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Image from a scanning tunnelling microscope (STM, left) and a scanning quantum dot microscope (SQDM, right). Using a scanning tunnelling microscope, the physical structure of a surface can be measured on the atomic level. Quantum dot microscopy can visualize the electric potentials on the surface at a similar level of detail – a perfect combination. Copyright: Forschungszentrum Jülich / Christian Wagner
The Jülich researchers owe the speed at which the complete sample surface can be measured to their partners from Otto von Guericke University Magdeburg. Engineers there developed a controller that helped to automate the complex, repeated sequence of scanning the sample. “An atomic force microscope works a bit like a record player,” says Wagner. “The tip moves across the sample and pieces together a complete image of the surface. In previous scanning quantum dot microscopy work, however, we had to move to an individual site on the sample, measure a spectrum, move to the next site, measure another spectrum, and so on, in order to combine these measurements into a single image. With the Magdeburg engineers’ controller, we can now simply scan the whole surface, just like using a normal atomic force microscope. While it used to take us 5–6 hours for a single molecule, we can now image sample areas with hundreds of molecules in just one hour.”
There are some disadvantages as well, however. Preparing the measurements takes a lot of time and effort. The molecule serving as the quantum dot for the measurement has to be attached to the tip beforehand—and this is only possible in a vacuum at low temperatures. In contrast, normal atomic force microscopes also work at room temperature, with no need for a vacuum or complicated preparations.
And yet, Prof. Stefan Tautz, director at PGI-3, is optimistic: “This does not have to limit our options. Our method is still new, and we are excited for the first projects so we can show what it can really do.”
There are many fields of application for quantum dot microscopy. Semiconductor electronics is pushing scale boundaries in areas where a single atom can make a difference for functionality. Electrostatic interaction also plays an important role in other functional materials, such as catalysts. The characterization of biomolecules is another avenue. Thanks to the comparatively large distance between the tip and the sample, the method is also suitable for rough surfaces—such as the surface of DNA molecules, with their characteristic 3D structure.
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