A novel gas sensor based on tunneling-field-ionization on whisker-covered gold nanowires

Typical gas ionization sensors (GISs) work by finger-printing the ionization breakdown voltages of the gases to be identified. In this work, we developed a GIS that operates by field-ionizing the unknown gas at exceptionally low voltages. The resultant field-ion current-voltage (I - V) characteristic was then used to identify the gas. Freestanding gold nanowires (AuNW), terminated with nanoscale whisker-like features, were employed as field-amplifiers to reduce the field ionization threshold voltages. Synthesis of the AuNWs was carried out by the template-assisted technique accompanied by two alterations: 1) polystyrene (PS) microspheres were incorporated to reduce the compactness of the pores, thus prevent the nanostructures from collapse, and 2) the template was impregnated by HAuCl₄ to form gold nanowhiskers during the electrochemical nucleation of AuNWs. The sensor was tested in three elemental gases: Ar, N₂ and He, in a pressure range of 0.01 < P < 100 torr. Each gas demonstrated a distinctive I - V curve, particularly in the field-limited regime. The threshold ionization voltages ranged from 1 to 10 V, almost three orders of magnitude lower than the voltages used in field-ion-microscopy. The low-voltage field ionization was attributed to the field-amplifying nanoscale whiskers on the AuNW tips, as well as the presence of residual amorphous alumina with semiconducting characteristics, due to incomplete removal of the porous anodized alumina (PAA) template.