Development of a new in situ x-ray diffraction technique for characterising embedded nanoparticles
thesisposted on 31.01.2017 by Zatsepin, Nadia Alexandrovna
In order to distinguish essays and pre-prints from academic theses, we have a separate category. These are often much longer text based documents than a paper.
A new non-destructive, high resolution X-ray diffraction technique is developed for the characterisation of ensembles of embedded nanoparticles. The method is based on reciprocal space mapping using an analyser crystal, making it sensitive to very low diffraction contrast between nanoparticles and their surrounds, and capable of encompassing a large volume, representative of the bulk material. The robustness of the technique is demonstrated by its lack of dependence on the X-ray coherence volume and optical stability. In addition, the use of a counting detector provides the necessary high dynamic range, and avoids the restrictions imposed by the finite pixel size of a direct space detector and loss of information due to a beamstop. We review the most widely used techniques for imaging on the nanometre scale and highlight their unique capabilities. We then demonstrate that no single technique alone is sufficient for model independent, non-destructive, nanoscale characterisation of embedded nanoparticles in a bulk material sample. In situ and real-time investigations are imperative for the understanding, and ultimately the control of nanoparticle nucleation and growth in technologically important alloys, colloidal suspensions and various nanomaterial specimens. In this thesis we make significant progress in addressing this crucial omission. We begin by presenting the particulars of scalar diffraction theory that enable us to mathematically describe kinematic diffraction from large ensembles of nanoparticles embedded in a matrix. The requirements of X-ray optics are then discussed, from the pertinent properties of synchrotron X-ray sources through high quality analysing and monochromating crystals. A method of simulating Fraunhofer diffraction and reciprocal space maps from a large, sparse ensemble of weakly diffracting Al-Cu nanoparticles is deduced from elementary coherence considerations. We then demonstrate that quantitative information regarding the nanoparticle ensemble polydispersity can be extracted from the reconstructions of nanoparticles from the Fraunhofer diffraction patterns of numerous such ensembles. In simulated reciprocal space maps we examine the effects of nanoparticle ensemble polydispersity and nanoparticle orientation with respect to the diffraction plane. Experimentally obtained reciprocal space maps of diffracted intensity from nanoparticles in an Al-Cu alloy are then presented, demonstrating the sensitivity of the technique to weakly diffracting embedded nanoparticles and their orientation relative to the diffraction plane. Here we also present the results of an iterative algorithm applied to reconstruct, with <10nm resolution, a two dimensional nanoparticle, representative of the ensemble. Practical considerations for an in situ, real-time X-ray diffraction investigation of the initial growth dynamics of embedded nanoparticles in a bulk material sample are explored in a pilot experiment. Finally, the results from an experimental demonstration of the first, real-time, in situ X-ray diffraction investigation of the early stages of nanoparticle growth (in Al-Cu alloys) are then presented and analysed in the context of clustering and dynamic strain in the sample. Simulations involving a simplified model of local strain are shown to be well correlated with the X-ray diffraction data, and a modal, representative nanoparticle size is determined, which agrees with data from transmission electron micrographs of the sample. In conclusion, current ongoing nanoparticle reconstruction efforts are discussed alongside the future directions suggested as an extension of the nanoparticle characterisation technique.