PerspectiveMaterials Science

Oriented Assembly of Metamaterials

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Science  10 Jul 2009:
Vol. 325, Issue 5937, pp. 159-160
DOI: 10.1126/science.1174401

Regular assemblies of colloidal particles have many potential uses from self-assembled electronics to biosensors. Recent advances in particle self-assembly suggest that such assemblies may also provide a simple route to metamaterials at infrared and visible length scales. Such metamaterials may, for example, be used to create cloaking devices or light-based circuits based on manipulations of local optical electric fields rather than on the flow of electrons (1).

Metamaterials are periodically structured composites with unit cells smaller than the wavelength used to interrogate them (2). By tailoring the unit cells to create a wide range of responses, technologies that were once the realm of fantasy—from computing with light to invisibility cloaks and superlenses—become reality. Such materials are relatively easy to create for use at radio frequencies, where the subunits need only be a few millimeters in size. However, use at optical and infrared wavelengths requires the assembly of three-dimensional micrometer- and nanometer-scale structures. This task is extremely challenging, but recent studies of particle self-assembly point the way to metamaterials at relevant length scales.

Metamaterials contain inclusions deliberately embedded in host media. The size, shape, and electromagnetic properties of the inclusions, along with inclusion density, arrangement, and alignment, determine their effective properties in a given host. Negative index of refraction metamaterials have been demonstrated in the optical range, made by serial approaches based on lithography (3, 4). If, instead, metamaterials were designed to incorporate anisotropic nanoparticles through self-assembly, cumbersome lithographic approaches could be avoided.

An extensive library of anisotropic microparticles and nanoparticles now exists (5), allowing inclusion size and shape to be readily selected. Particles can be synthesized from many different materials, which can be selected for their electromagnetic properties. However, incorporating the particles as inclusions in self-assembled metamaterials requires techniques for assembly with control over particle orientation and spatial arrangement in periodic structures.

Convective assembly is a promising technique for creating close-packed assemblies of particles. The method is easy, inexpensive, and amenable to creating relatively large, defect-free periodically ordered structures of spherical particles. New assembly methods are also being developed to promote oriented assembly of anisotropically shaped particles in close-packed structures and to control deposition of particles in prescribed spatial locations on substrates, with potential for creating non–close-packed structures.We review31 a few advances that are compatible with convective assembly, which enables mass production by continuous printing methods. The further development of these approaches is key to establishing robust methods for oriented assembly of metamaterials (6) (see the figure).

In convective assembly, a substrate is immersed in a colloidal suspension. The solvent forms a three-phase contact line where solvent, substrate, and vapor meet. As solvent evaporates, colloidal particles collect at the contact line. When particles protrude through the liquid-vapor interface, they create excess surface area and therefore excess surface energy. This energy diminishes as particles approach, creating capillary attraction that draws particles together to form an ordered structure (the nucleus). Continued evaporation collects particles near the nucleus, where they assemble into the growing colloidal crystal (7). Colloidal crystals are also grown from evaporating drops of suspensions in what is termed “drop-casting.”

Metamaterial formed by convective assembly of anisotropic particles.

Preferred locations can be lithographically defined or imposed by patterned evaporation. Oriented colloidal crystal nuclei could form by shape-dependent capillary interactions near the contact line (A). Oriented aggregation can be dictated by hydrophobic interactions created by selective surface functionalization (B) or by selective surface roughening to orient depletion attraction in bulk (C).

CREDIT: ERIC LEWANDOWSKI/UNIVERSITY OF PENNSYLVANIA AND MONIRAJ GHOSH/JOHNS HOPKINS UNIVERSITY

Ming et al. have recently used these techniques to make close-packed assemblies of anisotropic particles (nanorods, polyhedra, nanocubes, and bipyramids) into highly ordered structures in three dimensions (8). However, more systematic studies will be required to identify the conditions that promote such oriented assembly. To do so, two fundamental issues need to be addressed.

First, the particle orientation in colloidal crystal nuclei must be controlled. Solutions to this problem will likely use shape-dependent capillary interactions. Nuclei form near the contact line when particles protrude through the interface; particle shape and surface energy strongly influence the interfacial deformation fields and can be used to create orientation-dependent capillary interactions (see the figure). At free liquid-vapor interfaces, capillary interactions have been used to promote end-to-end or side-to-side assembly of elliptical microparticles (9). To promote formation of oriented nuclei in convective assembly, oriented capillary assembly of anisotropically shaped particles protruding through interfaces near contact lines should be studied (see the figure, panel A).

Second, means to dictate oriented assembly of particles in solution need to be developed. A recent numerical study predicts that elongated nanoparticles should readily form oriented liquid crystalline phases driven by the trade-off between translational and orientational entropy (10). In two recent experimental studies, particle orientation has been tuned by tailoring surface properties.

Rycenga et al. (11) tailored the surface energies of silver nanocubes by using self-assembled monolayers to render certain faces hydrophobic. When dispersed in an aqueous phase, the silver nanocubes form different structures depending on how many nanocube faces were functionalized. This approach could be used to orient any anisotropic particle (see the figure, panel B). Zhao and Mason (12) exploited depletion attraction and tailored surface roughness. In mixed suspensions of nanoscale and larger colloidal particles, the smaller particles are excluded from regions between the larger particles (see the figure, panel C). The resulting osmotic-pressure gradients push the larger particles together. Surface roughness increases this attraction by enhancing nanoparticle exclusion. Using these ideas, the authors assembled anisotropic particles with selected rough faces into highly ordered close-packed phases.

Methods have also been developed to deposit microscale and nanoscale particles in desired locations from evaporating suspensions to form non-close-packed structures. These will be useful for metamaterials with unit cells larger than the inclusion. For example, Aizenberg et al. have used lithographically defined attractive and repulsive regions on the substrate (13). When a drop of colloidal suspension evaporates on these surfaces, particles deposit on the attractive regions, and are drawn into ordered structures by capillary attraction. In another technique (14, 15), particles near the contact line are swept into grooves cut into solid substrates as the contact line recedes. Within the grooves, the particles assemble into aggregates as solvent evaporates.

In these methods, spacing between the aggregates is defined by the spacing between the lithographically defined attractive regions or between the grooves on the substrate (see the figure). A particularly novel method reported by Harris et al. exploits the dependence of surface tension on temperature and evaporative cooling of the interface. By using a mask to pattern evaporation, patterned surface tension gradients were created. These gradients drove patterned flows within the drop. Particles were convected and deposited in patterns dictated by the flow (16).

These advances in colloidal science suggest that oriented assembly of anisotropic materials with three-dimensional control over particle position and orientation will soon be feasible. Such approaches present important opportunities in metamaterials design in the infrared and optical regimes. The challenge now is to move from hit-or-miss assemblies of academic interest to the creation of technologically relevant devices that combine particle and patterned assembly via large-scale processes.

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