Thermionic emission from surface-terminated nanocrystalline diamond

Thermionic electron emission forms the basis of both electron sources for a variety of applications and a direct energy conversion process that is compact and scalable. The present study characterizes thermionic emission from boron-doped nanocrystalline diamond films with hydrogen and nitrophenyl surface termination layers. A hemispherical energy analyzer was used to measure electron energy distributions from the emitters at elevated temperatures. Thermionic emission energy distributions, acquired at temperatures ranging from 700 to 1100 °C, reveal that emission occurs from regions of differing work functions. The relative peak intensities, representing each work function, change with temperature indicating instability in the emitter's surface chemistry. Corresponding partial pressure measurements of pertinent gases present in the chamber during the experiment were collected by a residual gas analyzer and support the hypothesis of unstable surface chemistry. The lowest work functions measured for the hydrogen- and nitrophenyl-terminated films were 3.95 and 3.88 eV, respectively. After the initial heating cycle, the hydrogen-terminated sample's surface was regenerated by exposure to hydrogen plasma. The lower work function was restored by this process, and the resulting thermionic electron energy distributions again were indicative of surface desorption.