We describe nanocluster based tunneling sensors for detecting hydrogen. Pd clusters with diameters ranging from 3.5 to 6 nm are deposited between a pair of contacts until a predetermined resistance between the contacts is obtained. We demonstrate that the conduction through the cluster film is dominated by tunneling gaps. Upon exposure to hydrogen, the clusters expand, reducing the average size of the gaps and reducing the measured resistance. The sensor response as a function of external hydrogen pressure is described, for different cluster sizes and different operating temperatures, by a simple model with a single physically meaningful fitting parameter.
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