Synthesis and electron microscopy characterization of bimetallic nanoparticles and atomically controlled Au nanoclusters
The properties of metal nanoparticles are controlled by their composition, shape, size and crystalline structure. Nanoparticles and nanoclusters with controlled shape and size were synthesized and investigated using atomic resolution images from aberration corrected scanning/transmission electron microscopy (STEM) and mass spectrometry (MS). Gold-palladium (Au-Pd) core-shell nanocube and triangular nanoparticles were prepared by a seed-mediated growth process and the growth mechanism was studied by varying the volume of Pd precursors added to the Au seed solution. The atomic resolution STEM images revealed that the nanocube is formed from a single-crystal Au seed with rapid growth along <111> directions while the triangular nanoparticles were obtained with growth preferentially along <110> directions rather than <111> direction. The strain generated by the lattice mismatch between fcc -Au and fcc -Pd, is released by Shockley partial dislocations (SPD), combined with stacking faults (SF) that appear at the final (outer) Pd layer. Then, as the shell grows the SPDs and SFs appear at the interface and combine with misfit dislocations, which finally diffuse to the free surfaces due to the alloying of Au into the Pd shell. In related work, magneto-plasmonic gold-cobalt (Au-Co) nanoparticles of diameter 4-nm were generated by a phase-transfer process and investigated by STEM, where the Z-contrast imaging and energy dispersive x-ray spectroscopy (EDS) showed inhomogeneous alloying between Au and Co at the nanoscale. The observed ferromagnetic behavior carries significance in biomedical applications. In addition, selected metallic (Au144 (SR)60) and bimetallic (CuAu144) nanoclusters were obtained with thiolate-ligand protection and characterized using optical, MS, and STEM techniques. The optical spectrum and MS results established the monodispersity and purity of the nanoclusters. Another important aspect is that the emergence of broad strong plasmonic band centered near 520 nm (2.3-eV), by incorporation of single Cu atom into Au 144 nanoclusters, contrasts to the conventional view of 'non-plasmonic' response of sub-2.0-nm (Au, Cu) clusters. The high resolution STEM images and diffraction patterns, obtained from aberration corrected STEM, were used as an alternative technique to study the crystal structure of atomically defined Au 144 (SR)60 nanoclusters, where the images and diffraction patterns obtained, before they were altered by electron beam, were compared with theoretically simulated STEM images and diffraction patterns obtained from atomistic structural models derived from first principles density functional theory (DFT) calculations and confirmed the structure of Au 144 (SR) 60 as one featuring icosahedral shell packing. Finally, dodecane-thiolate protected gold (Au) nanoparticles of diameter ~ 4 ± 0.5 nm were prepared, in order to grow an ultrathin ordered film or superlattice of these nanocrystal-cores for investigation using STEM. The STEM imaging showed long-range hexagonally ordered superlattices of the nanocrystals, separated by the thiolate groups where the lattice constant determined by direct imaging are in good agreement with those determined by small-angle electron diffraction.