Raman Characteristic Peaks – A Short Summary

author: Peggy

2026-09-03

1. What Do Raman Peak Intensity, FWHM, and Shift Represent?

Peak shift (Raman shift change) corresponds to a change in the intrinsic vibrational frequency of a chemical bond. A red shift or blue shift typically reflects sample stress state, lattice distortion, temperature variation, doping effects, or differences in the chemical microenvironment.

Peak intensity reflects the signal strength of a given vibrational mode. It is directly related to material concentration, crystallinity / ordering, molecular orientation, and signal enhancement effects such as SERS or resonance Raman.

FWHM (Full Width at Half Maximum) indicates the homogeneity of lattice or molecular vibrations. A broader peak usually means lower crystallinity, more lattice defects, higher internal stress, or smaller grain size. A narrower peak suggests a more ordered structure, fewer defects, and better crystallinity.

2. Why Do Metals Generally Show No Raman Peaks?

Metals are crystals composed of a free electron gas and metal cations. Their electrons are highly delocalized, so there are no localized polar or non-polar covalent bond vibrations. Raman scattering fundamentally depends on changes in bond polarizability during vibration.

In addition, metals strongly reflect and absorb visible light, which severely quenches any Raman signal. The fast relaxation of free electrons prevents the formation of stable vibrational modes. As a result, pure bulk metals do not produce observable Raman characteristic peaks. Weak Raman signals may only appear from surface oxide layers, adsorbed molecules, or lattice defects on metals.

3. Summary of Common Raman Characteristic Peaks

Material	Characteristic Peaks (cm⁻¹)
Water (H₂O)	Weak signals around 1640, 3200
Ethanol	880, 1050, 1450, 2800–2900
Calcite (CaCO₃)	1086 (strong), 710, 280
Quartz (SiO₂)	464 (main peak), 795, 356
Anatase (TiO₂)	144, 395, 515, 635
Rutile (TiO₂)	143, 235, 447, 610
Diamond	1332 (single strong peak)
Graphite / Graphene	1350 (D band), 1580 (G band)
Polystyrene (PS)	1002 (very strong standard peak)