Colloidal lithography is a recently emerging field; the evolution of this simple technique is still in progress. Recent advances in this area have developed a variety of practical routes of colloidal lithography, which have great potential to replace, at least partially, complex and high-cost advanced lithographic techniques. This Review presents the state of the art of colloidal lithography and consists of three main parts, beginning with synthetic routes to monodisperse colloids and their self-assembly with low defect concentrations, which are used as lithographic masks. Then, we will introduce the modification of the colloidal masks using reactive ion etching (RIE), which produces a variety of nanoscopic features and multifaceted particles. Finally, a few prospective applications of colloidal lithography will be discussed.
Block copolymers, which consist of chemically distinct polymer blocks, exhibit a variety of self-assembled ordered nanophases such as spheres, cylinders, and lamellae depending on their compositions, volume fractions, and molecular weights.[1] These block copolymer self-assemblies have great potential as templates for fabricating functional devices with nanoscopic periodicities. [2] In particular, recent studies have demonstrated that block copolymers confined in two-and three-dimensional geometries self-assemble into ordered phase-separated domains that are observed in the bulk or anomalous microscopic phases such as helices and tori with improved directional order, [3][4][5][6][7][8][9][10][11][12][13][14] which is based on pioneering works on block copolymer self-assembly in one-dimensional confined geometries. [15][16][17][18][19] These previous studies have focused mainly on the commensurability between the characteristic length of the confining geometry and the block copolymer domain spacing. However, because methods for modifying the surfaces of such confining geometries have not been well-developed, many studies have not considered wall effects. [20][21][22] This is especially true of confining geometries with a mobile interfacial boundary, in which the dynamics of the interface may affect the shape of the confining geometry as well as the internal phase morphology. Here, we explored the interface-driven morphological evolution of a symmetric diblock copolymer of polystyreneblock-polybutadiene (PS-b-PB) confined in oil-in-water emulsion droplets. To control the surface preferences of the constituent PS and PB blocks at the emulsion interface, a mixture of two designed amphiphilic diblock copolymers, polystyrene-block-poly(ethylene oxide) (PS-b-PEO) and polybutadiene-block-poly(ethylene oxide) (PB-b-PEO), was used as a surfactant. In addition, we added PS homopolymer (hPS) in the emulsion phase to modify the nanoscopic features of the self-assembled morphologies. The emulsion drops with deformable interface-driven internal morphologies were solidified by evaporating the solvent, yielding blend particles of PS-b-PB and hPS with unique external shapes and internal morphologies (as illustrated schematically in Scheme 1). These colloidal particles with nanoscopic internal structures are potentially applicable to various particle-based technologies such as photonic bandgap materials, [23,24] conductive particles for anisotropic conductive films based on block copolymer/ metal nanoparticle composites, [25,26] porous particles, [27] optical actuation in microfluidic chips, [28] optochemical sensing devices, [29] and catalytic supports. [22,30] To the best of our knowledge, this is the first report on the structural evolution of a block copolymer driven by controlling the dynamics of the mobile interface and the commensurability of the block copolymer confined in emulsion droplets. In the present system, there are two compositional parameters: the volume fraction (F) of hPS out of the total volume of hPS and PS-b-...
A s dielectric structures with a submicrometer length scale can interact strongly with light, various remarkable optical responses can be designed and tailored depending on the types and parameters of their structures. Over the past few decades, the unusual optical properties of the periodic dielectric structures called photonic crystals have been investigated intensively [1]. Many research groups have endeavored to engineer the optical properties of photonic crystals, including photonic bandgaps, 'slow' photons, negative refraction and other properties, or to use them in practical applications. Two-dimensional (2D) structures, which are mostly prepared by conventional lithographic processes, were demonstrated initially, in which total internal refl ections were adopted for confi ning the light in a nonperiodic third direction, and their use has been investigated in some limited applications [2]. Th ree-dimensional (3D) structures have also been investigated intensively because of their complete photonic bandgaps in certain structures, a critical property for controlling light in 3D space. Research on such structures has been supported by the recent development of facile fabrication methods, including the selfassembly of simple monodisperse particles, also known as colloidal self-assembly [3], block copolymer self-assembly [4], the auto-cloning process [5] and holographic lithography [6]. Of these methods, colloidal self-assembly is the most promising for the low-cost production of 2D and 3D photonic crystals over large areas or with various shapes [7,8]. Schematic diagrams of the basic colloidal crystal structure along with inverse and short-range-ordered scattering structures for photonic applications are shown in Figure 1. Disordered dielectric structures of monodisperse particles called photonic glasses have begun to be investigated as another class of photonic nanostructures that can manifest some unusual optical phenomena such as random lasing, strong light localization and long-range intensity correlations. In this review article, we describe self-assembled colloidal photonic nanostructures in brief and summarize recent achievements in the fi eld of colloidal photonic nanostructures and their applications. Fabrication of photonic nanostructures by colloidal assembly Colloidal crystalsSince Vanderhoff 's serendipitous discovery of a synthetic method for preparing monodisperse polymer colloids [9], the method has been extended to the preparation of a variety of polymeric colloids and also to the processing of inorganic colloidal particles such as silica, titania and iron oxide. As long as particles are stable in liquid and their size distribution is suffi ciently narrow, they can be crystallized in a facecentered cubic (fcc) lattice by increasing their volume fraction through any concentration process, such as controlled evaporation, sedimentation or fi ltration. In general, the interparticle forces can be described by summing over the various potentials from diff erent origins, including intermolecular for...
Self-assembly of monodisperse colloidal particles into regular lattices has provided relatively simple and economical methods to prepare photonic crystals. The photonic stop band of colloidal crystals appears as opalescent structural colors, which are potentially useful for display devices, colorimetric sensors, and optical filters. However, colloidal crystals have low durability, and an undesired scattering of light makes the structures white and translucent. Moreover, micropatterning of colloidal crystals usually requires complex molding procedures, thereby limiting their practical applications. To overcome such shortcomings, we develop a pragmatic and amenable method to prepare colloidal photonic crystals with high optical transparency and physical rigidity using photocurable colloidal suspensions. The colloidal particles dispersed in a photocurable medium crystallized during capillary force-induced infiltration into a slab, and subsequent photopolymerization of the medium permanently solidifies the structures. Furthermore, conventional photolithography enables micropatterning of the crystal structures. The low index contrast between particles and matrix results in high transparency of the resultant composite structures and narrow reflection peaks, thereby enabling structural color mixing through the overlapping of distinct layers of the colloidal crystals. Multiple narrow peaks in the spectrum provide high selectivity in optical identification, thereby being potentially useful for security materials.
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