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@INPROCEEDINGS{Rosenauer:827191,
      author       = {Rosenauer, Andreas and Müller-Caspary, Knut and
                      Schowalter, Marco and Grieb, Tim and Krause, Florian F. and
                      Mehrtens, Thorsten and Béché, Armand and Verbeeck, Johan
                      and Zweck, Josef and Löffler, Stefan and Schattschneider,
                      Peter and Müller, Marcus and Veit, Peter and Metzner,
                      Sebastian and Bertram, Frank and Christen, Jürgen and
                      Schimpke, Tillmann and Strassburg, Martin and
                      Dunin-Borkowski, Rafal and Winkler, Florian and Duchamp,
                      Martial},
      title        = {{Q}uantitative {STEM} - {F}rom composition to atomic
                      electric fields},
      address      = {Weinheim, Germany},
      publisher    = {Wiley-VCH Verlag GmbH $\&$ Co. KGaA},
      reportid     = {FZJ-2017-01389},
      pages        = {552 - 553},
      year         = {2016},
      comment      = {European Microscopy Congress 2016: Proceedings},
      booktitle     = {European Microscopy Congress 2016:
                       Proceedings},
      abstract     = {The image intensity in high-angle annular dark field STEM
                      images shows a strong chemical sensitivity. As it is also
                      influenced by specimen thickness, crystal orientation as
                      well as characteristics of illumination and detector, a
                      standard-free quantification of composition requires a
                      comparison with accurate image simulation, for which we use
                      the frozen lattice approach of the STEMsim program taking
                      the non-uniform detector sensitivity into account. The
                      experimental STEM intensity is normalized with respect to
                      the incident electron beam. For the quantification of a STEM
                      image it is subdivided into Voronoi cells in which the
                      intensity is averaged. Analysis of the composition in a
                      ternary semiconductor layer such as InxGa1-xN requires
                      measuring the specimen thickness in regions with known
                      composition by comparison with the simulated STEM intensity.
                      Interpolation of the obtained thickness into the layer with
                      unknown composition yields a map of the specimen thickness.
                      Finally, specimen thickness and STEM intensity are compared
                      with simulations computed as a function of composition
                      resulting in a map of the In-concentration x. In alloys
                      containing atoms with different covalent radii (e.g. In and
                      Ga in InxGa1-xN) static atomic displacements occur, which
                      are computed with empirical potentials and included in the
                      simulation. As an application example Fig. 1a shows an array
                      of core-shell nanowires. One single nanowire is depicted in
                      Fig. 1b. The core-shell area marked by a yellow frame is
                      shown in Fig. 1c. Figs. 1d and 1f show high-resolution STEM
                      images of the core-shell regions corresponding to the top
                      and the bottom of a nanowire, respectively. The maps of the
                      measured In-concentration given in Figs. 1e and 1g reveal an
                      increasing thickness of the layer along the growth
                      direction. In the upper part, the layer shows variations of
                      the In-concentration clearly beyond the random-array
                      fluctuations as was shown by a comparison with image
                      simulation.In the second part of the talk we present results
                      on measurements of atomic electric fields. Differential
                      phase contrast STEM detects the field-induced angular
                      deflection of the electron beam with a segmented ring
                      detector (J. Chapman et al., Ultramicroscopy 3 (1978), 203)
                      assuming that the Ronchigram is homogeneously filled and
                      shifted as a whole in the presence of electromagnetic fields
                      (N. Shibata et al., Nat. Phys. 8 (2012), 611). These
                      assumptions were tested by simulation for 1.3 nm thick GaN.
                      Fig. 2b shows Ronchigrams simulated for 6x6 scan positions
                      within the region marked in Fig. 2a. The dominant effect of
                      the atomic electric field is a complex redistribution of
                      intensity within a Ronchigram. By fundamental quantum
                      mechanical arguments, we take the complex intensity
                      distribution in the Ronchigram into account (K. Müller et
                      al., Nat. Commun. 5 (2014), 5653). The intensity in a
                      certain pixel of the recorded Ronchigram is proportional to
                      the probability that the corresponding momentum is observed.
                      Thus, a center-of-gravity type summation yields the
                      expectation value for the momentum. To relate the electric
                      field in the specimen to the observed momentum transfer,
                      Ehrenfest's theorem is applied. For thin specimens, the
                      expectation value of the momentum is found to be
                      proportional to the projection of the electric field along
                      the optical axis, convolved with the intensity distribution
                      of the incident STEM probe. We demonstrate the potential of
                      this approach in both simulation and experiment. For the GaN
                      simulation in Fig. 2c we find the electric field depicted in
                      Fig. 2d. Atomic sites appear as sources of the field which
                      has a magnitude of up to 1.5 V/pm. As only the convolution
                      of the true field with the probe intensity can be measured,
                      the field strength decreases in the direct vicinity of
                      atomic sites. In a first experiment, 20x20 Ronchigrams of
                      SrTiO3 with a thickness of 2.5 nm have been recorded on a
                      conventional charge-coupled device (CCD), yielding the
                      electric field in Fig. 2e. We also report on pilot
                      experiments with the ultrafast pnCCD camera (K. Müller et
                      al., Appl. Phys. Lett. 101 (2012), 212110) which was
                      operated at read-out rates of up to 4 kHz. For example, Fig.
                      2f shows the momentum transfers recorded at a MoS2
                      mono/bilayer interface, demonstrating that fast detectors
                      are the key for atomic-scale materials analyses at a
                      reasonable field of view.},
      month         = {Aug},
      date          = {2016-08-28},
      organization  = {16th European Microscopy Congress (EMC
                       2016), Lyon (France), 28 Aug 2016 - 2
                       Sep 2016},
      cin          = {PGI-5 / ER-C-1},
      cid          = {I:(DE-Juel1)PGI-5-20110106 / I:(DE-Juel1)ER-C-1-20170209},
      pnm          = {143 - Controlling Configuration-Based Phenomena (POF3-143)},
      pid          = {G:(DE-HGF)POF3-143},
      typ          = {PUB:(DE-HGF)8 / PUB:(DE-HGF)7},
      doi          = {10.1002/9783527808465.EMC2016.8302},
      url          = {https://juser.fz-juelich.de/record/827191},
}