Multimode detection of hydrogen gas using palladium-covered silicon μ-channels

Palladium was electrodeposited onto lithographically patterened Si(100) "mu-channels" with dimensions of 2 mum (width) x 100 pm (length). The properties of these Pd-covered Si mu-channels for detecting dihydrogen gas were then evaluated. Pd electrodeposition was carried out under conditions favoring an instantaneous nucleation and growth mechanism. This strategy produced size-similar Pd particles at a coverage of (4-6) x 10(9) cm(-2) within the confines of the Sip-channel. When the mean particle radius, r(o), was smaller than a critical value (r(o) < r(c) = 70-85 nm), each Pd particle was well separated on the surface from adjacent particles, on average, and no response to H-2 gas attributable to the mu-channel was observed. As Pd particles were grown larger, to a mean radius of r(o) approximate to r(c), adjacent particles on the surface touched and the electrical resistance of they-channel dropped by several orders of magnitude. These "type 2" H-2 sensors exhibited a rapid (< 1 s), reversible decrease in their resistance in response to exposure to H-2 above 0.5%, but a minimum resistance was observed at 1-2%, and a resistance increase was seen at higher H-2 concentration. Ibis complex behavior resulted from the existence of three mechanisms for charge transport across the mu-channel. If still larger quantities of Pd were deposited, the Pd particle ensemble coalesced into an electrically continuous film. These "type 3" sensors became more resistive in the presence of H-2, not more conductive as seen for sensors of types 1 and 2, but the amplitude of this response was smaller than seen for type 2 sensors.