KIT Researchers Employ Two-Photon Polymerization 3D Printing for Better Tools in Atomic Force Microscopy

An atom is the smallest particle of an element. Known to retain the properties of the element from whence it sprung, study of these structures at the microscopic level can help researchers and engineers in any number of applications as they design new products and tools. Through the use of atomic force microscopy (AFM), scientists are able to employ extremely high resolution (up to 1,000 times better) to measure a range of different properties with probing technology as they examine the atomic surfaces.

Although extremely effective, this process relies on the use of pointed tips on a micromachined cantilever which are not only often in need of customization for proper research, but they are also expensive. The standard tips just don’t always do as thorough a job as scientists would prefer, although for more than 30 years, atomic force microscopy has been offering good results.

A research team at Karlsruhe Institute of Technology (KIT) in Germany finally decided to do something about the issue of these tips, however. With the advent of 3D technology and all the discussion of affordable customization, the team decided to work on making their own AFM tips through 3D direct laser writing. They outlined their findings in a recent article just out in AIP Publishing’s Applied Physics Letters. The paper, ‘Tailored probes for atomic force microscopy fabricated by two-photon polymerization,’ is authored by Gerald Göring, Philipp-Immanuel Dietrich, Matthias Blaicher, Swati Sharma, Jan G. Korvink, Thomas Schimmel, Christian Koos, and Hendrik Hölscher.

3D direct laser writing with two-photon polymerization was used to make custom-designed tips.

Pointing out that the performance of microscopy systems is often reliant on the construction of these tips, the KIT team believes that both that shape of the tip and the tip radius are of equal importance. When not just right, they can indeed limit research that involves imaging for deep trenches. The cantilever itself can be an issue as well, but there are numerous designs that have been created to improve on that. With these pre-fabricated designs, there is a need to also create tips that could be application specific, serving as an alternative to the cost-prohibitive micromachined tips.

The research team began experimenting with how to make hydrophobic tips that would offer a better result, and in their journey, latched onto using the two-photon polymerization (TPP) 3D printing process for ‘writing’ these tips. This includes the use of a strongly centered laser and UV-light-curable photoresist material. As the material initiates a reaction during two-photon absorption, components like these tiny AFM probes can be 3D written directly at the intended spot.

Dr. Hendrik Hölscher

“This concept isn’t new at the macroscopic scale: you can freely design any shape with your computer and print it in 3D,” said Dr. Hendrik Hölscher of the KIT team. “But at the nanoscale, this approach is complex.”

“To write our tips, we applied two-photon polymerization with an experimental setup, recently developed at KIT, which is now available from startup company Nanoscribe GmbH.”

In using TTP they were able to decrease tip radiuses, and used mold inserts for attachment of hydrogel probes. With 3D direct laser writing and TTP, the team was able to make completely customized tips rewarding the user with a range of choices.

“This approach offers a large variety of options to design tips in order to obtain optimal conditions for the imaging of surfaces by AFM,” stated the researchers in their paper.

Examples from the KIT team’s research paper, showing a variety of tips.

They tried making sharp tips, ‘exceptionally tall’ tips for particularly deep trenches, and also showed that rebar structures can be 3D written onto the cantilever as well for deeper resonance. As time went on, the team discovered that their 3D written tips are not only highly customizable and affordable, but most importantly, they have proven to be reliable.

“Writing parts via 3D printing is expected to become a big business at the macroscopic scale. But I was surprised by how nicely it works for nanoscale, too. When our group started with this project, we tried to continuously stretch the technology’s limits … but Ph.D. students Philipp-Immanuel Dietrich and Gerald Göring kept coming back from the lab with new successful results,” said Hölscher.

And while this solves a specific problem, improving processes greatly in AFM, experts in nanotechnology will also be able to take this TTP 3D printing method and translate it to other very important applications and research projects in areas such as optics, photonics, and biomimetics.

“We expect other groups working within the field of scanning probe methods to be able to take advantage of our approach as soon as possible,” said Hölscher. “It may even become an Internet business that allows you to design and order AFM probes via the web.”

Discuss this project further over in the 3D Printed Atomic Microscopy Tips forum at 3DPB.com.