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@INPROCEEDINGS{Zheng:827183,
      author       = {Zheng, Fengshan and Migunov, Vadim and Ramsperger, Urs and
                      Pescia, Danilo and Dunin-Borkowski, Rafal},
      title        = {{Q}uantitative measurement of the charge distribution along
                      a tungsten nanotip using transmission electron holography},
      address      = {Weinheim, Germany},
      publisher    = {Wiley-VCH Verlag GmbH $\&$ Co. KGaA},
      reportid     = {FZJ-2017-01381},
      pages        = {737 - 738},
      year         = {2016},
      abstract     = {Off-axis electron holography can be used to measure the
                      electron-optical phase shift associated with a charge
                      density distribution in the transmission electron microscope
                      (TEM). The charge density can then be recovered either by
                      integrating the Laplacian of the reconstructed phase1 or,
                      equivalently, by applying a loop integral2. Whichever
                      approach is used, the perturbed reference wave3 does not
                      affect the measurement of the projected charge density
                      inside the specimen so long as it does not itself contain
                      any charges. Here, we study a W nanotip, in which the charge
                      density distribution is of interest for applications in
                      field emission and atom probe tomography. We assess
                      artefacts and noise in the measurements.Figure 1(a) shows an
                      off-axis electron hologram of a W nanotip recorded at 300 kV
                      using an FEI Titan 60-300 TEM. The interference fringe
                      spacing is 0.318 nm, the nominal magnification is 140 000
                      and the voltage applied to the electrostatic biprism is 90
                      V. The apex of the nanotip has a diameter of approximately 5
                      nm and is covered with a layer of tungsten oxide. A voltage
                      of 50 V was applied between the nanotip and a flat electrode
                      positioned approximately 3 µm away from it. In order to
                      remove the contribution to the phase shift from the mean
                      inner potential, two holograms with and without a voltage
                      applied to the nanotip were recorded. The difference between
                      the two phase images was then evaluated after sub-pixel
                      alignment. Figures 1(b) and (c) show the resulting unwrapped
                      phase before and after adding phase contours of spacing
                      2π/3 radians. Figure 1(d) shows the charge distribution
                      calculated by applying a Laplacian operator to a
                      median-filtered version of the phase image. Figure 1(e)
                      shows cumulative charge profiles along the nanotip
                      determined both using a loop integral and by applying a
                      Laplacian operator to either an unwrapped phase image or the
                      original complex image wave. The integration region is
                      marked by a green dashed rectangle in Fig. 1 (b). The
                      measured charge profile is consistent between the three
                      approaches. Figure 1(f) shows an evaluation of noise in the
                      measurement obtained by performing a similar integration in
                      a region of vacuum indicated by the red dashed rectangle in
                      Fig. 1(b). Results such as those shown in Figs. 1(d) and (e)
                      can be used to infer the electric field and electrostatic
                      potential around the tip. Future work will involve comparing
                      the present approaches with using a model-based technique
                      for determining the charge density from a recorded phase
                      image.},
      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},
      doi          = {10.1002/9783527808465.EMC2016.6252},
      url          = {https://juser.fz-juelich.de/record/827183},
}