Individual Pd nanowire hydrogen sensors fabricated by electron-beam lithography

We report on the hydrogen gas (H-2) sensing performance of lithographically patterned Pd nanowires as a function of the nanowire thickness and H-2 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 mu m, and thickness t = 20-400 nm from continuous Pd films. The variation of the resistance and sensitivity at 20 000 ppm H-2 of Pd nanowires was found to be much lager than at 10 000 ppm H-2, which can be explained by an alpha-beta phase transition occurring at 20 000 ppm H-2. This is confirmed by the observation of hysteresis behavior in the resistance versus H-2 concentration for Pd thin films. The response time was found to decrease with decreasing thickness regardless of H-2 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 H-2 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 H-2 over a wide range of concentrations.