Synthesis and Characterization of Monodisperse Metallodielectric SiO₂@Pt@SiO₂ Core–Shell–Shell Particles

Metallodielectric nanostructured core−shell− shell particles are particularly desirable for enabling novel types of optical components, including narrow-band absorbers, narrow-band photodetectors, and thermal emitters, as well as new types of sensors and catalysts. Here, we present a facile approach for the preparation of submicron SiO2@Pt@SiO2 core−shell−shell particles. As shown by transmission and scanning electron microscopy, the first steps of this approach allow for the deposition of closed and almost perfectly smooth platinum shells onto silica cores via a seeded growth mechanism. By choosing appropriate conditions, the shell thickness could be adjusted precisely, ranging from ∼3 to ∼32 nm. As determined by X-ray diffraction, the crystalline domain sizes of the polycrystalline metal shells were ∼4 nm, regardless of the shell thickness. The platinum content of the particles was determined by atomic absorption spectroscopy and for thin shells consistent with a dense metal layer of the TEM-measured thickness. In addition, we show that the roughness of the platinum shell strongly depends on the storage time of the gold seeds used to initiate reductive platinum deposition. Further, using polyvinylpyrrolidone as adhesion layer, it was possible to coat the metallic shells with very homogeneous and smooth insulating silica shells of well-controlled thicknesses between ∼2 and ∼43 nm. After depositing the particles onto silicon substrates equipped with interdigitated electrode structures, the metallic character of the SiO2@Pt particles and the insulating character of the SiO2 shells of the SiO2@Pt@SiO2 particles were successfully demonstrated by charge transport measurements at variable temperatures.