Resumen de Structure nanoengineering of functional nanomaterials. Advanced electron microscopy study
Reza Zamani
In this report novel materials for advanced applications are studied by means of the latest microscopy technologies and methodologies which have had a dramatic impact on progress of materials science. The aim was to study phenomena such as polytypism, unusual morphologies, polytypic branching, cation ordering, polarity, epitaxial growth and interface, etc, in order to find adequate explanations for the influence of the phenomena on the properties and applications such as thermoelectricity, p-n junction functionality, photovoltaic efficiency, optoelectronic properties, and sensing response. Various semiconducting materials, i.e. complex chalcogenides, metal oxides, and III-V nanostructures were characterized for this purpose. Here, nanoengineered structures of functional materials at nanoscale are studied by means of advanced electron microscopy methods. Chapter 1 gives a brief introduction to the report; the main purpose of the work, state-of-the-art, challenges and possibilities. In chapter 2 the methodology is described. The results are provided in Chapter 3, 4, and 5, and Chapter 6 is the general conclusions and the outlook. Note that Chapters 3-5 have their own introduction and conclusion. Therefore, chapter 1 consists of a short introduction to the general idea of the study, its importance and the state-of-the-art, and a preface of the thesis. In Chapter 2, after a brief history and the basic concepts of EM, the TEM methodology is described; the advanced TEM techniques used to study the nanostructured semiconductors at atomic scale. In general, it consists of brief descriptions of basic principles of TEM techniques. As experimental results are corroborated by theoretical studies and simulations, these procedures (image processing, simulations, etc) are also described shortly. Chapter 3 is dedicated to nanoengineering crystal structure and morphology of nanocrystals of complex copper-based chalcogenide, from binaries to complex ternaries and quaternaries. In this chapter it is shown that there is a wide range of possibilities for engineering, as many elements can be substituted with the primary cations and anions. Advanced TEM studies are performed in order to figure out the physics behind the property modifications. Phenomena such as morphology change, polytypism, ordering, polarity, electronic band change, strain, etc are elaborately studied, and correlated to the physical properties such as thermoelectricity. CCTSe polypods are the case of a complete structure study to understand the branching mechanism. Therefore, by means of an aberration-corrected TEM the polarity and cation ordering was determined. Polarity-driven morphology and branching mechanism is explained. Moreover, electronic band structure in this polytypic structure is simulated. Chapter 4 is based on the study of nanojunctions in metal oxide heterostructured NWs, structures that can enhance the functionality of the targeted devices, such as photovoltaic cells, or gas sensors. Production of nanojunctions is a successful approach in the context. In this chapter it is shown how coaxial heterostructuring of NWs, e.g. formation of core-shell structures increase the efficiency of the solar cells or enhance the sensitivity/selectivity of the gas sensors. In chapter 5 almost the same approach was followed, nevertheless, this time with III-V NWs. The importance of axial heterostructures and fully-epitaxial and relaxed structure are emphasized. The optoelectronic properties of the GaN NWs, such promising p-n junctions, are examined. Polarity issue, as a remarkably influencing parameter, is precisely studied experimentally. Its effect on electronic band structure in the heterointerface is also proven by the theoretical simulations. In the end, a general conclusion of the whole work and room for further study and future work is discussed in Chapter 6. The ample freedom of structural nanoengineering in the materials, together with development of novel electron microscopy techniques, opens the way towards the new possibilities for the future work.
