Report
Teaser, summary, work performed and final results
Periodic Reporting for period 2 - ATOM (Advanced Holographic Tomographies for Nanoscale Materials: Revealing Electromagnetic and Deformation Fields, Chemical Composition and Quantum States at Atomic Resolution.)
Teaser
The ongoing miniaturization in nanotechnology and functional materials puts an ever increasingfocus on the development of three-dimensional (3D) nanostructures, such as quantum dot arrays,structured nanowires, nanoantennas or nanomagnetic textures such as skyrmions, which...
Summary
The ongoing miniaturization in nanotechnology and functional materials puts an ever increasing
focus on the development of three-dimensional (3D) nanostructures, such as quantum dot arrays,
structured nanowires, nanoantennas or nanomagnetic textures such as skyrmions, which permit
a better performance of optical or electronic devices in terms of speed and energy efficiency. To
develop and advance such technologies and to improve the understanding of the underlying
fundamental physical effects, the nondestructive and quantitative 3D characterization of
physical, e.g., electric or magnetic, fields down to atomic resolution is indispensable. Current
nanoscale characterization methods only inadequately convey this information, e.g., because they probe
surfaces, record projections, or lack resolution. AToM will provide a ground-breaking
tomographic methodology for current nanotechnology by mapping electric and magnetic
fields as well as crucial properties of the underlying atomic structure in solids, such as the
chemical composition, mechanical strain or spin configuration in 3D down to atomic
resolution. To achieve that goal, advanced holographic and tomographic setups in the Transmission
Electron Microscope (TEM) are combined with novel computational methods, e.g., taking into
account the ramifications of electron diffraction. Moreover, fundamental application limits are
overcome (A) by extending the holographic principle, requiring coherent electron beams, to
quantum state reconstructions applicable to electrons of any (in)coherence; and (B) by adapting a
unique in-situ TEM with a very large sample chamber to facilitate holographic field sensing down
to very low temperatures (6 K) under application of external, e.g., electric, stimuli. The joint
development of AToM in response to current problems of nanotechnology, including the previously
mentioned ones, is anticipated to immediately and sustainably advance nanotechnology in its
various aspects.
Work performed
The project is divided into two main areas: Holographic tomography of physical fields in materials and Quantum state reconstruction of inelastically scattered electrons for comprehensive analysis of dielectric response. Progress has been achieved in both, with the details pertaining the several subareas listed below.
Area 1:
In order achieve atomic-resolution in the 3D reconstruction of electrostatic fields we combined two ideas. First, we use to maximal information extractable from an elastic scattering experiment, i.e., the (holographically reconstructed) electron wave function and record a tilt series of that. Second, we identified a suitable (nonlinear) mapping of that holographic tilt series into a projection series linearly depending on the 3D electrostatic potential (based on Rytov approximation and diffraction tomography). These principles have been implemented in a very flexible reconstruction algorithm for atomic scattering potentials capable of dealing with large tomographic datasets. Key features of the implementation are the possibility to reconstruct from multiple non-orthogonal tilt series, the possibility to deliberately exclude certain parts of the data (e.g., corrupted by erroneous phase unwrapping or sample modifications during the tilt series), and the possibility to incorporate additional constraints (such as maximal or minimal potential values). This algorithm has been successfully applied to simulated tilt series of wave functions, which had been scattered on nanoobjects, such as WS2 nanotubes. More recently, we acquired atomic resolution holographic tilt series of W nanotips to be used as experimental proof-of-principle of the novel method. It turned out that the alignment of the atomic-resolution holographic tilt series as well as modification of the sample occurring while tilting (e.g., adsorbates) require additional efforts in the preprocessing, which we are currently addressing.
Several major breakthroughs have been accomplished in the development of magnetic vector field tomography for nanoscale magnetic textures. We developed dedicated multiple tilt holographic series acquisition routines, holographic and tomographic reconstruction algorithms, and a 3 translation + 3 rotation axis specimen holder; dedicated to the reconstruction of all three Cartesian components of the magnetization texture in non-trivial 3D nanomagnets (such as vortices, bubbles, skyrmions, Bloch points, etc.). Amongst others we were able to show that the current scheme of employing two perpendicular tilt series and Maxwell\'s third law imposes a non-isotropic band filter on the reconstructed vector field independent of the reconstruction algorithm (e.g., whether rooted in the field or potential calcul), which provided a major stimulus for the development of the unique 6-axis tomography holder, overcoming this fundamental restriction. We demonstrated the model-base-free reconstruction of all three Cartesian components of the magnetic induction as well as pertaining magnetic properties such as the bound current or parts of the exchange energy density in various magnetic nanowire configurations (Fig. 1) and made the first steps into extending the method to cryogenic conditions (as required for a large class of fundamental magnetic phenomena).
The problem of 3D strain field reconstruction as been tackled through a close collaboration with our ERC project partner from the TU Berlin, Prof. Dr. M. Lehmann, Dr. T. Niermann and PhD candidate Laura Meißner, who developed and tested tomographic schemes for the 3D reconstruction of strain fields based on the beam tilt method proposed in AToM. In a first step a precise control of the beam tilt (and hence the excitation error) has been developed. In a second step beam tilt series of various strained semiconductor structures (containing buried quantum dots and wells) have been acquired and evaluated employing dynamic scattering simulations and the analytic weighting function approximatio
Final results
Progress beyond the state of the art has been mainly achieved in two areas so far: (1) Development of vector field tomography as a unique and flexible tool for studying 3D magnetization textures such as vortices or Skyrmions at several nanometer resolution. The successful implementation of the technique includes several hardware and software developments (see above). The proof-of-concept has been carried out at the example of a complex magnetization texture in a magnetic Co-Cu stacked nanowire (Fig. 1). Currently, the technique is extended to be applicable under cryogenic conditions and under the application of a rotatable external magnetic field. With that we expect to resolve the 3D magnetization texture of topical nanomagnetic phenomena such as Skyrmions or Bloch point domain walls until the end of the project. These studies contribute to reveal the crucial impact of spatial confinement, surface and bulk anisotropies and various other parameters on the formation of 3D magnetization textures and their functionalization in devices. (2) Development of a novel nanoscale probing technique for transient electric and magnetic fields of (collective) charge excitations (i.e., surface plasmons). Here, we adapted the conventional differential phase contrast technique such to allow for spectrally resolving time-dependent fields. The technique has been successfully applied to characterize the plasmonic response of several plasmonic nanostructures, albeit with a rather strong artificial impact from geometrical aberrations of the beam forming optics (condenser). We currently work on aberration corrected setups to remedy the latter, facilitating a more quantitative application of the technique, e.g., for characterizing magnetic surface plasmon modes or peculiar magnetoelectric coupling effects in topological insulators (axion plasmonics).
In the field of atomic resolution reconstruction, cryogenic TEM and quantum state reconstruction main milestones have been achieved with respect to reconstruction algorithms, continuous-flow liquid He cooling and inelastic ptychography data acquisition and analysis, respectively. Major breakthroughs are therefore to be expected until the end of the project resulting from the further development and application of the developed methodology.