Double-gated isolated vertically aligned carbon nanofiber field emission and field ionization arrays

Abstract

Electron impact ionization (ElI) is used extensively in mass spectrometry for gas-phase analytes. Due to the significant amount of fragmentation generated by ElI, the spectrum is usually very noisy. In addition, the thermionic emission electron source used in ElI has a slow response time and consumes large amount of power. To address these two issues, double-gated vertically aligned carbon nanofiber (VACNF) field emission and field ionization arrays have been developed. These arrays were characterized as field ionizers, which could produce molecular ions without severe fragmentation; and as field emitters, which act as an electron source with a fast response time and consume less power than using thermionic emission. Self-aligned double-gated isolated VACNF arrays, which were fabricated using a photoresist-based planarization process, are reported. These arrays were designed such that the tip electric field is maximized and the shielding effect from the neighboring tips is minimized while the device is capable of handling large voltages during field emission and field ionization. Two types of arrays were fabricated: (1) CNFs with tips in-plane with the gate and (2) CNFs with tips 0.9tm below the gate.

(cont.) These arrays were characterized as a field emitter and a low field emission turn-on voltage of 24V is reported. These arrays were used as electron sources for ElI at pressures ranging from 5x10-6 to lx10-3 Torr. The ion current is linearly related to the product of the electron current and the ambient pressure. Thus, the device could be used as a gas pressure sensor in vacuum. Field ionization experiments were also conducted with double-gated VACNF arrays. The field ionization turn-on voltage was reduced from about 10kV, typical of ungated ionizers, to 350V.