Unraveling atomic-scale quantum and chemical processes with optically-coupled electron microscopy
Keywords: Materials Science and Engineering.

Light provides Earth with abundant energy resources, enables high-speed and high-fidelity information transmission, and facilitates the vast majority of biomolecular sensing and imaging technologies. Yet, with visible wavelengths of ~400-800nm, the diffraction limit prohibits interrogating optical processes at the nanometer-length scales characteristic of many optical devices. Here, we develop methods to study how light behaves and interacts with materials at the nano and atomic scale using optically coupled transmission electron microscopy (OTEM). We apply this method to two case studies spanning chemical and quantum science: 1) AgPd bimetallic nanoparticles for light-driven hydrogenation and 2) SiV centers in diamond for quantum sensing. First, we study light-driven hydrogenation reactions catalyzed by bimetallic AgPd nanoparticles. These bimetallic systems combine a strong light absorber (Ag) with a catalytically-active metal (Pd) to drive industrially-relevant reactions. We find that the addition of Ag to a Pd rich lattice shifts the particle's plasmonic response by over 1eV, while not altering the particle's scattering cross-section. Using in-situ environmental TEM, we investigate the thermodynamics of the AgPdH system at the single particle level and show that, contrary to pure Pd nanoparticles, the AgPd system exhibits phase coexistence within single crystalline nanoparticles in equilibrium, with distinctive thermodynamic properties from the bulk. By tracking the lattice expansion in real time during a phase transition, we see surface-limited hydrogenation, as well as a new secondary rate corresponding to the speed with which the hydrogenated phase can restructure. Then, we use OTEM to illuminate the AgPd particles in-situ, and discover that dehydrogenation nucleation sites can be controlled with light; by tuning the wavelength of the illumination, we demonstrate the potential for site-selective chemical transformations. Finally, en-route to monitoring molecular transformations, we develop a novel stimulated Raman spectroscopy technique in the OTEM that enables sub-diffraction-limited Raman mapping. We use coherent cathodoluminescence from gold plasmonic particles, generated with a condensed electron probe as the stimulating Stoke's laser source. We show that Raman enhancement can exceed 1000000 with dual electron and optical excitation of these gold nanoparticles. Importantly, the spatial resolution of this vibrational spectroscopy for electron microscopy is solely determined by the nanoparticle geometry and the plasmon mode volume. In the second part of thesis, I discuss applications of OTEM to quantum science. Here, we use OTEM to explore photon emission from the silicon vacancy defect (SiV-) in diamond. The SiV- exhibits high brightness, minimal phonon coupling, narrow optical line-widths, and high degrees of photon indistinguishability. Yet the creation of reliable and scalable SiV--based quantum sources has been hampered by heterogeneous emission of unknown origins. We show that different crystalline domains of diamond exhibit distinct zero-phonon line (ZPL) energies and differences in brightness. While certain grains are bright with blue shifted emission, others exhibit over 50\% reduced brightness, with red-shifted emission. 4D STEM, electron diffraction, and electron spectroscopy allow us to correlate these changes with local lattice expansion and defect density. Our work correlating SiV- emission with the defect's atomic-scale structure provides a foundation for eliminating SiV- heterogeneity through optimized nanodiamond synthesis