Weaving intricate, micro­scopic patterns of crystal or glass is now possible thanks to engineers at Rice University. The materials scientists are creating nano­structures of silica with a sophis­ticated 3D printer, demonstrating a method to make microscale electronic, mechanical and photonic devices from the bottom up. The products can be doped and their crystal structures tuned for various appli­cations. 

The electronics industry is built upon silicon, the basic semi­conducting substrate for micro­processors for decades. The study addresses the limi­tations of top-down manu­facturing by turning the process on its head. “It’s very tough to make complicated, three-dimensional geometries with traditional photo­lithography techniques,” Jun Lou, a professor of materials science and nano­engineering, said. “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,” he said. “That’s what we’re trying to demons­trate.”

The lab uses a two-photon polymeri­zation process to print structures with lines only several hundred nanometers wide. Lasers write the lines by prompting the ink to absorb two photons, ini­tiating free-radical polymeri­zation of the material. “Normal polymeri­zation involves polymer monomers and photo­initiators, molecules that absorb light and generate free radicals,” said graduate student Boyu Zhang of the process that commonly uses ultra­violet light in 3D printing and to cure coatings and in dental applications.

“In our process, the photo­initiators absorb two photons at the same time, which requires a lot of energy,” he said. “Only a very small peak of this energy causes polymerization, and that in only a very tiny space. That’s why this process allows us to go beyond the diffrac­tion limit of light.” The printing process required to develop a unique ink. Zhang and Xiewen Wen, a Rice alumnus, created resins containing nanospheres of silicon dioxide doped with poly­ethylene glycol to make them soluble. 

After printing, the structure is solidified through high-temperature sintering, which eliminates all the polymer from the product, leaving amorphous glass or poly­crystalline cristo­balite. “When heated, the material goes through phases from glass to crystal, and the higher the temperature, the more ordered the crystals become,” Lou said. The lab also demons­trated doping the material with various rare earth salts to make the products photo­luminescent, an important property for optical applications. The lab’s next goal is to refine the process to achieve sub-10 nanometer resolution. (Source: Rice U.)

Reference: X. Wen et al.: 3D-printed silica with nanoscale resolution, Nat. Mat. 20, 1506 (2021); DOI: 10.1038/s41563-021-01111-2

Link: Dept. of Materials Science and NanoEngineering, Rice University, Houston, USA