Scott T. Reed

Rapid prototyping of patterned functional nanostructures

Living systems exhibit form and function on multiple length scales and at multiple locations. In order to mimic such natural structures, it is necessary to develop efficient strategies for assembling hierarchical materials. Conventional photolithography, although ubiquitous in the fabrication of microelectronics and microelectromechanical systems, is impractical for defining feature sizes below 0.1 micrometres and poorly suited to pattern chemical functionality. Recently, so-called ‘soft’ lithographic approaches have been combined with surfactant and particulate templating procedures to create materials with multiple levels of structural order. But the materials thus formed have been limited primarily to oxides with no specific functionality, and the associated processing times have ranged from hours to days. Here, using a self-assembling ‘ink’, we combine silica–surfactant self-assembly with three rapid printing procedures—pen lithography, ink-jet printing, and dip-coating of patterned self-assembled monolayers—to form functional, hierarchically organized structures in seconds. The rapid-prototyping procedures we describe are simple, employ readily available equipment, and provide a link between computer-aided design and self-assembled nanostructures. We expect that the ability to form arbitrary functional designs on arbitrary surfaces will be of practical importance for directly writing sensor arrays and fluidic or photonic systems.

Hierarchically Structured Functional Porous Silica and Composite Produced by Evaporation-Induced Self-Assembly

Abstract Recently so-called soft lithography approaches [Angew. Chem. Int. Ed. 37 (1998) 550] have been combined with surfactant [Adv. Mater. 9 (1997) 811, Nature 390 (1997) 674] and particulate [Science 282 (1998) 2244] templating procedures to create oxides with multiple levels of structural order. But the materials thus formed have been limited primarily to oxides with no specific functionality, and the associated processing times have ranged from hours to days. Using self-assembling inks we have combined evaporation-induced (silica/surfactant) self-assembly [Adv. Mater. 11 (1999) 579] with rapid prototyping techniques like micro-pen lithography [Science 283 (1999) 661, Mat. Res. Soc. Symp. Proc. 542 (1999) 159], ink-jet printing [Adv. Mater. 11 (1999) 734, Mat. Sci. Eng. C5 (1998) 289], and dip coating on micro-contact printed substrates to form hierarchically organized structures in seconds. By co-condensation of tetrafunctional silanes (Si(OR) 4 ) with tri-functional organosilanes ((RO) 3 SiR ′ ) [Chem. Commun. (1999) 1367, Chem. Commun. (1997) 1769, J. Am. Chem. Soc. 119 (1997) 4090] or bridged silsesquioxanes (RO) 3 Si–R ′ –Si(OR) 3 ) or by inclusion of organic additives, we have selectively derivatized the silica framework with functional R ′ ligands or molecules. The rapid-prototyping procedures we describe are simple, employ readily available equipment, and provide a link between computer-aided design and self-assembled functional nanostructures. We expect that the ability to form arbitrary functional designs on arbitrary surfaces will be of practical importance for directly writing sensor arrays and fluidic or photonic systems.

Porous Optical Composites

Previous studies have shown that sol-gel matrices are excellent low temperature hosts for various optically-active materials, both organic and inorganic. Optical properties of these composites depend upon such factors as the structure of the matrix and size, shape, and degree of dispersion of the optically-active phase. We discuss factors that control the shrinkage and clarity of silicate aerogel host matrices and report on novel composites in which the optical properties are controlled by solid-vapor and/or solid-liquid reactions within the host matrix.

Sol—gel derived ceramic films — fundamentals and applications

Sol-gel processing begins with a colloidal dispersion, or sol, of particles or polymers in a liquid. Through subsequent chemical cross-linking, electrostatic destabilization, evaporation or some combination thereof, the fluid sol may be transformed into a rigid gel, which is a substance containing a continuous solid skeleton enclosing a continuous liquid phase. This sol-to-gel transition allows the solid phase to be shaped into films, fibers, microspheres or monoliths. Of these various forms, amorphous (or partially crystalline) thin films represent the earliest commercial application of sol-gel technology [1]. Thin films (normally less than 1 μm in thickness) use little in the way of raw materials and may be processed without cracking, overcoming the major disadvantage of sol-gel processing of bulk materials. Early applications of sol-gel coatings as optical films were reviewed by Schroeder [2]. Since then, many new uses of sol-gel films have appeared in electronic, protective, membrane and sensor applications [3–15]. Most often the as-deposited films are amorphous, but depending on composition and thermal history, they may subsequently crystallize: ferroelectric PLZT (lead lanthanum zirconate titanate) and nonlinear optic LiNbO3 are excellent examples of crystalline films derived from amorphous precursors [16–18].

Sol-gel silicate thin-film electronic properties

We have explored the effects of various processing parameters on the dielectric and electronic integrity of sol‐gel‐derived silicate thin films and have identified several factors that strongly affect the thin‐film electronic properties. We find that sol‐gel dielectrics can exhibit excellent dielectric integrity: viz., low interface trap densities and fairly good insulating properties approaching those of a thermally grown silicon dioxide film on silicon.

Deposition of high quality Sol-Gel oxides on silicon

We have fabricated high quality sol-gel derived silicate and aluminoborosilicate thin films deposited on silicon substrates. From capacitance vs voltage measurements we observe low interface trap densities (<1011/cm2eV) and very low densities of slow interface state (<1010/cm2) in most films investigated. We have been able to make significant improvements over previous sol-gel derived oxides on silicon by controlling some of the key factors which effect the structure of the sol-gel derived thin films.

Planarization of metal substrates for solar mirrors

The planarizing ability of sol-gel films was investigated on several as-rolled stainless steel substrates. The smoothing effect afforded by the films was evaluated, using optical techniques, following deposition of silver over the planarized substrates. The specular reflectance of various substrates, initially ranging from 0.36 to 0.90, could be improved to final reflectance values of /approximately/0.93 with sol-gel processing. This process is being used to prepare prototype foil mirrors for evaluation in the next generation of solar concentrators.

SOL-GEL AR Films for Solar Applications

Sol-gel derived antireflective films have been prepared for a variety of solar applications. The optical properties of the films are optimized by microstructure tailoring in solution by aging and/or in the film by heating and etching. The resulting film provides a quarter-wave, single layer interference surface with a reflectance minimum of

Sol-Gel Thin Film Electronic Properties

We have explored the effects of various processing parameters on the dielectric and electronic integrity of sol-gel derived silicate thin films and have identified several factors that strongly affect the thin film electronic properties. We find that sol-gel dielectrics can exhibit excellent dielectric integrity: viz., low interface trap densities and fairly good insulating properties approaching those of a thermally grown SiO 2 film on Si.

Development of an efficient large-aperture high damage-threshold sol-gel diffraction grating.

In order to develop the next generation of high peak intensity lasers, new grating technology providing higher damage thresholds and large apertures is required. The current assumption is that this technical innovation will be multilayer dielectric gratings, wherein the uppermost layer of a thin film mirror is etched to create the desired binary phase grating. A variant of this is explored with the upper grating layer being a lower density gelatin-based volume phase grating in either sol-gel or dichromated gelatin. One key benefit is the elimination of the etching step.