New Quantum Dot Microscope Shows Electric Potentials of Individual Atoms

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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.

Image Caption: Dr. Christian Wagner with a model of the PTCDA molecule, which serves as a quantum dot. Copyright: Forschungszentrum Jülich / Sascha Kreklau

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.”

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