Hongyou Fan

Evaporation-Induced Self-Assembly: Nanostructures Made Easy

As we look toward the next millennium, we envision new technologies based on nanoscale machines and devices. Key to the realization of this nanotech world are simple, efficient methods of organizing materials (molecules, molecular clusters, polymers, or, generally speaking, building blocks) into precise, predetermined nanostructures that can be preserved in a robust engineering form. Marine organisms like diatoms and radiolaria provide us with many examples of intricately organized architectures preserved in silica or calcium carbonate. Such natural microstructures are formed by biomineralization, a templated self-assembly process in which preorganized organic surfaces regulate the nucleation, growth, morphology and orientation of inorganic crystals. To date, a variety of synthetic pathways that mimic aspects of biomineralization have been explored to prepare patterned ceramic materials. In an early attempt to achieve antigen/ antibodyselectivity inaporousadsorbent,Dickey prepared silicagels inthepresenceofthetargetmoleculetobeadsorbed (in this case methyl orange). After methyl orange extraction, the resulting templated silicas showed preferential selectivity for methyl orange over its alkyl orange homologues. In the 1960s researchers at the Mobil Oil Corporation used alkylammonium ions as templates to control the pore size, shape and periodicity of zeolites, crystalline solids that define 1-, 2-, or 3-dimensional (1-, 2-, or 3-D, respectively) networks of microporous channels. More recently Kresge and colleagues at Mobil used longer-chain alkylammonium ions in an attempt to increase the maximum pore size of zeolites beyond ~1.2 nm. They observed honeycomb-like arrays of ~4 nm pores and, based on analogies with hexagonal liquidcrystalline systems, proposed a supramolecular liquid-crystalline templating mechanism. Although excellent progress has been made in the preparationofawidevarietyofpatternedceramicmaterials, current synthetic methods have several inherent drawbacks fromthestandpointofnanotechnology:First,mosttemplating procedures are conducted in time-consuming batch operations often employing hydrothermal processing conditions. Second, the resultant products are typically ill-defined powders, precluding their general use in thin film technologies. Third, procedures developed to date are often limited to forming patterns of pores. For many envisioned nanotechnologies, it would be desirable to create patterned nanocomposites consisting of periodic arrangements of two or more dissimilar materials. This article summarizes a simple evaporation-induced self-assembly (EISA) process, that enables the rapid production of patterned porous or nanocomposite materials in the form of films, fibers, or powders.

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.

Self-assembled, ordered gold nanocrystal/silica thin films for prism-based surface plasmon resonance sensors

Gold nanocrystal(NC)/silica films are synthesized through self-assembly of water-soluble gold nanocrystal micelles and silica by sol-gel processing. Absorption and transmission spectra show a strong surface plasmon resonance absorption peak at /spl sim/520 nm. Angular excitation spectra of surface plasmon show a steep dip in the reflectivity curve at /spl sim/65/spl deg/ depending on the thickness and refractive index of the gold NC/silica film. A potential SPR sensor with enhanced sensitivities can be realized based on these gold NC/silica films.

Surfactant Templated Mesoporous Hybrid Thin Films

Hybrid magnetic composites made up of calibrated maghemite nanoparticles well dispersed in an epoxide resin were obtained by polymerization of the resin inside stable organosols of maghemite. The sol stability was ensured by ligand adsorption onto the particle surface. Studies of phenylphosphonic acid adsorption are presented. They show a strong interaction between PPA ions and hydroxyl groups of particle surface and through bridging ≡Fe 2 O 2 POΦ species. Infrared and Mossbauer spectroscopies as well as quasi- elastic light scattering and transmission electron microscopy were used to characterize the materials.Organic/inorganic hybrid films exhibiting ordered mesophases were prepared by a simple dip-coating procedure. Beginning with a homogeneous solution of tetraethyl orthosilicate, organoalkoxysilane ((R Si(OR)3, R is a non-hydrolyzable functional ligand) surfactant, we relied on solvent evaporation to induce micellization and continuous self-assembly into hybrid silica-surfactant thin film mesophases. Surface acoustic wave (SAW)-based nitrogen sorption measurements indicate that the films have high surface areas and unimodal pore diameters after removal of surfactants. INTRODUCTION Organic-inorganic hybrid mesoporous silica (with controlled pore structure and tailored pore chemistry) could find many applications in new types of catalysis and separation, environmental and industrial processes, electronics, and sensors.[ 1Al A route to fi.mctionalized amorphous silica materials that has been widely investigated in sol-gel chemistry involves the co-condensation of organosilanes with silicate to produce hybrid organic-inorganic networks[5~ 61.In these materials, an organic fictional group, R, is covalently bound to siloxane that is hydrolyzed to form silica copolymer. Mann et al. [’71first used this concept in direct synthesis of ordered organic-inorganic mesoporous powders containing octyland phenylgroups in 1996, later extending to mesoporous materials containing mercapto-, amino-, epoxylgroups.[8J 91 At the same time, several other studies were reported about direct synthesis of hybrid functional mesoporous silica[2~ 10-121. A solvent extraction procedure was used to remove the surfactant, resulting in a functionalized mesoporous product with a hexagonal MCM-41-type architecture. In each case mentioned above, the hybrid mesoporous silica was in the form of powder, precluding its use in such promising applications as membranes and optically-based sensors that generally require transparent, defect-freed supported thin films. We recently reported on a rapid and continuous approach to form thin silica films with ordered mesoporous structures131. Films with 2-dimensional hexagonal, 3-dimensional hexagonal, cubic, or lamellar structures were prepared from initially homogeneous silica SOISby evaporation-induced surfactant enrichment during sol-gel dip-coating. In this paper, we extended our work to the preparation of hybrid organic/inorganic thin films with tailored pore surface chemistries. EXPERIMENTAL Precursor solutions were prepared by addition of surfactants (cationic, CTAB; C133(CH2)15~(CHJ3Bror non-ionic, Brij-56; CH3(CH2)15-(OCH2CH2) 10-OH ), organosilanes (R Si(OR)3, see Table 1), or organic molecules (see Table 1) to an acidic silica sol prepared from TEOS [Si(OCH2CH3 )4] (A2**). The acid concentration employed in the A2** synthesis procedure was chosen to minimize the siloxane condensation rate, thereby promoting facile selfassembly during printing. In a typical preparation, TEOS [Si(OCH2CH3 ) ethanol, water andDisilaoxacyclopentanes have proven to be excellent precursors to sol-gel type materials. These materials have shown promise as precursors for encapsulation and microelectronics applications. The polymers are highly crosslinked and are structurally similar to traditional sol-gels, but unlike typical sol-gels they are prepared without the use of solvents and water, they have low VOCs and show little shrinkage during processing.

Scalable Assembly of Patterned Ordered Functional Micelle Arrays: Final LDRD Report

In this project, we demonstrated the synthesis of polystyrene-polyvinylpyridyne (PS-PVP) micelles, functionalization of these micelles to form organic/inorganic composite nanoparticles, and template directed assembly of dynamic PS-PVP micelles into features defined via soft nanoimprint lithography. We demonstrated unique assembly properties of dynamic micellar nanoparticles by combining a top down lithographic nanopatterning technique with a solutionbased bottom up self-assembly. The templates for the directed self-assembly of the micelles consisted of arrays of cylindrical recess features fabricated by nanoimprint lithography. Silica was coated on this patterned substrate and subsequently selectively functionalized with a positively charged molecular monolayer (N-(3-Trimethoxysilylpropyl) diethylenetriamine) to regulate the micelle-surface interactions. The self-assembled block co-polymer poly(styrene-b4-vinyl pyridine), (PS480k – PVP145k ) micelles were approximately 325nm in diameter in aqueous solutions (pH = 2.5) and 50nm in diameter in the dry state. The average number of micelles assembled per feature increased from less than 1 to 12 with increasing feature diameter in the range of 200nm – 1μm. Using a 2D model for maximum packing of circles in circular host features, the effective sphere size of the micelles during assembly was calculated to be 250nm in diameter. Thus, the micelles exhibited three characteristic sizes during assembly, 325nm in bulk solution, 250nm during assembly, and 50nm in the dry state. This dramatic variation in nanoparticle diameter during the assembly process offers unique opportunities for forming nanometer scale, multidimensional arrays not accessible using hard sphere building blocks.

Nanomanufacturing : nano-structured materials made layer-by-layer.

Large-scale, high-throughput production of nano-structured materials (i.e. nanomanufacturing) is a strategic area in manufacturing, with markets projected to exceed

Nanoporous films for epitaxial growth of single crystal semiconductor materials : final LDRD report.

1T by 2015. Nanomanufacturing is still in its infancy; process/product developments are costly and only touch on potential opportunities enabled by growing nanoscience discoveries. The greatest promise for high-volume manufacturing lies in age-old coating and imprinting operations. For materials with tailored nm-scale structure, imprinting/embossing must be achieved at high speeds (roll-to-roll) and/or over large areas (batch operation) with feature sizes less than 100 nm. Dispersion coatings with nanoparticles can also tailor structure through self- or directed-assembly. Layering films structured with these processes have tremendous potential for efficient manufacturing of microelectronics, photovoltaics and other topical nano-structured devices. This project is designed to perform the requisite R and D to bring Sandias technology base in computational mechanics to bear on this scale-up problem. Project focus is enforced by addressing a promising imprinting process currently being commercialized.

Nanoporous Silica Templated HeteroEpitaxy: Final LDRD Report.

This senior council Tier 1 LDRD was focused on exploring the use of porous growth masks as a method for defect reduction during heteroepitaxial crystal growth. Initially our goal was to investigate porous silica as a growth mask, however, we expanded the scope of the research to include several other porous growth masks on various size scales, including mesoporous carbon, photolithographically patterned SU-8 and carbonized SU-8 structures. Use of photolithographically defined growth templates represents a new direction, unique in the extensive literature of patterned epitaxial growth, and presents the possibility of providing a single step growth mask. Additional research included investigation of pore viability via electrochemical deposition into high aspect ratio photoresist. This project was a small footprint research effort which, nonetheless, produced significant progress towards both the stated goal as well as unanticipated research directions.

Cell-directed assembly on an integrated nanoelectronic/nanophotonic device for probing cellular responses on the nanoscale.

This one-year out-of-the-box LDRD was focused on exploring the use of porous growth masks as a method for defect reduction during heteroepitaxial crystal growth. Initially our goal was to investigate porous silica as a growth mask, however, we expanded the scope of the research to include several other porous growth masks on various size scales, including mesoporous carbon, and the UV curable epoxy, SU-8. Use of SU-8 as a growth mask represents a new direction, unique in the extensive literature of patterned epitaxial growth, and presents the possibility of providing a single step growth mask. Additional research included investigation of pore viability via electrochemical deposition into high aspect ratio photoresist patterns and pilot work on using SU-8 as a DUV negative resist, another significant potential result. While the late start nature of this project pushed some of the initial research goals out of the time table, significant progress was made. 3 Acknowledgements This work was performed in part at the Nanoscience %40 UNM facility, a member of the National Nanotechnology Infrastructure Network, which is supported by the National Science Foundation (Grant ECS 03-35765). Sandia is multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United Stated Departmentmorexa0» of Energys National Nuclear Security Administration under Contract DE-AC04-94AL85000. This work was supported under the Sandia LDRD program (Project 99405). 4«xa0less