nanocomposites when they were dispersed in ethyl alcohol. The first one around 300 nm was caused by plasma resonance absorption of unoxidized iron particles; the second was shown to be due to the core–shell structure with different permittivities of the two regions and the third one was ascribed to a d–d transition. Detailed analyses of the second peak showed that the extracted values of electrical conductivity were below Mott’s minimum metallic conductivity for iron in the case of particles with diameters below ∼2.5 nm.