3D printing

Engineers create glass and crystal nanostructures with 3D printer

Image credit: Dreamstime

Engineers from Rice University have created nanostructures of glass and crystal for electronic and photonic devices, using a sophisticated 3D printer and unique ink.

The electronics industry is built on silicon: the basic semiconducting substrate for microprocessors. There are certain limitations of top-down semiconductor manufacturing, which this study sought to address using a bottom-up approach, harnessing additive manufacturing.

“It’s very tough to make complicated, three-dimensional geometries with traditional photolithography techniques,” said Professor Jun Lou, a materials science expert and lead author of the Nature Materials study. “It’s also not very green because it requires a lot of chemicals and a lot of steps. And even with all that effort, some structures are impossible to make with those methods.

“In principle, we can print arbitrary 3D shapes, which could be very interesting for making exotic photonic devices. That’s what we’re trying to demonstrate.”

Lou’s laboratory used a two-photon polymerisation process and a sophisticated 3D printer to create structures with lines only several hundred nanometres in width: smaller than the wavelengths of visible light. The engineers developed a unique ink for the method. They created resins using nanospheres of silicon dioxide doped with polyethylene glycol, which rendered them soluble.

3D printed nanostructures

The Nanomaterials, Nanomechanics and Nanodevices Lab/Rice University

Image credit: The Nanomaterials, Nanomechanics and Nanodevices Lab/Rice University

Lasers 'write' the lines by prompting the ink to absorb two photons, initiating free-radical polymerisation of the material.

Rice University graduate and co-lead author, Boyu Zhang, explained: “Normal polymerisation involves polymer monomers and photoinitiators: molecules that absorb light and generate free radicals.

“In our process, the photoinitiators absorb two photons at the same time, which requires a lot of energy. Only a very small peak of this energy causes polymerisation, and that in only a very tiny space. That’s why this process allows us to go beyond the diffraction limit of light.”

After printing, the structure is solidified via high-temperature sintering, which eliminates all the polymer, leaving amorphous glass or polycrystalline cristobalite. When heated, the material passes through phases from glass to crystal; the higher the temperature, the more ordered the crystals become.

Lou’s laboratory also demonstrated that the material can be doped with various rare earth salts, which makes the structures photoluminescent: a valuable adaptation for custom optical applications.

Next, the engineers will work to refine the process further, aiming at creating detail at a resolution of less than 10 nanometres.

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