Synthesis and characterization of germanium/Si-alkyl and germanium/silica core-shell quantum dots

Ge/Si-R and Ge/SiO(2) core-shell nanoparticles have been synthesized by a solution route from the stepwise reactions of the Zintl salt, Mg(2)Ge, with SiCl(4) and subsequently either RLi (R = butyl) or H(2)O(2). High-resolution transmission electron microscopy (HRTEM) images show that the core part of these Ge/Si-R and Ge/SiO(2) nanoparticles is crystalline and the d spacing of lattice fringes agrees with the {111} crystal pane (3.27 Angstrom) of Ge. The average sizes (diameter) are 5.8(2.3) and 6.8(2.8) nm for Ge/Si-R and Ge/SiO(2), respectively. Fourier transform-infrared (FTIR) spectra are consistent with alkyl groups bonded to the silicon surface in Ge/Si-R nanoclusters. Off-axis holography was performed for several Ge/SiO(2) nanoparticles. The characterization of the ratio of radii and geometry between the core part and the outer sphere of Ge/SiO(2) is presented. The reconstructed phase image from the hologram offers detailed information concerning the ratio of the shell thickness (R - r) to the radius of the core (r), Rr = (R - r)/r, by measuring the phase shift and the resultant phase contrast. The Rr value from the deconvoluted phase image of a 16 nm spherelike Ge/SiO(2) is 0.264 (0.065). Photoluminescence (PL) spectra of both Ge/Si-R and Ge/SiO(2) nanoclusters show a strong UV-blue PL with a peak centered between 400 and 410 nm. These results suggest that the UV-blue PL may be attributed to the core part of these nanoparticles.