Nanoscale imaging of low energy electronic excitations by fast electrons

Scanning/transmission electron microscope (S/TEM) is arguably the most powerfultool for materials characterization at the nanoscale. To image and understand the structure and composition of materials and nanoscale devices have been the main use of such instruments. The recent instrumental advance in monochromated STEM has improved the energy resolution of electron energy loss spectroscopy (EELS) to only a few meV. This has enabled low energy excitations, such as phonons and infrared plasmons, to be observed with high spatial resolution. In this dissertation, we explore low energy electronic excitations at the nanoscale, and address how these new observations can lead to better understanding of the structure-property relationships. We present examples of these new low energy electronic excitations, as well as the new understanding brought by the high spatial resolution. Among other things, we develop a novel method that allows real space imaging of free charge carriers at the nanoscale. This dissertation begins with an introduction of the electron microscope, including the main components that enable the high spatial and energy resolution. The theoretical background involved in interpretation of the low-loss EELS results will also be introduced. Next, we present a study of free carrier plasmons in free-standing films of Sn doped In2O3 (ITO). Studying this model system with well-known optical properties in a classic geometry allows fundamental understanding, such as the surface and bulk contributions of these plasmons, to be obtained. This is accomplished through the observation of surface plasmon interferences and by modeling with dielectric theory. We then analyze surface plasmon damping, in particular, its relation to the transport properties of a material. To this end, infrared surface plasmons sustained by the high mobility charge carriers in perovskite oxide BaSnO3 (BSO) are studied. The geometric dependence of the plasmon damping is investigated systematically and compared with plasmonic metals, which helps establish a relationship with the carrier mobility. Furthermore, we describe simultaneous imaging of charge carriers and dopants at the nanoscale. Combining low-loss and core-loss EELS, we visualize not only the spatial and statistical distribution of dopants and carriers at the nanoscale, but also quantify doping efficiency in individual nanocrystals of BSO. Finally, we conclude this dissertation with an outlook of future directions.Ph.D.Includes bibliographical reference