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>\mathrm{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
Fig. 2 Highly ordered I-<math>\mathrm{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>\mathrm{SiO_2}</math> film. C) A cleaved film reveals the growth direction along <110>. Inset: fcc-lattice model
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>\mathrm{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>\mathrm{SiO_2}</math> inverse opal film layer (shown magnified, Inset) deposited onto a sintered, macroporous Ti scaffold structure.

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 since cracking in "[...]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". 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> \mathrm {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-<math> \mathrm {SiO_2}</math>). 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.

Several observations were made: There's a critical TEOS-to-colloid ratio (approximately 0.15 mL TEOS solution per 20 mL PMMA suspension) beyond which crackfree films can be expected. Figure 2 shows a highly ordered crackfree film which was fabricated in optimized conditions: 1) 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%), 2) allowing the solvent to slowly evaporate at 65 °C (deposition rate = 2 cm/day), and 3) treating the composite structure at 500 °C for 5 h in air.

The film thickness (number of layers) is linearly proportional to the colloidal volume fraction for a given TEOS-to-PMMA ratio. There were no crack formation observed beyond a 20 layers. For thicker films crack free domains were as large as 100<math>/mu m</math> (Fig. 3A).

The growth direction was found to be along <110> direction as opposed to conventional film growth along <112> direction (Fig. 3C) and shows a fcc structure. Adding the silicate in the process seems to change the preferential growth direction along <110> and induces a "self-healing" of domains with a temporary different growth direction. The highest pore density in fcc films occurs along {111} planes where crack formation is most probable, which is confirmed for films with a thickness < 5<math>/mu m</math> (when cleaved). But for films > 5<math>/mu m</math> the formation occurs along <110> planes.

When explaining the effect of this new crackfree formation of films the authors refer to the phenomenon of the formation of large single crystals of calcite patterned at the micron scale: "[...]the transformation of the amorphous <math> \mathrm {CaCO_3}</math> film into a defect-free, porous calcite crystal is facilitated by the micropatterned substrate that not only determines the pattern of porosity of the final single crystal, but also provides interfacial sites for stress and impurity release during the amorphous-to-crystalline transition. Similarly in the current system, the formation of a colloidal crystal and the associated interfaces between the polymerizing sol-gel solution and the assembling colloidal spheres may provide sites for the relaxation of tensile stresses encountered during the gelation process. Controlled solvent release during the polycondensation reaction can also occur at these interfaces and be channeled through the interconnected porous network to evaporate at the surface."

This evaporative deposition is also suitable for multifilm inverse opal structures of different pore sizes as well as deposition on curved surfaces (Fig. 4). The multifilm is created via multiple depostions of template/matrix layers.

The I-<math> \mathrm {SiO_2}</math> films were also replicated in Si/MgO, Si and <math> \mathrm {TiO_2}</math> while maintaining the porous structure and minor cracking in some cases.