Microwave determination of sp2 carbon fraction in nanodiamond powders

We measured the bulk effective complex permittivities of various nanodiamond powders, of different average particle sizes (in the range from about 10 nm to 1 μm) using a microwave cavity perturbation technique at 2.49 and 5.72 GHz. We observed an increased permittivity with decreasing particle size, both in terms of the polarisation and microwave loss. In separate experiments we used Raman Spectroscopy to determine the amount of surface sp2 carbon present in the same samples. We also used X-Ray Diffraction to estimate the crystallite size and to determine the chemical compositions, whilst we measured overall particle size using Field Emission Scanning Electron Microscopy. We interpret the increased permittivity in the smaller particles as being due to larger volume fraction of sp2 hybridised carbon material, and verify this by comparison with a simple model that predicts that the amount of microwave absorption for fixed powder volume should be inversely proportional to the average particle size. Hence, we conclude that the microwave method is a fast, accurate and non-invasive way of estimating the relative proportion of sp2 carbon in nanodiamond samples.