Inelastic electron-solid interactions for the processing and characterization of soft materials

Energetic electrons interact strongly with solid matter. In addition to elastic interactions, which change the electron trajectory, inelastic interactions deposit energy into solid targets. Inelastic scattering lies at the heart of electron energy-loss spectroscopy (EELS), which is exploited in the scanning transmission electron microscope (STEM) to characterize the nanoscale chemistry of solids. EELS is particularly well suited to study soft matter, e.g. synthetic polymers and biological tissue, both because they are composed of light elements - C, N, O, etc - appropriate for core-loss spectroscopy and because they have rich valence electron structure appropriate for low-loss spectroscopy. However, the same energy-loss processes responsible for EELS spectral fingerprints also generate chemical changes in the solid target. The resulting electron-beam induced damage places important constraints on the achievable STEM spatial resolution. However, it also opens the opportunity to intentionally modify local structure, which is the basis of electron-beam lithography. This thesis explores both extremes. First, Monte Carlo methods are used to follow electron trajectories across ~100 nm films of poly(ethylene glycol) [PEG]. Energy deposition within 1 nm3 PEG voxels is tracked using the modified-Bethe model, and an energy threshold for PEG crosslinking is estimated using published G values. The model is used to predict the three-dimensional PEG crosslink density in response to a pulse of focused electrons as a function of dose (1 - 1000 fC) and incident electron energy (2 - 30 keV). The model shows that the resulting surface-patterned PEG microgels have a rich nanoscale structure consisting of a highly cross-linked core and a lightly cross-linked corona. This new understanding can explain both the important anti-biofouling behavior of surface-patterned PEG microgels as well as the remarkably good performance of microgel-tethered molecular beacon DNA probes. At the other extreme, spatially resolved EELS in the cryo-STEM is used to measure the intercellular water concentration in the stratum corneum of frozen-hydrated porcine skin. In order to avoid significant electron-beam damage, we restrict the spatial resolution to 12 nm. Higher resolution can be achieved by extracting reference spectra from the experimental datasets themselves. These experiments show that corneodesmosomes are enriched in water relative to the adjacent inter-cellular lipid-rich regions, consistent with the hypothesis that water plays a key role in the natural desquamation processes associated with healthy skin. Because of the nanoscale dimensions of a corneodesmosome (~30 nm thick x 150 nm diameter), such a nanoscale water measurement cannot be made by established methods such as infrared spectroscopy.