Among the most disordered of these materials is the CVD film called "diamond-like carbon," or DLC. This term is actually a misnomer. The lack of the sharp diamond band at 1332 cm^–1 argues against the assignment to diamond. The spectrum of DLC actually shows the G and D bands, enormously broadened and overlapped. However, oddly, the material has physical properties approaching those of diamond — optical semi-transparency, electronic insulation, and high hardness. Today it is understood that films of DLC are composed of covalent mixtures of sp² - and sp³ -bonded carbon (5), which can account for these physical properties. Because of these properties, DLC is used to coat every magnetic hard disk in every modern computer. Systematic analysis of the Raman spectra of DLC films by band-fitting has been used successfully to predict the tribological performance of the hard disk coatings (6).

Figure 1: Raman spectra of diamond (D), graphite (G), microcrystalline graphite (µG), disordered carbons (DC) in various states of disorder, and glassy carbon (GC).

Another material called "glassy carbon" is actually also a misnomer. Its spectrum shows the D band with higher intensity than the G band, and both bands are sharper than in other carbons, with high intensity in the D band. Glassy carbon is quite crystalline, but the crystallites are very small (7). Figure 1 shows the spectra of some of these common materials.

Fullerenes fall into two classes of materials. The so-called buckyballs are enclosed polyhedral balls, with C₆₀ and C₇₀ the most well known. The second class of material is the nanotube, which is composed of single- or multiwalled carbon nanotubes of rolled graphene sheets (graphene being a single layer of graphite). These tubes can be metallic, semiconducting, or insulating, depending upon the electronic structure, which is determined by the size, type, and chirality of the tube. (The chirality defines the axis of the graphene sheet around which the tube is conceptually formed.) The Raman spectra of these tubes have been characterized fully (8,9) in terms of their phonon behavior and double resonance with the possible electronic transitions. Raman has been found to be the technique of choice for characterizing the tubes because the frequency of the "radial breathing mode" in the region between 75 and 400 cm^–1 is predictive of the size, type, and chirality of the tube. In addition, the E_2g mode near 1580 cm^-1 is observed to split, presumably due to the curvature of the tube, which removes the twofold degeneracy.