Advanced Algorithms for Quantitative Electron Tomography
22 November 2017
Campus Groenenborger, U0.24 - Groenenborgerlaan 171 - 2020 Antwerpen (route: UAntwerpen, Campus Groenenborger
PhD defence Daniele Zanaga - Faculty of Science, Department of Physics
In the last decades researchers started developing the ability to manipulate matter at the nano and atomic scales. The characterization of these materials revealed the influence of size, structure and composition on the peculiar properties exhibited. A fundamental instrument aiding the development of new nanomaterials by enabling their observation is the Transmission Electron Microscope (TEM). However, conventional TEM only allows for two-dimensional imaging of specimens, often hindering a complete characterization. Combination of TEM and tomography overcomes this limitation, allowing to retrieve a three-dimensional reconstruction of the analyzed sample.
The increasing complexity of synthesized systems though, built in the attempt of achieving particular properties for applications in several fields such as catalysis, signal enhancement or drug delivery, poses new challenges to researchers involved in their characterization. The development of new methods, techniques and instruments is therefore necessary in such occasions in order to obtain a complete description of these samples.
An example of complex systems requiring a challenging characterization is given by nanoparticle assemblies. These structures, created by promoting the self-assembly of hundreds or thousands of nanoparticles, can extend for hundreds of nanometers or even microns, with either an ordered or disordered configuration. Their properties can be tuned by changing the positions of the building blocks, the type of packing and the inter-particles distances. A thorough quantitative characterization is therefore needed to study the relationship between structure and properties, and how changing the former can influence the latter.
Another fundamental problem that has been tackled extensively by several research groups in the recent years, is the determination of the three-dimensional elemental distribution in nanostructures, which can be achieved by combining Energy Dispersive X-ray Spectroscopy (EDXS) and tomography. The recent introduction of multiple detectors systems such as FEI Super-X detector, finally enabled this combination, but earlier attempts, although producing promising results, were still hampered by instrument limitations and lack of proper EDXS quantification methods.
My work as a PhD student at EMAT has been focused on the development of techniques for electron tomography, and specifically oriented at the quantitative analysis of nanoparticles assemblies as well as quantitative EDXS analysis of metal nanoparticles in 2D and 3D. For this reason, the thesis is divided into three main parts, an introduction on the techniques used, a part on the quantitative analysis of nanoparticle assemblies and finally a part on quantitative EDXS tomography.
In more detail the layout is as follows:
Part 1: Introduction
- Chapter 1: Historical perspective, presents the main historical events that brought to the development of the electron microscope.
- Chapter 2: Electron tomography principles, summarizes the main concepts and state-of-the-art of electron tomography, introducing the foundations, applications and limitations of the technique on which this thesis work is based on.
- Chapter 3: Electron tomography in practice, presents the experimental equipment used, and the steps involved in a typical electron tomography experiment, from a practical point of view.
Part 2: Quantitative tomography of nanoassemblies
- Chapter 4: Quantitative tomography of nanoparticle assemblies covers the development of the so called Sparse Sphere Reconstruction technique, aimed at the characterization of complex nanoparticles assemblies.
- Chapter 5: Sparse sphere reconstruction in materials science studies further introduces experimental studies where SSR was applied to perform a quantitative characterization of different systems, with a focus on the technique extension to the case of binary assemblies.
- Chapter 6: Conclusions and outlook on part II summarizes the results presented in this second part of the thesis, including a discussion on the novelty introduced by the technique and the outlook for future applications. A detailed list of own contributions to the work is provided at the end of this chapter.
Part 3: Quantitative EDXS tomography
- Chapter 7: Quantitative EDXS in 2D covers EDXS characterization and quantification of nanomaterials. In more detail, the ζ-factor method is introduced and a technique is developed for the measurement of ζ-factors from pure elemental nanoparticles. Experimental cases are shown as a validation of the method and as an example of quantitative EDXS studies.
- Chapter 8: Quantitative EDXS in 3D further extends the domain of quantitative EDXS to 3D. Here, a new method developed to obtain quantitative EDXS tomographic reconstructions is presented, showing in detail how limitations such as shadowing effects and low morphological resolution of EDXS are overcome by combining EDXS and STEM tomography in a synergistic approach.
- Chapter 9: Quantitative 3D EDXS tomography, materials science studies. The technique presented in the previous chapter is applied here to several materials science studies. Insights concerning the synthesis of these materials and their properties are obtained thanks to this characterization.
- Chapter 10: Conclusions and outlook on part III summarizes the results presented in this third part of the thesis, and discusses the novelties introduced by the techniques presented and the outlook for future applications. A detailed list of own contributions to the work is provided at the end of this chapter.