A novel miniature gas ionization sensor based on freestanding gold nanowires

Freestanding gold nanowires (AuNW) were incorporated to fabricate a miniaturized gas ionization sensor. The device exhibited improved sensitivity in sub-Torr pressure (P) regime compared to similar devices reported earlier, since the room temperature breakdown voltage (V_b) was further reduced in low gas pressures or concentrations (N). Also excellent selectivity was achieved for pressures up to 10 Torr, while V_b remained almost unaffected by pressure. In the V_b-P characteristic below 1 Torr, V_b was less when the AuNWs was configured as cathode, and it started to decline until a Paschen-like (V_b)_min was observed at very low pressures (0.3 < P < 0.5 Torr). The reduction of breakdown voltage in this case, was attributed to the creation of an abnormally large amount of secondary electrons at the nanowire tips due to the increase in Townsend's secondary ionization factor (ω/α) at very high reduced fields (E/N). The pre-breakdown I-V characteristic of the device, obtained at extreme low pressures showed polarity dependency. The field enhancement factor (β) of the AuNW array was estimated by operating the device in the ohmic discharge region. The average aspect-ratio of the nanowires, which was extracted from the I-V characteristic, showed good agreement with SEM observations.