Sulfur doped nanocrystalline diamond films as field enhancement based thermionic emitters and their role in energy conversion

Sulfur doped nanocrystalline diamond films, like other nanostructured carbon films, exhibit electron emission characterized by a spatial non-uniformity of the field enhancement factor. While field emission effects are observed at room temperature, an increase in emitter temperature is accompanied by an amplified emission current with a simultaneous drop in the threshold field. At low extraction fields a fit of the emission current to the Richardson equation indicates a material work function of ∼2.5 eV. The Schottky formula describes thermionic emission at a moderate field and is utilized to determine the work function at an electric field of 0.8 V/μm with a value of ∼1.7 eV and a concurrently reduced Richardson constant. This significant difference in the work function of 2.5 and 1.7 eV for 0.5 and 0.8V/μm, respectively can be attributed to field enhancement effects.