Inside Front Cover: The Elastic Properties and Plastic Behavior of Two‐Dimensional Polymer Structures Fabricated by Laser Interference Lithography (Adv. Funct. Mater. 10/2006)

The probing of the micromechanical properties within a two‐dimensional polymer structure with sixfold symmetry fabricated via interference lithography reveals a nonuniform spatial distribution in the elastic modulus “imprinted” with an interference pattern in work reported by Tsukruk, Thomas, and co‐workers on p. 1324. The image prepared by M. Lemieux and T. Gorishnyy shows how the interference pattern is formed by three laser beams and is transferred to the solid polymer structure. The elastic and plastic properties within a two‐dimensional polymer (SU8) structure with sixfold symmetry fabricated via interference lithography are presented. There is a nonuniform spatial distribution in the elastic modulus, with a higher elastic modulus obtained for nodes (brightest regions in the laser interference pattern) and a lower elastic modulus for beams (darkest regions in the laser interference pattern) of the photopatterned films. We suggest that such a nonuniformity and unusual plastic behavior are related to the variable material properties “imprinted” by the interference pattern.     

3D Micro‐ and Nanostructures via Interference Lithography

Interference lithography (IL) holds the promise of fabricating large‐area, defect‐free 3D structures on the submicrometer scale both rapidly and cheaply. A stationary spatial variation of intensity is created by the interference of two or more beams of light. The pattern that emerges out of the intensity distribution is transferred to a light sensitive medium, such as a photoresist, and after development yields a 3D bicontinuous photoresist/air structure. Importantly, by a proper choice of beam parameters one can control the geometrical elements and volume fraction of the structures. This article provides an overview of the fabrication of 3D structures via IL (e.g., the formation of interference patterns, their dependence on beam parameters and several requirements for the photoresist) and highlights some of our recent efforts in the applications of these 3D structures in photonic crystals, phononic crystals and as microframes, and for the synthesis of highly non spherical polymer particles. Our discussion concludes with perspectives on the future directions in which this technique could be pursued.     

Patterning Spherical Surfaces at the Two‐Hundred‐Nanometer Scale Using Soft Lithography

Two soft lithographic techniques—topographically directed photolithography (TOP) and near‐field contact‐mode photolithography—have been used to pattern spherical surfaces with features as small as 175 nm. Each technique has the ability to pattern more than a 60° arc of a spherical surface, albeit with distortions at the edge. Use as an optical polarizer demonstrates an application of these types of patterned surface.     

Bioinspired Fabrication of Superhydrophobic Graphene Films by Two‐Beam Laser Interference

Reported here is a bioinspired fabrication of superhydrophobic graphene surfaces by means of two‐beam laser interference (TBLI) treatment of graphene oxide (GO) films. Microscale grating‐like structures with tunable periods and additional nanoscale roughness are readily created on graphene films due to laser induced ablation effect. Synchronously, abundant hydrophilic oxygen‐containing groups (OCGs) on GO sheets can be drastically removed after TBLI treatment, which lower its surface energy significantly. The synergistic effect of micro‐nanostructuring and the OCGs removal endows the resultant graphene films with unique superhydrophobicity. Additionally, dual TBLI treatment with 90° rotation is implemented to fabricate superhydrophobic graphene films with two‐dimensional grating‐like structures that can effectively avoid the anisotropic hydrophobicity originated from the grooved structures. Moreover, the superhydrophobic graphene films become conductive due to the laser reduction effect. Unique optical characteristics including transmission diffraction and brilliant structural color are also observed due to the presence of periodic microstructures. As a mask‐free, chemical‐free, and cost‐effective method, the TBLI processing of GO may open up a new way to biomimetic graphene surfaces, and thus hold great promise for the development of novel graphene‐based microdevices.     

Three‐Dimensional SiCN Ceramic Microstructures via Nano‐Stereolithography of Inorganic Polymer Photoresists

We report a newly synthesized inorganic polymer photoresist with a high ceramic yield by the functionalization of polyvinylsilazane (KiON VL20) with 2‐isocyanatoethyl methacrylate via linkage or insertion reaction routes. The chemistry of the synthesis and the pyrolytic conversion as well as the mechanical evaluation were investigated by using various analytical instruments. We show for the first time that this photosensitive resin is a novel precursor for the fabrication of complex 3D SiCN ceramic microstructures with a 210 nm resolution via a two‐photon absorbed crosslinking process and subsequent pyrolysis at 600 °C under a nitrogen atmosphere. Moreover, the dimensional deformation during pyrolysis was significantly reduced by adding silica nanoparticles as a filler. In particular, the ceramic microstructures containing 40 wt % silica nanoparticles exhibited a relatively isotropic shrinkage owing to its sliding free from the substrate during pyrolysis.     

Laser Interference Lithography for the Nanofabrication of Stimuli‐Responsive Bragg Stacks

Dynamic structural coloration in Tmesisternus isabellae beetle elytra is a unique example of Bragg stack‐based wavelength tuning in response to external stimuli. The underlying principles could guide the design of quantitative optical stimuli‐responsive polymers. Existing nanofabrication techniques to create such materials are costly, time‐consuming, and require high expertise. This study reports a nanofabrication method to produce slanted Bragg stack structures in poly(acrylamide‐co‐poly(ethylene glycol) diacrylate) hydrogel films by combining laser interference lithography and silver halide chemistry in a cost‐effective and rapid process (≈10 min). The Bragg stacks consist of silver bromide nanocrystal multilayers having a lattice spacing of ≈200 nm. Upon broadband light illumination, the Bragg stacks diffract a narrow‐band peak at 520 nm at ≈10° with respect to the normal incidence. The lattice spacing of the hydrogel films can be modulated by external stimuli to shift the Bragg peak for dynamic quantitative measurements. To demonstrate the utility of this method, the Bragg stacks are functionalized with phenylboronic acid molecules. Bragg peak shift analysis allows reversible glucose sensing within a physiological dynamic range (0.0–20.0 mmol L^?1) having a sensitivity of 0.2 mmol L^?1. The developed Bragg stacks may have application in portable, wearable, and implantable real‐time medical diagnostics at point‐of‐care settings.     

Solution‐Processed Full‐Color Polymer Organic Light‐Emitting Diode Displays Fabricated by Direct Photolithography

The first full‐color polymer organic light‐emitting diode (OLED) display is reported, fabricated by a direct photolithography process, that is, a process that allows direct structuring of the electroluminescent layer of the OLED by exposure to UV light. The required photosensitivity is introduced by attaching oxetane side groups to the backbone of red‐, green‐, and blue‐light‐emitting polymers. This allows for the use of photolithography to selectively crosslink thin films of these polymers. Hence the solution‐based process requires neither an additional etching step, as is the case for conventional photoresist lithography, nor does it rely on the use of prestructured substrates, which are required if ink‐jet printing is used to pixilate the emissive layer. The process allows for low‐cost display fabrication without sacrificing resolution: Structures with features in the range of 2 μm are obtained by patterning the emitting polymers via UV illumination through an ultrafine shadow mask. Compared to state‐of‐the‐art fluorescent OLEDs, the display prototype (pixel size 200 μm × 600 μm) presented here shows very good efficiency as well as good color saturation for all three colors. The application in solid‐state lighting is also possible: Pure white light [Commision Internationale de l'Éclairage (CIE) values of 0.33, 0.33 and color rendering index (CRI) of 76] is obtained at an efficiency of 5 cd A^–1 by mixing the three colors in the appropriate ratio. For further enhancement of the device efficiency, an additional hole‐transport layer (HTL), which is also photo‐crosslinkable and therefore suitable to fabricate multilayer devices from solution, is embedded between the anode and the electroluminescent layer.     

Spontaneous Formation of Ordered Lateral Patterns in Polymer Thin‐Film Structures

The understanding of the lateral morphology stability of thin polymer devices is of fundamental importance. In this work, the lateral morphology in a model system consisting of thin polymer films capped with thin metal layers on a Si substrate is investigated. When the model system is heated above a critical temperature, a characteristic surface topographic structure is observed that has a well‐defined periodicity but random orientation. It is shown that the minimum temperature, T_min, required for the surface pattern to be observed decreases with increasing polymer‐film thickness. Increasing either the metal‐ or polymer‐layer thickness increases the characteristic wavelength of the topography. It is believed that the dominating driving force for the surface corrugated‐pattern formation is the thermal‐expansion‐coefficient mismatch of the capping layer and the substrate. A theoretical model based on local bending of a thin, stiff surface film on a thin, elastic medium is used to provide a quantitative analysis of the surface morphology. The calculated minimum temperature required for the surface morphology and the periodicity of the surface patterns to form are in strong agreement with the experimental results. By contrast, systems with prefabricated topographic patterns within any of the three layers (polymer, metal, substrate) produce highly anisotropic surface topographies aligned perpendicular to the prefabricated topographic structure. It is also found that, in a model system with pre‐patterned polymer films, a much higher critical temperature is required for the surface morphology to be observed. The changes in apparent stability and morphological orientation in the pre‐patterned systems can be understood as a result of the anisotropic release of the lateral surface stress during the heat treatment.     

Fabrication of Biofunctionalized Quasi‐Three‐Dimensional Microstructures of a Nonfouling Comb Polymer Using Soft Lithography

This paper describes a simple set of patterning methods that are applicable to diverse substrates and allow the routine and rapid fabrication of protein patterns embedded within a background that consists of quasi‐three‐dimensional microstructures of a cell‐resistant polymer. The ensemble of methods reported here utilizes three components to create topographically nonfouling polymeric structures that present cell‐adhesive protein patterns in the regions between the microstructures: the first component is an amphiphilic comb polymer that is comprised of a methyl methacrylate backbone and pendant oligo(ethylene glycol) moieties along the side chain, physically deposited films of which are protein‐ and cell‐resistant. The second component of the fabrication methodology involves the use of different variants of soft lithography, such as microcontact printing to create nonfouling topographical features of the comb polymer that demarcate cell‐adhesive regions of the third component: a cell‐adhesive extracellular protein or peptide. The ensemble of methods reported in this paper was used to fabricate quasi‐three‐dimensional patterns that present topographical and biochemical cues on a variety of substrates, and was shown to successfully maintain cellular patterns for up to two months in serum‐containing medium. We believe that this, and other such methods under development that allow independent and systematic control of chemistry, topography and substrate compliance will provide versatile “test‐beds” for fundamental studies in cell biology as well as allow the discovery of rational design principles for the development of biomaterials and tissue‐engineering scaffolds.     

Heterojunction Ambipolar Organic Transistors Fabricated by a Two‐Step Vacuum‐Deposition Process

Ambipolar organic field‐effect transistors (OFETs) are produced, based on organic heterojunctions fabricated by a two‐step vacuum‐deposition process. Copper phthalocyanine (CuPc) deposited at a high temperature (250 °C) acts as the first (p‐type component) layer, and hexadecafluorophthalocyaninatocopper (F₁₆CuPc) deposited at room temperature (25 °C) acts as the second (n‐type component) layer. A heterojunction with an interpenetrating network is obtained as the active layer for the OFETs. These heterojunction devices display significant ambipolar charge transport with symmetric electron and hole mobilities of the order of 10^–4 cm² V^–1 s^–1 in air. Conductive channels are at the interface between the F₁₆CuPc and CuPc domains in the interpenetrating networks. Electrons are transported in the F₁₆CuPc regions, and holes in the CuPc regions. The molecular arrangement in the heterojunction is well ordered, resulting in a balance of the two carrier densities responsible for the ambipolar electrical characteristics. The thin‐film morphology of the organic heterojunction with its interpenetrating network structure can be controlled well by the vacuum‐deposition process. The structure of interpenetrating networks is similar to that of the bulk heterojunction used in organic photovoltaic cells, therefore, it may be helpful in understanding the process of charge collection in organic photovoltaic cells.     

Enhanced Electro‐Optic Behavior for Shaped Polymer Cholesteric Liquid‐Crystal Flakes Made Using Soft Lithography

Polymer cholesteric liquid‐crystal (PCLC) flakes were investigated for their electro‐optical behavior under an applied alternating‐current field. Shaped flakes, fabricated using soft lithography and suspended in dielectric‐fluid‐filled cells, reoriented more uniformly than randomly shaped flakes made by fracturing of PCLC films. Extensive characterization found shaped flakes to be smooth and uniform in size, shape, and thickness. Reorientation in applied fields as low as tens of mV_rms μm^–1 was fastest for flakes with lateral aspect ratios greater than 1:1, confirming theoretical predictions based on Maxwell–Wagner polarization. Brilliant reflective colors and inherent polarization make shaped PCLC flakes of interest for particle displays.     

Soft‐Contact Optical Lithography Using Transparent Elastomeric Stamps and Application to Nanopatterned Organic Light‐Emitting Devices

Conventional photolithography uses rigid photomasks of fused quartz and high‐purity silica glass plates covered with patterned microstructures of an opaque material. We introduce new, transparent, elastomeric molds (or stamps) of poly(dimethylsiloxane) (PDMS) that can be employed as photomasks to produce the same resist pattern as the pattern of the recessed (or non‐contact) regions of the stamps, in contrast to other reports in the literature^[1] of using PDMS masks to generate edge patterns. The exposure dose of the non‐contact regions with the photoresist through the PDMS is lower than that of the contact regions. Therefore, we employ a difference in the effective exposure dose between the contact and the non‐contact regions through the PDMS stamp to generate the same pattern as the PDMS photomask. The photomasking capability of the PDMS stamps, which is similar to rigid photomasks in conventional photolithography, widens the application boundaries of soft‐contact optical lithography and makes the photolithography process and equipment very simple. This soft‐contact optical lithography process can be widely used to perform photolithography on flexible substrates, avoiding metal or resist cracks, as it uses soft, conformable, intimate contact with the photoresist without any external pressure. To this end, we demonstrate soft‐contact optical lithography on a gold‐coated PDMS substrate and utilized the patterned Au/PDMS substrate with feature sizes into the nanometer regime as a top electrode in organic light‐emitting diodes that are formed by soft‐contact lamination.     

Cover Picture: Three‐Dimensional SiCN Ceramic Microstructures via Nano‐Stereolithography of Inorganic Polymer Photoresists (Adv. Funct. Mater. 9/2006)

An inorganic polymer photoresist was successfully synthesized for the first time with high photosensitivity and a high ceramic yield. Furthermore, it is shown by Kim, Yang, and co‐workers on p. 1235 that a nano‐stereolithography process linked with careful pyrolysis of this resin paves the way for the fabrication of 2D and 3D ceramic microstructures (examples of which are shown on the cover) with high spatial resolution. We report a newly synthesized inorganic polymer photoresist with a high ceramic yield by the functionalization of polyvinylsilazane (KiON VL20) with 2‐isocyanatoethyl methacrylate via linkage or insertion reaction routes. The chemistry of the synthesis and the pyrolytic conversion as well as the mechanical evaluation were investigated by using various analytical instruments. We show for the first time that this photosensitive resin is a novel precursor for the fabrication of complex 3D SiCN ceramic microstructures with a 210 nm resolution via a two‐photon absorbed crosslinking process and subsequent pyrolysis at 600 °C under a nitrogen atmosphere. Moreover, the dimensional deformation during pyrolysis was significantly reduced by adding silica nanoparticles as a filler. In particular, the ceramic microstructures containing 40 wt % silica nanoparticles exhibited a relatively isotropic shrinkage owing to its sliding free from the substrate during pyrolysis.     

Pulsed Laser Deposition of Photoresponsive Two‐Dimensional GaSe Nanosheet Networks

Synthesis of functional metal chalcogenide (GaSe) nanosheet networks by stoichiometric transfer of laser‐vaporized material from bulk GaSe targets is presented. Uniform coverage of interconnected, crystalline, and photoresponsive GaSe nanosheets in both in‐plane and out‐of‐plane orientations are achieved under different ablation conditions. The propagation of the laser‐vaporized material is characterized by in situ ICCD‐imaging. High (1 Torr) Ar background gas pressure is found to be crucial for the stoichiometric growth of GaSe nanosheet networks. Individual 1–3 layer GaSe triangular nanosheets of ≈200 nm domain size are formed within 30 laser pulses, coalescing to form nanosheet networks in as few as 100 laser pulses. The thickness of the deposited networks increases linearly with pulse number, adding layers in a two‐dimensional (2D) growth mode. GaSe nanosheet networks show p‐type semiconducting characteristics with mobilities reaching as high as 0.1 cm²V^?1s^?1. Spectrally‐resolved photoresponsivities and external quantum efficiencies range from 0.4 AW^?1 and 100% at 700 nm, to 1.4 AW^?1 and 600% at 240 nm, respectively. Pulsed laser deposition under these conditions appears to provide a versatile and rapid approach to stoichiometrically transfer and deposit functional networks of 2D nanosheets with digital thickness control and uniformity for a variety of applications.     

Electrically Directed On‐Chip Reversible Patterning of Two‐Dimensional Tunable Colloidal Structures

In this work, we report a versatile approach to two‐dimensional colloidal patterning based on the lateral assembly of colloidal particles by an alternating electric field (AEF). Under the AEF, the lithographically templated electrodes provide an effective way to reversibly and rapidly assemble colloidal particles into some desirable patterns. By controlling the AEF and the electrode pattern geometry, various colloidal patterns with tunable lattice spacing and even with binary lattice spacing have been formed. Particularly, we demonstrate that well‐defined linear defects can be embedded inside the colloidal crystals, whereas the unwanted existing defects can be controllably relaxed by this patterning process. This novel patterning technique is amenable to both large scale on‐chip patterning and micro‐structural control with single‐particle resolution on a time scale of seconds. Furthermore, it introduces a new class of colloidal structures with the properties that can be finely tuned, reversibly switched, or permanently fixed, opening a new way for the engineering of novel materials and devices at micro levels.     

Pattern‐Transfer Fidelity in Soft Lithography: The Role of Pattern Density and Aspect Ratio

Using high‐aspect‐ratio nanostructures fabricated via two‐photon laser‐scanning lithography, we examine the deformation of elastomeric stamps used in soft nanolithography and the fidelity of patterns and replicas made using these stamps. Two‐photon laser‐scanning lithography enables us to systematically regulate the aspect ratio and pattern density of the nanostructures by varying laser‐scanning parameters such as the intensity of the laser beam, the scanning speed, the focal depth inside the resist, and the scanning‐line spacing. Two commercially available stamp/mold materials with different moduli have been investigated. We find that the pattern‐transfer fidelity is strongly affected by the pattern density. In addition, we demonstrate that true three‐dimensional structures can be successfully replicated because of the flexible nature of elastomeric poly(dimethylsiloxane).     

Transforming One‐Dimensional Nanowalls to Long‐Range Ordered Two‐Dimensional Nanowaves: Exploiting Buckling Instability and Nanofibers Effect in Holographic Lithography

Two‐dimensional nanowaves with long‐range order are fabricated by exploiting swelling‐induced buckling of one‐dimensional (1D) nanowalls with nanofibers formed in‐between during holographic lithography of the negative‐tone photoresist SU‐8. The 1D film goes through a constrained swelling in the development stage, and becomes buckled above the critical threshold. The degree of lateral undulation can be controlled by tuning the pattern aspect ratio (height/width) and exposure dosage. At a high aspect ratio (e.g., 6) and a high exposure dosage, nanofibers (30–50 nm in diameter) are formed between the nanowalls as a result of overlapping of low crosslinking density regions. By comparing experimental results with finite‐element analysis, the buckling mechanism is investigated, which confirms that the nanofibers prevent the deformed nanowalls from recovery to their original state, thus, leading to long‐range ordered two‐dimensional (2D) wavy structures. The film with nanowaves show weaker reflecting color under an ambient light and lower transmittance compared to the straight nanowalls. Using double exposure through a photomask, patterns consisting of both nanowaves and nanowalls for optical display are created.     

Morphology‐Controlled Growth of Large‐Area Two‐Dimensional Ordered Pore Arrays

A solution‐dipping template strategy for large‐area synthesis of morphology‐controlled, ordered pore arrays is reported. The morphology of the pore array can easily be controlled by concentration of the precursor solution and treatment conditions. With decrease of the concentration from a high level to a very low level nanostructured complex (pore–hole, and pore–particle) arrays, through‐pore arrays, and even ring arrays can, in turn, be obtained. The pore size is adjustable over a large range by changing the diameter of the template's latex spheres. This synthesis route is universal and can be used for various metals, semiconductors and compounds on any substrate. Such structures may be useful in applications such as energy storage or conversion, especially in integrated next‐generation nanophotonics devices, and biomolecular labeling and identification.     

Hierarchical Self‐Organization of Nanomaterials into Two‐Dimensional Arrays Using Functional Polymer Scaffold

Fabrication of two and three‐dimensional nanostructures requires the development of new methodologies for the assembly of molecular/macromolecular objects on substrates in predetermined arrangements. Templated self‐assembly approach is a powerful strategy for the creation of materials from assembly of molecular components or nanoparticles. The present study describes the development of a facile, template directed self‐assembly of (metal/organic) nanomaterials into periodic micro‐ and nanostructures. The positioning and the organization of nanomaterials into spatially well‐defined arrays were achieved using an amphiphilic conjugated polymer‐aided, self‐organization process. Arrays of honeycomb patterns formed from conjugated C₁₂PPPOH film with homogenous distribution of metal/organic nanomaterials. Our approach offers a straightforward and inexpensive method of preparation for hybrid thin films without environmentally controlled chambers or sophisticated instruments as compared to multistep micro‐fabrication techniques.     

Near‐Field Lithography by Two‐Photon Induced Photocleavage of Organic Monolayers

We prove that the enhanced electromagnetic near‐field around metallic nanostructures can be used for localized two‐photon induced activation of surfaces, obtaining a defined chemical pattern with nanometric resolution. Gold nanoparticles (Au‐NP) are deposited on glass slides that were modified with a polysiloxane layer containing a nitroveratrylcarbonyl (NVoc) photoremovable group. Upon illumination with a femtosecond laser, the NVoc entity is removed. Due to the electromagnetic field enhancement of the nanoparticles, the threshold of this process is lowered in the nm‐scale vicinity of the metal structures. Upon cleavage, an amine functional group is released, which can be used to site‐selectively bind species with complementary chemical functionality on the surface. This method can be utilized for sub‐wavelength chemical structuring.