Gas Sensing Properties of the Nanosized Pd and Cu x Pd Layers

Abstract

The adsorption properties of thin films of Pd and Cu x Pd, beginning with one monolayer (ML) of palladium and copper sequentially deposited on a tungsten single crystal surface in high vacuum (10− 9 Torr) at the dose of hydrogen adsorption about 3 ×10− 7 Torr, 10 min, have been studied. Thermal desorptionspectra (TDS) showed that the adsorbed hydrogen in such structures exists in three configurations: (1) relatively weakly bounded of surface chemical adsorption hydrogen (the activation energy of hydrogen desorption is e1 ≈ 1 eV), (2) chemisorbed hydrogen in the subsurface layer (the activation energy of hydrogen desorption is e2 ≈ 1.33 eV) and (3) absorbed hydrogen in the bulk of metal films and at the interface with tungsten substrate (the activation energy of hydrogen desorption is e3 ≈ 1.66 eV). The absorbed hydrogen causes of adsorboelectric effect [1, 2, 3] insensor structures based on metal-insulator-semiconductor (MIS) structures, changing its electrophysical parameters depending on the amount of adsorbed gas molecules, which should detect. Three desorption peaks of hydrogen A1, A2, A3, has been observed on thermal desorptionspectra of Cu(1 ML)/Pd(2 ML)/W, Cu(3 ML)/Pd(6 ML)/W and Cu(4.5 ML)/Pd(12 ML)/W. Thus, even monatomic copper layer on the Pd (2 ML) surface promotes the accumulation and a storing of hydrogen in the bulk of Pd film increasing of 3rd (A3 bulk) peak on TDS, where the spillover effect may also takes place. The activation energy of hydrogen desorption is estimated from the temperature of peak maximum (β1, β2, β3 ∼ 420 K, 550 K and 720 K, respectively). TDS of hydrogen from the Cu/Pd surface oxygen modified have also been studied. It was found that the oxygen pre-exposure leads to decrease of hydrogen desorption from both Pd(6 ML) and Cu(1.5, 3, 4.5 ML)/Pd(6 ML) monolayers.