Low-energy electron point source microscopy and electron holography

• Author / Creator

  Mutus, Josh Y

• Low energy electron point source (LEEPS) microscopy is the simplest embodiment of an electron microscope, consisting of only a source, a sample and a detector. In a specific regime, LEEPS may also be used to create in-line holograms; special interference patterns that contain the information about the entire electron wavefront, including the structure of the sample and electromagnetic field around it. This work describes the design, construction and characterization of a microscope designed to performs LEEPS microscopy and electron holography at the nanoscale. An overview of previous experimental apparatus are discussed. Also, the impact of spatial and energetic inhomogeneities of the electron source on the quality and resolution of the hologram, in terms of the numerical aperture of the microscope and the virtual source size of the electron emitter. The design of the microscope itself is presented including the system for isolating the microscope from contamination, mechanical vibration and electrical noise. Using scanning tunnelling microscopy (STM) the microscope is shown to be stable within 0.1 ̊A. Instructions for the maintenance of the system are presented for future users of the microscope and to illustrate many of the systems described in the design of the microscope. The source of electrons used in the LEEPS microscope is a tungsten tip sharpened so as to field-emit electrons from a single atom. The technique for crafting such tips by field- assisted etching with nitrogen is described along with a discussion of the parameters used to control the aspect-ratio of the tip. Several samples are investigated using LEEPS: a sharp silicon nitride edge, a carbon nanotube bundle and graphene. The sample preparation techniques are discussed for each sample. Also, simple models for describing the resulting fringe patterns are proposed. There are several benefits associated with using LEEPS, including the lack of beam induced morphological changes or contamination. The samples are used to elucidate many properties about the optical system of the microscope, most importantly the expected resolution of the system. The software designed for the microscope to acquire images with high fidelity and for post-processing and correcting data is demonstrated. The microscope is shown to have a virtual source size of 1.6±0.6 ̊A a figure that exceeds published results form similar instruments. Preliminary holographic reconstructions are shown. The work concludes with a discussion of the parameters to be optimized in order to reach atomic resolution.

• Subjects / Keywords

  • Coherence electron source
  • Holography
  • Graphene
  • Carbon nanotubes
  • Low energy electrons
  • Electron holography

• Graduation date

  Spring 2012

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3302J

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.