BaZrO 3 and BaCeO 3 Solid Solutions May Allow the Applicability of Doped Perovskites in Low-Temperature Fuel Cell Applic
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layer. These results distinguish hydrogenated diamond from other semiconductors. As reported in the October 16 issue of Physical Review Letters, the researchers used 10 different diamond samples, ranging from undoped chemical vapor deposition films, undoped homoepitaxial layers grown on single crystals, to various types of single crystals. The roomtemperature (RT) conductance of these samples, in the highly conducting hydrogenated state, was found to be between 10-6 and 10-4 Ω-1. In the first experiment, the homoepitaxial (100) diamond layer in a high conductance state was annealed in ultrahigh vacuum UHV at 410 ± 20°C for 15 min. The chemisorbed hydrogen was not desorbed due to annealing, as verified by the negative electron affinity property of the surface, but the conductance of the sample had dropped to 10-10 Ω-1. The researchers then masked half of the sample and removed the hydrogen from the unmasked surface by means of electronbeam-induced desorption. When this sample was brought back to air, the conductance of the masked part increased by four orders of magnitude within 20 min while the other part did not exhibit any change from its nonconducting state. The researchers also performed an annealing experiment on a plasma hydrogenated IIa C(100) single crystal in air while simultaneously monitoring the hydrogen coverage by looking at CH and CH 2 characteristic stretching modes using multiple internal reflection infrared spectroscopy (MIRIRS). After the sample had been annealed at 190°C, they found
that the hydrocarbon adsorbates CH 2 characteristic modes disappeared, while the sample still exhibited RT conductance of about 10-6 Ω-1. After annealing at 230°C the CH mode of the hydrogen termination also disappeared, and the RT conductance dropped to 10-11 Ω-1. Based on these findings, the researchers concluded that hydrogen termination is necessary for high surface conductivity in diamond, while hydrocarbon adsorbates play no role in it. An additional adsorbate from the atmosphere is needed to induce the surface hole accumulation layer, which is proposed to come from a thin water layer formed naturally on the diamond’s surface on exposure to the atmosphere. A redox reaction—2H 3 O + + 2e - ↔ H 2 + 2H2O—transfers electrons from the diamond to the water layer until the chemical potential of the water layer is equal to the Fermi level (EF) at the interface. This results in the accumulation layer of holes in the diamond that is responsible for the surface conductance. In their article, the researchers provide a quantitative account of this model, which also explains why this surface conductivity is observed only in diamond, and not in other semiconductors. WIRAWAN PURWANTO
BaZrO3 and BaCeO3 Solid Solutions May Allow the Applicability of Doped Perovskites in LowTemperature Fuel Cell Applications In oxides doped with M3+ ions and exposed to water vapor, H+ becomes the mobile species in both grain boundaries and bulk. Although the protons are always associated with oxygen ions, they
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