Quantum Dot Microscope Can Measure Electric Surface Potentials of Single Atoms

A new scanning quantum dot microscopy method can measure the electric potential of a sample at atomic accuracy. It was developed by a team from Forschungszentrum Jülich, working with researchers from two other institutions. The new technique has potential application for chip manufacturing and the characterization of biomolecules.

A quantum dot was attached to the tip of an atomic force microscope (AFM) to serve as a noncontact scanning probe. The quantum dot was so small that individual electrons from the tip of the AFM could be attached to it in a controlled manner. The quantum dot sensor and the joint electrostatic screening by tip and surface enabled quantitative surface potential imaging across all length scales down to individual atoms.

Image from a scanning tunneling microscope (STM, left) and a scanning quantum dot microscope (SQDM, right). Using a scanning tunneling 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. Courtesy of Forschungszentrum Jülich/Christian Wagner.

The reason for the very sharp images, the researchers said, is an effect that permits the microscope tip to remain roughly 2 to 3 nm from the sample — what would be an extremely large distance for a typical atomic force microscope.

According to the team, all elements of a sample generate electric fields that influence the quantum dot and that can therefore be measured. The microscope tip acts as a protective shield that dampens the disruptive fields from areas of the sample that are farther away. “The influence of the shielded electric fields thus decreases exponentially, and the quantum dot only detects the immediate surrounding area,” said researcher Christian Wagner. “Our resolution is thus much sharper than could be expected from even an ideal point probe.”

The researchers applied the technique to the characterization of a nanostructured surface, extracting workfunction changes and dipole moments for reference systems and validating the method as a tool for studying the building blocks of materials and devices down to the atomic scale.

“Not only can we visualize the electric fields of individual atoms and molecules, we can also quantify them precisely,” Wagner said. “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.”

To enable the complete sample surface to be measured quickly, engineers from the Otto von Guericke University Magdeburg developed a controller that helped to automate the repetitive sequence of scanning the sample. “In previous scanning quantum dot microscopy work ... 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,” Wagner said. “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 five to six hours for a single molecule, we can now image sample areas with hundreds of molecules in just one hour.”

The new quantum dot microscopy method is challenging: Preparing the measurements takes time and effort, and the quantum dot for the measurement has to be attached to the tip of the AFM beforehand — something that is only possible to do in a vacuum at low temperatures. In contrast, normal AFMs work at room temperature, with no need for a vacuum or complicated preparations.

However, there are many possible fields of application for quantum dot microscopy, said the researchers. Semiconductor electronics are pushing scale boundaries in areas where a single atom can make a difference for functionality. Electrostatic interaction plays an important role in functional materials such as catalysts. The characterization of biomolecules is another potential area of application. Because of the comparatively large distance between the tip and the sample, the quantum dot microscopy method is suitable for rough surfaces, such as the surface of DNA molecules.

The research was published in Nature Materials (https://doi.org/10.1038/s41563-019-0382-8).