A Raman spectrometer induces characteristic Stokes/anti-Stokes scattering by illuminating a sample with a high-power laser. A dichroic mirror separates weak Raman-shifted signals from strong elastically-scattered signals in order to identify molecules with a spectrometer.
Raman spectroscopy is a technique that provides a unique spectral fingerprint by detecting and identifying molecules through their characteristic vibrational and rotational energy level structure. It has become a popular technique in diverse fields like process and pharmaceuticals, explosives detection, semiconductor processing quality control, and biotechnology.
Raman spectroscopy is a technique that provides a unique spectral fingerprint by detecting and identifying molecules through their characteristic vibrational and rotational energy level structure. In contrast to fluorescence methods, Raman spectroscopy facilitates direct detection of a molecule without the need for sample preparation. For this reason, it has become a popular technique in diverse fields like process and pharmaceuticals, explosives detection, semiconductor processing quality control, and biotechnology. It is also used in many fields of research, including carbon nanomaterials.
Lasers are typically used as excitation sources for Raman applications because they deliver high power light in tightly focussed spots. A high-power beam is required to generate inherently weak Raman signals. When incident on the sample, the excitation light will incur:
· Elastic scattering (Rayleigh scattering)
· Inelastic scattering (Stokes and/or anti-Stokes Raman scattering)
The latter provides the characteristic information about the sample.
The ratio of Raman-to-Rayleigh scattered light is at most one part in a million. As a result, the intense, Rayleigh scattered excitation light can saturate the detector. A Rayleigh edge filter is placed in front of the detector to block Rayleigh signals and allow high transmission of the weak, inelastically scattered, and wavelength-shifted Raman signal. A sensitive detector is employed to detect these very faint signals known as Raman-shifts.
The intensity of Raman signals can be increased by illuminating the sample with a UV laser rather than a visible/IR laser like in standard Raman spectroscopy.
Resonance-enhanced Raman scattering
Visible or near-infrared (IR) lasers and detectors are commonly used for Raman spectroscopy because of their wide availability. Use of such wavelengths results in weak signals that are susceptible to intrinsic noise such as sample autofluorescence. This reduces the signal-to-noise ratio. The intensity of Raman signals can be enhanced through the use of components in the ultraviolet (UV) wavelength range. This is because UV photon energies usually lie within the electronic spectrum of a molecule. This effect is known as resonance-enhanced Raman scattering.
Surface-Enhanced Raman Spectroscopy (SERS)
In SERS, the analyte molecules are absorbed in a nanoscale-roughened metal surface such as silver or gold. Such a surface is developed by either metallically coating or electrochemically processing the substrate. The metal nanostructures vibrate when the surface is excited by a laser. This effectively ‘stimulates’ the material and creates surface charges, increasing the electric field encompassing the metal surface. The Raman signal is greatly enhanced when a molecule is situated in this large electric field.
Surface-Enhanced Resonance Raman Spectroscopy (SERRS)
The Raman intensity can be amplified further by combining SERS and resonance-enhanced Raman scattering approaches in a technique abbreviated to SERRS. A roughened metal surface is used in conjunction with a wavelength that provides the energy necessary to excite the analyte molecule to real electronic transitions.
Tip-Enhanced Raman Spectroscopy (TERS)
TERS uses a large electric field to amplify the Raman intensity similarly to SERS and SERRS. However, the increased fields are caused by a probe that is placed in close proximity to the specimen. The enhanced Raman signal is restricted to the size and location of the probe’s atomically sharp tip, usually coated with gold or silver. As a result, some TERS systems provide single-molecule sensitivity.
Raman imaging combines a CCD camera and specialised software with a Raman spectrometer and laser scanning system.
Firstly, the chemical compounds are identified and located by laser scanning the sample, as in confocal microscopy. In this process, Raman spectra are acquired from an array of sample points spanning the defined region of interest.
Then, detailed images based on the sample’s Raman spectrum are generated. The CCD camera, combined with a spectrometer, acquires the complete spectrum at every pixel. Using a look-up-table (LUT) of known Raman spectra, the raw data is translated to a false colour image related to the material’s composition and structure.