Materials: Functional Materials

The Chemical, Microsystem, and Nanoscale Technologies Group applies advanced chemical processing, fabrication, and nanostructuring methods to engineer materials with predesigned functional properties. Controlled surface texturing and chemical functionalization are two of the more prominent tools to achieve the desired material and device performance. 

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Scanning electron microscope image of a surface patterned in a dual-scale checkerboard geometry (arrays of 20 x 20 posts, each post placed at a pitch of 1200 nm). These arrays, fabricated in silicon and overcoated with silicon dioxide and a thin layer of a fluoropolymer, provide a very high degree of hydrophobicity, as shown by the sideview of a water droplet placed on the surface (contact angle of 160 degrees).

Actively controlled diffractive optical elements employing switchable liquid crystals have been designed and are being fabricated, with the goal of enabling broad optical functionality by orienting the director of the liquid crystal in predetermined complex geometries. In particular, the anchoring energy of the liquid crystal to the substrate is controlled by grooves patterned into the substrate using electron-beam lithography and inorganic resists, at dimensions of only a few tens of nanometers. Additional flexibility is added to the system by introducing photoalignment layers of organic material, which can be aligned by illumination with polarized light and which in turn is attached to the liquid crystal. Optimal flexibility in spatially and temporally controllable liquid-crystal orientation is achieved through combinations of applied electric fields at external electrodes, prepatterned surface nano-grooves, and applied aligning light.

In a separate set of projects, the Chemical, Microsystem, and Nanoscale Technologies Group is engineering spatial control of the wettability of given surfaces. Prepatterning the surface in various geometries, including designs of multiple spatial frequencies in two dimensions, enables local control of the surface energy and transition from a Cassie to a Wenzel state of wetting. For instance, hydrophilic regions can be prepared on a surface that is largely superhydrophobic, thus enabling multiple aqueous-based attachments and chemistries on an otherwise hydrophobic material. Furthermore, the addition of subsurface electrodes enables active switching from one state to another via electrowetting. This approach can have multiple applications in handling hazardous chemicals, as well as in the chemical analysis of minute samples.

Inorganic metal-oxide materials have been known to display electrochromic properties, whereby they change their color with the application of an external voltage. However, the utility of these materials is often limited by their slow response time, which can be as long as a minute. The Chemical, Microsystem, and Nanoscale Technologies Group has been exploring three-dimensional nano-structuring of nickel oxide and tungsten oxide as means to increase surface area and reduce response time. Various methods of such nano-structuring have been demonstrated, including templating with block copolymers, thermochemical thin-film metal oxidation, and plating. Employing these strategies, subsecond switching times have been achieved, thus overcoming one of the main deficiencies of these materials.

 

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