Structure nanoengineering of functional nanomaterials. Advanced electron microscopy study
[spa] En este trabajo hemos estudiado materiales avanzados con las últimas tecnologías y metodologías de microscopía electrónica, las que tienen un impacto importante en el desarrollo de la ciencia de materiales. El objetivo principal ha sido estudiar fenómenos como el politipismo, morfologías inusuales, ramificación, 'ramificación politípica', manipulación de la estructura de banda, ordenación de los cationes, polaridad, crecimiento e interfase epitaxial, alojamiento de una fase secundaria en una base, etc. para razonar la influencia de aquellos fenómenos en las propiedades y aplicaciones, por ejemplo la termoelectricidad, el funcionamiento de unión p-n, la eficiencia de las celdas solares, las propiedades optoelectrónicas, la respuesta de los sensores, etc. Distintos semiconductores han sido caracterizados: nanopartículas de calcogenuros complejos, nanohilos de óxidos de metales, y nanohilos del grupo III-V. Hemos estudiado los materiales en escala nanométrica por medio de métodos avanzados de microscopía electrónica de transmisión (TEM). El capítulo 1 es una breve introducción a la tesis, en la que se exponen los objetivos principales del trabajo, los últimos avances (state-of-the-art), los retos, y las nuevas posibilidades. En el capítulo 2 se explica la metodología de TEM utilizada para estudiar los semiconductores. Los capítulos 3 a 5 se componen de los resultados. El capítulo 3 está basado en el análisis de las nanopartículas de calcogenuros complejos. La sección de resultados contiene tres partes: monoestructurados, multiestructurados, y heteroestructuradas de tipo core-shell. En caso de nanopartículas cuaternarias de CCTSe, las nanopartículas ramifican y forman polipodes, que es el caso de un estudio elaborado porque el mecanismo de la ramificación es interesante. En capítulo 4 se trabaja con los nanohilos de óxidos de metales que sirven para muchas aplicaciones como celdas solares o sensores de gas. En nuestro caso, con el objetivo de mejorar la funcionalidad de los aparatos, hemos estudiado heteroestructuras. En el capítulo 5 prácticamente la misma aproximación está escogida, pero esta vez con nanohilos del grupo III-V. Aquí hemos enfatizado la importancia del crecimiento epitaxial de heteroestructuras. Por último, en el capítulo 6 hemos hablado de las conclusiones generales y las perspectivas para la investigación futura. ; [eng] 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.