| Abstrakt: |
Isolated palladium nanostructures expand when they are exposed to hydrogen gas, and the gaps between them become narrower, thereby decreasing the electrical resistance. This behavior is applicable for the hydrogen-gas sensing, and several types of nanogap structures have been developed. However, the resistance change is significantly small at a low hydrogen-gas concentration because of insignificant lattice expansion. In the present study, this problem is solved by using the palladium nanoclusters with extremely narrow gaps, which is achieved by our original method, resistive spectroscopy, and hydrogen-induced structural stabilization. The nanoclusters are fabricated by interrupting deposition just before forming the continuous film, in which palladium clusters are nearly touching each other, and exposing them to hydrogen gas. In conventional studies using nanoclusters, hydrogen gas is detected through a decrease in the surface electric resistance caused by gap narrowing/closing. However, in this study, we observe an increase in the resistance when the gap distance between the cluster is extremely small, which is attributed to the restriction of electron tunneling between the palladium nanoclusters because of hydrogen adsorption on their surface. We confirm that this mechanism allows ultrahigh sensitivity hydrogen-gas sensing, achieving a limit of detection of 0.25-ppm hydrogen gas. In addition, we find that an optimized structure for the present detection mechanism is different from those in conventional sensors based on the gap-narrowing/closing mechanism. [ABSTRACT FROM AUTHOR] |
