Ultrafast fluid transport through the core of single-wall carbon nanotube (SWNT) channels promises to advance membrane applications, from efficient water purification and low-cost separation of high-value components, to next-generation protective garments. For the last application, we recently demonstrated cm²-area, vertically-aligned, SWNT-polymer composite membranes that combined both high breathability and protection in a single functional material, thus breaking the typically elusive trade-off between the two.¹ As we seek to further enhance the performance of these membranes, we target vertically-aligned SWNT “forest” growth toward: a) minimizing SWNT diameter to maximize protection via size exclusion; b) maximizing SWNT number density to maximize breathability; and c) scaling up SWNT growth area to eventually incorporate into garments.

We perform low-pressure chemical vapor deposition (CVD) in an AIXTRON^® Black Magic cold-wall furnace, featuring a wafer-scale, local heater stage and gas showerhead. The combination of a low flux of carbon precursors with sub-nm Fe/Mo catalyst films on alumina-coated Si wafers maintains small-diameter SWNTs. High-resolution TEM and X-ray scattering² confirm that our forests contain >99% SWNTs below 3.5 nm diameter (on wafers up to 4 inches). Consistent with literature reports, 6 at% Mo optimally preserves small, densely packed particles, templating growth of forests with high number densities up to 1.4x10¹² cm^-2. In an effort to control the forest thickness and uniformity across large substrate areas, we investigate the area-dependent growth kinetics and temporal density decay. Despite exhibiting a growth rate that is initially uniform across the substrate, growth at the center self-terminates first, while edge growth continues at a sublinear rate for tens of minutes. Understanding these varying reaction lifetimes that arise on a single substrate is key to engineering large-area SWNT membranes with highly uniform transport characteristics.

To scale SWNT forest production and overcome non-uniformities in wafer-scale growth, we seek to simplify the catalyst film formulation. While wafer-scale growth from Fe films of equal thickness in the absence of Mo is markedly more robust over multiple sequential batches, the SWNT diameter distributions have larger means and tails, which are detrimental to the rejection properties of our membranes. Tuning the Fe thickness down renders smaller SWNTs, but the range of densities grown from these Fe-only films is 0.2-0.6x10¹² cm^-2, regardless of thicknesses studied (0.35-0.65 nm). This challenge of accessing a parameter space that co-optimizes growth yield of small, monodisperse diameter, high-density SWNT forests highlights a deficiency in conventional catalyst design and suggests a need for improved designs that could unlock a range of SWNT applications.

This work is supported by the Defense Threat Reduction Agency (DTRA) D[MS]2 project under Contract No. BA12PHM123 and was performed under the auspices of U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

1. Bui, N.; Meshot, E. R.; Kim, S.; Peña, J.; Gibson, P. W.; Wu, K. J.; Fornasiero, F., Ultrabreathable and Protective Membranes with Sub-5 nm Carbon Nanotube Pores. Advanced Materials 2016, 28 (28), 5871-5877.
2. Meshot, E.; Zwissler, D. W.; Bui, N.; Kuykendall, T. R.; Wang, C.; Hexemer, A.; Wu, K. J. J.; Fornasiero, F. Quantifying the Hierarchical Order in Self-Aligned Carbon Nanotubes from Atomic to Micrometer Scale. ACS Nano 2017, 11 (6), 5405-5416.