Investigation of Hg sorption and diffusion behavior on ultra-thin films of gold using QCM response analysis and SIMS depth profiling

Abstract

Using the quartz crystal microbalance (QCM) technique, we demonstrate that the contribution of Hg adsorption and absorption on the sensor response profile can be distinguished by studying the dynamic response curve of QCM based Hg vapor sensors that employ an ultra-thin film of Au in the range of 10 to 40 nm thickness as the sensitive layer. The response magnitudes of the QCMs were extrapolated to zero thickness (ETZT) in an attempt to determine the contribution of adsorbed Hg on the sensor response magnitude and response profile. In general, the ratio of adsorbed to absorbed Hg on Au films is found to decrease with increased Hg vapor concentration. Furthermore, the same ratio was observed to decrease with increasing Au film thickness. The 10 nm and 40 nm Au films for example were found to contain adsorbed Hg content of 43.8% and 16.4%, respectively, with the balance attributed to absorption/amalgamation, when exposed to Hg vapor concentration of 10.55 mg m⁻³ for a period of 14 hours and an operating temperature of 28 °C. In addition, the QCMs were characterized using secondary ion mass spectroscopy depth profiling in order to study the diffusion behaviour of Hg in the Au surfaces. It is deduced that in order to reduce Hg accumulation in Au thin films, a non-continuous type film (similar to the 10 nm ultra-thin Au sensitive layer morphology) would be more functional as a Hg sensitive layer where quick absorption and desorption processes are required.