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New approaches to nanofabrication: molding, printing, and other techniques.

Two-photon excitation provides a means of activating chemical or physical processes with high spatial resolution in three dimensions and has made possible the development of three-dimensional fluorescence imaging, optical data storage, and lithographic microfabrication. These applications take advantage of the fact that the two-photon absorption probability depends quadratically on intensity, so under tight-focusing conditions, the absorption is confined at the focus to a volume of order λ3 (where λ is the laser wavelength). Any subsequent process, such as fluorescence or a photoinduced chemical reaction, is also localized in this small volume. Although three-dimensional data storage and microfabrication have been illustrated using two-photon-initiated polymerization of resins incorporating conventional ultraviolet-absorbing initiators, such photopolymer systems exhibit low photosensitivity as the initiators have small two-photon absorption cross-sections (δ). Consequently, this approach requires high laser power, and its widespread use remains impractical. Here we report on a class of π;-conjugated compounds that exhibit large δ (as high as 1, 250 × 10−50 cm4 s per photon) and enhanced two-photon sensitivity relative to ultraviolet initiators. Two-photon excitable resins based on these new initiators have been developed and used to demonstrate a scheme for three-dimensional data storage which permits fluorescent and refractive read-out, and the fabrication of three-dimensional micro-optical and micromechanical structures, including photonic-bandgap-type structures.

Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication

Nanoimprint lithography (NIL) is a nonconventional lithographic technique for high-throughput patterning of polymer nanostructures at great precision and at low costs. Unlike traditional lithographic approaches, which achieve pattern definition through the use of photons or electrons to modify the chemical and physical properties of the resist, NIL relies on direct mechanical deformation of the resist material and can therefore achieve resolutions beyond the limitations set by light diffraction or beam scattering that are encountered in conventional techniques. This Review covers the basic principles of nanoimprinting, with an emphasis on the requirements on materials for the imprinting mold, surface properties, and resist materials for successful and reliable nanostructure replication.

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Nanoimprint Lithography: Methods and Material Requirements**

Natural and biomimetic artificial surfaces for superhydrophobicity, self-cleaning, low adhesion, and drag reduction

Flexible skin-attachable strain-gauge sensors are an essential component in the development of artificial systems that can mimic the complex characteristics of the human skin. In general, such sensors contain a number of circuits or complex layered matrix arrays. Here, we present a simple architecture for a flexible and highly sensitive strain sensor that enables the detection of pressure, shear and torsion. The device is based on two interlocked arrays of high-aspect-ratio Pt-coated polymeric nanofibres that are supported on thin polydimethylsiloxane layers. When different sensing stimuli are applied, the degree of interconnection and the electrical resistance of the sensor changes in a reversible, directional manner with specific, discernible strain-gauge factors. The sensor response is highly repeatable and reproducible up to 10,000 cycles with excellent on/off switching behaviour. We show that the sensor can be used to monitor signals ranging from human heartbeats to the impact of a bouncing water droplet on a superhydrophobic surface.

A flexible and highly sensitive strain-gauge sensor using reversible interlocking of nanofibres

We describe a novel UV-curable mold that is stiff enough for replicating dense sub-100-nm features even with a high aspect ratio. It also allows for flexibility when the mold is prepared on a flexible support such that large area replication can be accomplished. The composite material of the mold is inert to chemicals and solvents. The surface energy is made low with a small amount of releasing agent such that the mold can be removed easily and cleanly after patterning. In addition, the material allows self-replication of the mold. These unique features of the mold material should make the mold quite useful for various patterning purposes.

An Ultraviolet-Curable Mold for Sub-100-nm Lithography

Since the introduction of ultraviolet (UV)-curable polyurethane acrylate (PUA) materials in 2004, our group has extensively used the material in various applications ranging from nanopatterning, biomimetics, microfluidics, to tissue engineering. In this highlight, we aim to provide a brief overview of the properties and applications of PUA with particular focus on the ability of tuning modulus (20–320 MPa) and surface energy (20–60 mJ m−2). These unique features enable us to use the material in various patterning applications as a mold and a patterned film, respectively. In addition, one can benefit from several other features of PUA such as self-replication, partial curing, flexibility, biocompatibility, and transparency. These issues are also briefly described along with their applications.

Modulus- and surface energy-tunable ultraviolet-curable polyurethane acrylate: properties and applications

Modulus-tunable ultraviolet (UV) curing molds allow not only physical patterning of imprinting but also chemical patterning of microcontact printing. The mechanical properties of the mold can be tailored by the chain length of an acrylate modulator in the cross-linking reaction. This tunability can be utilized to obtain a proper balance that is needed for a given patterning technique between the rigidity requirement (tensile modulus = 320 MPa) of a mold for patterning a fine structure and the flexibility requirement (tensile modulus = 19.8 MPa) for a conformal contact. Successful low-pressure imprinting of sub-100-nm patterns and microcontact printing of submicrometer patterns verify the suitability of the mold for a wide spectrum of patterning techniques.

Unconventional Patterning with A Modulus-Tunable Mold: From Imprinting to Microcontact Printing

Rigiflex Lithography for Nanostructure Transfer

Directional and asymmetric properties are attractive features in nature that have proven useful for directional wetting, directional flow of liquids and artificial dry adhesion. Here we demonstrate that an optically asymmetric structure can be exploited to guide light with directionality. The Lucius prism array presented here has two distinct properties: the directional transmission of light and the disproportionation of light intensity. These allow the illumination of objects only in desired directions. Set up as an array, the Lucius prism can function as an autostereoscopic three-dimensional display.

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Se-Jin Choi

Papers

An Ultraviolet-Curable Mold for Sub-100-nm Lithography

Modulus- and surface energy-tunable ultraviolet-curable polyurethane acrylate: properties and applications

Unconventional Patterning with A Modulus-Tunable Mold: From Imprinting to Microcontact Printing

Rigiflex Lithography for Nanostructure Transfer

Arrays of Lucius microprisms for directional allocation of light and autostereoscopic three-dimensional displays