Chemical vapour sensors from functionalised single-walled carbon nanotube networks

Abstract

Solid-state gas detectors offer functional solutions in a broad range of applications, including non-invasive medical diagnosis, industrial health and safety, agricultural optimisation and environmental monitoring. Commercialisation of gas sensors requires devices that are highly sensitive and selective whilst remaining low-cost and portable with low power requirements. Carbon nanomaterials, specifically single-walled carbon nanotubes (SWCNTs), have shown excellent promise in gas sensors owing to their unique physical and chemical properties. SWCNTs are intrinsically sensitive to many stimuli; the challenge remains in achieving selective signal transduction. This thesis develops high performance gas sensors through a combination of surface functionalisations and assembly of novel hierarchical architectures. SWCNTs are exfoliated by reductive dissolution with sodium metal and naphthalene in n,n-dimetheylacetamide. The process yields solutions of highly individualised species without disrupting the sp2 lattice, whilst surface charges provide a convenient handle for in-situ functionalisation. Adjacent SWCNTs are crosslinked by functionalisation with dielectrophiles, forming molecular junctions that contain a selective binding site. Electrical transport through nanotube networks is limited by the junction resistance; by directing analyte binding to the crosslink locus, the junction resistance can be modulated. In contrast to conventional sensors, that utilise the high sensitivity of SWCNTs for signal transduction, this approach exploits the high aspect ratio of SWCNTs to wire together many molecular junctions. Through the reductive crosslinking reaction, porous aerogel thin films were deposited and subsequently characterised by UV-Vis, Raman and x-ray photoelectron spectroscopy, as well as scanning electron microscopy and N2 porosimetry. Aerogel film chemiresistors were highly sensitive to both carboxylic acid (<15 ppb) and amine (<100 ppb) analytes, exhibiting opposite sensing responses, indicating a charge transfer mechanism. Networks were then crosslinked using a series of metalloporphyrins, improving the binding of amine groups. Finally, a unique molecular nanolayer deposition has been developed that assembles SWCNT films systematically, using depletion of reactive surface charges to control the deposition of carbon material. Multilayer films were characterised by atomic force and helium-ion microscopy and UV-Vis, revealing the role of the crosslinking reaction in determining the deposition. The method permits tight control over film properties, with a thickness increase of 3 nm, and transmittance drop of 0.5%, observed per layer. As a whole, the thesis demonstrates a versatile and low-cost architecture that can be tailored to a broad spectrum of analytes, working towards distributable sensors. The work was directed to the study of volatile organics, relevant to personal care. The new devices successfully achieved detection limits in the low ppb range, well below the human odour detection thresholds. However, a wide range of future applications are envisioned for the technology.