Understanding Low Work Function Perovskite Thermionic Emission Materials.

다국어 초록 (Multilingual Abstract)

Work function, the surface energy barrier for electron migration towards vacuum, is one of the most important properties in science and technology involving material surfaces and interfaces such as vacuum and solid-state electronics, catalysis, energy harvesting, etc. However, there has been widely existing confusion on work function definition, measurement, and interpretation. Pursuing lower work function emitters is particularly crucial for thermionic emission cathodes used in vacuum electronics, as it will boost the emission performance, especially with intrinsically polar materials such as perovskite oxides, since it is possible with these materials to overcome shortcomings of contamination and lifetime existing in current dispenser cathodes. However, the experimentally measured work functions of the perovskites are generally much higher than the DFT predictions, leaving a question on whether such proposed polar material idea is practical. In this dissertation, deeper understanding of work function physics has been developed with an electrostatic energy treatment, together with the realization of the roles of external and patch fields. Such treatment clarifies the confusions in interpreting the work function definition and measurements. Also, the work function is understood as the benchmark for relative energy level alignment that is critical in a vast array of applications, and the two main strategies of tuning work functions, namely tuning bulk electronic structures and surface dipoles, are emphasized. The experimental demonstrations of low work function perovskites SrVO3 and BaMoO3 have benefited a lot from the development fundamental understanding of work function. Particularly, with the recognition and proper treatment on surface oxidation and patch field effect, SrVO3 has shown low 2.3 to 2.7 eV effective work function with thermionic emission, which is the lowest work function observed from a bulk monolithic conductive oxide material, promising for work function engineering including thermionic emission applications. BaMoO3 indicated multi-scale patch fields, with the micron-scale patch field being fully overcome, further proving the patch field theory. On the other hand, significant phase transformation that might be associated with Ba migration similar as dispenser cathodes is observed, possibly suggesting a simpler approach for cathode fabrication, although future research is still necessary to more thoroughly understand the properties of BaMoO3.