Raman microscope

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Overview

The confocal nature of the Raman micrsocope allows you to probe different layers

The Raman microscope is an optical microscope through which a monochromatic laser source is directed at a sample. The sample absorbs energy and scatters the light at different wavelengths which is then read with a CCD camera. Raman is based inelastic scattering due to characteristic vibrational modes of the material being studied. It is a sensitive spectroscopic technique which allows you to no destructively identify microscopic samples.

Energy level diagram showing the states involved in Raman signal. The line thickness is roughly proportional to the signal strength from the different transitions.

Significance

Graphene is a carbon containing material with sp2 hybridization. It can be made from exfoliated graphene from a highly ordered pyroforic graphene block and using the scotch tape method to flake off individual layers until there is a single level of graphene. Using the Raman microscope we optically probe the material to see if it is graphene, two layer graphene or graphite graphene

Raman is a non-linear application and it follows closely with IR absorption spectroscopy. In the dipole expression the absorption is dependent on the dipole moment. The first term in the dipole expression is the permanent dipole moment which is used for infrared absorption. So we go between vibrational energy levels with a photon of energy tuned to the gap between. If you want to have infrared absorption you need a permanent dipole moment.

In an aromatic compound such as benzene there are lots of delocalized electrons but there is not permanent dipole moment so it has no infrared absorption. Therefore this it is useful to use non-linear spectroscopy in which we examine the strength of the induced dipole in an electric field. Benzene has an extremely strong Raman signal.

IR spectroscopy and Raman spectroscopy are complementary; based on the rules of symmetry there are IR modes which are Raman inactive, and Raman modes which are IR inactive. Water is an IR absorbing material which limits the ability to study wet samples with IR spectroscopy. But water has a weak Raman absorption so we can use Raman to study wet materials or cells in growth media.

Selection Rules for Raman and IR

Raman is an inelastic scattering event. It uses a photon with high energy with respect to vibrational energy levels but low energy with respect to electronic transition. IR absorption is the absorption of a photon equivalent to the gap between vibrational energy levels. If we increase the energy we can get a a absorbed photon in the UV-Vis spectra that transitions between the ground level and the first excited state.

Raman uses an intermediate intensity and the electron bounces off a so-called virtual state and then back down to the ground state. The probability of this is somewhere between 1 in 10 to 1 in 1000 photons incident on the sample. Occasionally a photon scattered will come back with a different energy. It starts from the ground vibrational energy level and relaxes back to the first vibrational energy level and loses energy. This inelastic scattering causes a difference of energy resulting in a difference in color which can be measured with a grating monochromator. The Raman absorption is displayed as intensity (counts on the detector) on the y axis and the center wavelength in wavenumbers on the x axis. Wavelength of the scattered photon is inverse to energy. By subtracting the energy of the scattered photon from the energy of the exciting photons we get a unit of energy. This allows us to normalize for two different excitation sources. So a transition at 1600 wavenumbers will be consistent regardless of the exciting laser used.

Fluorescence is a broad band emission can be hundreds or thousands of wavenumbers wide and this has a probability of up to 100% in some laser dyes. This inelastic fluorescence will completely overwhelm any Raman signal. It is important to pick a laser that is blue enough to gain a substantial Raman cross section but not so blue as to excite fluorescence. Raman spectroscopy is based on the polarizability which is a fairly weak phenomenon. To get measureable signal we have to rely on a very intense light source. For this purpose lasers are very useful, they are intense, they are collimated, and they are able to be focused to a small spot.

In any given Raman spectrum you have four pieces of information.

  • The first is the center wavelength which is the energy at the center of the transition, this is particularly susceptible to changes in the local environment and this is the figure which tells you what sort of materials you have under study.
  • The second is intensity which is a function of counts which tells you how strong the transition is.
  • The third is the line width, how wide the transition is will tell you the degree of broadening you have in your sample.

Given that the line width is important it is important to start with a narrow source which is another convenient benefit of laser systems. They have very narrow line width which allows high resolution in the raman peaks.


Micro Raman spectroscopy is a useful technique for determining the number of layers of small samples of graphene.

Operation

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