3D Chemistry: Fabrication Technique Uses Light-Activated Molecules to Create Complex Microstructures

Known as "two-photon 3D lithography," the technique could compete with existing processes for fabricating microfluidic devices, photonic bandgap structures, optical storage devices, photonic switches and couplers, sensors, actuators, micromachines - and even scaffolds for growing living tissue.

Georgia Institute of Technology Researcher Seth Marder described the technique February 15 at the annual meeting of the American Association for the Advancement of Science (AAAS) in Seattle.

"We have developed a disruptive platform technology that we believe will provide broad new capabilities," said Marder, a professor in Georgia Tech's School of Chemistry and Biochemistry. "We believe this technique provides a real competitive advantage for making complicated three-dimensional microstructures."

The technique uses a family of organic dye molecules known as Bis-donor phenylene vinylenes that have a special ability to absorb two photons of light simultaneously. Once excited, the molecules transfer an electron to form a simple acid or a radical group that can initiate a chemical reaction - such as polymer cross-linking or ion reduction.

By adding small concentrations (0.1 percent) of the molecules to a resin slab containing cross-linkable acrylate monomer, for example, researchers can use a focused near-infrared laser beam to draw patterns and initiate cross-linking reactions only in material exposed to the light. The reactions can make that portion of the slab insoluble, allowing the remainder to be washed away to leave a complex three-dimensional structure.

The researchers have demonstrated the ability to create both positive and negative resists using two-photon activated reactions to alternatively create soluble or insoluble 3D patterns. Beyond polymers, Marder and collaborator Joseph Perry have demonstrated the fabrication of tiny silver wires from patterns written in materials containing silver nanoparticles and ions.

The molecules developed by Marder and Perry are hundreds of times more efficient at absorbing two photons than previous photoactive materials. That efficiency allows them to write 3D patterns in polymer slabs that are typically 100 microns thick, at light intensities low enough to avoid damaging the materials.

The laser writing process takes advantage of the fact that the chemical reaction occurs only where molecules have absorbed two photons. Since the rate of two-photon absorption drops off rapidly with distance from the laser's focal point, only molecules at the focal point receive enough light to absorb two photons.

"We can define with a very high degree of precision in the x, y and z coordinates where we are getting excitation," Marder explained. "Using 700-nanometer light, the patterning precision can be about 200 nm across by 800 nm in depth."

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