Microscopic images of the delicate structures printed by materials scientists at Rice University using their new technique. Sintering turns the structures into either glass or cristobalite. Image: The Nanomaterials, Nanomechanics and Nanodevices Lab/Rice University.
Microscopic images of the delicate structures printed by materials scientists at Rice University using their new technique. Sintering turns the structures into either glass or cristobalite. Image: The Nanomaterials, Nanomechanics and Nanodevices Lab/Rice University.

Weaving intricate, microscopic patterns of crystal or glass is now possible thanks to materials scientists at Rice University.

Using a sophisticated 3D printer, the researchers are able to create nanostructures of silica, demonstrating a method for making micro-scale electronic, mechanical and photonic devices from the bottom up. These products can be doped, and their crystal structures tuned, for various applications. The researchers, led by Jun Lou, a professor of materials science and nanoengineering, report their work in a paper in Nature Materials.

The electronics industry is built upon silicon, which for decades has been the basic semiconducting substrate for microprocessors. In this new study, the Rice researchers address the limitations of current top-down manufacturing by turning the process on its head.

“It’s very tough to make complicated, three-dimensional geometries with traditional photolithography techniques,” said Lou. “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.”

The lab uses a two-photon polymerization process to print structures with lines only several hundred nanometers wide, smaller than the wavelength of light. Lasers 'write' the lines by prompting a special liquid ink to absorb two photons, initiating free-radical polymerization.

“Normal polymerization involves polymer monomers and photoinitiators, molecules that absorb light and generate free radicals,” said Rice graduate student and co-lead author Boyu Zhang. “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 polymerization, and that in only a very tiny space. That’s why this process allows us to go beyond the diffraction limit of light.”

This printing process required the Rice lab to develop a unique ink. Zhang and co-lead author 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 researchers 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-10nm resolution.

This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.