microRaman

MicroRaman, or micro-Raman spectroscopy, is a technique that combines Raman spectroscopy with optical microscopy to obtain Raman spectra from micron- or sub-micron-scale regions of a sample. It enables chemical identification and spatial mapping of composition within small areas, using the diffraction-limited focus of a microscope objective. The method relies on Raman scattering, where photons interact with molecular vibrations and experience an energy shift. The resulting spectrum shows Raman shifts in wavenumbers (cm-1) and is typically dominated by Stokes lines; anti-Stokes signals are weaker at room temperature.

In a typical microRaman setup, a laser is focused onto the sample through a microscope objective. Scattered light is collected, passed through filters to reject elastically scattered light, and dispersed by a spectrometer onto a detector such as a CCD. A confocal arrangement improves lateral and axial resolution and reduces background from out-of-focus regions. By raster-scanning the sample or recording spectra at many points, microRaman produces chemical maps that reveal the distribution of different molecular species or crystal phases.

Common applications span materials science, semiconductors, carbon-based materials (graphene, carbon nanotubes), minerals, polymers, and thin films. It is used for phase identification, stress and strain analysis, quality control, and failure analysis. Biological samples can be studied, though fluorescence and sample damage must be managed.

Enhancements include surface-enhanced Raman scattering (SERS) for higher sensitivity and tip-enhanced Raman scattering (TERS) for nanoscale spatial resolution. Limitations include inherently weak signals, potential fluorescence background, and possible photodamage or heating of sensitive samples. Overall, microRaman is a versatile, non-destructive tool for localized chemical analysis.