Quantitative phase contrast imaging of arborescent graft polystyrene by off-axis transmission electron holography

Imaging nanoscale polymer objects in the Transmission Electron Microscope is difficult, because small polymeric objects interact only weakly with intermediate-energy electrons. Heavy element staining can induce significant amplitude contrast, but stains can introduce artifacts that complicate the structure determination at nanometer length scales. This paper explores transmission electron holography for phase contrast imaging of unstained arborescent graft polystyrene nanoparticles. Holography is able to recover significant phase contrast from these particles despite the fact that there is negligible amplitude contrast. Comparative imaging experiments show that off-axis holography provides substantially higher contrast than that generated by the traditional method of transferring phase information to amplitude information via defocus. This effect is a consequence of different lens contrast-transfer behavior in each of these two imaging approaches. Under kinematical conditions when the appropriate mean inner potential is known, the specimen's projected thickness can be directly mapped from the holographic phase image to give a measure of the specimen's three-dimensional shape. Such quantitative imaging shows that individual arborescent graft polystyrene nanoparticles, which are spherical in a good solvent, adopt a flattened shape when deposited on a carbon substrate and allowed to dry.