Positronium Annihilation Spectroscopy Characterization of Novel Nano-porous Materials with Advanced Functionalities

Abstract

Positron, the anti-particle of the electron, and its bound state with an electron, positronium, have found many applications in physics and chemistry. Due to the unique sensitivity to a materials local electron density, positrons can be used to investigate complex voids structures. As such, the development and characterization of materials with precisely engineered porous networks is a vibrant area of research. Applied studies with positron have found relevance in catalysis, opto-, nano- and microelectronics, gas sorption, separation, and sensors, among others. The technique relies on the rapid annihilation of the positrons with the electrons of the material under study. In contrary to scattering, transmission or equilibrium techniques, the positron is a truly local probe of its surroundings. The methods studying the annihilation in time, energy and position are grouped under the term Positron annihilation spectroscopy (PAS). Studies have shown sensitivity to the amount, distribution, and connectivity of single site defects, micro- and mesoporosity levels. Control over these parameters are essential to guide the design for future materials. The objective of this thesis was to expand the scope of PAS for the characterization of novel nanoporous materials with advanced functionalities. One aspect of this work was increasing the availability of positron beams. Different production schemes exploiting the recent advances and availability of cyclotrons were investigated. With the development of new kinds of radioactive thin-film sources, a step towards small lab scale positron beams was made. The field of possible applications was expanded with studies on state-of-the-art materials with the ETH slow positron beam. A study on the pore evolution of ZSM-5 zeolite emphasizes the unique sensitivity of PAS to the presence of guest species within the micropore network. This opens new doors to study the impact of targeted inclusion of pendant molecules on the textural properties of other porous materials. Another study focused on the distinct impact of chemical properties, e.g. acidity, on the positron annihilation characteristics. The ability to evaluate both porosity and acidity in functional materials will widen the scope of the technique for the analysis of functional materials. Moreover, proof of concept studies on different type of nanocrystals, surface- anchored metal-organic frameworks, defect engineered copper films and carbon nanotubes were made. For more wide-spread usage of PAS in the general scientific community, also a strong fundamental understanding and the development of a solid theoretical framework for data analysis is essential. An automated analysis to derive the desired structural information without requiring an involved knowledge of the technique is a particular challenge. Therefore, another core activity of the thesis was the development of improved numerical tools and models to account for the complexity in the pore architecture of functional materials. Concluding, the work presented in this thesis expanded the number of successful application of PAS. Furthermore, it highlighted a set of problems which need to be tackled towards a more wide-spread use and proposed solution approaches.