Origins of Unconventional Magnetism in Coinage Metal Nanomaterials
Creators: name: Marenco, Armando J.
Publisher: name: Graduate Studies
Year: 2017
Subjects: subject: Chemistry--Physical
subject: unconventional magnetism
subject: coinage metals
subject: nanomaterials
subject: ferromagnetism
subject: nanoparticles
subject: thin films
subject: nonstoichiometric oxidation
subject: magnetron sputtering

description: The studies presented in the dissertation focused on the unconventional magnetic properties of coinage metals - Cu, Ag, and Au - nanomaterials synthesized in the gas phase by sputtering. Unlike other common synthetic methods, gas-phase synthesis creates nanoparticles and thin films free of capping ligands allowing for pristine surface studies. The two types of nanomaterials synthesized were sub-12 nm diameter nanoparticles and thin films. These three parameters, namely the nature of the coinage metal, unmodified surfaces, and nanodimensionality, were the core effects independently studied. The unconventional magnetism of Cu nanomaterials has been studied in highly-pure and capping-ligand-free nanoparticles and thin films. Superconducting quantum interference device (SQUID) room-temperature (300 K) measurements displayed no size correlation to the ferromagnetic behavior observed in Cu nanoparticles ranging from 4.5 +/- 1.0 nm to 9.0 +/- 1.8 nm in diameter. Moreover, magnetic quartz crystal microbalance (MQCM) in situ tests of 4.5 +/- 1.0 nm nanoparticles under vacuum conditions showed magnetic behavior only after the onset of oxidation. SQUID analysis conducted on Cu thin films exposed to several heat treatments demonstrated minor oxidation inducing higher ferromagnetic responses compared to extended oxidation. Further analysis of nanomaterial samples exhibiting the highest magnetic responses indicated an atomic ratio of ~3-5:1 Cu:O suggesting non-stoichiometric oxidation as the source of the ferromagnetic signature. Similar ferromagnetic results were obtained for Ag nanomaterials. No size correlation to magnetism in Ag nanoparticles ranging from 3.3 +/- 0.9 nm to 7.8 +/- 1.3 nm was determined by SQUID magnetometry. Additionally, MQCM under vacuum conditions of 3.3 +/- 0.9 nm Ag NPs shows magnetic behaviour only after the onset of oxidation. The trend in SQUID magnetometry shows a higher saturation magnetization for samples exposed to oxygen compared to inert atmospheres, which is opposite to the Cu findings. However, this also indicates non-stoichiometric oxidation at the surface as the reason for the observed magnetism as supported by an ~8:1 Ag:O ratio from MQCM measurements. Finally, heat-treated Ag thin films present a lower saturation magnetization compared to those kept in oxygen without heating. This latter observation could be the result of driving the oxidation to stoichiometric surface oxides AgO and Ag2O which are known to be diamagnetic. For Au nanomaterials, our findings show some promise but are inconclusive. A single diameter was synthesized at 3.9 +/- 1.7 nm. Unlike the MQCM studies observed for Cu and Ag, Au NPs show no significant signal change which could be interpreted as ferromagnetic behaviour. However, SQUID magnetometry does provide a clear ferromagnetic signal. Further studies are required to determine for certain if oxidation plays a role in Au nanomaterials as determined for Cu and Ag.