Capillary Force Assembly Research Articles

Fabrication of Single-Nanocrystal Arrays.

To realize the full potential of nanocrystals in nanotechnology, it is necessary to integrate single nanocrystals into addressable structures; for example, arrays and periodic lattices. The current methods for achieving this are reviewed. It is shown that a combination of top-down lithography techniques with directed assembly offers a platform for attaining this goal. The most promising of these directed assembly methods are reviewed: capillary force assembly, electrostatic assembly, optical printing, DNA-based assembly, and electrophoretic deposition. The last of these appears to offer a generic approach to fabrication of single-nanocrystal arrays.

Control of Orientation, Formation of Ordered Structures, and Self-Sorting of Surface-Functionalized Microcubes at the Air-Water Interface.

The dependence of the orientation of microscale PS cubes, which are surface functionalized on only five faces, at the water/air interface and the ordered aggregates formed by capillary force assembly are reported. Depending on the wettability of the faces, the cubes were shown to adopt a preferred orientation that changes with decreasing wettability from face up to edge up and further to vertex up. Concomitantly, stable aggregates with different structures were formed by capillary force self-assembly. The unmodified bottom face of the cubes was localized by fluorescence labeling. Finally, self-sorting of differently surface functionalized microcubes was realized for the first time, due to the stronger capillary interactions of quadrupole-quadrupole and hexapole-hexapole interactions compared to quadrupole-hexapole interaction.

Tailored Combinatorial Microcompartments through the Self‐Organization of Microobjects: Assembly, Characterization, and Cell Studies

AbstractA new concept enables the generation of cell microenvironments by microobject assembly at an water/air interface. As the orientation of 30 μm sized polymer cubes and their capillary force assembly are controlled by the surface wettability, which in turn can be modulated by coating the initially exposed surfaces with gold and self‐assembled monolayers, unique niches in closely packed arrays of cubes with vertex up orientation can be realized. The random assembly of distinctly different cubes, prefunctionalized or surface‐structured exclusively on their top surface, facilitates the parallel generation of different microenvironments in a combinatorial manner, which paves the way to future systematic structure–property relationship studies with cells.

Tailored Combinatorial Microcompartments through the Self-Organization of Microobjects: Assembly, Characterization, and Cell Studies.

A new concept enables the generation of cell microenvironments by microobject assembly at an water/air interface. As the orientation of 30 μm sized polymer cubes and their capillary force assembly are controlled by the surface wettability, which in turn can be modulated by coating the initially exposed surfaces with gold and self-assembled monolayers, unique niches in closely packed arrays of cubes with vertex up orientation can be realized. The random assembly of distinctly different cubes, prefunctionalized or surface-structured exclusively on their top surface, facilitates the parallel generation of different microenvironments in a combinatorial manner, which paves the way to future systematic structure-property relationship studies with cells.

Asymmetric multifunctional 3D cell microenvironments by capillary force assembly

The fabrication and characterization of advanced 3D cell culture microenvironments that enable systematic structure–property relationship studies are reported.

High throughput sub-10 nm metallic particles organization on templates made by block copolymer self-assembly and nanoimprint

The combination of block copolymer (BCP) and nanoimprint lithography (NIL) presents a novel and cost-effective approach to achieve sub-50nm patterning resolution. Through this study, the authors demonstrate the placement of a few Au nanoparticles per hole using capillary force assembly (CFA). Polystyrene block poly(dimethylsiloxane) (PS-b-PDMS) with spherical morphology is used as etching mask, which is then transferred into Si by a plasma etching. Si posts with ∼35nm of diameter, ∼55nm of height and ∼51nm of periodicity are obtained. Si templates are then used as a NIL master mould to pattern a thermoplastic resist. An ultra-high resolution hole-array is created on a polymethyl-methacrylate (PMMA) substrate, where 10-nm Au nanoparticles are placed in each hole. The BCP-NIL method, presented here, is highly promising for the current semiconductor industry to build devices in the fields of opto-electronics, chemical sensors and energy conversion and storage.

Plasmonic Shaping in Gold Nanoparticle Three-Dimensional Assemblies

When a large number of similar gold particles are organized into complex architectures, the dipolar plasmon spectrum of the individual plasmonic entities gives rise to a broader, red-shifted feature centered around 750 nm. In this work, we show that superstructures fabricated using the convective assisted capillary force assembly method (CA-CFA) and excited at that wavelength display a subwavelength patterning of their optical field intensity that results from the self-consistent coupling between the colloidal nanoparticles. First, we demonstrate the fabrication of shape-controlled three-dimensional assemblies of metallic nanocrystals using the CA-CFA method. In a second step, the absorption band resulting from the mutual coupling between the metallic building blocks is exploited to excite a coupled plasmon mode and map the two-photon luminescence (TPL) by scanning a tightly focused light beam. Highly resolved TPL images show that the morphology of the plasmonic particle assemblies has a strong impact on their optical response. A model based on a rigorous optical Gaussian beam implementation inside a generalized propagator derived from a three-dimensional Green dyadic function accurately reproduces the TPL maps revealing the influence of interparticle separation and thus coupling between the individual particles. Finally, we show that the spatial distribution of the electric field intensity can be controlled by tuning the linear polarization of the optical excitation.

On-chip polychromatic visible light emitters obtained by 3D capillary force assembly

This work presents the fabrication of on-chip polychromatic light source from a colloidal suspension. Capillary force assembly of the colloidal mixture composed from red, blue and green fluorescent dye doped 100 nm particles is performed on a patterned substrate to create particles structure composed of ∼10,000 nanoparticles with a natural hexagonal close-packed arrangement. Under UV excitation, the particle structure exhibit a white light emission closed to the CIE characteristics of the ideal pure white.

Three-dimensional capillary force assembly: Fabrication of white light emitters

This work uses convective assisted capillary force assembly from a collection of 100 nm fluorescent polystyrene nanoparticles to fabricate 4 μm wide and 1 μm high localized light-emitting superstructures. Large scale superstructures of about 10 000 particles are transferred on free silicon surfaces by microcontact printing. Optical characterization reveals that these sources emit polychromatic visible light from 400 to 800 nm equivalent to a white spectrum. Finally, single objects are studied to characterize the spatial distribution of the emitted color.

Plasmonic coupling in nondipolar gold colloidal dimers

Nanoscale Gold colloidal dimers are built thanks to the convective capillary force assembly (CFA) technique. CFA efficiently demonstrates precise dimer localization, particle separation control, and object reproducibility. Darkfield microspectroscopy measurements combined with numerical modeling exhibit a redshift in the dimer surface plasmon resonance as the interdistance decreases. The study points out that metallic particles, which are not in the dipolar approximation (diameter ∼150 nm), present a similar optical behavior than that of smaller particles for the first resonance mode. Finally, local electric field simulations indicate that these dimers are valid candidates for sensing applications in the near-infrared regime.

Capillary force assembly of giant vesicles on a microstructured substrate

Convective capillary force assembly has shown to efficiently assemble nm to μm solid colloids onto a surface. We show here that this technique can also efficiently trap liposomes with 10μm mean size or films of lipid into microstructures. We hypothesize that whether liposomes or lipids are trapped in the template depends on the interfacial interaction of the objects with the surface. After solvent evaporation, ruptured and deflated liposomes are observed onto oxidized (hydrophilic) PDMS while uniform films of lipids are found in untreated (hydrophobic) PDMS microstructures. This assumption is supported by the analysis of the evaporation of a droplet containing a dispersion of liposomes in free environment.

Colloidal optical waveguides with integrated local light sources built by capillary force assembly

Colloidal waveguide fabrication based on the convective assisted capillary force assembly technique and their optical properties is presented in this work. A large variety in shape of colloidal waveguides is built with their own integrated local light sources. It is highlighted that these structures exhibit propagation of the visible light at the micrometric scale. Finally, the onset of the integration of a colloidal emitter on a classical photonic system is developed using a stamping method.

Assembly of Nanorods into Designer Superstructures: The Role of Templating, Capillary Forces, Adhesion, and Polymer Hydration

The assembly mechanism by which hundreds of thousands of two-segment gold-polypyrrole nanorods are assembled into kinetically controlled shape-directed superstructures is examined to predict the range of nanoparticle sizes and materials that can be utilized in their formation. Four processes are responsible for assembly: templating, capillary force assembly, adhesion, and polymer hydration. It is shown that templating, where rods are prepositioned for assembly, is scale invariant and that the energy-minimized state after this step is highly disordered. In addition, we predict that superstructures can be made independently from patterns of rods separated by a distance as small as six times the inter-rod spacing. Both modeling and experiment show that adhesion and polymer dehydration, which induces curvature in the superstructures, are applicable to other materials. However, the high surface energy and low elastic modulus of polypyrrole are advantageous toward generating three-dimensional structures, inducing curvature at gold/polypyrrole length ratios as large as 7:1.

Assisted convective-capillary force assembly of gold colloids in a microfluidic cell: Plasmonic properties of deterministic nanostructures

A new colloidal assembly technique in a microfluidic cell is proposed in this work. This technique is based on classic capillary force assembly (CFA) assisted by air-suction flow and temperature control during the assembly process. Their influence on the assembly macroscopic (triple contact-line speed, receding contact angle) and microscopic (colloidal motion at the triple contact line) parameters is examined. It is shown that the extent and the performance of the assembly process are significantly enhanced by this approach. Finally, the plasmonic properties of deterministic linear (2–5 colloid long) and tripod gold nanostructures created with this technique are investigated.

Separation of colloidal nanoparticles using capillary immersion forces

Capillary force assembly (CFA) of colloidal particles usually results in closed-packed films or particle aggregation within topographic features. In this work, it is shown that CFA can also be exploited to both localize and separate nanoparticles (d=50–200nm) when template shape and wettability are controlled. Well-defined geometric arrangements of one to four closely spaced particles (30–50nm separation) were realized in large arrays using this technique to demonstrate that particle aggregation during dewetting can be eliminated. Ordered SiO2 nanopillars in tight groupings were obtained by combining low-resolution e-beam lithography (>100nm) with CFA and etching. This approach provides a simple route to fast and precise placement of nanostructures using relatively low-resolution pattern making techniques.