Effect of Varying the Semiconducting/Metallic Tube Ratio on the Performance of Mixed Single-Walled Carbon Nanotube Network Gas Sensors

The sensing properties of mixed networks consisting of semiconducting and metallic single-walled carbon nanotubes (SWCNTs) have been found to largely vary depending on the ratio of semiconducting to metallic tubes. Solution-deposited 99% semiconductor-enriched nanotube networks exhibited a sensitivity of 1.908%/ppm, whereas the unenriched 66% and 90% enriched samples exhibited a sensitivity of 0.027%/ppm and 0.113%/ppm, respectively. These results suggest that it is extremely important to minimize the metallic pathways to achieve high sensitivity. After an oxygen plasma treatment, the unenriched 66% sample exhibited a 526% increase in sensitivity (0.142%/ppm) compared to the untreated one, whereas the 90% device demonstrated a sensitivity of 1.521%/ppm, which corresponds to an improvement in the sensitivity of 13.5 times the pristine 90% sample. In addition, the plasma-treated sensors exhibited a much faster response time than the untreated one. The significant improvement in the performance of the highly enriched network sensors was explained by the large increase in the anchoring sites for ammonia molecules on the surface of the semiconducting single-walled CNTs and the faster charge transfer from absorbed molecules