Estandarte, Ana Katrina C; (2017) Diffusion in Nanocrystals by Coherent Diffraction Imaging. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

My PhD involved the use of various imaging techniques in investigating the structure of nanocrystals and chromosomes. Results obtained from my work on the investigation of chromosome structure have previously been published. This thesis presents my work on the investigation of diffusion and alloying behaviour in individual gold nanocrystals using coherent diffraction imaging (CDI). Understanding how diffusion and alloying works for nanocrystals is of great importance for their stability and controlled synthesis. Existing methods for diffusion studies have limitations, thus requiring the development of new techniques that will allow the diffusion in individual nanocrystals to be investigated at the atomic scale in 3D. The CDI experiments were performed at the I-07 beamline at Diamond Light Source, UK. The diffraction pattern of individual gold nanocrystals was measured around the (11-1) Bragg peak of gold before and after metal deposition as a function of temperature and time. Phase retrieval algorithms were used to obtain real space reconstructions of the nanocrystals from the measured diffraction patterns. For the gold-iron system, alloying of iron with the gold nanocrystals at sample temperatures of 300°C-500°C and dealloying of iron from the gold nanocrystals at 600°C were observed. The volume of the alloyed region in the nanocrystals increased with increasing dose of iron. However, no time dependence was observed for the structure of the nanocrystals after the iron depositions suggesting that the nanocrystals reached equilibrium relatively quickly. The resulting phase pattern of the nanocrystals after the iron depositions suggested the contraction of the nanocrystals and formation of stacking faults due to the diffusion of iron into the nanocrystals. For the gold-iron-platinum alloy, its formation was not observed at 300°C but observed at 400°C. The results show that CDI is a useful technique in studying the 3D diffusion and alloying behaviour in individual nanocrystals at the atomic scale.

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