Ptychography & Quantification

For us, images and spectra are data-sets that should lead to quantitative measurements of materials parameters.  To achieve this we have developed a number of methods that enable measured data to be placed on a firm quantitative basis.  In addition, to applying this approach to established imaging approaches (for example annular dark-field imaging), we are also interested in developing new microscopy imaging modes that allow new types of measurement to be made.  In particular, we are interested in maximising the information obtainable at low-doses to unlock the power of electron microscopy for materials which have previously been challenging if not impossible to address.

An important strand of research is the use of Fast Pixelated Detectors in the STEM, which allows the collection of the scattered signal (CBED pattern) for each probe position (See Figure 1). This results in a 4D-dataset (2D CBED - 2D image scan), which contains all the information about the electron-sample interaction for the scanned area. Then, the analysis of this 4D-dataset allows the retrieval of information related to the material’s atomic structure and its composition, which can be linked to its physical and chemical properties. We are focusing this research on combining the high angle annular dark field (HAADF) signal with electron ptychography. HAADF STEM images deliver information of the atomic number Z of the atoms present in the material. However, for light elements, this signal is very weak. Electron ptychography is a phase imaging technique that allows visualisation of low atomic number Z materials. Then, the combination of information of both signals permits the full characterization of the atomic structure and composition of the material.

The project involves the design of appropriate experiments on the microscope, the development of image processing and computational techniques and the fundamental study of electron scattering processes. This research will lead to the development of electron microscopy techniques for the characterization of materials at the atomic level, which will allow one to extract quantitative information of the three-dimensional atomic structure and the identification of the atom types in the sample, as well as the correction of residual lens aberrations.

Figure 1: Schematic of the scanning transmission electron microscope when using an Annular Dark Field (ADF) detector in combination with a Fast Pixelated Detector (FPD).

The ADF detector integrates the signal that scattered at high angles, which is proportional to the atomic number Z of the atoms in the material. The FPD records the convergent beam electron diffraction pattern (CBED) for each probe position that is scanned over the sample. With this 4D dataset, electron ptychography can be performed.