Self-assembly of microspheres at the air/water/air interface into free-standing colloidal crystal films

Abstract

The air/water/air interface was employed as a platform for the self-assembly of colloidal microspheres, leading to planar and curved free-standing colloidal crystal films with sizes in excess of several square millimeters. In the resulting crystal films, few cracks were observed. The confinement of the air/water/air interface also led to a new type of cubic packing structure: stripes composed of cubic packed spheres were regularly separated by line defects, where spheres were hexagonally packed. Hence, the present colloidal crystallization method should be of interest in fundamental science and technical applications.

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Monodisperse colloidal particles can self-assemble into long-range, ordered, close-packed superstructures; so called colloidal crystals. The ability to manipulate the self-assembly of colloidal particles can lead not only to a better understanding of the physics of condensed matter, but also to advanced materials such as photonic crystals.¹ Accordingly, numerous techniques have been developed to organize colloidal particles, such as controlled solvent evaporation, electrophoresis, dip-coating, spin coating and crystallization in physically confined cells.^1,2 The evaporation of solvents is always accompanied with an increase of the volume fraction of colloidal crystals (up to 74% in the dry sample) and the shrinkage of their constituent particles. Since particles are pinned on the surface of solid substrates by the drying line, the shrinkage of colloidal crystals leaves behind a number of cracks in dried samples. Depending on the crystallization conditions, the cracks appear every 50–250 µm in solid colloidal crystal films.³ Cracks are expected to strongly scatter light, thus suppressing to a large extent the photonic application of colloidal crystals. Recently, liquid metals such as mercury and gallium have been used to get rid of the lateral pinning of colloidal particles on substrates, thus creating crack-free colloidal crystal films of sizes in excess of millimeters.⁴ Nonetheless, the liquid metals are toxic and it is not trivial to clean their surfaces. Herein we present an alternative approach—self-assembly of colloidal spheres at the air/water/air interface—to construct crack-free, free-standing, colloidal crystal films.

The self-assembly of surfactants at the air/water/air interface, well-known as Newton black films, has been extensively investigated.⁵ Ichinose and co-workers have recently succeeded in obtaining dried black films of surfactants by the deliberate control of the evaporation of water.⁶ Similar to surfactants, colloidal particles of sub micrometer sizes are readily attached to the interface between two immiscible fluids; the Pickering emulsion is a prominent example.⁷ Velikov and co-workers have studied the dynamics of self-assembly of colloidal particles within water/air thin films.⁸ Bohaty and Zharov generated suspended colloidal crystal films by sedimentation of silica spheres within frustum-shaped openings in silicon wafers.⁹ Nonetheless, their technique requires the peripheral support of a silicon wafer and it is hard to form colloidal crystal films of larger than 100 microns, which limits its applicability in photonics. Based on the similarity of the interfacial behavior between colloidal particles and surfactants, we were encouraged to conduct self-assembly of colloidal microspheres at the air/water/air interface (experimental details are shown in the ESI†). Monodisperse polystyrene (PS) and silica microspheres of sizes ranging from 100–1000 nm were used. The concentration of the sphere suspensions was set in the range of 5–52 wt%. In the current work, a copper ring with a diameter of 1 cm was dipped into a colloidal suspension and withdrawn, followed by drying at ambient temperature. The thin film of the colloidal suspension, confined by the copper ring, provided a platform for the spheres to self-assemble at the air/water/air interface, reminiscent of the formation of black films. After completely drying, a free standing thin film was obtained and enclosed by the copper ring (Fig. 1a and S1 in the ESI†). Note that the resulting free-standing films were rather fragile and the use of hydrophilic rings favored their formation. Scanning electron microscopy (SEM) was used to visualize the film structure. As shown in Fig. 1b,c, a long-range highly ordered packing of 470 nm PS spheres was clearly observed, consistent with the strong iridescence of the film. Low magnification SEM imaging (Fig. 1b) and optical photography (Fig. 1a) demonstrated the absence of cracks; the crystallite domains extended to millimeters. A number of point defects were visible, but they are expected to have only a negligible influence on the optical properties of the resulting colloidal crystal films.

Fig. 1 Photograph of a free-standing colloidal crystal film derived from self-assembly of 470 nm PS spheres at the air/water/air interface (a) and their low (b) and high (c) magnification SEM images.

As suggested in the literature, simple colloidal crystallization under gravity always leads to the coexistence of hexagonal and cubic packing. Intriguingly, besides the usual cubic packing structure, in the current work a new superlattice of cubic structures was observed (Fig. 2a). High magnification SEM shows that stripes of cubic packed spheres are regularly separated by line defects (Fig. 2b). The line defects are composed of hexagonally packed spheres. This suggests that the air/water/air interface (especially at the water layer center) provides a finite space for colloidal crystallization of the spheres to deviate their packing density from 0.74 (the maximal value for face-centered cubic packing).¹⁰

Fig. 2 (a) SEM image of cubic packed domains in a free-standing colloidal crystal film of 470 nm PS spheres. (b) High magnification SEM image of the domains highlighted by white squares in (a).

Note that the free-standing colloidal crystal films obtained were non-uniform multilayers across the films. The thickness of the crystal films increased with the concentration of the colloidal spheres. The thickness at the film center slightly increased from 3 layers of spheres to 6 layers when the concentration increased from 5 to 52 wt%. While, the thickness of the film edges increased dramatically from 7 to 20 layers. The resulting films exhibit a strong reflection, the so-called stop band (Fig. S2 in the ESI†).

When the suspensions of colloidal spheres were sufficiently concentrated, e.g. 50%, the removal of free salts led to crystallization of the spheres in the suspensions due to the strong electrostatic interaction between the spheres, evidenced by the beautiful iridescence of the suspensions. The drops of the concentrated colloidal suspensions were placed on hydrophobic substrates, followed by blowing air inside by a pipette. Thus, hemispherical bubbles were obtained. Note the hydrophobic substrate is necessary. After drying at ambient temperature, hemispherical, free-standing films of colloidal spheres were obtained, exhibiting a strong iridescent reflection (Fig. 3a,b). Similar to the planar ones obtained using copper rings, the curved free-standing films were composed of microspheres self-assembled into a fairly long range ordered array; no large cracks were observed (Fig. S3a,b in the ESI†). The stripes of cubic packed spheres, separated by line defects, were also observed in the curved free-standing colloidal crystal films (Fig. S3c in the ESI†). This may suggest that the new superlattice of cubic packed spheres is rather common for the self-assembly of colloidal spheres at the air/water/air interface, which should arise from quasi-two-dimensional confinement of the water/air thin film. In order to achieve a better elucidation, our current effort is devoted to an in situ study of the structures of self-assembly of colloidal spheres during thinning of the water/air thin films.

Fig. 3 (a) Photograph of a hemispherical colloidal crystal bubble obtained by blowing air into a droplet of the concentrated suspension of 470 nm PS spheres laid on a plastic substrate. (b) Photograph of the curved free-standing colloidal crystal films obtained by removing the crystal bubble from the substrate.

In conclusion, we succeeded in forming free-standing colloidal crystal films via self-assembly of microspheres at the air/water/air interface. This new strategy allows generation of crack-free colloidal crystals over several square millimeters. Self-assembly of microspheres at the air/water/air interface may give rise to new superlattices of microsphere self-assemblies, which has not otherwise been observed so far. The air/water/air interface is, therefore, envisaged as a new platform to control the dynamics of colloidal self-assembly, thus generating new structures. On the other hand, owing to the elimination of pinning on the solid substrates and, thus, the formation of cracks, free-standing colloidal crystal films should be of immense promise in photonic applications.

Acknowledgements

We thank for the financial support of DAAD, National Natural Science Foundation of China (No. 50533030 and 60121101) and the Max Planck Society. I. Ichinose and J. Jin (NIMS, Japan) and R. Krastev are acknowledged for helpful discussion.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, photograph, reflection spectrum and SEM images of a free-standing colloidal crystal film. See DOI: 10.1039/b611817h

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