Functionalized double-walled carbon nanotubes for integrated gas sensors

We have successfully fabricated gas sensors based on chemically functionalized double-wall carbon nanotubes (DWCNTs) using a robust and low cost process. The DWCNTs were synthesized by catalytic chemical vapor deposition (CCVD) method. They were then purified before functionalization (oxidation, amination, and fluorination). The sensor devices were fabricated by soft lithography using PDMS (Poly-DiMethylSiloxane) stencils and liquid phase pipetting of a suspension of chemically functionalized DWCNTs in deionized water, rinsing and finally drying in a nitrogen flow. Each device (1 cm x 2 cm) is equipped with a set of 7 DWCNT based resistors. Each resistor can accommodate a precise chemical functionalization for targeting a specific gas species, allowing a multiplexed (up to 7) detection. Due to their small size and the possibility to fabricate them on soft substrates, they could be used for many kinds of applications including wearable devices. The electrical resistance of the produced resistors turned out to decrease with temperature, suggesting fluctuations induced tunneling conduction through the disordered network of metallic nanotubes. However, we have shown in our work that for realistic applications, gas sensing can be achieved without any temperature regulation of our devices, because the variations of electrical conductance caused by gas molecules adsorption are significantly larger than those caused by possible temperature fluctuations. The as fabricated devices exhibit at room temperature a metallic conducting behavior. Devices with a resistance less than 100 kΩ were selected for gas detection. Because the sensing principle is based on the direct measurement of the resistance, our scheme ensures low power consumption (<1 μW). Raw (not functionalized) DWCNTs-based gas sensors exhibited a low sensitivity to the tested analytes, including ethanol, acetone, ammonia and water vapor. Functionalized DWCNTs-based gas sensors exhibited a moderate sensitivity to ethanol, acetone and water vapor but the response to ammonia, even in the presence of additional water vapor, was excellent. In particular, oxidized DWCNTs based gas sensors exhibited a high stability in the case of prolonged and repeated gas exposures. The oxidized DWCNTs gas sensors were also able to detect ammonia vapor at sub-ppm concentration in the presence of water vapor at high concentration. We believe that the functional groups grafted to the DWCNTs modify the interaction between gas molecules and DWCNTs and that the induced charge transfer is responsible for the modification of the electrical conductivity. We have built a simple phenomenological model for the analysis of the sensing response curve. This model includes two components for the variations of electrical resistance during gas exposure, an exponential regime and a linear one. In particular, the time constant extracted from the exponential part of the response was found to be informative on devices' sensitivity and selectivity. Finally, we tested our sensors for realistic applications such as trace detection of ammonia, which could be easily detected while far below the detection threshold of human nose (0.04ppm) Due to the high stability, ease of fabrication (very simple design, use of low-cost technologies, integration on flexible substrates), robustness (detection in the presence of a large excess of water vapor and resilient to temperature fluctuations) and extremely low amounts of carbon nanotube required, we expect these results to have some potential for a wide range of mass applications in the field of wearable gas sensors for Information and Communication Technologies (ICT) industry.