Gotta Look Sharp
Atomic force microscopy makes electrical measurements
Context: The rate of corrosion in devices like batteries and semiconductors is often dictated by nanometer-sized imperfections. Conducting atomic force microscopes (AFMs) can image these nanoflaws, but accurately measuring their electrical properties requires knowing how much of the microscope’s sharp conductive tip comes in contact with the active surface. Using a mathematical model, Ryan O’Hayre, an assistant professor in the Stanford University Department of Mechanical Engineering, and his colleagues have found a way to indirectly measure this contact area, overcoming a limit of conductive tip microscopy and improving quality control.
Methods and Results: Researchers used a platinum-coated AFM tip to monitor the reaction between hydrogen and oxygen at the surface of a polymer fuel cell membrane; the fuel cell was chosen to show that nanoscale measurements can correlate with macroscale results. The rate of the reaction depends on how much force the tip applies to the membrane: the force pushes the materials together, causing them to deform slightly, and thus increases the area of interaction between the two. Crucially, the researchers showed that the area of interaction can be estimated by determining the hardness of the membrane, accompanied by a few assumptions and mathematical tricks. The researchers experimented across three orders of magnitude of force between tip and sample, and their results were all consistent with conventional experiments, making them more credible.
Why it matters: Conducting AFM can give nanoscale resolution to electrical measurements of semiconductors, fuel cells, batteries, and other devices. But while it was possible to measure relative changes in properties like conductivity, capacitance, and impedance across the surface of a single sample of material, comparing such measurements between materials had been impossible. Conducting AFM, while capable of fi nding flaws, could not measure their absolute severity, since different materials interacted with the AFM tip in diff erent ways. This refinement may convert conductive AFM from a research instrument into a useful tool in a number of industries.
Source: O’Hayre, R. et al. (2004) Quantitative impedance measurement using atomic force microscopy. Journal of Applied Physics 96:3540-9.