Atomic Resolution Imaging
Our research focuses on theoretical aspects of the characterization of the structure of condensed matter down to the atomic level, mainly using electrons. Structure retrieval can proceed via electron imaging, by detecting inelastically scattered electrons or by measuring the X-rays emitted when electrons are inelastically scattered. This involves aspects of electron optics such as (1) aberration characterization and correction in software (“computer spectacles”), (2) multiple scattering (electrons interact strongly with matter) and (3) the modelling of inelastic scattering in solids (crystals) from first principles. Past and present work greatly facilitates structure retrieval in using energy dispersive X-ray analysis (EDX) and electron energy loss spectroscopy. Another rapidly developing strand of our research is based on our recent solution of the long standing phase and inversion problem in dynamical electron diffraction, a problem which had remained unsolved since 1928, when Hans Bethe first wrote down the forward scattering problem.
Our major research programs are:
- Aberration determination and correction in linear imaging systems
- The phase and inverse scattering problem for electrons multiply scattered by solids: Theoretical methods for the inversion of multiple scattering of electrons in both periodic and non-periodic solids are being investigated. These methods will extend the range of utility of atomic resolution electron microscopy and electron tomography. This work builds on our recent novel solutions to the phase problem from both images and diffraction patterns (needed as a prelude to the inversion) that are robust in the presence of discontinuities in the phase (such as vortices – a vortex with topological charge one is shown above.). These phase retrieval methods are also applicable in visible, X-ray, neutron and atom optics.
- Inelastic scattering of electrons in crystals: Scattering of fast electrons in crystalline solids; absorptive scattering; nonlocality in scattering, applications to microanalysis.
- Modelling of scanning transmission electron microscopy (STEM) images from first principles.
- Exit surface wave function retrieval in high-resolution transmission electron microscopy; aberration determination and correction.
A full list of the groups publications can be found here.
Recent Research Highlights
New directions for chemical maps
Nature Nanotechnology 3, 255-256 (2008)A new generation of scanning transmission electron microscopes will allow researchers to study the composition and bonding of all the atoms in a solid material. The first chemical mapping in two dimensions was recently demonstrated by the TCMP group and collaborators with an instrument corrected to third order in spherical aberration, and by Koji Kimoto and co-workers at NIMS and HREM Research in Japan (Kimoto, K. et al. Nature 450, 702–704 (2007).
The simultaneous measurement of structural and chemical information at the atomic scale provides fundamental insights into the connection between form and function, and with its Ångström-sized probe, a STEM is ideal for imaging nanostructures.