Fast, Easy, Functional Nanostructures

Some marine organisms provide examples of intricately organized architectures preserved in silica and calcium carbonate, according to Dr. Hongyou Fan, research scientist at UNM and Sandia. In order to mimic such natural structures and impart life-like qualities to man-made materials, it is necessary to develop efficient strategies for assembling hierarchical materials that exhibit form and function at multiple lengths and multiple locations. But until recently, templating procedures to create materials with multiple levels of structural order have been limited primarily to oxides with no specific functionality and processing times ranging from hours to days. However, the discovery of surfactant-templated silica mesophases in 1992 opened the way to the development of molecular-scale, organic schemes to effectively impart functionality to porous surfaces. Brinker's group has developed technology to introduce thousands of functions into ink that are rapidly printable, on the order of seconds.

Surfactants are detergents, which contain hydrophobic and hydrophilic parts. In aqueous solution above some critical micelle concentration (cmc), surfactants can self-assemble into micelles or micelle-based liquid crystals that maintain the hydrophilic parts in contact with water while shielding the hydrophobic parts within the micellar interior. To make the silica suitable as ink for printing, fluidity and stability are maintained by carefully controlling the acid concentration. Materials such as silica and polymers give structural soundness once out of solution. These form the basis of resists and low-k materials.

Patterned dot arrays have been created by inkjet printing on non-adsorbent and on silicon dioxide surfaces. Using computer aided design (CAD) and a standard Hewlett-Packard inkjet printer, the print rate was ~80 dots/sec, with resolutions of 300 dots/in. But these numbers are conservative. With state-of-the-art printers, several hundred dots per second with 1400 dots/in. may be achievable. According to Dr. Fan, the ability to selectively functionalize the ink suggests applications in display technologies, sensor arrays and photonic systems. •