Electron scattering contrast in unstained mesoscopic polymers.

摘要

Due to the fact that polymers scatter electrons weakly, heavy-element staining is usually required when preparing polymer specimens for transmission electron microscopy (TEM). Staining enhances image contrast but can introduce artifacts. These artifacts are especially problematic in high-resolution imaging of polymer nanostructures and interfaces. This thesis begins with an experimental demonstration of such staining artifacts. Phase-contrast imaging is one alternative method in TEM for imaging unstained polymer structures. This thesis compares two phase-contrast imaging techniques: off-axis transmission electron holography and focal-series imaging. The focal-series method retrieves the image phase from the recorded image intensity at different values of microscope focus. It can be understood in terms of image transfer theory. Off-axis transmission electron holography retrieves the image phase by interfering the image wave with a reference wave. A field-emission TEM is required, because of its high coherence. Using arborescent graft poly(styrene) nanoparticles as model specimens, phase-contrast images from both methods have been analyzed and compared. Holographic imaging shows significantly higher contrast of unstained polymer samples than does the focal-series method. Holography also provides 3-D information from 2-D projected TEM images when the specimen mean inner potential is known. High-resolution holography has significant opportunity to study nanospecimens like viruses, proteins, and dendritic polymers. This thesis concludes with a discussion of the possibility of using spatially-resolved electron energy-loss spectroscopy, an another alternative method of studying unstained polymer structure using chemical or bonding maps, to image polymer fine structure without staining.