Electron emission models for two-dimensional and topological materials

Electron emission from electrode into a diode is a fundamental process for beam physics, plasma science and technology. Depending on the energy used for electron emission, it can be broadly characterized into 3 different processes known as thermionic emission TE (by thermal energy), field emission FE (by quantum tunneling) and photoemission PE (by absorption of photons or optical tunneling). At high current regime, it becomes the space charge limited current (SCLC). The classical models for these processes (TE, PE, PE, SCLE) have been formulated many decades ago, known as the Richardson-Dushman (RD) law, Child-Langmuir (CL) law, Fowler-Nordheim (FN) law, and the Keldysh model, etc. With the development and fabrication of two-dimensional (2D) atomic scale and topological materials, the above-mentioned classical laws may require revisions to account for their unique material properties if such materials are used as electron emitters. This invited talk is focused on thermionc-field emission and will explain why the traditional RD and FN laws are no longer valid for these new quantum materials. The presentation will share some recent models to address the unique properties of the materials and the emission characterisitics. New scalings as a function of temperature and electric field will be shown for various classes of quantum electron emitters. A quick overview of recent experimental results from field emission from such materials will be presented to show its potential as compact electron emitter. The similarity of the emission physics to the charge injection in electrical contact will be discussed. Finally some unsolved questions will be suggested for future studies in order to have a better understanding and applications for the beam and plasma community. These electron emission laws will be useful PIC simulation or gun codes used widely in plasma physics and high power coherent radiation sources.