Raman expert Di Yan has created an educational series with us in which he explains Raman for beginners in an easy way.
The Raman fundamentals
Raman spectroscopy belongs into the category of vibrational spectroscopy. This means that it analyzes a sample chemically, by using light to create (excite) molecular vibration, and interpreting this interaction afterwards.
It is based on the inelastic scattering of light that occurs when matter is irradiated by light. As the change of wavelength is very small compared to the wavelength of the irradiating light, the change of wavelength is most easily observed when using monochromatic light sources.
After this (monochromatic) light has interacted with the sample, a very small part of it has changed its wavelength. This is change is called: the Raman effect. We can now collect that light and can use it gain information about the sample.
The Raman effect
For a better understanding it is important to know, that when photons (light) "strike" matter, most of the scattered light remains unchanged in its wavelength.
For example, if you point a green laser pointer at a wall, you will always see a green dot. The scattered light obviously has the same color and this phenomenon is called Rayleigh scattering.
However, also inelastic scattering processes can occur, which then lead to the emission of light with a different wavelength. This usually happens in relation to molecular vibration. This scattering phenomenon, which was predicted by Adolf Smekal in 1923 and discovered by C.V. Raman in 1930, is called the Raman effect.
Using the Raman effect for spectroscopy
Discovering and understanding the Raman effect opened the door to a new kind of spectroscopy. However, Raman spectroscopy did not really take off until the discovery of the laser, since the use of monochromatic light plays an important role.
Thus, the sample is irradiated with a laser and some of the scattered light is analyzed with a spectrograph (dispersive or FT technology). At the end we obtain a Raman spectrum that shows us characteristic signals or "bands" for the material under investigation.
Suprisingly, it is actually quite simple to build a Raman spectrometer! If you look to YouTube, you'll even find DIY videos that will show you how to setup a very basic experiment to acquire spectral data. That's why you'll find Raman spectrometers in almost any form: Raman handhelds, microscopes and process spectrometers.
To acquire Raman spectra, you just have to focus the laser onto the sample you want to investigate. That sample however, must not be showing fluorescence to the laser used for excitation. If that is the case, the fluorescence will cover most of the Raman effect, since it is so weak in comparison.
After the laser light has irradiated the sample, the scattered light is passed through a filter (to get rid of any light from the excitation laser). Then it is directed onto a grating, which distributes the inelastic parts like a prism and according to wavelength. At the end these rays are directed to a CCD sensor which then outputs a spectrum depending on the intensity.
At the beginning we mentioned that a Raman spectrum contains certain "bands" or signals. These are unique for certain functional groups and often also for substances. They provide information about the chemical composition of the substance, but also about crystallinity, polymorphism or changes in pressure and temperature.
A Raman spectrum is a powerful tool for materials research, the development of new pharmaceuticals and wherever chemical microanalyses down to the nanometer range is required. That's right, Raman can analyze samples down to 0.5 µm (500 nm). All you need is a confocal Raman microscope.
Typically, the laser light used in Raman spectroscopy is in the visible range, meaning it can freely pass the glass used in sampling slides or microscope lenses. Hence it is is quite feasible to integrate a Raman spectrometer into the optics of a standard microscope.
In fact, often a microscope is preferred to a classic benchtop Raman spectrometer, as it offers a "point and shoot" approach and doesn't require a lot of additional sample preparation. The sample (e.g. graphene fibers) are placed beneath the objective lense, targeted with the microscope and directly analyzed.
Simply put, a Raman microscope is a laser-based microscopic device for performing Raman spectroscopy. For Bruker, Raman microscopy and imaging plays a central role, which is why we have dedicated a special websites to it.
What is Raman spectroscopy?
Raman spectroscopy is based on the interaction of light with the chemical bonds of a substance. This yields detailed information about chemical structure, polymorphism, crystallinity and molecular dynamics.
What information is provided by Raman spectroscopy?
A Raman spectrum is like a chemical fingerprint that clearly identifies a molecule or material. And just like a human fingerprint, it can be compared with reference libraries to identify the material very quickly or distinguish it from others. Such Raman spectral libraries often contain hundreds of spectra with which the spectrum of a sample is compared to determine the analyte.
It provides insights into a sample's:
Are there sample requirements?
Raman is a universal sampling technique and therefore works for both, inorganic and organic materials. However, since it is based on the rather weak Raman effect, other spectroscopic effects and certain material properties can critically interfere.
In case of sample fluorescence, the sample won't yield a nice Raman spectrum. However, a switch to near infrared (NIR) lasers and FT-Raman technology is a viable solution. Another, more significant problem are strongly absorbing (e.g. black) samples, for example carbon filled polymers.
What's the time needed to obtain a Raman spectrum?
The time required for a Raman measurement depends on several factors, such as the desired spectral quality, the sample properties and of course the Raman spectrometer used. Typically, good quality Raman spectra can be acquired in a few seconds.
What are the applications of Raman spectroscopy?
Raman spectroscopy can be used in all areas where non-destructive (microscopic) chemical analysis and imaging is required. It delivers answers for qualitative and quantitative analytical questions.
In general, Raman is easy to use and quickly provides key information to characterize the chemical composition and structure of a sample. Basically, it matters little whether the samples are solid, liquid or gaseous.
Here are some applications of Raman spectroscopy: