Mark H. Wall
Thermo Fisher Scientific Inc.
Raman spectroscopy is a valuable measurement technique for characterizing carbon nanomaterials. However it is important to understand what is actually represented in the measurement before attempting to draw any conclusions from it. Typical samples of carbon nanomaterials include a range of related structures with diverse characteristics. This presentation will provide a good understanding of what is represented in a Raman spectrum of carbon nanotubes and explains how to optimize the measurement to ensure that you are capturing all the subtleties that may be present in a collection of carbon nanotubes. The physical structure of carbon nanotube samples has direct implications on the Raman measurement. Carbon nanotubes will almost always appear as loose, uniform black powders when they are in their bulk form. However, there is actually significant diversity within these samples which affects both the material properties and the Raman spectrum of the material. It is not uncommon for samples to have significant levels of carbonaceous impurities, defective nanotubes, and smaller levels of other irregularities such as residual catalyst. Beyond this, the number of nanotube walls can vary within the tube – single, double, or multiwall as well as tube diameter, chirality, and length. All of these variation sources will result in at least a small level of difference in the Raman spectrum. Given these factors range from sample to sample, reflecting all facets of the Raman spectrum of these materials is critical so we are not misled by a spectrum which only captures one variant of the nanotubes that are present. We will begin with the interpretation of the Raman spectrum of carbon nanotubes. Following this we will discuss some of the carbon nanotube properties that could detrimentally effect their Raman measurement with strategies to limit their impact. Two sampling methodology will be presented that will allow one to obtain an accurate assessment of a bulk collection of carbon nanotubes. Finally, we will demonstrate how these techniques may be used to gauge the separation of semiconducting carbon nanotubes from raw carbon nanotubes.