Highly sensitive hydrogen gas sensor based on a suspended palladium/carbon nanowire fabricated via batch microfabrication processes

We present a highly sensitive H2 sensor based on a single Pd-functionalized carbon nanowire suspended ∼10 μm above a substrate while monolithically bridging two carbon posts. The carbon nanowire and posts were fabricated using a batch carbon-microelectromechanical system (MEMS) process consisting of photolithography and pyrolysis. The pyrolysis process resulted in a significant volume reduction in which the nanowire diameter was reduced to ∼120 nm from the original 1 μm × 1.5 μm photoresist wire. Because of SiO2 eaves formed by isotropic etching of the Si substrate, as well as the suspended geometry of the carbon nanowire, a single palladium evaporation process enabled selective palladium coating on the carbon nanowire without any nano-patterning processes. The electrical conductivity of the carbon nanowire was modulated by varying the pyrolysis temperature so that the electrical resistance along the suspended Pd/C nanowire was affected predominantly by hydrogen chemisorption onto the thin Pd layer enclosing the carbon core. The suspended functionalized wire showed better sensing capability compared to a Pd nanowire fabricated on the substrate with similar dimensions, because of enhanced mass transport. The suspended nanowire-based H2 sensor quantified from 10 to 500 ppm H2 with percentile resistance changes of 5.9–129%.