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A Single-Gated CNT Field-Ionizer Array with Open Architecture

The micro gas analyzer project aims to develop the technology for portable, real-time sensors intended for chemical warfare and civilian air purity control. The device is composed of four micro-fabricated subsystems: an ionizer, a mass filter based on a quadrupole array [1], an electrometer [2], and a positive displacement pump [3]. We are developing a single-gated fieldionizer array based on gated carbon nanotubes (CNTs). The devices achieve species ionization by tunneling of outer shell electrons due to the presence of high electric fields that the device sets. We use CNTs as field enhancers because of their small radii and high aspect ratio while the gate proximity ensures high fields at low voltage. State-of-the-art ionizers use electron-impact ionization (thermionic cathodes), incurring in excessive power consumption, low current, current density, ionization efficiency, and short lifetime. The field-ionizer arrays (Figure 1) are able to soft-ionize species, thus achieving molecule ionization. The reliability and lifespan of the field-ionizer arrays are larger than the corresponding values for electronimpact ionizer arrays because the CNTs are biased at the highest potential in the circuit, thus making it unlikely for ionized molecules to back-stream. Figure 2 shows two SEM pictures of a single-gated CNT array that implements a selective CNT-growth process. This process reduces the fabrication complexity of the device because it grows CNTs from an un-patterned catalyst (Ni). Current research efforts concentrate on improving the device and data acquisition, including benchmarking the performance of the ionizer in low-pressure oxidizing environments.