Raman Spectroscopy of nanomaterials: How spectra relate to disorder, particle size and mechanical properties

The purpose of this review is to provide non-specialists with a basic understanding of the information micro-Raman Spectroscopy (μRS) may yield when this characterization tool is applied to nanomaterials, a generic term for describing nano-sized crystals and bulk homogeneous materials with a structural disorder at the nanoscale – typically nanoceramics, nanocomposites, glassy materials and relaxor ferroelectrics. The selected materials include advanced and ancient ceramics, semiconductors and polymers developed in the form of dots, wires, films, fibres or composites for applications in the energy, electronic and aeronautics–aerospace industries. The text is divided into five sections:
• Section 1 is a general introduction.
• Section 2 outlines the principles of conventional μRS.
• Section 3 introduces the main effects for nanomaterials, with special emphasis on two models that connect Raman spectra features to “grain size”, namely the Phonon Confinement Model (PCM) and the Elastic Sphere Model (ESM).
• Section 4 presents the experimental versatility of μRS applied to nanomaterials (phase identification, phase transition monitoring, grain size determination, defect concentration assessment, etc.).
• Section 5 deals with the micro-mechanical aspects of μRS (“Raman extensometry”). Special emphasis is placed on the relationship between the stress-related coefficients Sɛ/σ and the macroscopic response of the materials to the applied stress.