Ultra high resolution imaging of radiation-sensitive materials

Thesis

Electron radiation damage is an important topic in electron microscopy. A large proportion of materials or biological structures are still unable to be directly imaged by electron microscopes at high resolutions due to their radiation sensitivity, instead of by the instrumental resolution capacity of the microscopes. In fact, as the availability of aberration correctors was rapidly increased in the past decade, as well as the emergence of the next generation of chromatic aberration correctors, attention has been again focused on the control and reduction of radiation damage.

This thesis proposes a complete low-dose high resolution imaging strategy for characterising radiation-sensitive materials using an aberration-corrected transmission electron microscope. The microscope was quantitatively calibrated for electron dose and was operated under a strict low-dose condition to ensure maximum protection for the radiation-sensitive samples. Time series and focal series imaging were employed to allow other data processing techniques to be applied.

During the course of pursuing higher resolutions using low-dose imaging and data processing, several side problems have been explored. Image registration of low-dose image series was first tested using a variable dose time series of human tooth tissue. The impact of excessive image noise was reduced by modification of the registration algorithm. In the case of focal series image registration, a simulation-assisted registration procedure was developed. This demonstrated the ability to register focal series of cerium dioxide and silicon nitride at various doses, despite the contrast reversal problem when the series defoci moved from under focus to over focus. A quantitative evaluation metric, the IQ factor, was implemented to assess the signal-to-noise ratio in the power spectrum of an image and has proved to be a useful indirect criterion for measuring the quality of registration and that of exit wave reconstruction.

Exit wave reconstruction from low-dose HRTEM was investigated using this improved image registration and the resultant exit waves were quantitatively compared using the IQ factor. The influence of noise on exit wave reconstruction was an intertwined problem with the low electron dose and has also been addressed in this work.

By combining improvements in the various aspects of low-dose imaging and data processing, time/focal series of ZSM-5, an important type of zeolitic catalyst, was acquired. The low-dose time series of ZSM-5 was aligned by both the traditional rigid XCF image registration and a more sophisticated non-rigid image registration method. The low-dose focal series of ZSM-5 was used to restore the electron wave function at the exit plane of the sample. The restored exit wave was able to resolve the fine structure inside the ZSM-5 framework, which was not clearly resolved in the individual images of the focal series.

Authors: Huang, C

• DOI: 10.5287/ora-0zbrrkpzp
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford