Assembly of large-area, highly ordered, crack-free inverse opal films

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Birgit Hausmann


B. Hatton et. al. "Assembly of large-area, highly ordered, crack-free inverse opal films" PNAS 107 (23) 2010


Coassembly, colloidal assembly, crack-free films, inverse opals, nanoporous


A new synthesis of crack-free inverse opal films over cm length scales is presented. The two step process consists of a) an evaporative deposition of polymeric colloids in a hydrolyzed silicate sol-gel precursor solution and b) a colloidal/matrix coassembly. The preferential grwoth direction is <110>. The synthesis of multilayered hierarchical films are also demonstrated. Furthermore, the inverse opal films were replicated in other materials as porous Si and <math>TiO_2</math> while maintaining their morphology during the gas/solid displacement reaction.

Results and Discussion

Fig. 1 Schematic for inverse opal synthesis: 1) Colloids assemble from a sol-gel solution 2) template removal

The advantage of the presented process is that the infiltration of a preassembled porous structure is avoided, thus preventing the film from cracking during the drying step. Indeed, the fabrication technique consists only of two steps (Fig. 1): 1) Polymer colloids (e.g. polystyrene (PS) or poly methyl methacrylate (PMMA)) assemble in a sol-gel precursor solution (e.g. <math> Si(OH_4), Ti(OH_4), Ge(OH_4)</math>) during an evaporative deposition resulting in an opal film. 2) The template is removed. If the PMMA spheres are deposited from a hydrolyzed tetraethoxy silane (TEOS) solution, the interstitial spaces of the polymeric opal film are filled with silica gel matrix material which results in an inverse opal silica structure (I-SiO2). In that way defects as cracking, formation of domain boundaries and colloidal vacancies which are a main problem in conventionally assembled films can be omitted over a cm length scale.

The TEOS-to-colloid ratio was found to have a significant influence on the film structure and defect formation However,beyond a critical TEOS content (approximately 0.15 mL TEOS solution per 20 mL PMMA suspension), highly ordered, crackfree films were deposited. The coassembled I-SiO2 film (Fig. 2C) was produced under the following optimized conditions: (i) suspending a vertically oriented glass slide in a mixture comprised of 0.15 mL of a 28.6 wt% TEOS solution with a 20 mL suspension of 280 nm diameter PMMA spheres (approximately 0.125 vol%), (ii) allowing the solvent to slowly evaporate at 65 °C (deposition rate ¼ 2 cm?day), and (iii) treating the composite structure at 500 °C for 5 h in air. These conditions were found to generate nearly perfect, crack-free inverse opal films. The film thickness can be controlled by regulating the colloidal volume fraction (for a constant TEOS-to-PMMA ratio). The number of layers increased linearly with colloidal concentration (Fig. 3A). No cracks were observed in films with up to approximately 18–20 sphere layers (i.e., thicknesses up to approximately 5 ?m). Coassembled films with >20 layers begin to develop cracks (Fig. 3 C and D), but their characteristic triangular fracture is qualitatively different from the irregular, tortuous crack patterns in colloidal crystals. In particular, the distance between the cracks was in the order of approximately 100 ?m with no microcracks, thus producing defect-free regions that are 100× larger than those in the conventional films “self-healing” phenomenon it is apparent that the silicate additions induce the preferential growth in the h110i direction and help to increase the order of the resulting films. Cracks in fcc colloidal crystal films typically occur along the close-packed f111g planes, which represent the planes of weakness (49) (Fig. 2A). For an inverse opal structure, the f111g planes represent the highest pore density, and thus are the obvious candidates for crack propagation. Indeed, thin (<5 ?m) crack-free I-SiO2 films reveal the expected f111g fracture when cleaved intentionally (Figs. 2C and 3C). However, cracks in thick films (>5 ?m) show an unexpected, unique orientation along f110g planes of the colloidal crystal Cracking of conventional opal and inverse opal films tends to occur upon drying both at the colloidal assembly stage and at the infiltration stage due to a combination of dehydration and/or polymerization- induced contraction and associated local capillary forces we propose that the process might be similar to the formation of large single crystals of calcite patterned at the micron scale Multilayer inverse opal structures with varying pore sizes can be created by the successive deposition of template/matrix composite layers Herein we demonstrate the evaporative coassembly of a sacrificial colloidal template with a matrix material in a single step to yield a colloidal composite, thereby avoiding the need for liquid infiltration into a preassembled porous structure.

Fig. 2 Highly ordered I-<math>siO_2</math> films formed from PMMA/sol-gel coassembly (Left scale bar is <math>10\mu m </math> and right scale bar is <math>1\mu m </math>)
Fig. 3 A) The film thickness is directly proportional to the colloidal concentration. The threshold thickness for cracking is indicated. B) A 1.5cm I-<math>siO_2</math> film. C) A cleaved film reveals the growth direction along <110>. Inset: fcc-lattice model

Cracking seems to occur along {111} planes for thin films, which is consistent with conventional evaporative deposited films, whereas thicker films seem to crack along {110} planes.

Fig. 4 <math>SiO_2</math> inverse opal structures formed by colloidal coassembly. Schematics (left) vs. SEM images (right). (A) Synthesis of multilayered, hierarchical films with different pore sizes by successive layer deposition prior to template removal. (The top left and bottom left SEM images show the interface between layers before and after calcination, respectively.) (B) <math>SiO_2</math> structures grown on topologically patterned substrates (Left), SEM fractured cross section of inverse opals grown in 4 μm wide, 5 μm deep channels on a Si substrate (Right). (C) Coassembly onto curved surfaces (Left), and SEM images (Right) of a <math>SiO_2</math> inverse opal film layer (shown magnified, Inset) deposited onto a sintered, macroporous Ti scaffold structure.