There is a strongly increasing demand for highly sensitive gas detecting devices in numerous applications ranging from industrial process control to disease diagnostics in medicine. However, most commercial gas sensors are not compatible with CMOS technology, which limits their use in integrated, low-power devices. Metal oxide based gas sensors, which rely on changes of electrical conductance due to the interaction with the surrounding gas, have been thoroughly investigated in the past decades. A most powerful strategy to improve sensor performance is the implementation of single-crystalline nanowires as sensing elements, which have a high surface to volume ratio and thus a strong interaction between the surrounding gas and the material.
Cupric oxide (CuO) as a non-stoichiometric p-type semiconductor with a narrow band gap of about 1.2 eV is a promising candidate for the realization of novel nanowire gas sensor devices. Thermal oxidation was employed for nanowire synthesis, as it is a convenient method for the growth of single-crystalline structures with high aspect ratios. Two different approaches (heating in a furnace and resistive heating) were chosen for the oxidation of different copper substrates at various temperatures. The as-synthesized nanowires reached lengths between 0.5 m and 100 m and diameters ranging from 8 nm to 200 nm. Scanning electron microscopy, transmission electron microscopy and electron energy loss spectroscopy were used for subsequent characterization. Furthermore, nanowires were transferred to silicon substrates and contacted by a structured metalization layer in order to investigate transport properties of single nanowires. In particular high aspect ratio nanowires were employed as gas sensing elements, which showed promising results for the detection of water vapor and low concentrations of hydrogen sulfide and nitrogen oxide.