Nanoscale Electron Microscopy Study of Phase Transitions and Thermal Properties in Functional Materials.

Analytical high-resolution scanning transmission electron spectroscopy (HR-STEM) combined with in-situ heating enables us to acquire atomic resolution images as well as low-loss and high-loss electron energy loss (EEL) spectra of the material at various temperatures. Using the temperature dependence from plasmon energy, obtained from low-loss EEL spectra, various thermal properties such as thermal expansion coefficient, melting point, and phase transition temperature of the material can be calculated. By analyzing the change in shape and relative intensities of ionization edges from core-loss EEL spectra with respect to the temperature, the evolution of the material’s phase can be studied. In this thesis, I have combined low-loss and core-loss Electron Energy Loss Spectroscopy (EELS) and HR-STEM imaging with in-situ heating to study thermal expansion, melting and phase transition of various functional materials. The thermal expansion coefficient (TEC) of the bulk material can be determined by using Michelson interferometry by measuring the temperature-induced change in sample dimensions. For nanoparticles, high-temperature x-ray diffraction (XRD) can extract TEC from the XRD patterns acquired at various temperatures. These methods are not appropriate to determine the TEC of interfaces or nanoparticles at the nanoscale due to limitations in spatial resolution and sample requirements. Scanning transmission electron microscopy (STEM) can be used to determine the TEC of individual nanoparticles, interfaces as well as grain boundaries by probing the characteristics plasmon energy of the materials with high spatial resolution. While direct HR-STEM imaging can be useful in some scenarios in detecting structural phase changes, the material is subjected to a high electron dose rate in the process. The mechanical and thermal vibrations introduced to the sample during heating may distort the images making acquisition of high-quality images challenging. Furthermore, the requirement of thin samples (< 50 nm) makes it unsuitable to study thicker nanoparticles. In such a context, the low-loss EELS method can be handy for studying physical as well as structural phase transition indirectly. The low-loss EELS can be acquired for much thicker samples (< 300 nm). The dose rate can be easily controlled by changing dwell time and pixel size. This gives a lot more flexibility in designing and studying a wide range of materials. This thesis aims to develop the techniques to study the thermal expansion coefficient, the melting point, and the phase transition temperature at the nanoscale with the help of in-situ electron energy loss spectroscopy (EELS).