Quantitative imaging and alternative proposals on image simulation and reconstruction in atomic resolution transmission electron microscopy

Abstract

Quantitative imaging is an active area of research in atomic resolution transmission electron microscopy. Both aberration-corrected high-resolution transmission electron microscopy (HRTEM) and atomic resolution off-axis electron holography provide a unique opportunity to study phase information at the atomic and sub-atomic length scale. These methods can be utilized to extract a wide range of information on materials, e.g., atom types, distribution, electronic bonding, etc. The thesis starts with an introduction to aberration-corrected atomic resolution HRTEM and off-axis electron holography techniques. In chapter 2, two different atomic resolution techniques: Off-axis electron holography and HRTEM, have been employed to count atoms on individual columns of Zn and O in ZnO epitaxial thin film. Results show that the reconstructed phase from both the side and the central bands and the corresponding number of Zn (Z = 30) and O (Z = 8) atoms are in close agreement with the systematic increase in the number of atoms for a sample area less than the extinction distance. However, complete disagreement is observed for the sample area more than the extinction distance. On the other hand, the reconstructed phase obtained via in-line holography shows no systematic change with thickness for the same sample. Phase detection limits and the atomic model used to count the atoms are also presented. In chapter 3, an alternative approach to the image simulation in HRTEM is introduced after a comparative analysis of the existing image simulation methods. The alternative method is based on considering the atom center as an electrostatic interferometer akin to the conventional off-axis electron biprism within few nanometers of focus variation. Simulation results are compared with the experimental images of 2D materials of MoS2 and BN recorded under the optimum combination of third-order spherical aberration 𝐶𝑠=−35 m and defocus Δ𝑓= 1, 4, and 8 nm and are found to be in good agreement. In chapter 4, An alternative reconstruction method is proposed for retrieving the object exit wave function (OEW) directly from the recorded image intensity pattern in HRTEM. The method is based on applying a modified intensity equation representing the HRTEM image. A comparative discussion is provided between the existing methodologies of the reconstruction of OEW in HRTEM, off-axis electron holography, and the present proposal. Phase shift extracted from the experimental images of MoS2, BN, and ZnO are found to be in excellent agreement with the theoretical reference values. Additionally, it is shown that the Fourier series expansion of diffraction pattern is effective in retrieving the isolated and periodic image functions of a specific form. However, for aperiodic object information, e.g., defects, dopants, edges, etc., the first method works its entirety. A future perspective is provided based on the results presented in Chapter 5 of the thesis.