(357g) Nucleation and Growth of Spontaneously Aligned Regions in Carbon Nanotube Thin Films: A Morphological Analysis

In the ongoing search for scalable organization of carbon nanotubes, deposition of dispersed nanotubes on a filter membrane via pressure-driven flow has long been a promising candidate. Thin films of uniform thickness, from sparse sub-percolation networks to thick freestanding buckypapers, can be fabricated by this method. Minimal apparatus is required and the films can be readily transferred to various substrates.

The films constructed by this method do not generally exhibit any organization or ordering (aside from uniform thickness). However, recent work [1-4] has shown that under specific process conditions, short and long-range ordering of the nanotubes does occur.

As fluid moves across the membrane, the dispersed nanotubes are unable to pass. This results in elevated concentration of tubes near the membrane surface. Furthermore, the velocity of the fluid medium presumably confines the tubes to orient themselves parallel to the plane of the membrane. Tubes near the membrane eventually form domains of uniform local alignment. While the fluid velocity and concentration of the nanotube dispersion are important factors for the emergence of aligned domains, it seems that formation time (time required for the entire volume of fluid to pass through the membrane, leaving dry nanotube membrane behind) is the critical parameter, with organization in the films increasing for longer formation times.

We prepared films with a uniform nanotube density of 0.46 µg/mm2 â?? approximately enough to form a monolayer of nanotubes on the membrane. Formation times were varied by altering fluid velocity, and by altering the sample concentration. In the first case, the solution volume was held constant at 100 mL and vacuum pressure was varied. In the second, vacuum pressure was held constant (near the maximum level used in constant volume series) and the sample was diluted to volumes of 100 to 800 mL.

Domains â?? regions of uniform nanotube orientation â?? were identified from SEM images by a two-step algorithm. SEM images of the sample were converted to orientation maps, then k-means segmentation was applied to the maps, and continuous regions of the clusters which met threshold criteria for shape and coherency were identified as domains.

Visual inspection of the images and trends in domains identified by the algorithm show that in all cases, the film morphology transitions from many small domains to fewer, and larger domains. However, the effect is more pronounced in the constant fluid velocity case.
Nearest neighbor statistics of the domains show that, for the constant fluid velocity samples, increasing formation time causes the film morphology to transition from one with small domains which are relatively different from their neighbors in terms of orientation angle and size, to films of larger domains, more similar to their neighbors. The same trends in nearest neighbor statistics were not present in the samples of varying fluid velocity, though average domain area did still increase.

Thus it appears that the formation of films with large domains is in part due to the reconciliation of orientation vectors among the small domains which occur when the process is first initiated. This effect was present in the constant fluid velocity samples, but absent in the long formation-time samples with very low fluid velocities.

Further investigation with these methods into other factors such as surfactant concentration will help pinpoint the driving forces behind organization of the nanotubes and guide development of large-scale aligned nanotube films.

References

1. Dan, B., Ma, A. W. K., Hároz, E. H., Kono, J., & Pasquali, M. (2012). Nematic-Like Alignment in SWNT Thin Films from Aqueous Colloidal Suspensions. Industrial & Engineering Chemistry Research, 51(30), 10232â??10237. doi:10.1021/ie3001925

2. King, B., & Panchapakesan, B. (2014). Vacuum filtration based formation of liquid crystal films of semiconducting carbon nanotubes and high performance transistor devices. Nanotechnology, 25(17), 175201. doi:10.1088/0957-4484/25/17/175201

3. Oh, J. Y., Yang, S. J., Park, J. Y., Kim, T., Lee, K., Kim, Y. S., â?¦ Park, C. R. (2015). Easy preparation of self-assembled high-density buckypaper with enhanced mechanical properties. Nano Letters, 15(1), 190â??197. doi:10.1021/nl5033588

4. He, X., Gao, W., Xie, L., Zhang, Q., Lei, S., Li, B., â?¦ Adams, W. W. (2016). Wafer-scale monodomain films of spontaneously-aligned single-wall carbon nanotubes, (April). doi:10.1038/nnano.2016.44