A study on the aging of ultra-thin Palladium films on SAW hydrogen gas sensors

Abstract

Traditionally, low-powered, room temperature sensing of gaseous hydrogen (H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) is difficult. With renewed interest in H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> as a source of energy, there is a need for reliable, energy-efficient sensors. A potential solution can be found in using surface acoustic wave (SAW) devices, which have been implemented as passive, wireless RFID tag-sensors. Thus, in concept, it is advantageous to develop a SAW device with H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> sensing capabilities. Prior experiments have successfully demonstrated a passive SAW-based H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas sensor by placing an ultra-thin Palladium (Pd) film (<;50 Å) in the propagation path. These sensors have an instantaneous response and a significant fractional change in SAW propagation loss; however, the lifetime of these sensors are still unknown. Hence, the objective of this study was to examine the influence of aging of ultra-thin Pd films on the usable life of passive SAW H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas sensors. The aging behavior of a thin film is highly dependent on its morphology and the environment in which it is used. Particularly, during growth, ultra-thin Pd films may form a discontinuous network of nano-sized atomic islands-nano-clusters. These nano-clusters may be amorphous or have a highly defective crystalline structure. When used in ambient air (1atm, 79% N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , 21% O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , 44% humidity, 21°C), these films are susceptible to oxygen adsorption (i.e., blocking H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> sorption and decreasing the H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> reactivity of the film), which reduces the useable life of the sensor. This paper presents data demonstrating that ultra-thin Pd films suffer from oxygen adsorption when exposed to ambient air. The results of the study provide promising solutions to the aging problem, such as encapsulation and film annealing. These solutions may accelerate the practical implementation of passive, wireless, SAW H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas sensors in various environments.