Raman spectroscopy identifies materials through their unique molecular vibrations. When integrated with scanning electron microscopy through CLEM (Correlative Light and Electron Microscopy), it bridges the gap between high-resolution morphological imaging and chemical identification—providing information that neither technique can deliver alone. This CLEM approach enables simultaneous optical and electron-beam analysis at the exact same sample location.

How Raman Spectroscopy Works

A monochromatic laser beam illuminates the sample, and most photons scatter elastically (Rayleigh scattering). However, a small fraction of photons exchange energy with molecular vibrations, shifting to different wavelengths (Raman scattering). These wavelength shifts create a spectrum that acts as a molecular fingerprint, unique to each compound’s chemical bonds and structure.

Unlike infrared spectroscopy, Raman works well with aqueous samples and requires minimal preparation. It can analyze samples through glass or plastic containers and works on samples ranging from bulk materials to sub-micron particles.

Why CLEM: Combining SEM and Raman?

Key Capabilities

Molecular Identification

Every material has a characteristic Raman spectrum. Compare unknown spectra against databases to identify compounds, polymers, minerals, and contaminants. Distinguish between polymorphs, isomers, and different crystalline phases of the same compound.

Carbon Material Analysis

Raman is the gold standard for carbon materials. Distinguish between:

• Diamond: Sharp peak at 1332 cm^-1
• Graphite: G band at 1580 cm^-1, D band indicates disorder
• Graphene: 2D band shape indicates layer count
• Carbon Nanotubes: Radial breathing modes reveal tube diameter
• Amorphous Carbon: Broad D and G bands with varying ratios

Polymer Identification

Identify plastics, coatings, and organic materials that appear similar in SEM images. Raman spectral libraries contain thousands of polymer references for rapid matching. Detect polymer degradation, contamination, and additive distribution.

Pharmaceutical Analysis

Identify crystalline forms (polymorphs) that affect drug bioavailability. Map active ingredient distribution in tablets. Detect counterfeit medications through spectral comparison.

Applications

Failure Analysis

• Contaminant Identification: Identify organic and inorganic contamination on surfaces
• Coating Analysis: Measure coating thickness, composition, and degradation
• Corrosion Products: Identify specific corrosion phases and mechanisms

Materials Research

• 2D Materials: Characterize graphene, MoS₂, and other 2D materials
• Composites: Map filler distribution in polymer matrices
• Thin Films: Analyze stress, crystallinity, and composition

Geology & Mineralogy

• Mineral ID: Rapid identification without sample preparation
• Inclusions: Identify fluid and solid inclusions in host minerals
• Gemstones: Authenticate gems and detect treatments

Life Sciences

• Microplastics: Identify and characterize microplastic particles
• Biological Tissues: Map lipid, protein, and mineral distributions
• Biofilms: Study composition and structure of microbial communities

Raman vs EDS: The Power of CLEM

Raman and EDS are complementary, not competing techniques. CLEM brings them together:

• EDS: What elements are present? Great for metals, alloys, inorganics
• Raman: What compounds/molecules? Great for organics, polymers, carbon materials
• CLEM Integration: Complete elemental AND molecular characterization from the same sample location simultaneously

Practical Considerations

• Fluorescence: Some samples fluoresce under laser excitation, masking Raman signal. Different laser wavelengths (532nm, 633nm, 785nm) help mitigate this
• Sample Heating: Dark or absorbing samples may heat under laser; reduce power if needed
• Spatial Resolution: Optical diffraction limits Raman to ~0.5-1 μm resolution
• Non-destructive: With appropriate power settings, Raman is generally non-destructive