S.J. Park, A.J. Schmidt, M. Bedewy, A.J. Hart. Measurement of carbon nanotube microstructure density by optical attenuation and observation of size-dependent density variationsPhysical Chemistry Chemical Physics 15:11511-11519, 2013. [http://dx.doi.org/10.1039/C3CP51415C]

TOC Figure Optical Density

Engineering the packing density of CNT forest microstructures is vital to applications such as electrical interconnects, micro-contact probes, and thermal interface materials. For CNT forests on centimeter-scale substrates, weight and volume can be used as a measure of bulk density. However, this is not suitable for smaller samples, including individual microstructures, and moreover does not enable mapping of spatial density variations within the forest. We demonstrate that the packing density of CNTs within individual microstructures can be measured by optical attenuation, with spatial resolution equaling the size of the focused spot. For this, a custom optical setup was built to measure the transmission of focused laser beam through CNT microstructures. The transmittance was correlated with the thickness of the CNT microstructures by Beer-Lambert Law to calculate the attenuation coefficient. We reveal that the density of CNT microstructures grown by CVD can depend on their size, and that the overall density of arrays of microstructures is affected significantly by run-to-run process variations. Further, we use the technique to quantify the change in CNT packing density due to capillary densification. This is a useful and accessible metrology technique for CNTs in future microfabrication processes, and will enable direct correlation of density to important properties such as stiffness and electrical conductivity.