Phase Retrieval Imaging for Soft Materials at Low-Voltage

The imaging of soft and biological materials has long challenged researchers due to their low inherent contrast and susceptibility to beam damage even at low electron dose. Cryo-transmission electron microscopy (TEM) and advances in related hardware, software and sample preparation methods have greatly improved resolution and signal-to-noise for the imaging of soft structures. However, achieving sufficient contrast is generally dependent on lens aberrations or the introduction of phase plates to induce phase contrast [1], or on algorithms for class averaging from highly homogeneous samples. Scanning transmission electron microscopy (STEM) can be used to observe mass-thickness contrast, but the high localized dose can be prohibitive for radiation-limited structures. Imaging at lower voltage can improve contrast due to the increased scattering cross-section but also introduces elevated radiation damage [2]. The imaging of materials that are non-homogeneous or flexible in solution as well as beam-sensitive requires extended methods to improve contrast without increasing electron dose. One such method is ptychography, a phase retrieval imaging technique popularized for scanning x-ray diffraction microscopy [3]. It has recently proven useful in electron microscopy, especially for low-atomic number, easily damaged materials.