Mechanosynthesis Group, Department of Mechanical Engineering, University of Michigan, 2350 Hayward Street, Ann Arbor, Michigan 48109, USA.
Phys Chem Chem Phys. 2013 Jul 21;15(27):11511-9. doi: 10.1039/c3cp51415c. Epub 2013 Jun 10.
Engineering the density of carbon nanotube (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 to calculate 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 relative mass density of individual CNT 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 a focused laser beam through CNT microstructures. The transmittance was correlated with the thickness of the CNT microstructures by Beer-Lambert-Bouguer 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 microstructure 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.
工程化碳纳米管 (CNT) 森林微观结构的密度对于电互连、微接触探针和热界面材料等应用至关重要。对于厘米级衬底上的 CNT 森林，可以使用重量和体积来计算密度。然而，这对于较小的样本（包括单个微结构）并不适用，而且无法实现森林内空间密度变化的映射。我们证明了可以通过光衰减来测量单个 CNT 微结构的相对质量密度，其空间分辨率与聚焦光斑的大小相等。为此，我们构建了一个定制的光学装置来测量聚焦激光束通过 CNT 微结构的传输。根据比尔-朗伯-玻尔定律，将传输率与 CNT 微结构的厚度相关联，以计算衰减系数。我们揭示了 CVD 生长的 CNT 微结构的密度可以取决于其尺寸，并且微结构阵列的整体密度受到运行到运行过程变化的显著影响。此外，我们使用该技术量化了由于毛细凝结导致 CNT 微结构密度的变化。这是未来微制造工艺中 CNT 的一种有用且易于使用的计量技术，并且将能够将密度与刚度和电导率等重要性质直接相关联。
