Synthesis of SixGe1-x Nanocrystals Using Hydrogen Silsesquioxane and Soluble Germanium Diiodide Complexes

We report an investigation into the formation of SixGe1-x alloy nanocrystals (64 < x < 100) synthesized from mixing GeI2:PR3 adducts with hydrogen silsesquioxane (HSQ). The use of triallcylphosphine adducts allows GeI2 and HSQ to be homogeneously coprecipitated, improving control over the size and composition of the resulting SixGe1-x nanocrystals. This approach yields oxide-embedded and freestanding materials with near-infrared photoluminescence (PL) comparable in quantum efficiency to similarly prepared Si nanocrystals. The formation of bimodal populations of Si-rich and Ge-rich nanocrystals was observed, with homogeneous distribution of Ge within each population. Through changes to precursor stoichiometry and annealing temperature and time, control over particle size and composition was demonstrated. The impact of these factors on the near-IR PL was evaluated. We propose a multistep formation mechanism to account for the formation of separate Si-rich and Ge-rich populations and present indirect evidence for the participation of Ge in the emission process. Materials were analyzed using Fourier transform infrared spectroscopy, Raman spectroscopy, powder X-ray diffraction, PL spectroscopy, high-resolution transmission electron microscopy and spatially resolved energy-dispersive X-ray spectroscopy.