Low-Voltage Ionization of Air with Carbon-Based Materials

Polycrystalline diamond and carbon nanotubes (CNTs) exhibit excellent vacuum field emission properties, characterized by low turn-on voltage and high current density. Their atmospheric field emission and ionization capabilities are reported in this paper. Highly graphitic polycrystalline diamond (HGPD) film was grown in a plasma-enhanced chemical vapour deposition process, and its ability to ionize atmospheric air was characterized and compared against CNTs. The HGPD sample was activated by applying a moderate voltage bias (340 V) for an extended period across a 10µm electrode gap. After activation, a turn-on voltage of 20V and a sustainable current of 10µA were observed with the same gap. Results also indicate that field emission helps to create a moderate ionization effect without catastrophic air breakdown. A hydrogen plasma treatment is shown to restore emission current back to or even exceeding the original level, which suggests an important role of surface termination in the electron emission process. CNTs were grown and tested but did not perform as well under similar conditions