Hydrogen sensors based on percolation and tunneling in films of palladium clusters

Abstract

We describe the formation of hydrogen sensors by deposition of Pd clusters onto silicon dioxide coated silicon substrates with electrical contacts defined by a simple shadow masking technique. The clusters are prepared by sputtering in a gas aggregation source. The sensors are characterized by exposure to hydrogen in a simple flow chamber and by measuring the temperature dependence of the sensor resistance. Sensors with cluster coverage greater than the percolation threshold form "thin film" type sensors which exhibit a small increase in resistance on exposure to hydrogen, consistent with the increase in resistivity of bulk Pd on absorption of hydrogen. Sensors with coverage smaller than the percolation threshold form sensors which exhibit a much larger decrease in resistance on exposure to hydrogen. The response of these "percolating-tunneling" sensors is due to the absorption of hydrogen by the Pd clusters, which causes the tunnel gaps in the film to decrease in size, leading to an increase in conductance. Finally we describe tunneling sensors, where gold islands are grown on the substrate prior to cluster deposition, and which exhibit similar characteristics to the percolating-tunneling sensors.