The key principle of physical sciences is the comparison between theory and experiment. My research is motivated by applying this principle to electron microscopical experiments. This requires both quantitative experiments and theoretical modeling. A further area of interest is electron holography.

Quantitative Microscopy

• Sufficiently calibrated and characterized microscope and detectors including noise transfer
• Perform measurement with a spread of measurement conditions (e. g. beam tilt in conventional imaging) to mitigate systematic uncertainties.
• Metadata curation of all relevant experimental parameters for repeatable measurement setups
• Deterministic data evaluation: all steps during data evaluation should be reproducible down to numeric precision.
• Automated measurements: more throughput (thus more reliable statistics) and less error prone

Computative Microscopy

• Numerical verification of assumptions inferred from first order approximations (e. g. kinematic scattering) to be valid for actual experimental conditions (e. g. in the dynamical scattering regime).
• Scattering theory and numerical scattering calculations: Multislice, multi-beam Howie-Whelan, Bloch wave, hybrids and beyond.
• Inverse methods
• Development of PyTEM: our in-house software suite for scattering simulations, data evaluation and processing, as well as metadata curation.

Applications & Methods

• Electric potential and field measurements in semiconductors (electron holography, 4D-STEM).
• Strain measurements (dark field electron holography, 4D-STEM, high-resolution TEM/STEM).
• Atomically resolved measurements (electron holography, STEM, HRTEM)
• Phase reconstruction in general
• Time resolved holography

• Accuracy of polarization field measurements by electron holography in InGaN quantum wells
  T. Niermann, L. Niermann, M. Narodovitch, and M. Lehmann
  Phys. Rev. B, 103, 075306 (2021)
  DOI: 10.1103/PhysRevB.103.075306
• Numerical simulation of TEM images for In(Ga)As/GaAs quantum dots with various shapes
  A. Maltsi, T. Niermann, T. Streckenbach, K. Tabelow, T. Koprucki
  Optical and Quantum Electronics, 52, 257 (2020)
  DOI: 10.1007/s11082-020-02356-y
• Refined structure model of rare earth silicide nanowires on Si(001)
  S. Appelfeller, J. Heggemann, T. Niermann, M. Lehmann, M. Dähne
  Applied Physics Letters 114, 093104 (2019)
  DOI: 10.1063/1.5086369
• Scattering of fast electrons by lattice vibrations
  T. Niermann
  Phys. Rev. B 100, 144305 (2019)
  DOI: 10.1103/PhysRevB.100.144305
• Dynamical diffraction effects on the geometric phase of inhomogeneous strain fields
  L. Meißner, T. Niermann, D. Berger, M. Lehmann
  Ultramicroscopy 207, 112844 (2019)
  DOI: 10.1016/j.ultramic.2019.112844
• Electron holography on HfO2/HfO2-x bilayer structures with multilevel resistive switching properties
  G. Niu, M.A. Schubert, S.U. Sharath, P. Zaumseil, S. Vogel, C. Wenger, E. Hildebrandt, S. Bhupathi, E. Perez, L. Alff, M. Lehmann, T. Schroeder, T. Niermann
  Nanotechnology 28, 215702 (2017)
  DOI: 10.1088/1361-6528/aa6cd9
• Gated interference for time-resolved electron holography
  T. Niermann, M. Lehmann, T. Wagner
  Ultramicroscopy 182, 54-61 (2017)
  DOI: 10.1016/j.ultramic.2017.06.017
• All metalorganic chemical vapor phase epitaxy of p/n-GaN tunnel junction for blue light emitting diode applications
  S. Neugebauer, M.P. Hoffmann, H. Witte, J. Blasing, A. Dadgar, A. Strittmatter, T. Niermann, M. Narodovitch, M. Lehmann
  Applied Physics Letters 110, 102104 (2017)
  DOI: 10.1063/1.4978268
• Holographic focal series: differences between inline and off-axis electron holography at atomic resolution
  T. Niermann, M. Lehmann
  J. Phys. D: Appl. Phys. 49, 194002 (2016)
  DOI: 10.1088/0022-3727/49/19/194002
• Strain field of a buried oxide aperture
  F. Kießling, T. Niermann, M. Lehmann, J.-H. Schulze, A. Strittmatter, A. Schliwa, and U. W. Pohl
  Phys. Rev. B 91, 075306 (2015)
  DOI: 10.1103/PhysRevB.91.075306
• Nanometer-scale tomographic reconstruction of three-dimensional electrostatic potentials in GaAs/AlGaAs core-shell nanowires
  A. Lubk, D. Wolf, P. Prete, N. Lovergine, T. Niermann, S. Sturm, H. Lichte
  Phys. Rev. B 90 (2014) 125404
  DOI 10.1103/PhysRevB.90.125404
• Averaging scheme for atomic resolution off-axis electron holograms
  T. Niermann, M. Lehmann
  Micron 63 (2014) 28–34
  DOI 10.1016/j.micron.2014.01.008
• Impact of electron irradiation on electron holographic potentiometry
  J. B. Park, T. Niermann, D. Berger, A. Knauer, I. Koslow, M. Weyers, M. Kneissl, M. Lehmann
  Applied Physics Letters 105 (2014) 094102
  DOI 10.1063/1.4894718
• A new linear transfer theory and characterization method for image detectors. Part I: Theory
  T. Niermann , A. Lubk, F. Röder
  Ultramicroscopy 115 (2012) 68–77
  DOI 10.1016/j.ultramic.2012.01.012
• A new linear transfer theory and characterization method for image detectors. Part II: Experiment
  A. Lubk, F. Röder , T. Niermann, C. Gatel, S. Joulie, F. Houdellier , C. Magén, M. Hÿtch
  Ultramicroscopy 115 (2012) 78–87
  DOI 10.1016/j.ultramic.2012.01.011
• State of the art in atomic resolution off-axis electron holography
  M. Linck, B. Freitag, S. Kujawa, M. Lehmann, T. Niermann
  Ultramicroscopy 116 (2012) 13–23
  DOI 10.1016/j.ultramic.2012.01.019