Coherent Single-Electron Gun

Electron microscopes have been widely used by material scientists, biologists, and industrial scientists to study the composition and chemical structure of materials with high spatial and temporal resolution. Aberration-corrected instruments can image individual defects and interfaces at atomic resolution, and continued advances in electron energy-loss spectroscopy (EELS) have made elemental analysis possible down to the atomic level. Low energy electron microscopes (LEEM) provide an exquisitely sensitive surface imaging technique, capable of imaging single atomic layers with high contrast. Pulsed techniques, e.g. ultrafast electron diffraction and dynamic transmission electron microscopy have been developed to resolve structural dynamics on the ultrafast timescale. Commonly used electron sources limit the performance of these techniques: electrons are emitted with a relatively large energy spread (0.25-1 eV), which limits the energy resolution of spectroscopic techniques like EELS and makes techniques like LEEM susceptible to chromatic aberrations. Monochromators have been developed to reduce the energy spread; however, the filtering of the energy distribution also dramatically reduces the beam current and thereby its brightness. As a result, these instruments suffer from long acquisition times, which constrain their practical applications to niche areas. In this Phase I project, we will design a novel coherent single-electron gun (CSEG) that reduces the energy spread of emitted electrons into the single meV range while maintaining a high beam current in the nA range. The CSEG utilizes a quantum-dot electron emitter that is currently being developed in a collaborative effort at Lawrence Berkeley National Laboratory (LBNL).The electron emitter will produce a continuous, “single-file” beam of electrons with an energy spread in the range of 1-10 meV. The acceleration and focusing of these uniquely emitted electrons by the electromagnetic fields of the gun extractor and lenses will be analyzed in detail using state-of-the-art simulation software. Particular attention will be paid to the Coulomb interactions inside the gun, where the electrons start from rest and are accelerated to the final beam energy. A detailed analysis of the trade-offs between the achievable beam current, energy, and gun geometry will be carried out for both electrostatic and magnetic lenses. The goal of the phase I research is to provide a detailed electron-optical design of a CSEG that can be prototyped in phase II. During phase II, the CSEG will be built and its performance will be characterized in collaboration with the research group developing the electron source at LBNL. The simultaneous reduction in the energy spread and increase in the beam current of the incident electron beam will enable the direct imaging of vibrational modes using EELS, the study of band gaps and band- gap defects in semiconductors with sub-nanometer resolution, as well as the detailed study of low-loss structures in materials such as metal nanoparticles, solar cells, and organic materials. The reduction in the energy spread would also moderate the impact of chromatic aberrations in Low-voltage SEMs and LEEMs to improve their spatial resolution into the sub-nanometer range.