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 resolution. Commonly used electron sources limit the performance of these techniques because they emit electrons with a relatively large energy spread (0.25-1eV). This spread limits the energy resolution of spectroscopic techniques like electron energy-loss spectroscopy (EELS) and makes techniques like low energy electron microscopy 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. 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 a novel superconducting Nb electron emitter with intrinsically low energy spread. These emitters are currently being developed in a collaborative effort with Dr. Minor’s group at Lawrence Berkeley National Laboratory. In phase II, EOI produced a detailed mechano-optical design of the CSEG and is currently assembling the initial prototype. Concurrently, Dr. Minor’s group improved and evaluated the novel Nb emitter. In particular, their experimental work has produced electron beams with extremely low energy spreads in the range of 20 to 70 meV at liquid helium temperatures. While we successfully completed the emitter region design, we encountered difficulties in minimizing vibrations of the cryogenic, high voltage and ultra-high vacuum interfaces. Furthermore, researchers at LBNL discovered that the energy spread of the emitted beam is intimately linked to the nanostructure of the tip apex. We now understand that we must add vibration suppression elements as well as develop a tip formation procedure. EOI plans to finish building the initial prototype by the end of phase II, incorporate the Nb emitter into the CSEG and prepare it for testing. In the subsequent Phase IIA, EOI will complete the CSEG prototype by implementing vibration controls and emission diagnostics features similar to techniques we have successfully used before. We will fully characterize the performance of the CSEG at both room and cryo- genic temperatures. Researchers in Dr. Minor’s group will focus on the refinement of the current method to arrive at a more robust procedure for reliably producing the Nb tip nanostructure for generating beams with energy spreads in our target 10-20 meV range. The simultaneous reduction in the energy spread and a more than 10x increase in the beam current has the potential to dramatically expand the use of electron spectroscopy by enabling the direct imaging of vibrational modes using EELS, the study of band gaps and defects in semiconductors, as well as the detailed study of low-loss structures in materials such as metal nanoparticles, solar cells, and organic materials. Furthermore, the CSEG will greatly benefit novel techniques that demand high coherence, such as multi- pass-transmission electron microscopy and quantum electron microscopy.