3D Direct Laser Writes AFM Probes

3-D direct laser writing based on two-photon polymerization can be used to create custom-designed tips. Here a spherical tip with diameter of 10 microns (Source: KIT)

3D direct laser writing based on two-photon polymerization can be used to create custom-designed tips. Here a spherical tip with diameter of ten microns (Source: KIT)

Atomic force micro­scopy AFM allows researchers to analyze surfaces at the atomic scale, and it’s based on a surpri­singly simple concept: A sharp tip on a canti­lever senses the topo­graphy of samples. While this technique has been successfully used for more than thirty years, and you can easily buy standard micro­machined probes for experi­ments, standard-sized tips aren’t always exactly what you need. Researchers frequently desire tips with a unique design, a specific tip apex shape or extremely long tips that can reach the bottom of deep trenches. Preparing nonstandard tips via micro­machining is possible, but it’s often expensive.

But now, a group of Karlsruhe Insti­tute of Techno­logy KIT researchers report that they have developed a method to tailor tips for specific applications via 3D direct laser writing based on two-photon polyme­rization. Two-photon polymerization is a 3D printing process that provides struc­turing with extremely high resolution. It involves using a tightly focused infrared femto­second laser to expose an ultraviolet-light-curable photo­resist material, which causes two-photon adsorp­tion that, in turn, triggers a polyme­rization reaction. In this way, freely designed parts can be written exactly at the place of their purpose, even nano­scale objects such as AFM tips on canti­levers.

“This concept isn’t new at the macro­scopic scale: you can freely design any shape with your computer and print it in 3D,” explained Hendrik Hölscher, head of the scanning probe techno­logies group at KIT. “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.”

Tips with radii as small as 25 nano­meters and arbitrary shapes can be attached to conven­tionally shaped micro­machined canti­levers. Long-term scanning measure­ments show low wear rates that demonstrate the relia­bility of these tips. “We were also able to prove that the resonance spectrum of the probe can be tuned for multi-frequency appli­cations by adding rein­forcing structures to the cantilever,” Hölscher said. The key signi­ficance of the group’s work is that the ability to design optimal tips or probes opens the door to endless options for analyzing samples with greatly enhanced reso­lution.

“Writing parts via 3D printing is expected to become a big business at the macro­scopic scale,” Hölscher said. “But I was surprised by how nicely it works for nano­scale, too.” As for near-term future appli­cations, two-photon polyme­rization will become widely available for nano­technology researchers. “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,” he noted. “It may even become an Internet business that allows you to design and order AFM probes via the web.” The group will continue to optimize their approach and apply it to research projects ranging from bio­mimetics to optics and photonics. (Source: KIT)

Reference: G. Göring et al.: Tailored probes for atomic force microscopy fabricated by two-photon polymerization, Appl. Phys. Lett. 109, 063101 (2016); DOI: 10.1063/1.4960386

Link: Inst. for Microstructure Technology IMT, Karlsruhe Institute of Technology KIT, Eggenstein-Leopoldshafen, Germany • Nanoscribe GmbH, Karlsruhe, Germany 

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