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

Core/shell Si/SiO _x nanoparticles (Si/SiO _x -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/SiO _x -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 P_b centres with the concentration up to 4 × 10¹⁸ particles/g. At the annealing temperature, T _an > 900 °C, a rapid growth of nanocrystal sizes takes place, and, simultaneously, a rapid growth of paramagnetic P_b 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/SiO _x -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 T _an at the stage I rises from 350 to 1100 °C. At relatively low T _an = 350 °C, the nanoparticles with monocrystalline Si cores are mainly formed, while at T _an > 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.