The field of carbon nanomaterials is growing rapidly and Raman spectroscopy is gaining popularity as a characterization tool
because of the wealth of information that it can provide about these materials. A few factors are important to consider before
you get started characterizing carbon nanomaterials with Raman spectroscopy. One of the most important considerations is understanding
the impact that excitation laser power can have on carbon nanomaterial samples. It is particularly important that you have
precise control over your excitation laser power. This column installment will discuss these important factors further and
will provide guidelines on how to accurately characterize carbon nanomaterials with Raman spectroscopy.
One of the most important factors to consider in characterizing carbon nanomaterials with Raman spectroscopy is the impact
that excitation laser power can have on samples. The impact of laser power is twofold. First, it is important to be aware
that with some materials it is possible to damage or alter the sample with the excitation laser. This damage can be very obvious
in extreme cases in which the laser burns a hole in the sample, although in other cases the damage can be more subtle and
if you have not taken care to avoid damaging the sample it may result in spectra that do not represent the true sample and
could easily be misinterpreted. Figure 1 provides an example of one such situation with a sample of C60 fullerene. Here we can see that the C60 begins to break down into other structures, probably amorphous carbon, with as little as 0.5 mW of laser energy applied to
the sample. It turns out that C60 is one of the more sensitive carbon nanomaterials, but even when dealing with more laser-tolerant materials such as carbon
nanotubes, you have to exercise some caution as it is possible that surface modifications to these materials may not be as
laser-tolerant as the base materials.
Figure 1: Effect of increasing laser power on C60 (532-nm excitation laser).
The second way that laser power can impact samples of carbon nanomaterials is by changing the temperature of the sample. The
Raman spectra of many carbon nanomaterials can be very sensitive to even small temperature changes. Figures 2 and 3 provide
examples of multiwall carbon nanotubes and single-wall carbon nanotubes, respectively, that demonstrate the effect that relatively
small changes in laser power can have on the Raman spectra by inducing small temperature changes in the samples. Most carbon
nanomaterials are black in color and will absorb significant amounts of visible light. Most of this absorbed energy will be
converted to heat and this will change the temperature of the sample in the locality where the excitation laser is applied.
Figure 2: Effect of thermal softening with increasing laser power on multiwall carbon nanotubes (532-nm excitation laser).
In both of these examples we see significant shifts in the G-band, and in the multiwall carbon nanotube example we also see
some shifting of the D-band. This is the effect of thermal softening of the planar graphene configuration of the tubes. As
the temperature increases from the laser excitation the bonds become somewhat looser, which results in a lower vibrational
energy and hence a shifting of the G-band to lower wavenumbers. The band shifts that we can observe here are certainly significant
to any attempt to characterize the samples based on a fine interpretation of the Raman spectrum, and in some cases they may
even be significant for simple quality assessments of the sort that are routinely done by comparing the ratio of the intensity
of the D-band to the intensity of the G-band. In the example presented in Figure 2, the D-band/G-band intensity ratio decreases
by 6% as we increase the laser power from 1 mW to 2 mW and then decreases by 3% when we further increase the laser power from
2 mW to 3 mW. This may or may not be significant to the quality assessment depending on how tight the tolerances are, but
in any case it certainly magnifies the variation that is present in the measurement.
Figure 3: Effect of thermal softening with increasing laser power on single-wall carbon nanotubes (780-nm excitation laser).