Synthesis and characterization of magnetic-optical Co-Au core-shell nanoparticles

We report the successful synthesis of Co-Au core-shell nanoparticles by reducing an organo-gold compound onto cobalt seeds with a weak reducer in a nonpolar solvent. The core-shell morphology was unequivocally confirmed by complementary structural, magnetic, and optical property measurements. High-resolution transmission electron microscopy and Z-contrast imaging shows a gold shell composed of multiple grains. Electron-energy loss spectroscopy confirms the chemically distinct characteristics of the core (Co) and shell (Au). Temperature-dependent magnetic property measurements confirm that the particles are superparamagnetic with a blocking temperature, T_B ∼ 55 K, consistent with a magnetic diameter ∼6 nm. The UV-visible absorption spectra of these nanoparticles show a red shift (relative to pure gold nanoparticles) in agreement with a Au-shell morphology.