Ion beam formation and modification of Cobalt nanoparticles

Abstract

This thesis has investigated the structural and vibrational properties of ion beam synthesized Co nanoparticles (NPs) and the influence of ion irradiation on the size, shape, crystallographic phase and structure. The evolution of the aforementioned properties were determined using a combination of laboratory and synchrotron based techniques, including cross-sectional transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS) and X-ray absorption spectroscopy (XAS). The results of this thesis have revealed a rich array of shape and structural transformations in Co NPs and highlights the effectiveness of a combined ion implantation/ion irradiation procedures in controlling and tuning the NP size, shape, crystallographic phase and structure. The structural and vibrational properties of Co NPs embedded in Si0{u2082}, formed by ion implantation were investigated as a function of the implantation concentration and post implantation annealing conditions. The implantation concentration and annealing temperature governed the spherical NP size and phase. X-ray absorption near-edge spectroscopy (XANES) was used to quantify the RCP, FCC and oxide fractions. The structural properties were characterized by extended X-ray absorption fine structure spectroscopy (EXAFS) and finite-size effects were readily apparent. With a decrease in NP size, an increase in structural disorder and a decrease in both the coordination number and bondlength were observed. The surface tension of Co NPs calculated using a liquid drop model was more than twice that of bulk material. The size-dependent vibrational properties were probed with temperature-dependent XAS measurements. Using a correlated anharmonic Einstein model and thermodynamic perturbation theory, Einstein temperatures for both NPs and bulk material were determined. Compared to bulk Co, the mean vibrational frequency of the smallest NPs was reduced as attributed to a greater influence of loosely-bonded, under-coordinated surface atoms relative to the effect of capillary pressure generated by surface curvature. Elemental Co NPs embedded in Si0{u2082} were irradiated with low energy Au ions and the evolution of the NP size and structure were then studied as a function of ion fluence. Upon irradiation, SAXS and TEM show that there was no change in the average NP size, in contrast to the short and long-range order where significant changes were observed. The coordination number decreased further while the mean value (bondlength), variance (Debye-Waller factor) and asymmetry (third cumulant) of the interatomic distance distribution all increased, as consistent with theoretical predictions for an amorphous elemental metal. Furthermore, the experimentally determined interatomic distance distribution for the irradiated Co NPs were in excellent agreement with molecular dynamics simulations for bulk amorphous Co and the observed structural changes were thus attributed to the formation of an amorphous phase. Such a crystalline-to-amorphous phase transformation is not readily achievable in bulk material (or large Co NPs) and the perturbed structural state prior to irradiation and the amorphous host matrix both contribute to nucleating and stabilising the amorphous phase in irradiated Co NPs. The Einstein temperature calculated from temperature-dependent XAS measurements for the unirradiated NPs showed a decrease in Einstein temperature. In contrast, that of the irradiated amorphous NPs was substantially higher than the bulk value. this apparent bond stiffening is attributed to the influence of the rigid surrounding matrix. When subjected to swift heavy ion irradiation, embedded FCC Co NPs were found to rapidly transform at low fluence to the HCP phase prior to any changes in size or shape. The crystallographic phase was identified with XAS and electron diffraction and quantified, as functions of the irradiation energy and fluence, with the former. The transformation was complete at low fluence and was governed by the electronic-energy-loss of the incident ions. A direct-impact mechanism was identified with the transformation interaction cross-section correlated with that of a molten ion track in amorphous Si0{u2082}. The swift heavy-ion irradiation-induced phase transformation was attributed to the large shear stress resulting from the rapid thermal expansion about an ion track in the Si0{u2082}. The size, shape and structural evolution of embedded Co NPs subject to high fluence swift heavy ion irradiation were also investigated over a wide energy region. Various electronic-energy-loss-dependent shape and structural changes in the Co NPs were observed. Depending on the irradiation energy, NPs below 4-7 nm remained spherical in shape and progressively decreased in size with fluence due to dissolution. NPs with sizes above a threshold diameter readily transformed into ellipsoids with their major dimension parallel to the incident ion beam direction. Modifications of the atomic-scale structure were characterised with XAS. Prior to irradiation, all Co atoms were in a metallic state. After SHIl, however, Co atoms were found in different atomic environments including: large elongated NPs, small spherical NPs and a large fraction of isolated Co atoms bonded to O in the matrix. The work presented in this thesis has resulted in the identification and understanding of a number of fundamental and technologically important effects in the formation and irradiation of Co NPs embedded in a Si0{u2082} matrix. Results demonstrate that the the size, shape and structure of Co NPs formed by ion implantation and thermal annealing in Si0{u2082} can be readily controlled by ion irradiation. This methodology represents an effective means of controlling the NP properties to best suit specific technological applications.