Using a technique known as two-photon polymerization, researchers led by Hendrik Hölscher of Karlsruhe Institute of Technology have printed AFM tips of their own design with a resolution in the neighborhood of 25 nm (Appl. Phys. Lett. 2016, DOI: 10.1063/1.4960386).
Scientists first created AFM tips with two-photon polymerization more than 10 years ago, but the Karlsruhe team is the first to couple the technique’s resolution with the design flexibility of 3-D printing. The researchers have tuned their process and their printer to the point where they can make virtually any tip that they can design on a computer, Hölscher says.
As a demonstration, they printed a microscopic replica of the Matterhorn, the famous Swiss Alp, on the end of an AFM cantilever. But the true power of the technique lies in the on-demand printing of tip geometries that are optimized for the sample being investigated, Hölscher says.
For example, the team printed long, narrow tips that could better trace the steep peaks and valleys of a rose petal’s surface than some standard tips. Although commercial tip providers offer a range of tip geometries and can accommodate custom orders, getting the best tip for a unique specimen can be costly, Hölscher explains.
The two-photon technique used by Hölscher and his colleagues is similar to standard stereolithography, which exposes photosensitive polymers called photoresists to ultraviolet light to controllably create 3-D shapes. The two-photon technique offers higher resolution by constraining where curing happens using ultrafast laser pulses and photoresists that must simultaneously absorb two photons, rather than a single photon, to polymerize.
Although the printed polymer tips aren’t as sturdy as standard silicon or silicon nitride tips, they are surprisingly hardy, says Santiago D. Solares, who leads the scanning probe microscopy lab at George Washington University.
Solares, who was not involved in the study, adds that the printing method could be especially useful for designing tips for multifrequency AFM work. A single multifrequency scan can shed light on multiple sample properties, such as surface topography and elasticity. Printing customized tips “opens up many opportunities for both routine and specialized AFM users,” Solares says.