Characterization of Carbon Nanomaterials Using Raman Microscopy

There is increasing growth in the field of carbon nanomaterials. Raman spectroscopy is gaining a lot of popularity due to the large amount of information it can provide. It is important to consider certain facts before beginning the characterization of carbon nanomaterials with Raman spectroscopy.

Firstly, the impact of excitation laser power on carbon nanomaterial samples needs to be understood. Accurate control over excitation laser power is essential. DXR Raman Instruments from Thermo Scientific offer the required control.

Impact of Laser Power

There are two types of impact of laser power:

Firstly it is essential to be aware that with certain materials, the sample may be altered or damaged with the excitation laser. The laser may burn a hole in the sample or may cause subtle damage. This may cause spectra that are not representative of the true sample. Figure 1 shows an example of one such situation using a C60 fullerene sample. When a minimal amount of energy of just 0.5 mW of laser energy is applied, C60 begins to breakdown into other structures such as amorphous carbon. C60 is probably a highly sensitive carbon nanomaterial, however surface modifications to materials like carbon nanotubes may not be as tolerant to lasers as the base materials.

Figure 1. Effect of increasing laser power on C₆₀ (532 nm excitation laser)

Secondly laser power may change the sample temperature. The Raman spectra of a number of carbon nanomaterials is highly sensitive to even very minute temperature changes. Two examples of multiwall carbon nanotubes and singlewall carbon nanotubes are shown in figure 2 and 3 which illustrate how very small changes in laser power can impact the Raman spectra by inducing temperature changes in the sample. Since most of these carbon nanomaterials are black, they will absorb considerable amounts of light that get converted to heat hence changing the sample temperature.

Figure 2. Effect of thermal softening with increasing laser power on multiwall carbon nanotubes (532 nm excitation laser)

Figure 3. Effect of thermal softening with increasing laser power on singlewall carbon nanotubes (780 nm excitation laser)

It is observed that in both these examples, there are significant shifts in the G-band, and there is some shifting of the D-band in the multiwall carbon nanotube example.

In the example illustrated in Figure 2, as the laser power is increased from 1 mW to 2 mW, the D-band/G-band intensity ratio decreases by 6% and when the laser power is further increased from 2mW to 3mW, the intensity ratio decreases by 3%. This may or not impact the quality assessment, based on how tight the tolerances are, but in any case it increases the variation that is present in the measurement.

Solutions

There are certain relatively simple solutions to controlling these impacts.

Initially when new materials or materials with new modifications are being tested, it is wise to begin with very low power. In case a number of samples of the same material are expected, it is recommended that a small area of one sample is tested to check for laser tolerance. Spectra at a range of laser powers are collected to determine how much power can be applied before there is any damage in the Raman spectrum. Once it is clear on what is the upper laser power limit that can be safely handled by the material, the measurements can be conducted confidently.

The second aspect to take care of is controlling the temperature. It is important to avoid excitation of a sample area larger than the area over which emissions will be collected and focused by the detector. One must understand that any section of the sample exposed to the laser but out of view of the detector is not generating a Raman signal but only generating heat.

DXR Raman Instruments from Thermo Scientific ensure that the laser spot size on the sample is comparable to the area observed by the detector or slightly smaller than the same. This kind of an optical design provides excellent control of temperature effects. A patented auto alignment system is also included in the DXR Raman systems insuring that these areas are always closely aligned.

Sample temperature can also be controlled by having precise control over the laser power at the sample and adjusting the same in small increments. DXR Raman instruments include a laser power regulator. This unit uses a gradient-neutral density filter, which can be adjusted with high precision. This is combined with a laser power meter that is calibrated to the sample position in a way that the laser power is monitored actively and adjusted to make sure the set power and delivered power match. Thus the laser power can be finely controlled within 0.1mW increments offering both confidence and a large amount of flexibility to optimize measurement conditions.

The concept of the laser power regulator is shown in Figure 4

Figure 4. Principle of operation of Thermo Scientific Laser Power Regulator

Finally, a number of carbon nanomaterial samples necessitate operating with very low excitation laser power. If the amount of heat generated on the sample is reduced, it is easy to control thermal effects. As laser power, and hence the heat being generated at the sample is increased, more variability in the spectra due to these temperature effects are observed.

In case the laser power is kept low and a large amount of heat is not generated, the sample will dissipate the heat more efficiently hence stabilizing spectral variations. As it is important to work with low laser power and low Raman emissions there needs to be a system that will perform well under these conditions. The key to achieving good sensitivity under low Raman emission conditions is the ability to maintain a system in a well aligned state.

Figure 5. Thermo Scientific DXR Raman microscope

Figure 6. Thermo Scientific DXR SmartRaman spectrometer

Conclusions

Raman spectroscopy is definitely a very powerful tool used to characterize carbon nanomaterials however it is important to take care to accurately control the laser power for preventing the risk of sample damage and the risk of introducing additional variability to the measurement. Thermo Scientific DXR Raman instruments offer a superior level of laser power control and high sensitivity with low laser power combining all the benefits offered by Raman with the accurate control over measurement parameters required for the confidence required in the results.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Materials & Structural Analysis.

For more information on this source, please visit Thermo Fisher Scientific – Materials & Structural Analysis.