Stress in densely packed adsorbate layers and stress at the solid–liquid interface — is the stress due to repulsive interactions between the adsorbed species?

Sign-inverted response of aluminum work function to tangential strain

We have investigated the response of the work function, W, of low-index aluminum surfaces to tangential strain by using first-principles calculations based on density functional theory. This response parameter is a central quantity in electrocapillary coupling of metal electrodes relating to the performance of porous metal actuators and surface stress based sensing devices. We find that Al surfaces exhibit a positive response for all orientations considered. By contrast, previous studies reported negative-valued response parameters for clean surfaces of several transition metals. We discuss separately the response of W to different types of strain and the impact of the strain on the Fermi energy and the surface dipole. We argue that the reason for the abnormal positive sign of the Al response parameter lies in its high valence electron density.

Molecular interactions in self-assembly monolayers on gold-coated microcantilever electrodes

An electrochemical microcantilever (EMC) was used to study the intermolecular interaction of self-assembly monolayers (SAMs) with different n-alkanethiols chain lengths (n = 0, 4, 6, 8, 12, 16) on a Au-coated microcantilever surface. Comparing potential cycling and steps in NaClO(4) solution within the same potential range, the deflection rate of bare microcantilevers is much smaller for the former which revealed that potential excitation, i.e. the surface charge, played the dominant role in driving the instant and large deflection of the bare microcantilever, while the smaller deflection amplitude of the former implied that adsorption of ClO(4)( - ) had an adverse effect on the potential-induced stress. Upon adsorption of SAMs, the deflection amplitude of the microcantilever under the potential step was much smaller than that of a bare microcantilever, and linearly decreased with the chain length increasing for n ≤ 8 (the linear correlation coefficient and the slope are 0.98 and about - 10.4 nm per CH(2) unit, respectively), following a transition (8 ≤ n ≤ 12) to a stable state (n ≥ 12). The decrease of deflection amplitude and faster decay of deflection rate of the SAMs modified microcantilever under the potential step implyed increasing compactness of the SAMs with longer chains.

Cantilever-based sensing: the origin of surface stress and optimization strategies

Many interactions drive the adsorption of molecules on surfaces, all of which can result in a measurable change in surface stress. This article compares the contributions of various possible interactions to the overall induced surface stress for cantilever-based sensing applications. The surface stress resulting from adsorption-induced changes in the electronic density of the underlying surface is up to 2-4 orders of magnitude larger than that resulting from intermolecular electrostatic or Lennard-Jones interactions. We reveal that the surface stress associated with the formation of high quality alkanethiol self-assembled monolayers on gold surfaces is independent of the molecular chain length, supporting our theoretical findings. This provides a foundation for the development of new strategies for increasing the sensitivity of cantilever-based sensors for various applications.

Surface stress-charge response of a (111)-textured gold electrode under conditions of weak ion adsorption

We report a cantilever bending investigation into the variation of surface stress, f, with surface charge density, q, for (111)-textured thin films of gold in aqueous NaF and HClO 4. The graphs of f(q) are highly linear, and the surface stress-charge coefficients, d f/d q, are -1.95 V for 7 mM NaF and -2.0 V for 10 mM HClO 4 near the potential of zero charge. These values exceed some previously published experimental data by a factor of 2, but they agree with recent ab initio calculations of the surface stress-charge response of gold in vacuum.

Microcantilever-Based Sensors: Effect of Morphology, Adhesion, and Cleanliness of the Sensing Surface on Surface Stress

The surface stress response of micromechanical cantilever-based sensors was studied as a function of the morphology, adhesion, and cleanliness of the gold sensing surface. Two model systems were investigated: the adsorption of alkanethiol self-assembled monolayers at the gas-solid interface and the potential-controlled adsorption of anions at the liquid-solid interface. The potential-induced surface stress, on a smooth and continuous polycrystalline Au(111)-textured microcantilever in 0.1 M HClO4, is in excellent agreement with macroscopic Au(111) single-crystal electrode results. It is shown that ambient contaminants on the sensing surface dramatically alter the surface stress-potential response. This observation can be misinterpreted as evidence that for polycrystalline Au(111) microcantilever electrodes, surface stress is dominated by surface energy change. Results for anions adsorption on gold are in contrast to the gas-phase model system. We demonstrate that the average grain size of the gold sensing surface strongly influences the magnitude of the surface stress change induced by the adsorption of octanethiol. A 25-fold amplification of the change in surface stress is observed on increasing the average gold grain size of the sensing surface from 90 to 500 nm.