Collective Mechanisms Limiting the Indefinite Growth of Carbon Nanotube Assemblies

While carbon nanotubes (CNTs) have been produced industrially in ton-scale quantities for nearly two decades, scalable manufacturing processes that precisely control the structure, length, and alignment of CNTs are needed to realize the exceptional properties of CNTs at larger scales. Specifically, vertically aligned CNT forests are a model system for further understanding what limits the growth of indefinitely long CNTs, and are building blocks for novel microstructures and multifunctional thin films. This paper will present our current understanding of the limiting mechanisms of CNT forest growth by chemical vapor deposition (CVD). Interactions among a population of CNTs govern their collective growth behavior, and in combination with the performance of individual catalyst particles, prevent indefinite CNT growth using known methods. To understand these effects, we examine the dynamics of catalyst formation, CNT forest self-organization, steady growth, and termination using a holistic approach, combining in situ and ex situ X-ray scattering with spatiotemporal mapping of CNT forest mass, height, and density. While indefinite CNT growth may remain a dream, the present findings have enabled scalable production of CNT forests and horizontally-aligned thin films via continuous CVD, rolling, and printing schemes. These technologies have been realized in our laboratory using prototype bench-scale machines and show promise for cost-effective manufacturing of largearea, organized CNT films.