Computational Materials Group

Controlling Electron Emission From Surfaces – Morgan

We build on ab initio methods to understand materials structure-property relationships governing electron emission, including work function physics, surface dipole effects, and secondary electron emission, and use this understanding to develop new materials. Some of our major results include:

  1. Understanding surface structures of Ba-O-Sc / W thermionic emission cathodes: In close collaboration with Prof. John Booske at UW we initiated some of the foundational ab initio studies on Ba-O-Sc coated W electron emitter cathodes about 15 years ago. We recently successfully integrated ab initio work function predictions with surface characterization and electron emission simulations to provide the first microstructurally informed physics-based model for thermionic emission from a commercial dispenser cathode2. This model provided a milestone for structure-property relationships in electron emission, yielded a better understanding of role of patch fields in the smooth crossover from temperature-limited to full-space-charge-limited emission (explained in more detail in Ref3), and will serve as a foundation for more accurate modeling and optimization of microstructure and surface structure of cathodes for enhanced performance.
  2. Understanding of oxide work functions and discovery of new perovskite thermionic emission materials: In close collaboration with Dr. Ryan Jacobs and Prof. John Booske at UW we have developed an ab-initio based approach that led to discovery of new low work function perovskites with potential use as thermionic electron emitters. We demonstrated correlation of work function with a simple bulk p-band descriptor and using these correlations to discover promising stable, conducting, low work function oxides that could be used for thermionic emissions electrodes4. This work also led to a patent on some of these new materials5. We performed detailed studies on one promising perovskite, SrVO3, and demonstrated significant stable high-temperature electron emission, suggesting potential use as a thermionic electron emitter1. This work represents the first discovery of a new thermionic emission material with ab initio methods and demonstrates the potential for such ab initio based design. More specifically, this work suggests the use of polar conducting oxides as a promising materials family for further exploration as electron emitters.
  3. Development of ab initio based secondary electron yield (SEY) prediction: SEY measures the electrons liberated from a surface by an incoming high energy particle and is relevant for many applications, ranging from scanning electron microscopy to multipactor breakdown in microwave devices. In collaboration with Dr. Maciej Polak we have developed the first fully ab initio SEY simulation by integrating with quantum mechanical simulations. This innovation means that SEY can be calculated for orders of magnitude more systems, potentially much more consistently and accurately, than previous semi-empirical approaches. We also improved SEY simulations by introducing excitations of secondary electrons from the full density of states (including semi-core excitations) and treatment of alloys with a virtual crystal approximation. These SEY tools have been made available to the community in the first open source code for SEY determination (MAST-SEY)6 and used to demonstrate several new results, including that alloy SEY can deviate significantly from a simple linear interpolation from its constituents and that there is a robust trend in the maximum SEY of elements across the periodic table (work in preparation). This tool opens the door to exploration of SEY physics in increasingly complex systems and can help make practical the rational design of high and low SEY materials.
The work function measurement and thermionic emission test results on SrVO3 pellets. a) The UPS measured bias-corrected secondary electron cutoffs on an as-sintered pellet, and a pellet after Ar+ ion sputter-etching plus annealing at 700 °C for 0.5 h, and after cooling down, suggesting 3.45, 4.06, and 4.07 eV, respectively. It is not unexpected that these values do not match the DFT prediction due to surface contamination and patch field effect. b) The thermionic emission J–V plots from an over-reduced and cleaved sample at different temperatures, suggesting an increasing emission current as A-K voltage becomes larger, consistent with the patch field effect. c) The thermionic emission J–T plots of several over-reduced-and-cleaved samples within the temperature-limited regime, with datasets #1 to #7 collected at 1.5 kV A-K voltage, and the A-K voltage of #8 unknown. These datasets indicate an effective 2.7 eV work function under the testing condition together with a few samples showing an effective work function close to 2.3 eV. Figure from Ref. 1

References

 

1              L. Lin, R. Jacobs, D. Z. Chen, V. Vlahos, O. Lu-Steffes, J. A. Alonso, D. Morgan, and J. Booske, Demonstration of Low Work Function Perovskite Srvo3 Using Thermionic Electron Emission, Advanced Functional Materials 32 (41) (2022).

2              D. Chen, R. Jacobs, J. Petillo, V. Vlahos, K. L. Jensen, D. Morgan, and J. Booske, Physics-Based Model for Nonuniform Thermionic Electron Emission from Polycrystalline Cathodes, Physical Review Applied 18 (5), 054010 (2022).

3              D. Z. Chen, R. Jacobs, D. Morgan, J. Booske, and Ieee, presented at the 22nd International Vacuum Electronics Conference (IVEC), Electr Network, 2021 (unpublished).

4              R. Jacobs, J. Booske, and D. Morgan, Understanding and Controlling the Work Function of Perovskite Oxides Using Density Functional Theory, Advanced Functional Materials 26 (30), 5471-5482 (2016).

5              R. Jacobs, D. Morgan, and J. Booske, Perovskites as Ultra-Low Work Function Electron Emission Materials, United States Patent No. 62278813 (2016).

6              M. P. Polak and D. Morgan, Mast-Sey: Material Simulation Toolkit for Secondary Electron Yield. A Monte Carlo Approach to Secondary Electron Emission Based on Complex Dielectric Functions, Computational Materials Science 193 (2021).