Sensitive NH₃OH and HCl Gas Sensors Using Self-Aligned and Self-Welded Multiwalled Carbon Nanotubes

We present the design and fabrication of sensitive and responsive and HCl sensors based on multiwalled carbon nanotubes (MWCNT). The sensors consisted of self-aligned and self-welded MWCNT bridges grown between 18 mum-high silicon posts with 2-6 mum gaps using metal-catalyzed chemical vapor deposition technique (CVD). The 5-10 MWCNTs spanning the gap showed excellent ohmic electrical contacts and mechanical bonding strengths in excess of 0.1 muN/CNT. The conductivity of the MWCNTs changes as oxidizing and reducing gases are physically adsorbed on their large surface-to-volume ratio structures. Since carbon atoms in CNTs do not have any dangling bonds, the interaction between CNTs and gas molecules does not lead to chemical reactions and the process is reversible. Provided that the starting MWCNTs have high resistivity, they have nearly exponential sensitivity in detecting gases and owing to their very small volume and reversible interaction with gases, they can have a very fast response time. In our experiment, upon exposure to ammonia and hydrochloric gases at room temperature, the electrical conductivity of the self-welded carbon nanotube bridges exhibited a dramatic change. The growth process, test results, and the gas sensitivities will be discussed.