Synthesis of photoluminescent Si/SiOx core/shell nanoparticles by thermal disproportionation of SiO: structural and spectral characterization

Core/shell Si/SiOx nanoparticles (Si/SiOx-NP) having bright red-infrared photoluminescence were obtained by a three-stage synthesis based on the thermal disproportionation of microdispersed SiO. Transformation patterns of structure and spectroscopic properties of the material during passage through all process stages (starting from initial SiO microparticles and up to the Si/SiOx-NP sols) have been revealed by using Raman, photoluminescence and ESR spectroscopy, XPS, XRD, and electron microscopy. Thermal annealing of SiO microparticles (stage I) results in formation of amorphous-crystalline Si nanophase in the matrix of SiO2, as well as generation of paramagnetic Pb centres with the concentration up to 4 × 1018 particles/g. At the annealing temperature, Tan > 900 °C, a rapid growth of nanocrystal sizes takes place, and, simultaneously, a rapid growth of paramagnetic Pb centre concentration occurs. Elimination of SiO2 from the annealed sample by etching in HF (stage II) stimulates further crystallization of amorphous-crystalline core, caused by stress relaxation inside the Si core when removing SiO2 matrix. Functionalization of nanoparticle surface (stage III) allows obtaining core/shell Si/SiOx-NP with a bright red-infrared photoluminescence and their sols. Average size of the crystalline Si core increases from 4.7 to 11.1 nm when Tan at the stage I rises from 350 to 1100 °C. At relatively low Tan = 350 °C, the nanoparticles with monocrystalline Si cores are mainly formed, while at Tan > 1100 °C, a large number of polycrystalline Si nanoparticles are also observed. Our TEM images have revealed the existence of monocrystalline Si nanoparticles having significantly different contrast even at comparable nanoparticle sizes. We attribute that to the formation of both bulk (with a high TEM contrast) and flat (2D) Si nanocrystals (with a low TEM contrast) in the course of SiO annealing.