The development and applications of STEM ptychography using direct electron detectors

Thesis

Since the introduction of direct electron detectors to scanning transmission electron microscopy (STEM), electron ptychography - a technique which utilises the interference in diffraction patterns to reconstruct the sample-induced phase changes of a transmitted electron wave - has significantly extended the capabilities of electron microscopy. However, a number of limitations to electron ptychography exist, namely the poor contrast transfer for low and high spatial frequencies to the phase reconstruction, and the relatively slow detector speeds used to acquire the ptychographic data (~ 1,000 fps). In this thesis, a number of strategies are introduced to further improve the robustness and dose-efficiency of focused-probe electron ptychography (FPP), after which several applications of FPP techniques are demonstrated.

Firstly, the contrast transfer properties of single side-band (SSB) ptychography are experimentally measured from an amorphous carbon sample in order to determine the optimal experimental parameters for ptychography. It is demonstrated that the probe convergence semi-angle can be used to tune the phase-contrast transfer function (PCTF) for each experiment, such that the relevant sample information is transferred with high contrast. Furthermore, careful consideration of the noise in the ptychographic data can provide an enhanced PCTF which broadens the transfer window in the image plane. These strategies are combined with a 1-bit fast (12,500 fps) acquisition scheme to enable the atomic-resolution phase reconstruction of a beam-sensitive zeolite sample using a low electron dose of 1.0 x 10^(5) e nm^(-2). By implementing these experimental and analytical strategies, the efficiency of FPP techniques can be signifificantly improved.

At the end of this thesis, several experimental challenges common to STEM are overcome using electron ptychography. Firstly, the precision of phase reconstructions are improved considerably by increasing the electron dose via multi-frame image acquisition, hence avoiding the slow-scan instabilities inherent to long STEM acquisition times. Furthermore, three-dimensional analysis of an unknown graphene defect is performed using a single ptychographic data set. Finally, electron ptychography is used to visualise oxygen vacancies in uranium dioxide for the first time.

Authors: O'Leary, C

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Electron Microscopy; Ptychography; Contrast transfer functions; STEM
• Subjects: Physics; Imaging; Crystallography; Condensed matter; Electron optics; Diffraction; Materials science