Thermionics in Topological Materials

Thermionic emission is fundamental to many technologies and devices, including thermionic energy converters, X-ray tubes, scanning electron microscopes, and transmission electron microscopes. The discovery of topological materials, particularly graphene, has significantly advanced thermionics research. Thermionic emission in these materials deviates from the Richardson-Dushman equation due to their linear energy dispersion. Various models are developed to accurately describe thermionic emission. Graphene, with its dangling bond-free surface, can be stacked either vertically or laterally with materials to form heterostructures. The Schottky barrier height at the interface of heterostructures can be tuned from a few millielectronvolts to several electronvolts by selecting appropriate materials or adjusting the Fermi level of graphene. This low and tunable barrier height gives rise to a great potential in developing thermionic energy converters and photodetectors. While free-standing single-layer graphene exhibits high electron mobility, its thermionic emission capability is constrained by the low density of states. This constraint can be alleviated by using 3D Dirac materials, which also possess linear energy dispersion. Thermionic emission in 3D Dirac materials is further enhanced by the emergence of nodal-ring semimetals and Weyl semimetals that exhibit linear-like energy dispersion. This review highlights recent progress in thermionic emission and devices in graphene structures and other topological materials.