Researchers have developed an imaging technique that uses a tiny, super sharp needle to nudge a single nanoparticle into different orientations and capture 2-D images to help reconstruct a 3-D picture. The method demonstrates imaging of individual nanoparticles at different orientations while in a laser-induced excited state.

Conventional STM image of a quantum dot, top, compared with an image produced using the new excited-state SMA-STM technique. (Graphic courtesy Martin Gruebele)

Nanostructures like microchip semiconductors, carbon nanotubes, and large protein molecules contain defects that form during synthesis that cause them to differ in composition from one another. However, the term “defect” is a bit of a misnomer. For example, semiconductors are manufactured with intentional defects that form the holes that electrons jump into to produce electrical conductivity. Having the ability to image those defects enables them to be better characterized in order to control their production.

As advances in technology allow for smaller and smaller nanoparticles, it is critical for engineers to know the precise number and location of these defects to assure quality and functionality.

The study focused on a class of nanoparticles called quantum dots. These dots are tiny, near-spherical semiconductors used in technology like solar panels, live cell imaging and molecular electronics – the basis for quantum computing. The team observed the quantum dots using a single-molecule absorption scanning tunneling microscope fitted with a needle sharpened to a thickness of one atom at its tip. The needle nudges the individual particles around on a surface and scans them to get a view of the quantum dot from different orientations to produce a 3-D image.

The researchers said there are two distinct advantages of the new single-molecule absorption-scanning tunneling microscope (SMA-STM) method when compared with the current technology — cryogenic electron tomography (CryoET). Instead of an image produced using an average of thousands of different particles, as is done with CryoET, SMA-STM can produce an image from a single particle in about 20 different orientations. And it is not required to chill the particles to near-absolute zero temperatures — the particles can be captured at room temperature, rather than frozen and motionless.

The researchers looked at semiconductor quantum dots for this study, but SMA-STM can also be used to explore other nanostructures such as carbon nanotubes, metal nanoparticles, or synthetic macromolecules. They believe the technique can also be refined for use with soft materials like protein molecules.

The group is now working to advance SMA-STM into a single-particle tomography technique, meaning that they will need to prove that method is noninvasive. For SMA-STM to become a true single-particle tomography technique, they will need to prove that their nudges do not damage or score the nanoparticle in any way while rolled around. Knocking off just one atom can fundamentally alter the defect structure of the nanoparticle.

For more information, contact Martin Gruebele at 217-333-1624, This email address is being protected from spambots. You need JavaScript enabled to view it..


Photonics & Imaging Technology Magazine

This article first appeared in the March, 2019 issue of Photonics & Imaging Technology Magazine.

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