Difference between revisions of "Assembly of large-area, highly ordered, crack-free inverse opal films"

From Soft-Matter
Jump to: navigation, search
Line 16: Line 16:
 
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.
 
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.
  
 +
[[Image:Hatton2010 2.png|400px|thumb|left|'''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>)]][[Image:Hatton2010 3.png|500px|thumb|right|'''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.
  
  
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.
 
  
  
[[Image:Hatton2010 2.png|400px|thumb|left|'''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>)]][[Image:Hatton2010 3.png|500px|thumb|right|'''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.   
 
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.   

Revision as of 14:09, 13 September 2010

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.