Ph.D. Defense: Jessica Alexander, High-Resolution Electron Energy-Loss Spectroscopy of Beam-Sensitive Functional Materials

Abstract

By making electron energy-loss spectroscopy (EELS) measurements in a scanning transmission electron microscope (STEM), the optoelectronic properties of a material can be determined with nanometer spatial resolution. Since these optoelectronic properties can be related to the electronic structure of a material, STEM-EELS can also probe the local bonding environment at the interface of two materials. Such measurements could be key in developing more efficient P3HT:PCBM bulk-heterojunction organic photovoltaics (P3HT = poly(3-hexylthiophene), PCBM = [6,6] phenyl C61butyric acid methyl ester), as understanding the local electronic structure/local bonding environment at P3HT/PCBM interfaces should provide insight into charge generation/transport. Traditionally, these measurements have not been made for beam-sensitive materials, such as those used in organic photovoltaics, as their susceptibility to damage under the electron beam makes it difficult to collect reliable data. In this work, it was demonstrated that, via a beam damage-minimization EELS acquisition method, reliable high-resolution low-loss STEM-EELS data could be collected for electron beam-sensitive materials, such as those used in organic photovoltaics. Using these techniques, standard low-loss EELS spectra were collected for P3HT, PCBM, copper phthalocyanine (CuPc), and C60. Spatially-resolved EELS data was also collected for a CuPc/C60bilayer structure in order to prove that reliable EELS data could be collected at the interface between two organic materials, and preliminary measurements were conducted for a P3HT:PCBM bulk-heterojunction organic photovoltaic, as will be discussed during the presentation. These methods were then applied to an organic-based ferrimagnetic semiconductor (vanadium tetracyanoethylene, V[TCNE]x~2) and two lead-free halide double perovskites (Cs2AgBiBr6and Cs2AgBiCl6) in order to extract their optoelectronic properties, which were previously unknown. Additionally, core-loss EELS measurements were used to determine the V oxidation state in V[TCNE]x~2and how it varies throughout the thin film.