Raman spectroscopy is one of the vibrational spectroscopic techniques used to provide information on molecular vibrations and crystal structures. This technique uses a laser light source to irradiate a sample, and generates an infinitesimal amount of Raman scattered light, which is detected as a Raman spectrum using a CCD camera. The characteristic fingerprinting pattern in a Raman spectrum makes it possible to identify substances including polymorphs and evaluate local crystallinity, orientation and stress.
Raman spectroscopy has some unique advantages such as:
- Non-contact and non-destructive analysis
- High spatial resolution up to sub-micron scale
- In-depth analysis of transparent samples using a confocal optical system
- No sample preparation needed.
- Both organic and inorganic substances can be measured
- Samples in various states such as gas, liquid, solution, solid, crystal, emulsion can be measured
- Samples in a chamber can be measured through a glass window
- Typically, only 10 msec to 1 sec exposure to get a Raman spectrum
- Imaging analysis is possible by scanning the motorized stage or laser beam
Because of these advantages, Raman spectroscopy plays an important role in both R&D and QA/QC in a variety of industries and academic fields such as semiconductors, polymers, pharmaceuticals, batteries, life sciences and more.
In this article, the basic principles of Raman spectroscopy are explained and typical applications are shown to give specific images of what Raman spectroscopy is used for.
Raman scattering (Raman effect)
When light is scattered by matter, almost all of the scattering is an elastic process (Rayleigh scattering) and there is no change in energy. However, a very small percentage of scattering is an inelastic process, thus a scattered light has different energy from incident light. This inelastic scattering of light was predicted theoretically by Adolf Smekal in 1923 and first observed experimentally by Chandrasekhara Venkata Raman in 1928, which is why this inelastic scattering is called Raman scattering (Raman effect).
Figure 2 shows the energy diagram of Rayleigh and Raman scattering. The incident light interacts with the molecule and distorts the cloud of electrons to form a “virtual state”. This state is not stable and the photon is immediately re-radiated as scattered light. Rayleigh scattering is a process in which an electron in the ground level is excited and falls to the original ground level. It does not involve any energy change so Rayleigh scattered light has the same energy as incident light (which means both lights have the same wavelength). Raman scattering can be classified as two types, Stokes Raman scattering and anti-Stokes Raman scattering. Stokes Raman scattering is a process in which an electron is excited from the ground level and falls to a vibrational level. It involves energy absorption by the molecule thus Stokes Raman scattered light has less energy (longer wavelength) than incident light. By contrast, anti-Stokes Raman scattering is a process in which an electron is excited from the vibrational level to the ground level. It involves an energy transfer to the scattered photon thus anti-Stokes Raman scattered light has more energy (shorter wavelength) than incident light.
The dominant process is Rayleigh scattering, and Raman scattering is an extremely weak process in that only one in every 106 - 108 photons scatters. The ratio of Stokes Raman and anti-Stokes Raman scattering depends on the population of the various states of the molecule. At room temperature, the number of molecules in an excited vibrational level is smaller than that of in the ground level, thus generally the intensity of Stokes Raman light is higher than anti-Stokes Raman light. In standard Raman measurement, Rayleigh scattered light is rejected using a filter and only the Stokes Raman scattering is recorded for simplicity. The intensity of anti-Stokes Raman light increases relative to Stokes scattering as the temperature rises, thus the intensity ratio of anti-Stokes and Stokes light can be used to measure the temperature of a sample.
Figure 3 shows a Raman spectrum of ethanol. The Raman spectrum is expressed in a form of intensity of scattered light versus wavenumber (the reciprocal of wavelength, called Raman shift). For example, the Raman peak at 547.14 nm obtained by a 532 nm excitation wavelength can be converted into a wavenumber as below;
Light is often characterized by wavelength, however in Raman spectroscopy, wavenumber is commonly used because it is linearly related with energy and makes the form of the Raman spectrum independent of excitation wavelength. For example, the Raman peak of crystalline silicon always appears at a wavenumber of 520 cm-1 whatever excitation wavelength is used. However, if you use wavelength as a orizontal axis unit, the Raman peak of silicon appears at 547.14 nm when 532 nm excitation is used and 818.41 nm with 785 nm excitation.
Raman scattering arises from molecular vibration causing a change in polarizability. This means that intense Raman scattering occurs from symmetric vibrations which induce a large distortion of the electron cloud around the molecule. A peak appearing in the Raman spectrum will be derived from a specific molecular vibration or lattice vibration. Peak position shows the specific vibrational mode of each molecular functional group included in the material. The same vibrational modes for each functional group will show a shift in peak position due to the nearby environment surrounding the functional group, thus it is said the Raman spectrum shows the "molecular fingerprint" of the target.
The shape of a Raman peak is important, not just its position. Whether there is much or little crystallinity can be read from the width of the peak. Any residual stress inside the crystal can also be evaluated from the direction and amount of any shift of the Raman peak.
What is Raman spectroscopy used for?
In the wake of the emergence of high-performance Raman imaging equipment, Raman spectroscopy is now used in a variety of fields. Year after year, new applications arise as new markets and industries develop. But in spite of this, the level of awareness of Raman spectroscopy is not as high as that of the analytical methods that everyone knows. In order to introduce the diverse characteristics of Raman spectroscopy, specific examples are incorporated in this chapter.
When foreign material is discovered on the surface of manufactured goods or inside a transparent film, a spectrum can be taken for identification. Raman spectroscopy can analyze buried material, or particles of foreign material smaller than 1 micron which cannot be measured by FTIR. Because Raman spectroscopy is also sensitive in lower frequency regions of the spectrum, identification of inorganic material is also possible.
Figure 4 shows an analysis of foreign matter in a polymer film. When embedded foreign matter is measured, a spectrum of the foreign body can be obtained, mixed with that of the normal material. Under these circumstances, by taking the difference between the mixed spectrum and the spectrum from only the normal portion, the spectrum of the foreign matter itself is obtained, allowing for analysis of the foreign material.
Crystal polymorphism of a material occurs when the chemical formula is the same but the crystal structure of the material is different. Raman spectroscopy allows for analysis of crystal polymorphism, with similar but slight differences in position and intensity ratios of particular peaks between polymorphs, sufficient for identification. Hydrated and anhydrated materials, salt and salt-free materials can also be distinguished in a similar manner.
Figure 5 shows the Raman spectrum of the pseudo-polymorphism of CaCl2 and the crystal polymorphism of mefenamic acid. The waveforms are similar, but discriminating between them can carried out by looking at the subtle differences in peak positions and intensity ratios.
Both polymerization and damage to the molecular structure are accelerated via irradiation by ultraviolet rays. By recording the Raman spectrum over time, one can follow changes in the molecular structure of the sample. For example, in a polymer the C = C bond can be formed by the irradiation of ultraviolet rays, thus changes can be tracked by focusing on the change in the corresponding peak intensity.
An ultraviolet curable resin is a polymer which is composed of monomers and then hardened by the monomers crosslinking with each other due to the absorption of ultraviolet rays. If one compares the Raman spectrum after irradiation and before ultraviolet irradiation, the peak intensity dwindles with irradiation; thus you can see that the cure is progressing.
In a substance where the crystallinity changes with heating, changes can be analyzed using the Full Width at Half Maximum (FWHM) of the peak appearing in the Raman spectrum. By performing peak fitting, it is possible to quantify the FWHM and thus carry out quantitative analysis.
If the material is the same (PET) but with a different crystalline structure, that difference will show up in the FWHM of the C = O peak (peak corresponding to the C = O bond vibrations). Since there is a good correlation between the FWHM of the peak and the density of material, it is possible to evaluate the density of PET by Raman spectroscopy.
Under the addition of pressure or stress, a crystal structure becomes distorted. Raman peaks corresponding to a particular crystal structure and molecular vibration of the substance will shift under tensile strain and compressive strain. (Tensile → shift lower; compressive → shift higher) Since the peak wavenumber shifts by an amount proportional to stress, by measuring the exact wavenumber shift, the amount of strain can be accurately measured.
Silicon has an optical mode at 520cm-1, where the peak position will be shifted toward the low-wavenumber side under tensile stress, and shifted to the high-wavenumber side under compression stress. By using a formula for conversion relating the amount of shift and stress, it is possible to evaluate the magnitude of residual stress.
Stretching a film in a certain direction will orient the polymer chains in that direction and preferentially orient any splitting properties. With polarized Raman measurements you can evaluate the orientation and extent of the chains’ alignment. Raman spectroscopy can in addition analyze opaque samples, with the advantage of obtaining the ratio of the sample’s orientations conveniently as a numerical value.
Figure 9 shows a comparison of the Raman spectra measured with polarization of the laser light perpendicular and parallel. When parallel, the peaks corresponding to the C-C and CH2 activations in the macromolecular framework are strong. By comparison, when the polarization is perpendicular, the C=O and NH peaks are strong instead.