Raman Spectroscopy & Microscopy

Felix Bachmair, winter semester 2018/19

Raman Spectroscopy & Microscopy is a powerful spectroscopically tool for spatially resolved material analytics and characterisation. It’s based on inelastic scattering effects (Raman Effekt), when monochromatic light interacts with molecules or solids.

History

Raman spectroscopy was named in honour of C.V. Raman, who demonstrated the Raman-Effect the first time, together with K.S. Krishnan, and was honoured with the Nobel Prize in physics, as the only Indian scientist. ^[1]

Physical Background

In general, all spectroscopic methods studying their interaction of electromagnetic radiation with matter and are based on emission, absorption, fluorescence or scattering phenomena’s. The Raman spectroscopy is a scattering technique which is based on the Raman-Effect and analyses the interaction of a monochromatic incident radiation with the molecular vibrations or crystal phonons. ^[1]

The electromagnetic radiation, which is used for Raman spectroscopy, is monochromatic radiation with certain frequency $//<![CDATA[ \begin{array}{l}\nu_\text{0}\end{array} //]]>$. When this radiation incident on a system, whether it is gas, solid, liquid or glass, most of the radiation is transmitted through the system, without any change in frequency. Some of the electromagnetic radiation is scattered, $//<![CDATA[ \begin{array}{l}\nu`\end{array} //]]>$, by the molecules or atoms ($//<![CDATA[ \begin{array}{l}\sim 1\end{array} //]]>$ photon of $//<![CDATA[ \begin{array}{l}10^7\end{array} //]]>$) while inducing an electric dipole moment, which deforms the molecule. Due to this deformation the molecules start to vibrate with a characteristic frequency $//<![CDATA[ \begin{array}{l}\nu_\text{m}\end{array} //]]>$.

$//<![CDATA[ \begin{array}{l}\nu`= nu_\text{0} \pm nu_\text{m}\end{array} //]]>$ ^[2]

Scattering can happen by a change in vibrational, rotational or electronic energy of the molecule. The interacting phonons can then be scattered in three different ways with difference in the energy which is scattered back. ^[2]

$//<![CDATA[ \begin{array}{l}E=h \nu=\frac{hc}{\lambda}\end{array} //]]>$ (with: $//<![CDATA[ \begin{array}{l}h\end{array} //]]>$ is Planks constant, $//<![CDATA[ \begin{array}{l}\lambda\end{array} //]]>$ the wavelenght and $//<![CDATA[ \begin{array}{l}c\end{array} //]]>$ the speed of light)

Rayleigh Scattering

When incident photon, interacts with molecular vibrations or crystal phonons, it will be absorbed, and the molecule gets excited in an energetically higher level.(Figure 1) When the molecule returns into his energetically ground state by emitting light with the same energy or frequency $//<![CDATA[ \begin{array}{l}\nu_\text{0}\end{array} //]]>$ like the incoming photon, this is called elastic scattering or the so-called Rayleigh-Scattering and is the dominate scattering interaction.

$//<![CDATA[ \begin{array}{l}E=h\nu_{0}\end{array} //]]>$

$//<![CDATA[ \begin{array}{l}\Delta E=0\end{array} //]]>$

This occurs if the dimensions of the molecule are less than the wavelengths of the incoming beam (no active Raman-mode). ^[3]

Raman Scattering or Effect

The Raman-Effect differ from the Rayleigh scattering effects, in such a way, that a part of the scattered photons will undergo a slight shift in frequency. If scattered photons have a different energy compare to incident photons, the scattering process is called inelastic. ^[2] This shift in energy or frequency ($//<![CDATA[ \begin{array}{l}\Delta E\end{array} //]]>$), results from transitions between different vibration or energetically levels, which are the consequence of polarization processes of molecules. ^[3] (Figure 1)

$//<![CDATA[ \begin{array}{l}E = h \nu_\text{0} \pm \Delta E\end{array} //]]>$

with: $//<![CDATA[ \begin{array}{l}\Delta E=h\nu_\text{m}\end{array} //]]>$^[4]

This shift can occur in 2 different ways.

Stokes & Anti-Stokes frequency:

A photon, which is in an energetically ground state ($//<![CDATA[ \begin{array}{l}\nu=0\end{array} //]]>$) can absorb another photon with the energy $//<![CDATA[ \begin{array}{l}h\nu_\text{0}\end{array} //]]>$ and scatter it back with a different energy $//<![CDATA[ \begin{array}{l}h(\nu_\text{0}-\nu_\text{m})\end{array} //]]>$. If the frequency of the inelastic scattered radiation is lower as the excitation radiation, the process is called Stokes-Scattering. (Figure 1)

$//<![CDATA[ \begin{array}{l}E=h\nu_\text{0}-\Delta E\end{array} //]]>$ ^[3]

Molecules or atoms, which are already in an excited state, are also able to do inelastic scattering of radiation. If this scattered radiation has a higher frequency compare to the incoming, this is called Anti-Stokes-Scattering. (Figure 1)

$//<![CDATA[ \begin{array}{l}E=h\nu_\text{0}+\Delta E\end{array} //]]>$ ^[1]

The frequency can shift by $//<![CDATA[ \begin{array}{l}(\nu_\text{0}+\nu_\text{m})-\nu_\text{0}=\nu_\text{m}\end{array} //]]>$ or $//<![CDATA[ \begin{array}{l}(\nu_\text{0}+\nu_\text{m})-\nu_\text{0}=\nu_\text{m}\end{array} //]]>$, which correspond to the vibration frequency of the molecule. ^[3]

Raman Spectrum

The spectral shift or Raman shift, $//<![CDATA[ \begin{array}{l}\Delta \nu\end{array} //]]>$, can be measured by the spectrometer and is given by the wavenumber and is characteristic for each molecule or atom.

$//<![CDATA[ \begin{array}{l}\Delta \nu=\frac{1}{\lambda_\text{laser}}+\frac{1}{\lambda_\text{spectral}}\end{array} //]]>$ ^[2]

The resulting spectra is presented in form of the intensity of the scattered photon vs. the shift in wavelengths in $//<![CDATA[ \begin{array}{l}[nm]\end{array} //]]>$ or wavenumber in $//<![CDATA[ \begin{array}{l}[cm^-1]\end{array} //]]>$. ^[2], ^[5] (Figure 2)

The intensity or strength of Raman signals is more or less the scattering or excitation efficiency. It depends, in complex manner, on the polarizability of the molecules, concentration of molecules, scattering properties of the sample and mainly on the intensity of the radiation source, which depends on the choice of laser. ^[3] The Raman scattering efficiency is indirect proportional to the fourth power of the wavelengths of incoming beam. ^[6]

$//<![CDATA[ \begin{array}{l}P_\text{scattered} \propto \frac{1}{\lambda^4}\end{array} //]]>$ ^[6]

Working Principle

Instrumentation & Measurement

The Raman machine consist out of a spectrometer and an optical microscope, with different magnifications. This gives the operator the opportunity to do spatially resolved measurement (approximately $//<![CDATA[ \begin{array}{l}1\mu m\end{array} //]]>$) plus imaging of small samples. (Picture 1)

The spectrometer is composed of a light source, a sample holder, monochromator and detector. (Figure 3)

Before measuring with Raman spectrometer has to be calibrated with a reference material like, silicon or diamante. The measurement itself is compared to other analytical methods, very fast. The spot of interest is focused by the optical microscope and then illuminated or excited with a, by pinhole diaphragm, focused beam on a few $//<![CDATA[ \begin{array}{l}\mu m\end{array} //]]>$ area. The scattered light is collected with lenses and send through a monochromator, which filter out the elastic scattered radiation. The rest is detected by a CCD detector and shown by a Raman spectrum. ^[8]

To measure Raman scattering with a good signal-to-noise ratio, the used light source should have a relatively high intensity. Therefore, modern spectrometers are equipped with lasers, with different wavelengths and intensities, to adapt the excitation energy to the sample, in order to avoid problems, like fluorescence or destructive effects. (Table 1) The wavelength of the excitation beam should also be beyond the adsorption area of the analyte. ^[2]

Table 1: Example of different wavelengths for different samples ^[2]

Due to the fact, that the excitation efficiency can be expressed by the number of scattered photons, selecting of the right laser wavelength, for specific material identifications, plays an important role. As lower the wavelengths as higher is the number of scattered photons or as higher is the efficiency, which gives a better resolution in shorter time. Lower wavelengths are preferred for inorganic materials but leading to higher fluorescence. For example, IR-Laser, with 785 nm, provides the best balanced between efficiency and low fluorescence, which makes them applicable 90% of Rama active materials. With increasing wavelength, the probability to damage the sample is increasing but decrease fluorescence phenomena, which fit to materials like dyes, oils or coloured polymers. ^[6]

Types of measurement

There are different types of measurement executable. The microscope can focus at different depths into the sample, to obtain a depth profile of the material. The simplest one is single point measurement of an arbitrary spot. With a moveable sample holder, it’s possible to perform automatically several single point measurements. This enables the user to make line or even mapping measurements to show for example chemical distribution of a selected area. ^[8]

Hand held spectrometers for non-destructive fieldwork testing

There are several commercially hand-held Raman spectrometers available, which gives the opportunity to measure in the field, without any sampling. They only have one laser source (mostly 785 nm) and are still not as precise, under certain conditions, as the stationary ones. Therefore, their range of applications limited, but can be further improved for applications in the field of non-destructive testing. (Example: Mira 1, Metrohm)

Samples

One of the big Advantages of this method, is the not needed sample preparation, like you need it for other analytical methods. In general, you can use all kinds of samples form, which fits on the sample table. Better results can be reached with polished samples, like for example thin sections. There is a brought spectra of suitable materials, solid or liquid, which can be analysed by this method: ^[7]

• Minerals, rocks, crystals
• Mineral inclusions
• Glasses
• Fluids
• Alloys
• Cells
• Blood
• Bone material
• ……

Metals or metallic compounds are not suitable or show a weak Raman effect.

Evaluation of the spectrum & possible information’s

Raman spectrum are like a “fingerprint” of a specific material or functional group, due to the unique vibrational characteristic. The evaluation is done over determination of the peak position, to compare their characteristics with reference known material of a database. (Figure 4) As a result, the availability of such a reference is important to facilitate the analysis of materials and should be further improved. ^[9]

This is used to get very fast information´s about the composition of a material. By further evaluation of other peak characteristics, it’s possible to get additional information about the structure, crystallinity, modification and element concentration. (Figure 5) It´s also possible to distinguish between different polymorphs, which have the same chemistry, but a different atomic structure, which is a big advantage compared with other analytical tools, which only can give information’s about the chemistry. ^[7]

Problems

Possible problem during the measurement can be the heating and burning of the sample due to a too high power of the laser beam, which can also lead to re-crystallisation. The Raman Effect can be hide by fluorescence of the sample, which can be inhibited by increasing the wavelengths of the laser.

Applications

Geo- & Material science

Example:

Characterisation of minerals, their structure and modifications, degree of crystallisation, differentiation between amorphous and crystalline materials, investigation of fluid inclusions ^[2] Device inspections of modern electronic and optoelectronic devices, like crystalline quality, doping level and defect analysis of semiconducting materials, like graphene. ^[10]

Archaeology & Art History

Examples:

Characterisation of pigments and gems ^[11] Determination of ages examining the Raman spectra of paintings and artefacts, like pottery. Distinguish of original work from fakes. ^[8]

Bioscience & Pharmaceutical Research

Example:

Quality management by chemical analysis of drug composition or information’s about the precipitation and crystallization characteristics.^[12] Possibility of in-Situ measurements of biological samples, like living cells in pharmaceutical area tablets, inside polymer containers. Studying of biological molecules. ^[8]

Constructions industry & conservation

Example:

Material analytics for cement: Presentation and chemical identification of cement phases. Investigation of the effect of changes in production conditions for the composition and properties of the different cement phases. ^[12]

Damage analysis in preservation of historical monuments: Inventory and status detection for restauration processes. Material characterisation of natural stones, mortal and paints on plaster; damage-assessment for humidity and saline load ^[13]

Advantages & Disadvantages

Table 2: Advantages and Disadvantages ^[7]

Literature

1. Bumbrah, G. S., & Sharma, R. M. (2016). Raman spectroscopy–Basic principle, instrumentation and selected applications for the characterization of drugs of abuse. Egyptian Journal of Forensic Sciences, 6(3), 209-215.
2. Neuville, D. R., de Ligny, D., Henderson, G. S.: Advances in Raman Spectroscopy Applied to Earth and Materials Sciences. Reviews in Mineralogy & Geochemistry (2014); Vol. 78, p. 509-541.
3. Skoog, D. A., Holler, F. J., Crouch, S. R.: Instrumentelle Analytik, Grundlagen-Geräte-Anwendungen (6. Auflage). Springer pupl., Berlin Heidelberg (2013), p. 475-490.
4. Demtröder, W.: Laserspektroskopie: Grundlagen und Techniken (5.Aufalge). Springer pupl., Berlin Heidelberg New York (2007). P. 345 -366.
5. https://www.nanophoton.net/raman/raman-spectroscopy.html, 14.12.2018, 14:00
6. Zhou, P., & Tek, W. : Choosing the Most Suitable Laser Wavelength For Your Raman Application (2015). vol. 1, p. 1-6.
7. Kaliwoda, M., Vorlesungsscript (Power-Point): Raman Spectroscopy. Mineralogische Staatssammlung München, SNSB
8. Smith, E., Dent, G.: Modern Raman Spectroscopy, A Practical Approach. John Wiley & Sons, Ltd (2005), p. 23-91.
9. Rostron, P., Gaber, S., Gaber, D.: Raman Spectroscopy, Review. International Journal of Engineering and Technical Research (2016), p. 1-16.
10. Zoubir, A. (ed.), Raman Imaging, Springer Series in Optical Science 168 (2012), Chapter 2: Tiberji, A., Camassel, J.: Raman Imaging in Semiconductor Physics: Applications to Microelectronic Materials and Devices. DOI:10.1007/978-3-642-28252-2_2, Springer-Verlag Berlin Heidelberg, p. 39-83.
11. Ropret P. & Madariaga J. M.: Applications of Raman spectroscopy in art and archaeology, J. Raman Spectrosc. (2014), 45; pages 985–992, doi: 10.1002/jrs.4631.
12. https://www.laborpraxis.vogel.de/raman-imaging-die-anwendungsfelder-werden-breiter-a-554967/, 12.12.2018, 13:30.
13. Zöldföldi, J., Viefhaus, T., Ullmann, D.: Was geht? Zerstörungsfreie Untersuchungsmethoden in der Denkmahlpflege, Möglichkeiten und Grenzen mobiler schwingungsspektroskopischer Methoden in der Schadensanalitik am Bauwerk. IFS Bericht 55/2018, p. 43-5.