Individual Pd nanowire hydrogen sensors fabricated by electron-beam lithography.

We report on the hydrogen gas (H2) sensing performance of lithographically patterned Pd nanowires as a function of the nanowire thickness and H2 concentration. A combination of electron-beam lithography and a lift-off process has been utilized to fabricate four-terminal devices based on individual Pd nanowires with width w = 300 nm, length l = 10 microm, and thickness t = 20-400 nm from continuous Pd films. The variation of the resistance and sensitivity at 20 000 ppm H2 of Pd nanowires was found to be much lager than at 10 000 ppm H2, which can be explained by an alpha-beta phase transition occurring at 20 000 ppm H2. This is confirmed by the observation of hysteresis behavior in the resistance versus H2 concentration for Pd thin films. The response time was found to decrease with decreasing thickness regardless of H2 concentration due to a higher surface-to-volume ratio and a higher clamping effect. A single Pd nanowire with t = 100 nm was found to successfully detect H2 at a detection limit of 20 ppm. Our results suggest that lithographically patterned Pd nanowires can be used as hydrogen gas sensors to quantitatively detect H2 over a wide range of concentrations.