17-Jun-2019 - Forschungszentrum Jülich GmbH

New Quantum Dot Microscope Shows Electric Potentials of Individual Atoms

A team of researchers from Jülich in cooperation with the University of Magdeburg has developed a new method to measure the electric potentials of a sample at atomic accuracy. Using conventional methods, it was virtually impossible until now to quantitatively record the electric potentials that occur in the immediate vicinity of individual molecules or atoms. The new scanning quantum dot microscopy method, which was recently presented in the journal Nature Materials by scientists from Forschungszentrum Jülich together with partners from two other institutions, could open up new opportunities for chip manufacture or the characterization of biomolecules such as DNA.

The positive atomic nuclei and negative electrons of which all matter consists produce electric potential fields that superpose and compensate each other, even over very short distances. Conventional methods do not permit quantitative measurements of these small-area fields, which are responsible for many material properties and functions on the nanoscale. Almost all established methods capable of imaging such potentials are based on the measurement of forces that are caused by electric charges. Yet these forces are difficult to distinguish from other forces that occur on the nanoscale, which prevents quantitative measurements.

Four years ago, however, scientists from Forschungszentrum Jülich discovered a method based on a completely different principle. Scanning quantum dot microscopy involves attaching a single organic molecule – the “quantum dot” – to the tip of an atomic force microscope. This molecule then serves as a probe. “The molecule is so small that we can attach individual electrons from the tip of the atomic force microscope to the molecule in a controlled manner,” explains Dr. Christian Wagner, head of the Controlled Mechanical Manipulation of Molecules group at Jülich’s Peter Grünberg Institute (PGI-3).

The researchers immediately recognized how promising the method was and filed a patent application. However, practical application was still a long way off. “Initially, it was simply a surprising effect that was limited in its applicability. That has all changed now. Not only can we visualize the electric fields of individual atoms and molecules, we can also quantify them precisely,” explains Wagner. “This was confirmed by a comparison with theoretical calculations conducted by our collaborators from Luxembourg. In addition, we can image large areas of a sample and thus show a variety of nanostructures at once. And we only need one hour for a detailed image.”

The Jülich researchers spent years investigating the method and finally developed a coherent theory. The reason for the very sharp images is an effect that permits the microscope tip to remain at a relatively large distance from the sample, roughly 2–3 nanometres – unimaginable for a normal atomic force microscope.

In this context, it is important to know that all elements of a sample generate electric fields that influences the quantum dot and can therefore be measured. The microscope tip acts as a protective shield that dampens the disruptive fields from areas of the sample that are further away. “The influence of the shielded electric fields thus decreases exponentially, and the quantum dot only detects the immediate surrounding area,” explains Wagner. “Our resolution is thus much sharper than could be expected from even an ideal point probe.”

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.

Facts, background information, dossiers

  • scanning quantum do…

More about Forschungszentrum Jülich

  • News

    Alzheimer’s Research: New Insights into the Formation of Toxic Protein Clumps

    Small aggregates of proteins known as Aβ oligomers are suspected as the main cause for the development of Alzheimer’s disease. However, it is not yet clear where and under what conditions these toxic aggregates form. Researchers from Heinrich Heine University Düsseldorf and Forschungszentru ... more

    Increasing the Activity of Catalysts

    A layer as thin as a single atom makes a huge difference: On the surface of an electrode, it doubles the amount of water split in an electrolysis system without increasing the energy requirements. Thus, the ultrathin layer also doubles the amount of hydrogen produced without increasing cost ... more

    Talent Scout in the Cell Factory

    They’re small, but mighty: microorganisms. The industry known as “white biotechnology” takes advantage of their potential in a variety of ways, for example to produce chemicals, medicines, or dietary supplements. The little powerhouses’ work can be found in a whole series of products, the n ... more

  • q&more articles

    Macromolecular environments influence proteins

    The high-intensity interaction of proteins with other macromolecules can cause signifi cant changes to protein properties such as translational mobility, for example, or their conformational states. Accordingly, the study of proteins in macromolecular environments that typically very closel ... more

    Caffeine Kick

    Caffeine is the most widely consumed psychoactive substance worldwide. It supplies the active ingredient in beverages such as coffee, tea and energy drinks. Caffeine can focus vigilance and attention, reduce drowsiness and enhance the ability to perform cognitive functions. Its neurobiologi ... more

  • Authors

    Prof. Dr. Jörg Fitter

    Jörg Fitter studied physics at the University of Hamburg. After completing his doctoral studies at FU Berlin, he worked in neutron scattering and molecular biophysics at the Hahn Meitner Institute in Berlin and Jülich Research Center. He completed his habilitation in physical biology at Hei ... more

    Dr. David Elmenhorst

    studied medicine in Aachen before receiving his doctorate in sleep research from the German Aerospace Centre (Deutsches Zentrum für Luft- und Raumfahrt, DLR) in Cologne. During 2008/2009, he was a visiting researcher at the Brain Imaging Centre in Canada’s Montreal Neurological Institute an ... more

    Prof. Dr. Andreas Bauer

    studied medicine and philosophy in Aachen, Cologne and Düsseldorf, where he received his doctorate in the field of neuroreceptor autoradiography. After specialist medical training at Cologne University Hospital he completed his habilitation in neurology at the University of Düsseldorf. Sinc ... more

q&more – the networking platform for quality excellence in lab and process

The q&more concept is to increase the visibility of recent research and innovative solutions, and support the exchange of knowledge. In the broad spectrum of subjects covered, the focus is on achieving maximum quality in highly innovative sectors. As a modern knowledge platform, q&more offers market participants one-of-a-kind networking opportunities. Cutting-edge research is presented by authors of international repute. Attractively presented in a high-quality context, and published in German and English, the original articles introduce new concepts and highlight unconventional solution strategies.

> more about q&more

q&more is supported by: