Jul 21, 2019 | Updated: 09:46 AM EDT

Measuring an Atom's Electric Potential Possible Through New Quantum Dot Microscope

Jun 18, 2019 09:30 AM EDT

Quantum Dot
(Photo : By Antipoff at en.wikipedia, CC BY-SA 4.0)

Collaborators from Julich and the University of Magdeburg have developed a new method that quantitatively measures the sample's electric potentials at atomic accuracy. The new method has provided a solution to the problem that is concerned with recording quantitatively the electric potentials that happen in the immediate vicinity of atoms or molecules.

The researchers developed the new scanning quantum dot microscopy method and they published their findings in the journal Nature Materials. This development opens up the possibilities in the characterization of biomolecules as well as chip manufacture.

Electric potential fields are produced by the positive atomic nuclei and negative electrons. Current methods do not have the capacity to measure quantitatively on these small-area fields that are responsible for various nanoscale material properties and functions. Force measurements caused by electric charges are the basis for imaging of these potentials by current methods. The problem is the indistinguishability of these forces on the nanoscale that prevents measuring these quantitatively.

Forschungszentrum Jülich researchers established a method four years ago based on the quantum dot in the scanning quantum dot microscopy. The single organic molecule or the quantum dot is attached to the tip of an atomic force microscope which 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).

A patent application was filed by the researchers. "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 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."

A coherent theory was established by the researchers who spent years in investigating the method. The microscope tip can obtain very sharp images at 2-3 nanometers from the sample which is not normal for atomic force microscopy.

"In this context, it is important to know that all elements of a sample generate electric fields that influence 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," according to Eureka Alert. 

"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."

Scientists from Otto von Guericke University Magdeburg are the ones responsible for the speed at which the complete sample surface can be measured. A controller was made that automated the process.

"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."

The method has its disadvantages like measurement preparation. It requires a vacuum at low temperatures to attach the organic molecule that serves as the quantum dot for measurement which is contrary to current normal atomic force microscopes that work at room temperature and no need for vacuum 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."

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