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Stanford researchers 3D print nanoparticles for shapeshifting materials

New research made possible by nanoprinting technology to produce elusive Archimedean truncated tetrahedrons (ATTs)

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Just last week Stanford Bio-X researchers published a study on 3D printing a million of microscopic parts (using a version of Carbon’s technology), and another Stanford team made the news for using a 3D nanoprinting technique to produce one of the most promising shapes known – Archimedean truncated tetrahedrons. These are micron-scale tetrahedrons with the tips lopped off.

In the paper, Wendy Gu, an assistant professor of mechanical engineering at Stanford University, and her co-authors describe how they nanoprinted tens of thousands of these challenging nanoparticles, stirred them into a solution, and then watched as they self-assembled into various promising crystal structures. More critically, these materials can shift between states in minutes simply by rearranging the particles into new geometric patterns.

“If we can learn to control these phase shifts in materials made of these Archimedean truncated tetrahedrons it could lead in many promising engineering directions,” she said.

Stanford researchers nano 3D print Archimedean truncated tetrahedrons (ATTs) nanoparticles for shapeshifting materials
Optical images of truncated tetrahedrons forming two large hexagonal grains at an anti-phase boundary (left), and transforming into a quasi-diamond phase that initiated at the anti-phase boundary (right). Scale bars are 25 um. (Image credit: David Doan & John Kulikowski)

Elusive prey

In nanomaterials, the geometry of the particle in the material defines the physical characteristics of the resulting material. Archimedean truncated tetrahedrons (ATTs) have long been theorized to be among the most desirable of geometries for producing materials that can easily change phase, but until recently were challenging to fabricate – predicted in computer simulations yet difficult to reproduce in the real world.

Gu’s team is not the first to produce nanoscale Archimedean truncated tetrahedrons in quantity, but they are among the first, if not the first, to use 3D nanoprinting to do it. “With 3D nanoprinting, we can make almost any shape we want. We can control the particle shape very carefully,” Gu explained. “This particular shape has been predicted by simulations to form very interesting structures. When you can pack them together in various ways they produce valuable physical properties.”

ATTs form at least two highly desirable geometric structures. The first is a hexagonal pattern in which the tetrahedrons rest flat on the substrate with their truncated tips pointing upward like a nanoscale mountain range. The second is perhaps even more promising, Gu said. It is a crystalline quasi-diamond structure in which the tetrahedrons alternate in upward- and downward-facing orientations, like eggs resting in an egg carton. The diamond arrangement is considered a “Holy Grail” in the photonics community and could lead in many new and interesting scientific directions.

Stanford researchers nano 3D print Archimedean truncated tetrahedrons (ATTs) nanoparticles for shapeshifting materials
Confocal images of truncated tetrahedrons forming several quasi-diamond grains (left). Bond order analysis shows different quasi-diamond grains through alternating colors (right). Neighboring tetrahedrons that have alternating colors (i.e. blue and red/brown, or dark blue and yellow) indicate that they have the same grain orientation. Scale bars are 20 um. (Image credit: David Doan & John Kulikowski)

Most importantly, however, when properly engineered, future materials made of 3D printed particles can be rearranged rapidly, switching easily back and forth between phases with the application of a magnetic field, electric current, heat, or other engineering method.

Gu said she can imagine coatings for solar panels that change throughout the day to maximize energy efficiency, new-age hydrophobic films for airplane wings and windows that mean they never fog or ice up, or new types of computer memory. The list goes on and on.

“Right now, we’re working on making these particles magnetic to control how they behave,” Gu said of her latest research already underway using Archimedean truncated tetrahedron nanoparticles in new ways. “The possibilities are only beginning to be explored.”

Additional co-authors of the work are PhD students David Doan and John Kulikowski. Gu is also a member of Stanford Bio-X.

This work was funded by the National Science Foundation, a Stanford Graduate Fellowship. DD, JK, the Hellman Foundation, and the National Science Foundation. Part of this work was performed at the Stanford Nano Shared Facilities, which is supported by the National Science Foundation, and at the Stanford Cell Sciences Imaging Facility.

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Davide Sher

Since 2002, Davide has built up extensive experience as a technology journalist, market analyst and consultant for the additive manufacturing industry. Born in Milan, Italy, he spent 12 years in the United States, where he completed his studies at SUNY USB. As a journalist covering the tech and videogame industry for over 10 years, he began covering the AM industry in 2013, first as an international journalist and subsequently as a market analyst, focusing on the additive manufacturing industry and relative vertical markets. In 2016 he co-founded London-based VoxelMatters. Today the company publishes the leading news and insights websites VoxelMatters.com and Replicatore.it, as well as VoxelMatters Directory, the largest global directory of companies in the additive manufacturing industry.

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