Direct Transcription of Two‐Dimensional Colloidal Crystal Arrays into Three‐Dimensional Photonic Crystals

A simple protocol for the fabrication of three‐dimensional (3D) photonic crystals in silicon is presented. Surface structuring by nanosphere lithography is merged with a novel silicon etching method to fabricate ordered 3D architectures. The SPRIE method, sequential passivation reactive ion etching, is a one‐step processing protocol relying on sequential passivation and reactive ion etching reactions using C₄F₈ and SF₆ plasma chemistries. The diffusion of fresh reactants and etch product species inside the etched channels is found to play an important role affecting the structural uniformity of the designed structures and the etch rate drift is corrected by adjusting the reaction times. High quality photonic crystals are thus obtained by adding the third dimension to the two‐dimensional (2D) colloidal crystal assemblies through SPRIE. Careful adjustments of both mask design and lateral etch extent balance allow the implementation of even more complex functionalities including photonic crystal slabs and precise defect engineering. 3D photonic crystal lattices exhibiting optical stop‐bands in the infrared spectral region are demonstrated, proving the potential of SPRIE for fast, simple, and large‐scale fabrication of photonic structures.     

Patterning Colloidal Crystals and Nanostructure Arrays by Soft Lithography

As one of the most robust and versatile routes to fabricate ordered micro‐ and nanostructures, soft lithography has been extensively applied to pattern a variety of molecules, polymers, biomolecules, and nanomaterials. This paper provides an overview on recent developments employing soft lithography methods to pattern colloidal crystals and related nanostructure arrays. Lift‐up soft lithography and modified microcontact printing methods are applied to fabricate patterned and non‐close‐packed colloidal crystals with controllable lattice spacing and lattice structure. Combining selective etching, imprinting, and micromolding methods, these colloidal crystal arrays can be employed as templates for fabrication of nanostructure arrays. Realization of all these processes is favored by the solvent swelling, elasticity, thermodecomposition, and thermoplastic characteristics of polymer materials. Applications of these colloidal crystals and nanostructure arrays have also been explored, such as biomimetic antireflective surfaces, superhydrophobic coatings, surface‐enhanced Raman spectroscopy substrates, and so on.     

Artificial Defect Engineering in Three‐Dimensional Colloidal Photonic Crystals

Artificial defect engineering in 3D colloidal photonic crystals is of paramount importance in terms of device applications. Over the past few years, we have carried out a great deal of research on introducing artificial defects, including point, line, and planar defects, in 3D colloidal photonic crystals by using “bottom‐up” self‐assembly in combination with “top‐down” micromachining techniques. In this Feature Article, we summarize our research results regarding the engineering of artificial defects in self‐assembled 3D photonic crystals, along with other important research breakthroughs in the literature. The significant advancements in the engineering of defects as reviewed here together with the encouraging reports on the fabrication of perfect colloidal crystals without unwanted defects will collectively lead to technological applications of self‐assembled 3D photonic crystals in the near future.     

Rapid Fabrication of Two‐ and Three‐Dimensional Colloidal Crystal Films via Confined Convective Assembly

We have developed a self‐assembly method for fabricating well‐ordered two‐dimensional (2D) and three‐dimensional (3D) colloidal crystal films. With a minute amount of a polystyrene colloidal suspension and without any special equipment, the proposed method can be used to rapidly deposit high‐quality colloidal crystal films over a large surface area. By controlling the lift‐up rate of the substrate, we modulate the meniscus thinning rate, which determines whether the colloidal particles are assembled into two or three dimensions. The proposed method can be used to fabricate not only monolayered colloidal crystals with colloidal particles of various sizes, but also multilayered colloidal crystals. In addition, the method enables us to fabricate binary colloidal crystals by consecutively depositing large and small particles.     

Large‐Scale Fabrication of Three‐Dimensional Surface Patterns Using Template‐Defined Electrochemical Deposition

A new strategy to achieve large‐scale, three‐dimensional (3D) micro‐ and nanostructured surface patterns through selective electrochemical growth on monolayer colloidal crystal (MCC) templates is reported. This method can effectively create large‐area (>1 cm²), 3D surface patterns with well‐defined structures in a cost‐effective and time‐saving manner (<30 min). A variety of 3D surface patterns, including semishells, Janus particles, microcups, and mushroom‐like clusters, is generated. Most importantly, our method can be used to prepare surface patterns with prescribed compositions, such as metals, metal oxides, organic materials, or composites (e.g., metal/metal oxide, metal/polymer). The 3D surface patterns produced by our method can be valuable in a wide range of applications, such as biosensing, data storage, and plasmonics. In a proof‐of‐concept study, we investigated, both experimentally and theoretically, the surface‐enhanced Raman scattering (SERS) performance of the fabricated silver 3D semishell arrays.     

Three‐Dimensionally Ordered Macroporous Polymeric Materials by Colloidal Crystal Templating for Reversible CO2 Capture

The design and preparation of porous materials with controlled structures and functionalities is crucial to a variety of absorption‐ or separation‐relevant applications, including CO₂ capture. Here, novel functional polymeric materials with three‐dimensionally ordered macroporous (3DOM) structures are prepared by using colloidal crystals as templates using relatively simple, rapid, and inexpensive approaches. These ordered structures are used for the reversible CO₂ capture from ambient air by humidity swing. Typically, the colloidal crystal template is synthesized from polymer latex particles of poly(methyl methacrylate) (PMMA) or polystyrene (PS). To maintain the functionality of the material, it is important to prevent the porous structure collapsing, which can occur by the hydrolysis of the ester bonds in conventional crosslinkers under basic conditions. This hydrolysis can be prevented by using a water‐soluble crosslinker containing two quaternary ammonium moieties, which can be used to prepare stable porous crosslinked polymers with the monomer (vinylbenzyl)trimethylammonium chloride (VBTMACl) and using a PMMA‐based colloidal crystal template. The hydroxide‐containing monomer and dicationic crosslinker are synthesized from their chloride precursors, avoiding the ion‐exchange step which causes shrinkage of the pores. An analysis of different methods for infiltrating the monomer solution into the colloidal crystal template shows that infiltration using capillary forces leads to fewer defects than infiltration under a partial vacuum. In addition, functional macroporous films with micrometer thickness are prepared from a template of PS‐based colloidal crystals in a thin film. In general, the colloidal crystal templated materials showed improved CO₂ absorption/desorption rates and swing sizes compared to a commercially available material with similar functional groups. This work could easily be extended to create a new generation of ordered macroporous polymeric materials with tunable functionalities for other applications.     

Exploiting Nanoroughness on Holographically Patterned Three‐Dimensional Photonic Crystals

The fabrication of three‐dimensional (3D) diamond photonic crystals with controllable nanoroughness (≤120 nm) on the surface from epoxy‐functionalized cyclohexyl polyhedral oligomeric silsesquioxanes (POSS) is reported. The nanoroughness is generated on the 3D network due to microphase separation of the polymer chain segments in a nonsolvent during the rinsing step in holographic lithography process. The degree of roughness can be tuned by the crosslinking density of the polymer network, which is dependent on the loading of photoacid generators, the exposure dosage, and the choice of developer and rinsing solvent. Because the nanoroughness size is small, it does not affect the photonic band gap position of the photonic crystal in the infrared region. The combination of periodic microstructure and nanoroughness, however, offers new opportunities to realize superhydrophobicity and enhanced dye adsorption in addition to the photon management in the 3D photonic crystal.     

Highly Ordered Nanometer‐Scale Chemical and Protein Patterns by Binary Colloidal Crystal Lithography Combined with Plasma Polymerization

Surfaces with micro‐ and nanometer‐scale patterns have many potential applications, particularly in lifescience. This article reports on a versatile, straightforward, and inexpensive approach for the creation of chemical patterns using fabricated binary colloid crystals, consisting of small and large particles, as masks for the deposition of an amino‐functionalised ultrathin film by plasma polymerization. After removal of the binary colloidal mask, the characterization techniques [scanning electron microscopy (SEM) and atomic force microscopy (AFM)] reveal a surface contrast that depicts an ability of the small particles to allow diffusion of the plasma to the substrate. A plasma‐polymer film is created under the small particles and the region of substrate in direct contact with the large particle remains uncoated. Numerous types of patterns and feature heights can be produced with good fidelity over areas of several cm² by appropriate tuning of the binary colloid crystal mask morphology and the plasma‐polymer deposition time. Finally, the amine groups of the patterned surface are used for covalent grafting poly(ethylene glycol) propionaldehyde (PEG‐PALD) by reductive amination under conditions of reduced solubility to produce a patterned surface for directed adsorption of protein. AFM investigations show that the proteins are preferentially attached to the nanometer‐scale regions of the pattern without PEG‐PALD.     

Layer‐by‐Layer Growth of Multicomponent Colloidal Crystals Over Large Areas

Self‐assembly of different sized colloidal particles into multicomponent crystals results in novel material properties compared to the properties of the individual components alone. The formation of binary and, for the first time, ternary colloidal crystals through a simple and inexpensive confined‐area evaporation‐induced layer‐by‐layer (LBL) assembly method is reported. The proposed method produces high quality multicomponent colloidal crystal films over a broad range of particle size‐ratios and large surface areas (cm²) from silica/polystyrene colloidal suspensions of low concentration. By adjusting the size‐ratio and concentration of the colloidal particles, complex crystals of tunable stoichiometries are fabricated and their structural characteristics are further confirmed with reported crystal analogues. In addition, complex structures form as a result of the interplay of the template layer effect, the surface forces exerted by the meniscus of the drying liquid, the space filling principle, and entropic forces. Thus, this LBL approach is a versatile way to grow colloidal crystals with binary, ternary, or more complex structures.     

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.     

Fabrication and Characterization of Two‐Photon Polymerized Features in Colloidal Crystals

The fabrication and characterization of two‐photon polymerized features written within and outside of colloidal crystals is presented. Two‐photon polymerization (TPP) response diagrams are introduced and developed to map the polymerization and damage thresholds for features written via modulated beam rastering. The use of tris[4‐(7‐benzothiazol‐2‐yl‐9,9‐diethylfluoren‐2‐yl)phenyl]amine (AF‐350) as an initiator for TPP is demonstrated for the first time and TPP response diagrams illustrate the polymerization window. These diagrams also demonstrate that the polymerization behavior within and outside of colloidal crystals is similar and electron microscopy reveals nearly identical resolution. Fluorescence confocal microscopy further enables visualization of non‐self‐supporting, three‐dimensional TPP features within self‐assembled photonic crystals. Finally, microspot spectroscopy is collected from a two‐photon feature written within a colloidal crystal and this is compared with simulation.     

Wafer‐Scale Fabrication of Ordered Binary Colloidal Monolayers with Adjustable Stoichiometries

Colloidal monolayers with high order and increased complexity beyond plain hexagonal packing geometries are useful for 2D templating of surface nanostructures and lithographic applications. Here, binary colloidal monolayers featuring a close‐packed monolayer of large spheres (L) with a superlattice of small particles (S) are prepared in a single step using a Langmuir trough. Adjustment of the stoichiometry of the two particle types at the air–water interface leads to a high degree of control over the occupation of the interstitial sites in the close‐packed layer of large spheres by the small colloids. Thus, large areas of binary 2D crystals with LS₂, LS₆, and LS₉ structures are fabricated in a controlled way. The process allows the formation of binary crystals over a wide range of particle size ratios from 0.19 to 0.40. The pH value of the subphase can be used to enhance the crystallization process by changing the contact angle of the particles at the interface. An interfacial polymerization of butyl cyanoacrylate is used to directly image the contact angle of the colloids at the interface. Transfer to solid substrates is achieved by a surface lowering technique. A variety of substrates with arbitrary topographies can thus be decorated with colloidal monolayers. Applied to a lithographic process, such monolayer architectures allow the generation of complex patterns, not accessible with conventional close‐packed monolayers.     

Direct Writing of Three‐Dimensional Polymer Scaffolds Using Colloidal Gels

Polymer scaffolds intended to provide a substrate for cell attachment and proliferation benefit if the geometric architecture, mechanical properties, and surface chemistry are controllable within the range applicable for the target tissue. Such scaffolds may be made bioinductive through the inclusion of surface proteins and release of growth factors. Furthermore, the polymer support may be formed of biodegradable polymers for use as tissue‐engineering scaffolds. In this study, a new scaffold‐fabrication technique based on the direct writing of polymer colloidal‐gel‐based inks is described. The colloidal approach allows for the modular design of inks where the structure and composition of the colloidal particles, surface adsorbed molecules, and dissolved species may be easily controlled. Polyacrylate latex particles are formulated into colloidal gels by using a thermoreversible gel‐forming poly(ethylene oxide)–poly(propylene oxide) block‐copolymer adsorbed layer. The resulting colloidal gels are laced with the model protein bovine serum albumin (BSA) either dissolved in the solvent phase of the ink or dispersed in chitosan nanoparticles as a second colloid. Ink development and rheological characterization are presented along with demonstration of assembly of mesoporous scaffolds. After assembly and drying of the scaffold structure, the drug‐release kinetics are measured upon re‐exposure to an aqueous environment. Protein activity appears to be unaffected by the processing route of these scaffolds. Finally, the assembly of heterogeneous scaffolds is demonstrated to illustrate the possibilities for staged or heterogeneous drug release. This approach to scaffold fabrication offers a new route for scaffold assembly from water‐insoluble polymers while allowing the inclusion of sensitive biomolecules without risk of denaturation.     

Fabrication of Stable Metallic Patterns Embedded in Poly(dimethylsiloxane) and Model Applications in Non‐Planar Electronic and Lab‐on‐a‐Chip Device Patterning

This article describes the fabrication of durable metallic patterns that are embedded in poly(dimethylsiloxane) (PDMS) and demonstrates their use in several representative applications. The method involves the transfer and subsequent embedding of micrometer‐scale gold (and other thin‐film material) patterns into PDMS via adhesion chemistries mediated by silane coupling agents. We demonstrate the process as a suitable method for patterning stable functional metallization structures on PDMS, ones with limiting feature sizes less than 5 μm, and their subsequent utilization as structures suitable for use in applications ranging from soft‐lithographic patterning, non‐planar electronics, and microfluidic (lab‐on‐a‐chip, LOC) analytical systems. We demonstrate specifically that metal patterns embedded in both planar and spherically curved PDMS substrates can be used as compliant contact photomasks for conventional photolithographic processes. The non‐planar photomask fabricated with this technique has the same surface shape as the substrate, and thus facilitates the registration of structures in multilevel devices. This quality was specifically tested in a model demonstration in which an array of one hundred metal oxide semiconductor field‐effect transistor (MOSFET) devices was fabricated on a spherically curved Si single‐crystalline lens. The most significant opportunities for the processes reported here, however, appear to reside in applications in analytical chemistry that exploit devices fabricated using the methods of soft lithography. Toward this end, we demonstrate durably bonded metal patterns on PDMS that are appropriate for use in microfluidic, microanalytical, and microelectromechanical systems. We describe a multilayer metal‐electrode fabrication scheme (multilaminate metal–insulator–metal (MIM) structures that substantially enhance performance and stability) and use it to enable the construction of PDMS LOC devices using electrochemical detection. A polymer‐based microelectrochemical analytical system, one incorporating an electrode array for cyclic voltammetry and a microfluidic system for the electrophoretic separation of dopamine and catechol with amperometric detection, is demonstrated.