Nonequilibrium Green's function study of Pd4-cluster-functionalized carbon nanotubes as hydrogen sensors

Pd-cluster-functionalized carbon nanotubes (CNTs) have been shown experimentally to be effective hydrogen sensors. Semiconducting CNTs exhibit much higher sensitivity than ensemble (mixed) ones. Using the nonequilibrium Green's function method combined with the density-functional theory, we simulate and contrast the (8,0) semiconducting and the (5,5) metallic CNT model systems. We find that the electron localization effect plays a crucial role in determining electron transport. Pd clusters and hydrogen adsorption cause opposite effects on electron localization in the CNT backbone for the semiconducting CNT-based systems. Consequently Pd functionalization dramatically increases the conductance, but then it is strongly suppressed by hydrogen absorption. For the metallic CNT-based systems, there is a tiny shift of the transmission peak near the Fermi energy. These results offer a consistent explanation for the experiments.