Atomic structure and chemical analysis of metal nanoparticles by scanning transmission electron microscopy

[摘要] This thesis explores the use of aberration-corrected Scanning Transmission Electron Microscope(ac-STEM) to characterise the size, atomic structure and chemical composition of different types of nanoparticles, including Ag clusters produced in a matrix assemblym cluster source (MACS), chemically-synthesised monolayer protected Au clusters, metal oxide nanoparticles and bimetallic nanoparticles. The size and density of clusters produced in the MACS with different experimental parameters are characterised by STEM High-Angle Annular Dark Field (HAADF) imaging, shedding light on the capabilities of this new cluster source.A statistical investigation is conducted into the size and atomic structure of nominally Au144(SR)60 clusters synthesised by two different research groups. The clusters are “weighed” against size-selected clusters (i.e. atom counting) and the structures are compared to several model structure simulations. A ring-dot feature, characteristic of a local icosahedral order, is frequently observed. The oxidation states of CeOx, CuOx, CoOx and FeOx nanoparticles are investigated by Electron Energy Loss Spectroscopy (EELS) in the STEM. All these samples show a mixing oxidation states. HAADF images are used to analyse the elemental distribution of Cr-Pt bimetallic nanoparticles. Comparison between the experimental and simulated HAADF intensity profiles indicates that Cr and Pt atoms are mixed. EELS mapping and energy dispersive X-ray spectroscopy (EDX) are used to characterise hydrothermally-prepared FeCo oxide nanoparticles. The results show that the Fe-Co nanoparticles synthesised with iron nitrate precursors are less crystallised and the elemental distributions of Fe and Co are not uniform. While the nanoparticles synthesised with ammonium iron citrate are well crystallised and the elemental distributions are homogenous.

[发布机构] University:University of Birmingham;Department:School of Physics and Astronomy

[关键词] Q Science;QC Physics