Microwave impedance spectroscopy of dense carbon nanotube bundles

We have performed impedance spectroscopy of dense carbon nanotube (CNT) bundles in the broad frequency range from 10 MHz to 67 GHz. Dense CNT bundles were formed on sharp tips from aqueous suspension by ac dielectrophoresis and incorporated into on-wafer test structures. The frequency response of the bundles can be fit to a model with frequency-independent elements in the entire frequency range up to 67 GHz strongly suggesting that CNT properties do not depend on the frequency throughout the whole frequency range. The measurements at microwave frequencies allowed separate characterization of the bundle/metal electrode contacts and the bundle bulk. Effects of different CNT fabrication and suspension processing routes on bundle characteristics were identified. We have also made a preliminary estimation of the average inductance per current carrying shell in the bundles. For good quality nanotube bundles, the inductance has been found to fall within the range from approximately 3.5 to 40 nH/μm. With decreasing nanotube quality, the implemented estimation procedure yields higher values with a large uncertainty. Systematic measurements of devices with individual nanotubes are required to provide more accurate data.