Chemical Microscope Enabled: Imaging Of Materials At The Nanometer Scale

The modern way of microscopy relies on a tiny tip scanning over a surface to sense feeble currents or forces, to build a relief image in the computer. That the tip can also sense the infrared vibrations of the surface material, thus revealing the material's chemical composition, is described by researchers at the Max Planck Institute for Biochemistry (Max-Planck-Institut für Biochemie), Martinsried/Germany, in their letter to Nature appearing on May 13. The results lay the foundation to develop a generally usable infrared microscope with at least 100 nm resolution, possibly 10 nm or even better.

The photos shows how the infrared microscope reveals the chemical identity of a small polystyrene (PS) grain embedded in a polymethylmetacrylate (PMMA) matrix. The image sizes are 2x2 micrometers. The topography image (left) indicates a shallow depression but can not tell whether this area contains the same or a different material. The infrared images, however, exhibit a different absorption relative to the surrounding, telling us that there is a different type of material. The two infrared images are taken with infrared laser illumination at two nearby wavelengths, chosen to be in resonance with either only PS (middle) or only PMMA (right). Since the inclusion is dark in the former and bright in the latter illumination, the depression area is positively assigned as PS embedded in PMMA. Full size image available through contact

The classical optical microscope is limited in its resolution by the wavelength, in practice to just below one micrometer. To visualize nanoscale structures - which are of growing interest in electronics, materials and biology - one can use the electron microscope which needs however high vacuum and special sample preparation. More generally usable are the scanning tip microscopes such as the tunneling or atomic force microscope, who function by moving a sharp sensor tip along a surface. Feeble electric currents or mechanic forces are registered and compilated in a computer to form the surface's relief image or topography. These techniques can however not find out which chemical substances are present where on the surface.

The physicists at the Max Planck Institute show that infrared waves can accomplish just this. They employ a well-known material response, the infrared vibrational absorption, to identify the chemical composition of the surface. While the use of infrared spectroscopy for the identification of macroscopic samples has been a standard technique in chemical and semiconductor plants, as well as in physics and biology research, the surprising news is that this long-wavelength technique should be combinable with the scanning tip microscope principle.

In the experiment, a metal tip moves close to a polymer film composed of different compounds, and the surface topography is recorded in usual AFM (atomic force microscope) manner. At the same time the tip is illuminated by an infrared beam. Much as by an antenna the infrared wave is concentrated at the very apex of the tip where it senses the local absorption of the closeby sample. The compiled infrared image overlaid with the topography shows where the more absorbing substance is, by being darker. When a different infrared wavelength is employed the infrared image changes its contrast accordingly.

The letter accomplishes demonstrating how to identify material composition on a nanoscale, using a scanning metal tip. This includes a theoretical description of the infrared near-field interaction which is predicted to be enhanced, an effect already partly verified in the experiment. The enhancement is due to the metal electrons of the tip lending absorption strength to the sample vibration. The enhancement mechanism strongly amplifies the potential of the new microscope because weaker absorbers can be identified and the resolving power can be increased.

Other techniques of identifying material composition on a nanoscale are not available. The optical near-field microscope is color-blind concerning the material-specific absorption resonances because these occur at infrared wavelengths in the range of 3 to 30 micrometers.

The results lay the foundation to develop a generally usable infrared microscope with at least 100 nm resolution, possibly 10 nm or even better. The next steps are to achieve a dual or multifrequency simultaneous imaging, to be followed by broadband infrared operation. Suitable infrared lasers exist. The chemical microscope will be complete once a full infrared spectrum can be mapped at each pixel, revealing the local material's identity. Apart from widening the infrared illumination spectrum the development will have to focus on sharper tips, sharper optical alignment and tighter mechanical control.

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