Fabrication of Planar Colloidal Clusters with Template-Assisted Interfacial Assembly

Abstract

The synthesis of nanoparticle clusters, also referred to as colloidal clusters or colloidal molecules, is being studied intensively as a model system for small molecule interactions as well as for the directed self-assembly of advanced materials. This paper describes a technique for the interfacial assembly of planar colloidal clusters using a combination of top-down lithographic surface modification and bottom-up Langmuir− Blodgett deposition. Micrometer sized polystyrene latex particles were deposited onto a chemically modified substrate from a decane−water interface with Langmuir−Blodgett deposition. The surface of the substrate contained hydrophilic domains of various size, spacing, and shape, while the remainder of the substrate was hydrophobic. Particles selectively deposited onto hydrophilic regions from the decane−water interface. The number of deposited particles depended on the size of each patch, thereby demonstrating that tuning cluster size is possible by engineering patch geometry. Following deposition, the clusters were permanently bonded with temperature annealing and then removed from the substrate via sonication. The permanently bonded planar colloidal clusters were stable in an aqueous environment and at a decane−water interface laden with isotropic colloidal particles. The method is a simple and fast way to synthesize colloidal clusters with few limitations on particle chemistry, composition, and shape. ■ INTRODUCTIONSubstantial effort has been invested in developing assembly techniques for the synthesis of colloidal clusters of varying shape.1−11 A colloidal cluster assembled from individual particles is often referred to as a "colloidal molecule", especially when aggregation is sufficiently controlled such that the cluster structure resembles molecular structure. For example, a dimer of identically sized spherical colloidal particles is analogous to a molecule of H 2 .4,7 Not only do colloidal clusters serve as a model system for studying small molecules whose interactions are dictated by cluster geometry and variations in surface chemistry, but such clusters can also serve as components in the directed self-assembly of advanced materials.12 Anisotropic colloidal clusters in either 3D (i.e., in the bulk) or 2D (i.e., on a surface) have been fabricated in a number of ways, including tuning depletion interactions in "lock and key" colloids, 13 assembly via spatial templating of the substrate, 2,6 controlled evaporation of one phase in an emulsion, 1,9 and irreversible attraction between particles or between particles and a substrate that have been modified to have a specific colloidal attraction.14−20 These studies have illustrated the wide structural diversity and utility of geometrically complex colloidal clusters.The 2D directed self-assembly of colloidal particles on a substrate is a closely related research area that is often used for the fabrication of colloidal clusters. There is a large body of literature in this area focused on the assembly of various colloidal particles, i...

The Fluid Joint: The Soft Spot of Micro‐ and Nanosystems

“…We specifically refer to the subtype of fluid bridges where at least one of the areas of contact on the supporting surfaces is constrained laterally (besides axially) as to the fluid joint (Figure e). Lateral liquid confinement on a surface (i.e., contact line pinning) can be enforced chemically, i.e., through the juxtaposition of surfaces of contrasting wetting properties ( Figure a,b), or topographically, i.e., through the machining of solid edges (Figure c,d). In both cases, liquid confinement at the pinning borders results from an extension of the canthotaxis sector.…”

“…The rich literature on the physics of the coffee stain, including ways to suppress it, provides substantial knowledge in the domain of colloidal deposition and organization, whereby substrate withdrawal techniques such as Langmuir–Blodgett and dip‐coating are well established. In this context, the fluid joint is central to recently devised techniques for controlled particle deposition, which can be described by analogy as horizontal dip‐coating or a sliding coffee stain.…”

“…In this case, selective dewetting by the receding colloidal meniscus leaves lenses of colloidal solution within superficial traps defined by sharp wetting contrast (Figure a,b). After solvent evaporation, however, random clusters of particles are more easily obtained in this way (Figure d). The precise size, packing, layering, and internal organization of the clusters are hardly repeatable, and very sensitive to dynamic instabilities, the particle volume fraction, and contingent laboratory conditions .…”

“…Particle sequestration is thereby controlled by the ratio between the particle size and height of the lens coated within the lyophilic patch, itself proportional to the patch diameter and the solution's receding contact angle . The pitch of periodic patch arrays may also correlate to the deposited cluster size . Importantly, the final position and orientation of the particles within both superficial and recessed trapping sites can be significantly influenced by drying‐mediated adhesion and capillary immersion forces .…”

“…d) Clusters of microparticles deposited on lyophilic superficial patches arrayed over a lyophobic substrate. (Reproduced with permission . Copyright 2015, American Chemical Society).…”