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

Birgit Hausmann

Reference

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

Keywords

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

Overview

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.

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

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.

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.