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. Commonly used electron sources limit the performance of these techniques because they emit electrons with a relatively large energy spread (0.25-1 eV). This spread limits the energy resolution of spectroscopic techniques like electron energy-loss spectroscopy (EELS) and makes techniques like low energy electron microscopy (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 program, Electron Optica, Inc. (EOI) is developing a novel coherent single-electron gun (CSEG) that reduces the energy spread of emitted electrons while maintaining a high beam current. The CSEG utilizes novel quantum dot and superconducting electron emitters with intrinsically low energy spread. These emitters are currently being developed in a collaborative effort at Lawrence Berkeley National Laboratory (LBNL). The CSEG is predicted to produce a continuous beam of electrons with an energy spread in the range of 10-20 meV, while delivering high beam currents in the nanoamp range. During phase I, EOI focused on establishing the feasibility of key aspects required for this approach. In close collaboration with Dr. A. Minor and his group at LBNL, EOI devised a design of a CSEG gun that can be prototyped in phase II. Phase I results have demonstrated that the proposed CSEG design can pack 10 nA into a 1 nm probe at 5 keV with a 10-20 meV energy resolution. Furthermore, this concept is extendable to STEM applications at 100 keV aiming for a 1 Å probe with ultralow energy spread provided that an aberration corrector is used to eliminate the objective lens spherical aberration coefficient. In phase II, EOI aims to develop a compact 5 keV CSEG that will utilize the novel electron emitter currently under development at LBNL. EOI will produce a detailed opto-mechanical design of the CSEG gun. EOI will then build the components, assemble, integrate, and test a prototype using a standard emit- ter. Concurrently, Dr. Minor’s group will continue improving and testing the novel electron emitter. EOI will then work with Dr. Minor’s group to incorporate their emitter into the CSEG and verify experimentally its performance at LBNL. The simultaneous reduction in the energy spread and increase in the beam current of the probing electron beam will enable the direct imaging of vibrational modes using EELS, the study of band gaps and 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 high coherence of the CSEG will benefit the novel techniques of multi-pass-transmission electron microscopy and quantum electron microscopy.