A Study of Imaging Modes in the Scanning Transmission Electron Microscope

Su, Yongjie (1989) A Study of Imaging Modes in the Scanning Transmission Electron Microscope. MSc(R) thesis, University of Glasgow.

Abstract

The efficiency of transmitted electron utilization is low in the standard phase contrast imaging mode in the scanning transmission electron microscope (STEM). This leads to serious difficulties when, for example, radiation sensitive material is under investigation. A significant improvement is achieved by using the differential phase contrast (DPC) imaging mode in STEM where a non-rotationally symmetric detector system is employed. To systematically investigate image formation in the DPC mode, a two-dimensional calculation of the DPC transfer function for an aberrated STEM imaging system is undertaken. The results show that the effect of spherical aberration increases markedly with the semiangle of the objective aperture, and a properly chosen defocus is necessary to balance the effect. From the calculations the optimum instrumental parameters pertaining to the VG-HB 5 STEM at Glasgow University are provided. Further improvement in efficiency is achieved by the use of a segmented annular detector leading to a modified DPC imaging mode. Variation of the annular detector geometry can lead to either a high signal-to-noise ratio or a flat DPC transfer function and signal-to-noise ratio band. In addition, computer image simulation techniques provide a flexible way to select conditions for imaging radiation sensitive material. Both the DPC and modified DPC modes are used in a set of simulations applicable to small pigmentary particles. Of particular interest are the topography and internal structure of the particles. The results substantiate the transfer function calculations.

• Item Type: Thesis (MSc(R))
• Qualification Level: Masters
• Keywords: Optics
• Date of Award: 1989
• Depositing User: Enlighten Team
• Unique ID: glathesis:1989-77936
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 30 Jan 2020 15:47
• Last Modified: 30 Jan 2020 15:47
• URI: https://theses.gla.ac.uk/id/eprint/77936