1meV Electron Monochromator

C53-13a-271127Electron 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. 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. Commonly used electron sources limit the performance of these techniques because the electrons are emitted with a relatively large energy spread (0.25-1eV), 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 to as low as 5-10 meV; however, the filtering of the energy distribution also dramatically reduces the beam current to below 1 pA. As a result, these instruments suffer from long acquisition times, which constrain their practical applications to niche areas. Furthermore, there is great interest in reducing the energy spread further, down to 1 meV, which would open up new opportunities in the spectroscopic characterization of materials. The monochromator proposed in this project incorporates a novel electron source that is currently being developed in a collaborative effort with researchers at Lawrence Berkeley National Laboratory (LBNL) and Kimball Physics. Initial results have shown that this electron source equipped with a superconducting Nb emitter can produce a beam of electrons with an energy spread as low as 15 meV. The novel electron source is combined with an energy-dispersive, magnetic beam separator with an electrostatic electron mirror and a knife-edge aperture to filter the electron beam. In phase I, a detailed opto-mechanical design of a monochromator was completed. The results have demonstrated that the proposed monochromator will reduce the energy spread of emitted electrons to 1 meV while delivering a beam current exceeding 10 pA, and to 3 meV while delivering a beam current of more than 300 pA: beam current values that are 2-3 orders of magnitude higher than those achievable by state-of-the-art approaches. In phase II, EOI aims to build, assemble, and integrate the components of the monochromator monochromator and test the prototype using a standard emitter. EOI will then work with Dr. Minor’s group to integrate the monochromator with the liquid He instrumentation required for the superconducting Nb emitter and verify experimentally its performance utilizing LBNL's high resolution electron spectrometer. 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 monochromatic beam will benefit novel techniques of multipass-transmission electron microscopy and quantum electron microscopy.