Disorder and Dissipation in the Microwave Conductivity Spectra of Carbon Nanotube and Silicon Nanowire Arrays

Reports of unconventional DC electronic transport behavior such as ballistic or Luttinger conduction, as opposed to standard Drude conduction, in carbon nanotubes (CNTs) and silicon nanowires (SiNWs) have led to suggestions that such nanomaterials may have unusual and potentially very attractive microwave conductance properties. Recent theoretical and experimental literature have generated evidence both for and against exotic AC characteristics such as dissipationless microwave conductance and the presence of a kinetic inductance at room temperature.

We have carried out an extensive series of experiments measuring the broadband microwave conductance on arrays of single-wall CNTs and doped SiNWs at frequencies from 0.01 to 50 GHz and at temperatures from room temperature to 4 K. The nanomaterial arrays consisted of order 1,000 to 10,000 nanotubes or wires self-assembled on microwave waveguide electrodes using dielectrophoretic techniques. Magnitude and phase of the reflection and transmission coefficients from the waveguides, both before and after nanomaterial assembly, were measured with precision, signal-to-noise, and systematic reproducibility significantly better than reported in the existing literature. For both CNTs and doped SiNWs a frequency-dependent dissipation is always observed, with the real part of the AC conductance increasing with frequency as w^s, where s~0.3 for SiNWs and s~0.7 for CNTs. This frequency dependence cannot be explained by the Drude model but is consistent with theoretical expectations for AC conductance in a randomly disordered conductor. In particular, we find that the independently measured imaginary part of the AC conductance follows the form and magnitude of the disorder theory prediction using only the same fitting parameter values obtained from the real part of the conductance. Some possible microscopic mechanisms of the disorder and their potential impact on microwave applications of these nanomaterials will be discussed.