Source: Laboratory of Dr. Ryoichi Ishihara — Delft University of Technology

Raman spectroscopy is a technique for analyzing vibrational and other low frequency modes in a system. In chemistry it is used to identify molecules by their Raman fingerprint. In solid-state physics it is used to characterize materials, and more specifically to investigate their crystal structure or crystallinity. Compared to other techniques for investigating the crystal structure (e.g. transmission electron microscope and x-ray diffraction) Raman micro-spectroscopy is non-destructive, generally requires no sample preparation, and can be performed on small sample volumes.

For performing Raman spectroscopy a monochromatic laser is shone on a sample. If required the sample can be coated by a transparent layer which is not Raman active (e.g., SiO₂) or placed in DI water. The electromagnetic radiation (typically in the near infrared, visible, or near ultraviolet range) emitted from the sample is collected, the laser wavelength is filtered out (e.g., by a notch or bandpass filter), and the resulting light is sent through a monochromator (e.g., a grating) to a CCD detector. Using this, the inelastic scattered light, originating from Raman scattering, can be captured and used to construct the Raman spectrum of the sample.

In the case of Raman micro-spectroscopy the light passes through a microscope before reaching the sample, allowing it to be focused on an area as small as 1 µm². This allows accurate mapping of a sample, or confocal microscopy in order to investigate stacks of layers. Care has to be taken, however, that the small and intense laser spot does not damage the sample.

In this video we will briefly explain the procedure for obtaining a Raman spectra, and an example of a Raman spectrum captured from carbon nanotubes will be given.

1. Turn on the required laser and select the correct optics for the wavelength used. Let the laser warm up to get a stable emission over time.
2. Perform the required calibration of the Raman spectroscope. This depends on the instrument, but here an internal Si reference sample is used to calibrate the Raman shift to the known position of the crystalline Si Raman peak. Si is often used as it gives a strong sharp peak at a known position which is insensitive to the laser wavelength. First, the Raman spectrum of the reference sample

The Raman spectrum taken from multi-walled carbon nanotubes using a 514 nm laser is shown in Figure 1. The linear baseline has been removed and the data has been normalized to the most intense feature around 1,582 cm^-1.

Several peaks can be observed, which originate from different crystalline features of the sample. The D-peak at 1,350 cm^-1 originates form double resonance elastic phonon scattering with a defect in

Raman spectroscopy can be applied in a wide range of fields, ranging from (bio)chemistry to solid-state physics. In chemistry, Raman spectroscopy can be used to investigate changes in chemical bonds and identify specific (organic or inorganic) molecules by using their Raman fingerprint. This can be done in either the gas, liquid, or solid-state phase of the material. It has been, for instance, used in medicine to investigate the active components of drugs, and Raman gas analyzers are used for real-time monitoring of resp