High spatial resolution and three-dimensional measurement of charge density and electric field in nanoscale materials using off-axis electron holography
| Main Author: | Zheng, Fengshan |
|---|---|
| Format: | info publication-thesis Journal |
| Bahasa: | eng |
| Terbitan: |
Forschungszentrum Jülich GmbH
, 2020
|
| Subjects: | |
| Online Access: |
https://zenodo.org/record/3928733 |
Daftar Isi:
- The ability to make local measurements of charge density in nanoscale materials and devices is essential for understanding many material properties. The charge density can then be used to infer the electric field or electrostatic potential within and around the specimen. This information is important for scientists working on subjects such as field electron emission and atom probe tomography. Off-axis electron holography is a powerful technique that can be used to record the phase shift of a high-energy electron wave travelling through an electron-transparent specimen in a transmission electron microscope. Information about the charge density within the specimen can be retrieved from the measured phase with high spatial resolution. In this thesis, charge density and electric field measurements are performed, both in projection and in three dimensions, with a primary focus on samples that have a needle-shaped geometry. Three approaches are used: an analytical model-dependent approach, a model-independent approach and an approach based on numerical model-based iterative reconstruction. The model-based iterative approach allows a priori information, such as the shape of the object and the influence of charges that are located outside the field of view, to be taken into account. More importantly, it also allows for the reconstruction of three-dimensional charge density distributions from incomplete tomographic tilt of phase images without the artefacts that would be present if conventional tomographic reconstruction algorithms were used. In this thesis, a W5O14 nanowire is investigated experimentally in the presence of an applied electrical bias and the charge distribution along it is evaluated. A carbon fibre needle-shaped specimen is then studied, in order to demonstrate the capability of the model- based iterative approach to measure the three-dimensional charge density, electric field and electrostatic potential both inside and around it. Finally, a systematic investigation of electron-beam-induced charging in a needle-shaped specimen with an insulating Al2O3 apex is presented, including the dependence of the results on electron dose rate, total dose, temperature, primary electron energy and the surface state of the sample. Great care is required with the acquisition and interpretation of the results, in particular because charging phenomena are sensitive to the electrical conductivity of the sample, the presence of contact potentials and the presence of (unknown) surface states.