A fingerprint from scattered light
When laser light hits a material, almost all of it scatters with the same color it arrived with. A tiny fraction scatters with slightly different energy, because the light has exchanged energy with vibrations of the atoms in the material. Raman spectroscopy records that small shift. Plotting intensity against the energy shift (conventionally in wavenumbers, written cm⁻¹) gives a spectrum whose peaks correspond to the material’s vibrational modes.
Since vibrations depend on how atoms are bonded and arranged, each form of a substance has its own spectrum. That makes Raman a fingerprinting method, and carbon is a particularly good case, because its forms differ greatly in bonding while being chemically identical. Carbon allotropes such as diamond and graphite differ only in atomic arrangement, yet their spectra look nothing alike.
Why it suits carbon
Raman has several practical advantages for carbon work:
- It usually needs no sample preparation and does not damage the sample at sensible laser power.
- With a microscope attached, it can examine spots a micrometer or so across, so it can map a coating or a tool edge.
- It can be done through transparent windows or in air.
- It responds strongly to the carbon-carbon bonds of interest.
Reading a carbon spectrum
Diamond shows one sharp peak near 1332 cm⁻¹. This single narrow line reflects the regular network of bonds in the diamond crystal structure. In high-quality crystal the peak is very narrow; in material with defects, strain or small grains it broadens or shifts slightly.
Graphite shows a strong feature near 1580 cm⁻¹, called the G band, which arises from in-plane bond stretching in the sp2 carbon sheets. Disordered graphite also shows a D band near 1350 cm⁻¹, tied to defects and edges, and the relative size of the D and G bands is widely used as a rough measure of disorder. A second-order feature at higher shift, around 2700 cm⁻¹, changes in shape with the number of layers, which is how graphene sheets are distinguished from thicker graphite.
Amorphous carbon, including diamond-like carbon, produces broad humps in the region of the D and G bands instead of sharp peaks. The position and shape of the humps vary with the proportion of diamond-like and graphite-like bonding, so careful analysts can estimate that balance, though with some uncertainty. This is how coatings sold as hard carbon films are separated from true diamond.
Carbon nanotubes have their own signs, including a G band with a characteristic shape and low-shift “breathing” modes whose positions relate to tube diameter.
Quality of diamond films
For chemical vapor deposition diamond, the spectrum is routinely used as a quality check. A sharp peak at 1332 cm⁻¹ with little background suggests a high proportion of good diamond. A broad band near 1500 cm⁻¹ or a visible G band suggests non-diamond carbon, often at grain boundaries. In fine-grained films, such as nanocrystalline diamond, those features are more prominent simply because boundaries make up more of the volume.
The peak position also carries information about stress. Compressive stress tends to shift the diamond line to a higher wavenumber and tensile stress to a lower one, so a precisely calibrated instrument can give a picture of residual stress in a film or at a coating edge. Such values need careful calibration and are usually presented as estimates.
Practical cautions
Raman is not a cure-all, and several issues can mislead.
- Fluorescence. Some samples, including diamonds with certain defects, emit light strongly under the laser, which can bury the Raman signal. Changing the laser wavelength often helps.
- Laser heating. Dark, absorbing carbon can heat and even burn at high power, altering what is being measured.
- Different sensitivity. Graphite-like carbon scatters much more strongly than diamond for the same amount of material, so a visible G band does not mean the sample is mostly graphitic.
- Small area. The spot sampled may not represent the whole part, so several locations should be measured.
- Interpretation. Peak fitting and assignment require judgment, and the numbers quoted in the literature depend on the laser used.
Raman works best alongside other methods. X-ray diffraction confirms crystal structure over larger volumes, and electron microscopy shows what the microstructure looks like, so together they give a fuller picture of any carbon material.