Fast Pixel Detectors: a paradigm shift in STEM imaging

The research proposed here aims to develop entirely new ways of imaging in the electron microscope, and to use these methods to study real-world materials problems. To illustrate the power of the methods we propose to develop, we have selected proof-of-principle materials problems from the areas of carbon nanotechnology, life sciences and electronic device structures.
Over the past couple of decades a particular type of electron microscope, the scanning transmission electron microscope (STEM) has become increasingly popular due to its high-spatial and energy-resolution. Much of our understanding of how macroscopic materials properties relate to atomic structure and bonding, and how we can control properties by manipulating these, is a result of the development of techniques to characterise materials on very short length scales. The STEM is not only capable of imaging atoms and observing the structure and crystallographic details of materials, but also in performing spectroscopy on single atoms, allowing atom-by-atom chemistry to be determined. A further key development is the spherical aberration corrector, which has revolutionised the performance of these instruments by overcoming the earlier limitations of electron lenses.
The principle of STEM is the use of electron lenses to focus a beam of electrons to form a small illuminating spot or probe. The probe can be scanned across a sample using a beam deflector. A thin, electron-transparent sample is used, and transmitted electrons can be detector, and the intensity of those detected plotted as a function of the probe position during a two-dimensional scan to form an image. The mostly commonly used detectors are an annular dark-field (ADF) detector which is a detector in the form of a broad annulus that detects relatively high angles of scatter, and bright-field (BF) detectors that collect the unscattered and low-angle scattered electrons. Both these detectors collect the total scattering over a range of scattering angles, and the total intensity is used to form an image. Such an approach neglects the rich information that is contained in the fluctuation in intensity that occurs as a function of scattering angle.
The overarching aim that forms the basis of the current proposal is to use fast pixelated detectors to record the intensity as a function of scattering angle in the detector plane of a STEM, which is effectively a diffraction pattern. By recording each two-dimensional diffraction pattern as a function of probe position in a two-dimensional scan, a four-dimensional data set can be recorded that is the ultimate STEM imaging experiment. Such a rich dataset contains information about the phase shift that results from transmission, about the composition of the sample, the strain in the sample and the three-dimensional ordering in the sample. We propose to develop the methods to record this 4D data set, using fast pixelated detectors, and by developing an optimised direct-detection system, together with the methods to process such datasets to enable physically useful measurements to be made.
We believe the approach we are taking will create a paradigm shift in STEM imaging, and in time will become the standard approach to record data in the STEM. To illustrate the power of the approach, we have identified key materials science questions that we will address with the methods we develop. The applications are: (i) imaging charge transfer in doped nanomaterials; (ii) imaging of soft and radiation sensitive materials, (iii) imaging of electric and magnetic fields in magnetic nanostructures, (iv) 3D composition and structural ordering effects in ceramics; (v) interdiffusion in ceramic and semiconductor heterointerfaces.
The methods developed will be disseminated widely, particularly through their implementation at the EPSRC National Facility for Aberration-Corrected STEM (SuperSTEM) through which a wide range of users will be able to access the new methods.

We have identified a number of pathways by which the outputs of the proposed research will impact more widely than the more obvious academic research implications. These pathways are identified in the diagram presented in the attached Impact Plan, and are summarised here. The pathways may be identified as being direct (i.e. having an immediate, direct causal potential benefit) or indirect (where a link occurs indirectly by supporting other areas of science research). An obvious direct impact of the proposed research is the further training and development of two post-doctoral researchers in the field of advanced STEM. There has been substantial recent capital investment in STEM instrumentation in the UK (approximately £10M EPSRC funding over the past 5 years alone). Projects that support the training of a generation of researchers capable of getting results of the highest quality from such instruments is critical to fully exploit the capital investment.

The economic direct impact of this research is the potential for commercialisation of its outputs as follows:

(i) Commercialisation of detector technologies. A crucial part of the proposed research is to identify, then design and manufacture a bespoke STEM detector optimised for STEM phase contrast imaging. It is likely that a manufacturer would be interested in licensing such a design. Indeed JEOL, Gatan and Deben are all providing support for this project (see letters of support).
(ii) Commercialisation of data acquisition and processing methods A substantial part of the project will involve creating software to control the instrument during data acquisition, and then to process the large amount of data produced to produce images that can be interpreted in terms of object parameters. Such software would undoubted by of interest to commercial suppliers, and licensing agreements would be sought.

The outputs of this research can also be identified as impacting indirectly in the areas of environment, economy and education.

(i) Economy and environment. Electron microscope imaging is used by researchers in industry and academia across a wide range of materials research, and indirectly impacts upon the EPSRC challenge themes of energy, digital economy, healthcare technologies and living with environmental change through the powerful characterisation capabilities it can provide for materials that are used in these areas.

For example, developments in nanomaterials form the foundations for the many technologies that are addressing the challenge themes described above. Such developments are impossible without the high-resolution characterisation enabled by electron microscopy. Economic activity in nanotechnology already stands at many billions of $ annually worldwide and a current EPSRC portfolio of £170M of grant funding in the socio-economic theme of nanotechnology.

(iii) Education. Both Oxford Materials and SuperSTEM already engage significantly in outreach activities, described further at http://outreach.materials.ox.ac.uk/index.php. The details of these activities, and how they will be strengthened by the research proposed here, are described further in the Impact Plan.