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 ε₁ ≈ 1 eV), (2) chemisorbed hydrogen in the subsurface layer (the activation energy of hydrogen desorption is ε₂ ≈ 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 ε₃ ≈ 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.