The surface stress induced during the formation of alkanethiol self-assembled monolayers (SAMs) on gold from the vapor phase was measured using a micromechanical cantilever-based chemical sensor. Simultaneous in situ thickness measurements were carried out using ellipsometry. Ex situ scanning tunneling microscopy was performed in air to ascertain the final monolayer structure. The evolution of the surface stress induced during coverage-dependent structural phase transitions reveals features not apparent in average ellipsometric thickness measurements. These results show that both the kinetics of SAM formation and the resulting SAM structure are strongly influenced both by the surface structure of the underlying gold substrate and by the impingement rate of the alkanethiol onto the gold surface. In particular, the adsorption onto gold surfaces having large, flat grains produces high-quality self-assembled monolayers. An induced compressive surface stress of 15.9 ± 0.6 N/m results when a c(4×2) dodecanethiol SAM forms on gold. However, the SAMs formed on small-grained gold are incomplete and an induced surface stress of only 0.51 ± 0.02 N/m results. The progression to a fully formed SAM whose alkyl chains adopt a vertical (standing-up) orientation is clearly inhibited in the case of a small-grained gold substrate and is promoted in the case of a large-grained gold substrate.

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Two cantilever-based chemical sensors are reported. First, a differential cantilever-based chemical sensor capable of sensitive surface stress measurements has been designed and implemented. The system uses two commercially available atomic force microscope cantilevers. The surface of one cantilever is functionalized to make it receptive to specific target analytes, while the second, passivated cantilever is used as a reference by subtracting undesired signals, resulting from mechanical and thermal noise, from the signal of interest. The second system is a combined cantilever-based sensor and ellipsometer capable of simultaneous in situ surface stress and film thickness measurements. Both sensors, operated in static mode, achieve a cantilever deflection measurement sensitivity of 0.2 nm and a surface stress resolution of 5ϫ10 Ϫ5 N/m. Molecular monolayer thicknesses are measured simultaneously with a 0.1 nm resolution. The real-time formation process of alkanethiol self-assembled monolayers on gold was investigated as a model system to characterize these instruments.

Most atomic force microscopes and cantilever-based sensors use an optical laser beam detection system to monitor cantilever deflections. We have developed a working model that accurately describes the way in which a position sensitive photodetector interprets the deflection of a cantilever in these instruments. This model exactly predicts the numerical relationship between the measured photodetector signal and the actual cantilever deflection. In addition, the model is used to optimize the geometry of such laser deflection systems, which greatly simplifies the use of any cantilever-based instrument that uses a laser beam detection system.