Bonding assembled colloids without loss of colloidal stability

Abstract

In recent years the diversity of self-assembled colloidal structures has strongly increased, as it is fueled by a wide range of applications in materials science and also in soft condensed-matter physics.[1–4] Some potential applications include photonic bandgap (PBG) crystals, materials for plasmonic devices, high-efficiency energy conversion and storage, miniature diagnostic systems, desalination, and hierarchically structured catalysts.[1–4] Three dimensional colloidal crystals with mostly close-packed (randomly or face-centered cubic (fcc) stacked) structures have been fabricated via various methods, some of which are able to impose the orientation of these crystals, e.g., sedimentation,[5] colloidal epitaxy,[6] evaporative or “flow controlled” deposition,[7] shear flow,[8,9] and spin-coating.[10] Fewer methods have been reported to generate non-close-packed colloidal crystal structures, for instance, by a physical or chemical immobilization of colloidal arrays with a readily polymerizable monomer, which is dissolved in the dispersion,[11–14] and by a combination of thermal sintering and etching of close-packed colloidal crystals.[15] Many methods are currently being developed further to fabricate more diverse crystal symmetries and non-close-packed structures by tuning the interaction between the particles, e.g., oppositely charged interactions,[16,17] external electric fields,[18–20] and/or non-spherical shapes.[21,22] However, the structures thus formed are vulnerable to capillary forces that arise when the solvent is evaporated and to many other post-treatment steps, especially when the particles are non-close-packed.[1–4] For example, to obtain a PBG in the near infrared, the artificial opals must be dried, then infiltrated with a high-refractive-index material, and the spheres must subsequently be selectively removed by chemical etching[15,23] or a thermal treatment (calcination or pyrolysis).[1,2,4,14,24 Here, we present a facile and flexible one-step, in situ, thermal annealing method to permanently fix non-close-packed and close-packed polymeric structures so that they easily survive a subsequent drying step without loss of colloidal stability. We first demonstrate the concept with fluorescently labeled and sterically stabilized polymethylmethacrylate (PMMA) particles[25] because this system can be readily index and density matched, allowing us to compare their structures in 3D real-space before and after the treatment by means of confocal laser scanning microscopy. In unrelated work, we have already demonstrated this method for creating 1D colloidal bead chains by the application of an external electric field,[26] but it is shown in the present paper that the method presented is quite general and can be applied on many other self-assembly schemes. Moreover, we show that the shape and volume fraction of the particles after bonding can be changed by varying the heating time. Here, we implement our method to three different non-close-packed assemblies and one close-packed structure: i) ionic colloidal crystals of oppositely charged particles with a CsCl morphology, ii) external electric field induced body-centered tetragonal (bct) crystal structures, iii) labyrinthine or maze-like sheet structures induced by external electric fields, and iv) random hexagonal close-packed crystals.