EELS and ELNES Investigation of Titanate and Zirconate Perovskites
Electron energy-loss spectroscopy (EELS) is a powerful technique, which can be used to analyze materials on the nano-scale. EELS spectra are generated by inelastic interaction of atoms in a target material and a beam of fast electrons. The energy transferred from the fast electrons excites core atomic electrons to higher energy states above the Fermi level. This results in the presence of ionization edges within the associated energy loss spectrum. The energy of each edge onset can be related to a specific electronic transition and so the chemical identity of atoms in a given sample may be ascertained. Also, the local chemical and crystallographic environment of the excited atom affects its electronic states, and thus, also affects the energy-loss near-edge structure (ELNES) present in the energy region immediately following an ionisation edge.
ELNES is formed due to a variety of complex interactions within a sample and they are not currently well understood. Therefore direct interpretation of the fine structure arising in complex systems is not possible. In order to improve understanding of ELNES, data is generally collected in a systematic study from a series of related materials which have similar unit cell structures. In this way the effect of changes to the local crystallographic environment, or even the effect of altering the chemical identity of a neighbouring atom in the unit cell, to the final near-edge structure may be found. Comprehension of ELNES may also be improved though quantum mechanical calculations which simulate both the electronic structure and ionization edge ELNES. Modifications to the unit cell contents or parameters are easily incorporated into such calculations and so the resulting effect on EELS spectra may be obtained.
Perovskite materials are complex functional materials which have generated wide ranging interest due to the diversity of their electrical properties. The aim of this project was to study a series of titanate and zirconate perovskite materials (of general formula ABO3) via EELS, in order to deepen our understanding of the fundamental interactions that cause ELNES. The oxygen K-edge from each material has been acquired on a Tecnai F20 electron microscope and two individual quantum mechanical codes are used to model the associated near-edge structure.
A framework with which to understand the oxygen K-edge ELNES of undoped ternary perovskite powders has been constructed, and subsequently applied, to a doped, quaternary, perovskite system, in the form of a thin film. Information from the standard powders is used to probe variations to the basic structure and composition within the film. The majority of the results given for this investigation have been obtained using EELS analysis. However, additional information on the samples studied has also been acquired using x-ray diffraction (XRD), energy-dispersive x-ray (EDX) and electron microscopy. Each of these techniques help to provide a balanced understanding of the nature of the samples being studied.
Chapter 1 provides an introduction to the three basic components of this investigation. Initially, principles of electron microscopy, with a specific emphasis on EELS, are discussed. Second, perovskite materials and their associated properties. Both traditional solid state synthesis methods and comparatively newer, wet chemical methods, which have resulted in significant reductions in crystallization temperatures and improvements to sample homogeneity, are detailed. Finally, an introduction is given into the theoretical simulation codes that have been used to calculate the electronic structure of each material.
Key components and operational modes on the Tecnai F20 electron microscope and EELS spectrometer used to collect EELS data are considered in chapter 2 along with details of how the data was collected and processed. The basic principles behind EELS and ELNES analyses are also provided in this chapter. Modelling of the near-edge structure has been carried out by both reciprocalspace band structure, and real space multiple scattering calculations. Each of these is discussed in chapter 3 with details of how each type of calculation relates the electronic structure of a material to the near-edge structure of an ionisation edge. Details of the actual calculations performed are given. while some examples of the type of information that can be obtained are also provided. Chapter 4 details both the syntheses, and characterization methods used for each of the perovskites included in this investigation. Both x-ray diffraction (XRD) and (EDX) experiments have been utilized to assess the homogeneity and overall purity of the powder samples before EELS investigation. Some unreacted material has been observed using EDX in some samples that was not found in the original XRD spectra. Consequently, each of the samples were sent for further XRD analysis using a longer data acquisition times in a more modern instrument. These results confirmed that the overall purity levels in some samples were not adequate for EELS analysis and they were subsequently replaced by newer samples, formed by different syntheses methods.
Results from EELS analyses of the oxygen K-ELNES are given in chapter 5 and these spectra are compared to simulated data by both codes described in chapter 4. Only EELS data collected from powders found in chapter 4 to contain a homogeneous composition, and to be free from impurity, are presented. Reasonable agreement has been found between theory and experiment for each of the samples, with one exception. Data simulated for the oxygen near-edge structure associated with BaTiO3 shows significant discrepancies to the experimentally collected data. However, spectra produced by both simulation methods exhibit a similar form to each other, suggesting that something in the actual material is not being properly accounted for in the calculations. Several hypotheses for this disagreement are investigated.
Chapter 6 presents results from the investigation of a thin film perovskite by electron microscopy. Techniques such as Z-contrast scanning transmission electron microscopy (STEM) imaging, high resolution – transmission electron microscopy (HR-TEM) and EDX are used to characterise the film before EELS analysis.
Chapter 7 is dedicated to EELS analysis of the same thin film. EELS data has been quantified to study variations in the elemental composition over each sol-gel deposited layer within the thin film, while ELNES data has been used to study the corresponding alterations to the perovskite unit cell. Interpretation of the ELNES variations within the doped film is aided using results discussed in chapter 5 for each of the end members of the PZT solid solution, PbTiO3 and PbZrO3, as a framework.
Finally, the results of this investigation are discussed in chapter 8. Further work that could be performed to improve upon these findings is also considered.