Highly-ordered nanostructured films hold promise to improve photovoltaic, thermoelectric, low dielectric constant, and catalytic materials. A lost-cost bottom-up approach to films with extremely small and regular feature sizes (sub 10 nm) is via “evaporation induced self-assembly” (EISA) of surfactants with metal oxide precursors. In this process, film formation proceeds dynamically during solvent evaporation after dip-coating, spin-coating, or casting a liquid film. The successful development of new materials and devices using this EISA approach is highly dependent upon the final film topology, which is found to be strongly effected by thermodynamics of self-assembly, the kinetics of silica condensation, and mass transport. Recent studies on dip-coated films have revealed the importance of the evaporation rate on the formation of the final nanostructure.[1-3] However, precise control over the evaporation rate during dip-coating experiments is difficult due to the lack of control over the gas-phase mass-transport.. Further, there is only one experimental variable available for control of the mass transport rate, the humidity. In contrast, spin coating[4,5] provides multiple experimental variables for controlling the mass transport rate; humidity, angular velocity, and the size/shape of the substrate.

Here, we show the results from spin-coating experiments to elucidate the effect of mass transport on self-assembly. The coating solutions used in these experiments yield cubic, 2D hexagonal, tricontinuous double-gyroid, and lamellar phases when dip coated from a solution with an Si:EO ratio of 1.10.[6] The double gyroid derived tricontinuous phase is unique in that it has been shown to yield accessibility to the substrate for ions in solution. Accessibility is a key feature that makes the tricontinuous phase promising for several applications. The spin coated films were prepared using 1-inch diameter circular discs at angular velocities from 1000 rpm to 6000 rpm. At these velocities, a well-defined laminar flow profile develops over the surface of the film[7] enabling the use of well-established mass transfer correlations.[8] The relative humidity of the spin coating chamber was controlled at different set points between 0% and 90% using a forced flow mixture of saturated and dry air streams.[3] The resulting nanostructures were examined by grazing incidence small angle x-ray scattering to determine the nanostructure symmetry, orientation, and topology.[9, 10] Comparison of the nanostructures at different values of humidity and angular velocity reveal an effective mass transport rate and coefficient. From this information, processing diagrams independent of the synthesis technique (ie. dip coating, spin coating, spray casting, etc.) are developed, which may then be used to predict the final nanostructure phase using any synthesis technique. These experiments reveal a sensitive interplay between the rate of mass transfer and the ultimate structure of the film. Analysis of the various time scales inherent in the process reveals a new fundamental understanding of the self-assembly of such films that may play a key role in the ability to scale-up devices to commercial applications.

REFERENCES

1. Grosso, D., F. Cagnol, G. Soler-Illia, E. L. Crepaldi, H. Amenitsch, A. Brunet-Bruneau, A. Bourgeois and C. Sanchez, Fundamentals of mesostructuring through evaporation-induced self-assembly, Advanced Functional Materials, 14, pp. 309-322 (2004).

2. Urade, V. N., L. Bollmann, J. D. Kowalski, M. P. Tate and H. W. Hillhouse, Controlling Interfaction Curvature in Nanoporous Silica Films formed by Evaporation Induced Self-Assembly from Nonionic Surfactants: II Effect of Processing Parameters on Film Structure, Langmuir, 23, pp. 4268-4278 (2007).

3. Tate, M. P., B. W. Eggiman, J. D. Kowalski and H. W. Hillhouse, Order and Orientation Control of Mesoporous Silica Films on Conducting Gold Substrates Formed by Dip-Coating and Self-Assembly: A Grazing Angle of Incidence Small-Angle X-ray Scattering and Field Emission Scanning Electron Microscopy Study, Langmuir, 21, pp. 10112-10118 (2005).

4. Ogawa, M., Formation of Novel Oriented Transparent Films of Layered Silica-Surfactant Nanocomposites, Journal of the American Chemical Society, 116, pp. 7941-7942 (1994).

5. Ogawa, M., A simple set-gel route for the preparation of silica-surfactant mesostructured materials, Chemical Communications, 1, pp. 1149-1150 (1996).

6. Urade, V. N., T.-C. Wei, M. P. Tate, J. D. Kowalski and H. W. Hillhouse, Nanofabrication of Double Gyroid Thin Films, Chemistry of Materials, 19, pp. 768-777 (2007).

7. Bornside, D. E., C. W. Macosko and L. E. Scriven, Sping coating: One-dimensional model, Journal of Applied Physics, 66, pp. 5185-5193 (1989).

8. Kreith, F., J. H. Taylor and J. P. Chong, Heat and Mass Transfer From a Rotating Disk, Journal of Heat Transfer, pp. 95-105 (1959).

9. Tate, M. P., V. N. Urade, J. D. Kowalski, T. C. Wei, B. D. Hamilton, B. W. Eggiman and H. W. Hillhouse, Simulation and Interpretation of 2D Diffraction Patterns from Self-Assembled Nanostructured Films at Arbitrary Angles of Incidence: from Grazing Incidence (above the critical angle) to Transmission Perpendicular to the Substrate, Journal of Physical Chemistry B, 110, pp. 9882-9892 (2006).

10. Tate, M. P. and H. W. Hillhouse, General Method for Simulation of 2D GISAXS Intensities for Any Nanostructured Film Based on a Discrete Fourier Transform: Implementation of the Distorted Wave Born Approximation, Journal of Physical Chemistry C, Articles ASAP, (2007).