Observation Of Real-Time Thin Film Evolution Using Microcantilever Sensors
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Observation Of Real-Time Thin Film Evolution Using Microcantilever Sensors Alan M. Schilowitz and Dalia G. Yablon Corporate Strategic Research, ExxonMobil Research and Engineering Annandale, NJ 08801 ABSTRACT Adsorption and desorption kinetics of thin film formation on metal surfaces has been directly monitored in real-time by optically measuring the deflection of activated atomic force microscope microcantilevers. Microcantilever deflection is caused by stress generated during the formation of an adsorbate layer on one side of the microcantilever. In this work, rapid adsorption of carboxylic acid in hydrocarbon solvent onto a gold surface was directly observed as a compressive stress developed on the microcantilever substrate. Upon exposure to alkylthiol, acid desorbed and was displaced by alkylthiol; this process was continuously monitored in real-time. Experiments suggest that some film organization and intermolecular interaction are required before substantial surface stress can be detected. INTRODUCTION Microcantilevers (MC) developed for atomic force microscopes have been shown to be useful tools for sensitive sensing of adsorption phenomena [1,2]. MC sensors are generally operated in either dynamic or static mode. In dynamic mode, cantilevers can detect mass change and viscous damping by monitoring changes that occur in the cantilever's vibrational resonance spectrum. Adsorption induced resonance changes have also been observed that are due to changes in the cantilever's force constant [1,3,4]. Static mode cantilevers coated on one side with an activating layer such as gold bend due to surface stress caused by preferential adsorption [5,6,7,8]. These applications of MC sensors all depend on selective adsorption. The impact of competing adsorption processes on MC sensor deflection is a relevant issue that has not had much attention and is especially relevant to sensing in multicomponent systems or cases where the cantilevers become contaminated before use. Cantilever deflection affords sensitive, real-time, in situ observation of competing adsorption/desorption phenomena in real-time. Our interest in MC sensors derives from the potential for using these devices in static mode for detecting contaminants that might occur in hydrocarbons. Small mercaptans (e.g. sulfur containing molecules with less than 10 carbons) and carboxylic acids are potential species of interest. Application of such sensors will require them to function in environments where significant surface contamination will be impossible to prevent. The effect of pre-contamination to improve sensor selectivity is discussed. Calibration of MC deflection with concentration is also discussed. Finally, the use of MC sensors to monitor real-time adsorption and desorption phenomena is demonstrated. EXPERIMENTAL Silicon contact mode microcantilevers (Budget Sensors) with a nominal resonance frequency of 13 kHz and a nominal force constant of 0.2 N/m were used in all experiments. A 25 nm thick gold layer was deposited onto one side of the cantilever in a vap
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