Development of Maximum Efficiency Phase Contrast Electron Microscopy

Microscopy has helped enhance our understanding in many fields of science, advancing societies capabilities from technology to medicine. The goal of this project was to investigate new techniques in electron microscopy and employ microscopy to study novel materials. Many materials can handle only limited doses before being significantly altered or destroyed, and in such cases the practical resolving power of the microscope can depend as much on the signal to noise obtained before the sample is damaged as on the imaging optics. This project investigated the use of ptychography to extract information as efficiently as possible.

In ptychography a pixelated detector is used to record the details of the electron scattering at low angles, where most of the transmitted electrons can be found, in a scanning transmission electron microscope (STEM). At every probe position an image of this scattering is recorded, creating a fourdimensional dataset. This dataset is then processed digitally to determine the phase and amplitude of the spatial frequencies transmitted by the specimen. It is then possible to construct a phase contrast image by interfering all frequencies and transforming back to real space. Such ptychographic phase contrast imaging was first used to overcome the limitation of spherical aberrations in the electron optics before the advent of aberration correction in hardware. This was achieved by using a small portion of the signal in Fourier space where the aberrations in the interfering disks cancels out. Now however aberration correctors are available that allow atomic resolution at relatively low accelerating voltages. Therefore it is possible to rely on the hardware aberration correction and make use of as much of the signal as possible in Fourier space.