Nanoscale scale imaging of photoexcited states using electron microscopy
Keywords: Mechanical Engineering.

Seeing is believing. The ability to directly visualize things greatly deepens people's knowledge and advances researches in many fields. Apart from resolving tiny things, optical imaging can also provide spectroscopy information which offers fundamental insights into the energy states of matter. As research develops at the nanoscale, these energy states are often affected by nanostructuring and local defects of the sample. An imaging tool that can provide optical information with nanometer-scale spatial resolution will offer fundamental insights, greatly enhance our ability to design novel materials, and advance research in a wealth of areas. Optical spectroscopy and imaging techniques like Raman, photoluminescence, and infrared spectroscopy are widely used for materials characterization. Visible photons have energies (meV to eV) match with those of the energy states inside the material and thus process excellent spectral selectivity. However, the spatial resolution of traditional optical techniques is diffraction-limited by the wavelength of light used. Although various super-resolution techniques have been developed to overcome this diffraction limit and reached ~10 nm resolution, these techniques require fluorescent labels or a sharp scanning tip, which limits their application. On the other hand, modern scanning or transmission electron microscopes (SEM or TEM) can readily achieve nanometer and angstrom spatial resolution using 1-300 keV high-energy electrons. However, the energy mismatch between such high-energy electrons and the energy states inside the sample makes high spectral resolution challenging for electron microscopes. Only very recently can some state-of-the-art electron monochromators achieve meV energy resolution, but this requires expensive and specialized instruments. Nanometer and atomic resolution label-free imaging with optical information has remained a major scientific challenge. In the work presented in this thesis, we developed a new imaging technique named PhotoAbsorption Microscopy using ELectron Analysis (PAMELA). PAMELA combines the high spectral selectivity of photoexcitation and the high spatial resolution of electron microscopes to offer nanometer-scale imaging with optical information. We implement PAMELA on two platforms, an SEM and a TEM, to demonstrate optical imaging first below the optical diffraction limit and eventually at the atomic scale resolution. For PAMELA-SEM, we experimentally demonstrate spectrally specific photoabsorption imaging with sub-20 nanometer spatial resolution using various semiconductor and metal nanoparticles. The photoabsorption-induced contrast mechanism is attributed to surface photovoltage which modulates the secondary electron emission. Theoretical analysis and Monte Carlo simulations are performed to explain the trends of the signal observed. For PAMELA-TEM, we discuss the possibility of imaging photoexcited states with atomic-scale resolution. We design an experimental set-up based on high-resolution TEM (HRTEM) and use ab initio together with HRTEM simulations to calculate the imaging conditions required for a few model systems, including defects in hexagonal boron nitride (h-BN) and core-shell quantum dots. We believe PAMELA will offer new opportunities for nanometer-scale optical spectroscopic imaging and material characterization