Controlled solution-based synthesis offers a path towards nanomaterials with designed structure and functionalities. PbSe quantum dots self-assembled into superlattices at liquid-liquid interfaces, for example, can connect epitaxially to nearest neighbors to form confined-but-connected structures in with new electronic state have been predicted. Self-assembly of amphiphilic block copolymers and surfactant molecules in solution is also used to template for the production of tailored nanostructures. The initial steps in these formation processes are often critical. Cryo-transmission electron microscopy (TEM) allows direct observation of specimens snap-frozen in solution and is especially valuable for characterization of the earliest stages of solution-based formation processes. In the first example, I will discuss how cryo-TEM can be used to study the formation process of hybrid organic-inorganic silica nanorings, 10 nm in diameter, formed by the condensation of silica onto organic micelles. We have captured steps in the synthesis of these particles by snap-freezing the structures in solution, revealing the evolution from isolated micelles to aligned silica rings.<br/><br/>The second part of the talk will focus on recent developments that have enable atomic resolution spectroscopic mapping of nanostructures at cryogenic temperatures. Technical advances in spectroscopic imaging using electron energy loss spectroscopy (EELS) have allowed a material’s chemical and electronic structure to be resolved at the atomic scale. While this can now be achieved almost routinely for radiation-hard materials studied at room temperature, cryogenic applications are hampered by the reduced stability of side-entry cryo-holders. This is particularly challenging for spectroscopic mapping as it requires pixel dwell times that are orders of magnitude larger than typical imaging dwell times. Using a spectroscpy enabled direct electron detector with improved detector quantum efficiency, point spread function, and signal-to-noise ratio compared to indirect, scintillator-based detectors, we demonstrate atom-resolved spectroscopic mapping at liquid nitrogen temperature. Operating at cryogenic temperatures not only allows imaging of nanostructures snap-frozen in solution, but also enables the study of reactive materials and suppresses carbon built-up during imaging which is particularly important for nanoparticles that are not robust to standard cleaning methods.

Advances in electron imaging and spectroscopy of nanomaterials at cryogenic temperatures

256th National Meeting (2018)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.