Microscope Records Chemical Makeup

Robert C. Pini

Researchers at Max Planck Institut fur Biochemie have combined the atomic force microscope's ability to define surface features with the infrared absorption techniques of spectroscopy to record chemical composition.

Atomic force microscopy not only imaged the topography of polystyrene in polymethylmethacrylate (left), but also spectrographic information at two IR wavelengths: 9.7 and 10.2 µm (middle and right, respectively). Courtesy of Fritz Keilmann.

The hybrid instrument promises imaging results that could benefit research in biology, materials science and electronics, where scientists need to detect nanometer-scale structures.

Normally an atomic force microscope picks up weak mechanical and electrical signals from its scanning tip. In their experiment, the researchers at Planck focused an IR beam on the scanning tip. Besides the surface features, they recorded the level of vibrational absorption to produce a gray-scale image that contrasted the presence of the two materials. The differential absorption defined the sample's chemical topography.

Fritz Keilmann, a research physicist at the institute, developed the process with graduate student Bernhard Knoll. Keilmann said he was surprised to find how the metal scanning tip concentrated the infrared beam at the apex of the tip, enhancing the vibrational absorption and response.

Multiwavelength imaging

The researchers used a Cassegrain objective made by the former Ealing Electro-Optics of Watford, UK, to focus the infrared beam from a CO₂ laser made by MPB Technologies Inc. in Pointe Claire, Quebec, Canada. Freestanding metal grid elements from Lasnix in Berg, Germany, attenuated and polarized the beam. Infrared and atomic force microscope images were recorded simultaneously with a HgCdTe detector from Infrared Associates in Stuart, Fla.

Keilmann used only one IR frequency to create an image. Increasing wavelengths will improve contrast, he said.

One potential application is the study of proteins on a cell membrane. Researchers measuring chemicals in suspension and materials scientists building nanostructures also could benefit from the device.