Manufacturing thin films of densely packed horizontally aligned carbon nanotubes

Dense packing of carbon nanotubes (CNTs) over long-range dimensions is necessary to replicate their outstanding properties in functional thin films. We present a continuous method for transforming pillars of vertically aligned (VA) CNTs into densely packed, horizontally aligned (HA) CNT ribbons and sheets, which can be directly used on wafer-scale dimensions and/or patterned by photolithography and plasma etching to achieve feature dimensions down to the micron scale. In this process, a small roller is used to ``topple'' millimeter-tall VA-CNT microstructures and to simultaneously compress them, thus increasing the packing fraction of CNTs from 2{\%} to 60{\%}. We formulate design guidelines for selection of pattern geometry, roller diameter and material, and the kinetics of the rolling motion. This enables precise control of the HA-CNT film topography and thickness, and the packing density and orientation of the CNTs. Nanoindentation of the HA-CNT films reveals that the initial tortuosity of the VA-CNT forest determines the ultimate achievable densification. Electrical conductivity of ribbons is characterized using dc-four-point testing of lithographically-patterned CNT ribbons with Au contacts. The HA-CNT structures are easily transferred to other substrates, enabling integration with CMOS and MEMS fabrication, and with alternative substrates such as flexible plastics.