Researchers from imec
and the University of Michigan have reported a new technology to fabricate complex
three-dimensional microstructures, with intricate bends, twists, and multidirectional
textures, starting from vertically aligned carbon nanotubes (CNT).
The resulting assemblies have a mechanical stiffness exceeding that of microfabrication
polymers, and can be used as molds for the mass production of 3D polymer structures.
The method is straightforward, in that it requires only standard two-dimensional
patterning and thermal processing at ambient pressure.
Complex surfaces with precisely fabricated nanosized features are needed in,
for example, metamaterials, substrates for cell culture and tissue engineering,
smart active surfaces, and lab-on-a-chip systems. But existing methods of fabricating
3D microstructures all have their drawbacks, requiring tradeoffs in feature
geometry, heterogeneity, resolution, and throughput. This new method, which
the researchers have termed ‘capillary forming’, promises a path
to robust, deterministic fabrication of intricate structures with high mechanical
The approach to capillary forming of CNTs starts with patterning a catalyst
layer on a silicon wafer, using optical lithography. Second, that layer is used
to grow microstructures made of vertically aligned CNTs – CNT forests
– through thermal chemical vapor deposition (CVD) at atmospheric pressure.
Next, a solvent such as acetone is condensed on the substrate. This is done
by positioning the substrate, with the CNT patterns facing downward, over a
container with the boiling solvent. The solvent vapor rises through the container
and condenses on the substrate. Due to capillary rise, the solvent is drawn
into each CNT microstructure independently. After the substrate has been exposed
to the vapor for the desired duration, it is removed from the container. As
a result of the process of infiltration and evaporation of the solvent liquid,
capillary forces will bundle the CNTs, resulting in a transformation of the
initial 2D geometries into intricate 3D structures.
With this method, it is possible to construct robust 3D assemblies of filamentary
nanostructures. The researchers have demonstrated this method through the fabrication
of a library of diverse CNT microarchitectures. A bending motion, for example,
can be combined into twisting and bridge-shaped architectures which cannot be
made using standard lithography. This new approach to manipulate nanoscale filaments
using local mechanical deformations makes it easier to deterministically design
and fabricate 3D microarchitectures with complex geometries as well as nanotextured
surfaces. Yet it only requires a standard patterning and thermal processing
at ambient pressure.
This work received the Robert M. Caddell award for outstanding research in
materials and manufacturing.
Michael De Volder, Sameh H. Tawfick, Sei Jin Park, Davor Copic, Zhouzhou Zhao,
Wei Lu, A. John Hart, “Diverse 3D Microarchitectures Made by Capillary
Forming of Carbon Nanotubes”, Advanced Materials Volume 22, Issue 39,
pages 4384–4389, October 15, 2010.