Raman Spectroscopy for Chemical Analysis
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- 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.
- 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 is obtained using an appropriate exposure energy and time. The wavenumber of obtained spectrum is compared to values from the literature (in the case for Si a strong peak at 520.7±0.5 cm-1 should be observed). In case of a mismatch, the position of the CCD with respect to the monochromator (often a grating) has to be changed. Most commercially available Raman tools include calibration routines to achieve this.
- Place the sample underneath the microscope and focus on the layer which has to be investigated. In general, a close-able dark enclosure is used to remove stray light. Make sure the path of the laser is not obstructed by light absorbing or Raman active layers in order to obtain a clean spectrum. In the literature Raman spectra can be found taken from many materials, which can be used to determine which materials might influence the experiment. If unknown peaks appear beside the peaks known to originate from the sample, they can either by cosmic rays (which are generally only a few wavenumbers wide and very intense), or other layers interfering with the measurement. If the layer is thin compared to the attenuation length of the laser in the material, it is likely that the substrate underneath will also be probed.
- Select the range of wavenumbers which should be scanned by the monochromator. This is highly sample dependent. Generally in the literature the regions at which the interested Raman peaks will appear can be found. For completely unknown samples a wide range (e.g., 100–2,000 cm-1) test scan can be performed. Extended scans will consume more time, though. Select a laser intensity which produces sufficient signal, but which doesn't damage the crystal lattice of the material under investigation (e.g., if amorphous Si is investigated a high intensity laser can crystallize the sample). This can be checked by imaging the same spot twice, if the spectrum changes damage might have occurred. If the signal is too weak the exposure time can be increased.
- Acquire the spectrum of the sample. This is generally done automatically by the instrument while scanning the monochromator and reading the CCD output. No background scans have to be performed if the sample is in a completely dark enclosure, otherwise stray light will influence the measurement.
- Investigate the data using appropriate software and by using the literature. This can include the removal of cosmic rays, which appear as very sharp and intense lines in the spectrum and can generally be completely removed.. Interference with the substrate or contaminants can result in a baseline, which can be removed by fitting a appropriate curve (e.g., linear line or spline) to the regions of the spectrum which are expected to be flat (i.e. do not contain Raman peaks originating from the sample). For some materials the different Raman peaks can appear so close to each-other that peak deconvolution might be necessary, for this check the literature on the material.
Source: Laboratory of Dr. Ryoichi Ishihara — Delft University of Technology
Raman spectroscopy is a technique for analyzing vibrational and other low…
Chapters in this video
0:00
Overview
0:59
Principles of Raman Spectroscopy
4:23
Performing Raman Spectroscopy
6:44
Results
7:34
Applications
8:54
Summary
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