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

Abstract

Whereas considerable interest exists in self-assembly of well-ordered, porous "inverse opal" structures for optical, electronic, and (bio)chemical applications, uncontrolled defect formation has limited the scale-up and practicality of such approaches. Here we demonstrate a new method for assembling highly ordered, crack-free inverse opal films over a centimeter scale. Multilayered composite colloidal crystal films have been generated via evaporative deposition of polymeric colloidal spheres suspended within a hydrolyzed silicate sol-gel precursor solution. The coassembly of a sacrificial colloidal template with a matrix material avoids the need for liquid infiltration into the preassembled colloidal crystal and minimizes the associated cracking and inhomogeneities of the resulting inverse opal films. We discuss the underlying mechanisms that may account for the formation of large-area defect-free films, their unique preferential growth along the 110 direction and unusual fracture behavior. We demonstrate that this coassembly approach allows the fabrication of hierarchical structures not achievable by conventional methods, such as multilayered films and deposition onto patterned or curved surfaces. These robust SiO(2) inverse opals can be transformed into various materials that retain the morphology and order of the original films, as exemplified by the reactive conversion into Si or TiO(2) replicas. We show that colloidal coassembly is available for a range of organometallic sol-gel and polymer matrix precursors, and represents a simple, low-cost, scalable method for generating high-quality, chemically tailorable inverse opal films for a variety of applications.

Fig. 1.

Schematics of “conventional” colloidal template self-assembly (A), and coassembly of colloids with a soluble matrix precursor (B), for the syntheses of inverse opal thin films. The conventional method typically involves the sequential steps of colloidal self-assembly, matrix infiltration, and template removal. The use of such conventional colloidal self-assembly to generate large-area films has been plagued with the formation of defects, such as overlayer coatings, multiple domains, and significant cracking. Colloidal coassembly combines the steps of template self-assembly with matrix infiltration into one process in which colloids are allowed to assemble directly from the sol-gel solution, to yield robust inverse opal films with no overlayer, very large (mm to cm) ordered domains, and no cracks, due to the “gluing” action of the sol-gel matrix.

Fig. 2.

SEM images of colloidal and inverse opal films. (A) PMMA colloidal crystal film (as synthesized), showing cracks propagating along the close-packed {111} planes and spaced at the 5–10 μm scale); (B) I-SiO₂ (inverse opal) film by sol-gel infiltration (calcined to remove PMMA template), showing an overlayer coating, cracking in both the inverse opal and overlayer coating, and a region of SiO₂ infiltration into a preexisting template defect (white arrow); (C) I-SiO₂ film by PMMA/sol-gel coassembly, showing highly uniform films with no cracks. Note that the thickness of the coassembled film is comparable (and higher) than the thickness of the cracked film shown in (A). Left scale bars = 10 μm, right scale bars = 1 μm.

Fig. 3.

Well-ordered SiO₂ inverse opal films produced by colloidal coassembly. (A) Linear relationship between film thickness (number of sphere layers) and colloidal concentration (volume of PMMA stock solution in 20 mL H₂O), and threshold thickness for crack formation; (B) optical photograph of an I-SiO₂ film deposited on glass (substrate width approximately 1.5 cm); (C) a fracture cross section of a crack-free I-SiO₂ film on a Si wafer, showing the 〈110〉 growth orientation, as illustrated by an fcc model (Inset); (D) a FIB-cut cross section of a film showing the {110} plane (a model is shown in the inset); (E) optical photograph of the triangular crack pattern in a thick I-SiO₂ film; (F) SEM image of the corner cracked region indicated in (E), with schematic illustration and higher magnification SEM image showing fracture along the {110} planes. All of the films in this figure are oriented with the vertical growth direction aligned from top to bottom.

Fig. 4.

Novel SiO₂ inverse opal structures enabled by colloidal coassembly. Schematics of the processes are shown on the left and the representative SEM images are shown on the right. (A) Schematic presentation of the synthesis of multilayered, hierarchical films with different pore sizes by successive layer deposition prior to template removal (Left), and a SEM cross section images of a bilayer SiO₂ structure produced using 300 nm and 720 nm colloidal spheres (Right). The top left and bottom left SEM images show the interface between layers before and after calcination, respectively. (B) Schematic presentation of the oriented SiO₂ structures grown on topologically patterned substrates (Left), and an SEM fractured cross section of inverse opals grown in 4 μm wide, 5 μm deep channels on a Si substrate (Right). (C) Schematic presentation of the coassembly onto curved surfaces (Left), and SEM images (Right) of a SiO₂ inverse opal film layer (shown magnified, Inset) deposited onto a sintered, macroporous Ti scaffold structure.

Fig. 5.

Morphology-preserving reactive transformation of inverse opal SiO₂ films into MgO/Si, Si, and TiO₂. SEM images (left column), TEM images (center two columns), and SAED analysis (right column) are shown for: (A) a starting I-SiO₂ film, (B) a Si/MgO replica after reaction with Mg(g) at 850 °C for 4 h, (C) a Si replica film after selective MgO dissolution, and (D) a TiO₂ replica film after reaction with at 200 °C for 2 h and with water vapor at 400 °C for 5 h.