Hong Yee Low

Direct imprinting of high resolution TiO2 nanostructures

We demonstrate a different approach to direct nanoimprint lithography of oxides, in particular TiO(2), using the metal methacrylate route which not only gives very high resolution ( approximately 20 nm) but also provides yields of approximately 100% over areas > 1 cm x 1 cm. TiO(2) was imprinted using a polymerizable liquid TiO(2) resin consisting of a mixture of titanium methacrylate, ethylene glycol dimethacrylate, and azobis-(isobutyronitrile). The resin underwent free radical polymerization when imprinted using a silicon mold at 110 degrees C with pressures as low as 10 bar. Polymerization strengthens the imprinted structures, thereby giving approximately 100% yield after demolding. Heat-treatment of the imprinted structures at 400 degrees C resulted in the loss of organics and their subsequent shrinkage ( approximately 75%) without the loss of integrity or aspect ratio, and converted them to TiO(2) nanostructures as small as approximately 20 nm wide. Furthermore, our method demonstrates that large imprinted areas of sub-100-nm features can be achieved by sub-micron molds which translate into huge cost savings with the added flexibility of direct patterning of urinary as well as multi-component oxides.

Direct nanoimprinting of metal oxides by in situ thermal co-polymerization of their methacrylates

The use of polymerization to solidify, strengthen and imprint liquid organic materials is the basis of ultraviolet (UV) nanoimprint lithography. In spite of these advantages, the use of polymerization to pattern materials in thermal nanoimprint lithography is almost non-existent. In this study, we demonstrate a facile and general method to directly imprint a host of unary metal oxides (Fe2O3, ZrO2, TiO2, Nb2O5 and Ta2O5) at a very high resolution viain situ thermal free radical co-polymerization of various metal methacrylates in the presence of cross-linker ethylene glycol dimethacrylate using a silicon mold. Polymerization during nanoimprinting rigidly shapes the patterns, traps the metal atoms, reduces the surface energy and strengthens the structures, thereby giving ∼100% yield after demolding. It was found that the higher oxidation state of metal resulted in excessive cracking of imprinted structures. This could be due to a higher degree of cross-linking of the precursor leading to shrinkage-related stress. Optimization of the resin composition by partially replacing ethylene glycol dimethacrylate with methyl methacrylate alleviated this problem. Heat-treatment of the imprinted structures resulted in the loss of organics, their subsequent shrinkage and converted the patterns to their corresponding metal oxides with line-widths as small as 25 nm.

A state-of-the-art review and analysis on the design of dry adhesion materials for applications such as climbing micro-robots

Miniaturization of robotic systems has led to a demand for an alternative adhesive for use as the footpad of robots, with primary requirements of minimizing energy expenditure and satisfying performance and operational scenarios such as surveillance and reconnaissance. Inspired by nature, the dry adhesive concept as seen in climbing lizards such as the gecko has drawn significant interest from researchers. Adhesion in geckos is attributed to micro/nano fibrils found on its feet that rely on van der Waals forces to adhere to a surface, hence the terminology of dry adhesive. While immense progress has been made in the design and fabrication of multiscale hierarchical adhesive structures, the robustness, durability and endurance (ability to adhere to surfaces for an extended period of time) of gecko-foot mimetic dry adhesives still lags behind their biological counterparts. In this review article, we highlight the design considerations for the development of robust and durable bio-inspired synthetic adhesives. Current challenges and future directions are also highlighted for the design and development of robust and durable dry adhesive structures.

Direct nanoimprint lithography of Al2O3 using a chelated monomer-based precursor

Nanostructuring of Al₂O₃ is predominantly achieved by the anodization of aluminum film and is limited to obtaining porous anodized aluminum oxide (AAO). One of the main restrictions in developing approaches for direct fabrication of various types of Al₂O₃ patterns, such as lines, pillars, holes, etc, is the lack of a processable aluminum-containing resist. In this paper, we demonstrate a stable precursor prepared by reacting aluminum tri-sec-butoxide with 2-(methacryloyloxy)ethyl acetoacetate, a chelating monomer, which can be used for large area direct nanoimprint lithography of Al₂O₃. Chelation in the precursor makes it stable against hydrolysis whilst the presence of a reactive methacrylate group renders it polymerizable. The precursor was mixed with a cross-linker and their in situ thermal free-radical co-polymerization during nanoimprinting rigidly shaped the patterns, trapped the metal atoms, reduced the surface energy and strengthened the structures, thereby giving a ~100% yield after demolding. The imprinted structures were heat-treated, leading to the loss of organics and their subsequent shrinkage. Amorphous Al₂O₃ patterns with line-widths as small as 17 nm were obtained. Our process utilizes the advantages of sol-gel and methacrylate routes for imprinting and at the same time alleviates the disadvantages associated with both these methods. With these benefits, the chelating monomer route may be the harbinger of the universal scheme for direct nanoimprinting of metal oxides.

Gecko-Inspired Dry Adhesive Based on Micro–Nanoscale Hierarchical Arrays for Application in Climbing Devices

The unusual ability of geckos to climb vertical walls underlies a unique combination of a hierarchical structural design and a stiffer material composition. While a dense array of microscopic hierarchical structures enables the gecko toe pads to adhere to various surfaces, a stiffer material (β-keratin) composition enables them to maintain reliable adhesion over innumerable cycles. This unique strategy has been seldom implemented in engineered dry adhesives because fabrication of high-aspect-ratio hierarchical structures using a stiffer polymer is challenging. Herein, we report the fabrication of high-aspect-ratio hierarchical arrays on flexible polycarbonate sheets (stiffness comparable to that of β-keratin) by a sacrificial-layer-mediated nanoimprinting technique. Dry-adhesive films comprising the hierarchical arrays showed a formidable shear adhesion of 11.91 ± 0.43 N/cm2. Cyclic adhesion tests also showed that the shear adhesion of the adhesive films reduced by only about 20% after 50 cycles and remained nearly constant until about 200 cycles. Most importantly, the high-aspect-ratio hierarchical arrays were integrated onto the feet of a miniature robot and the locomotion on a 30° inclined surface was demonstrated.

Novel soft stamp development for direct micro- and nano-patterning of macroscopic curved surfaces

Surface topographical patterning is a simple way to functionalize surfaces without changing material chemistry. Topographical patterning of nonplanar surfaces has remained a challenge, despite sought after applications in microfluidics, optics, and biomedical technologies. Here the authors develop transparent, reusable soft molds that allow facile micro- and nanopatterning of macroscopically curved surfaces. The authors use bilayer molds with a soft backing and a hard pattern carrying layer to overcome challenges that arise from the opposing need for mold compliance (to allow conformal contact with nonflat substrates) and rigidity (to maintain patterned feature resolution and fidelity). With our approach, high yield curved surface patterning (>98%) over large (2 × 1u2009cm) area can be effectively achieved. Structure replication down to 80u2009nm resolution is demonstrated.

Large area sub-100 nm direct nanoimprinting of palladium nanostructures

Direct imprinting of metals is predominantly achieved by using polydimethylsiloxane (PDMS) molds to pattern metal nanoparticles and subsequently melting them to form continuous structures. Although such a combination can successfully imprint metals, the yield and reproducibility are usually low when sub-100 nm features over large areas are desired. In this work, we demonstrate a simple method involving the addition of a cross-linker ethylene glycol dimethacrylate (EDMA) to a palladium metal precursor, and its in situ free radical polymerization during imprinting, which not only dramatically increases the yield to ∼100% but also enables high reproducibility. Palladium mercaptide resist was formed by dissolving acetoxy(benzylthio)palladium, EDMA and azobis-(isobutyronitrile) in an organic solvent mixture. The resist underwent polymerization when imprinted using a silicon mold at 120 °C with pressures as low as 30 bar. Polymerization rigidly shapes the imprinted patterns, traps the metal atoms, reduces the surface energy and strengthens the structures, thereby giving ∼100% yield after demolding. Heat-treatment of the imprinted structures at 330 °C resulted in the loss of organics and their subsequent shrinkage without the loss of integrity or aspect ratio and converted them to palladium nanostructures as small as ∼35 nm wide, over areas >1 cm × 1 cm. With suitable precursors, our technique can potentially be extended to pattern noble metals such as platinum, gold and silver.