Raman spectroscopy of defects in materials

Raman spectroscopy is a tool for physicochemical analysis of a wide variety of media (solid or liquid, organic or mineral, semiconductor or insulator, crystal or glass, polymer, etc.), providing information on structure: chemical bonds, crystalline arrangement, symmetry, phase... As such, the Raman spectrum provides a fingerprint of the compound analyzed, enabling identification of a substance in a mixture or heterogeneous material. As a result, it is used in a wide range of applications: chemistry, biology, physics, archaeology and even on Mars.

As the structure of a medium is sensitive to the environment (stress, temperature, etc.), Raman spectroscopy can also be used to extract a physical parameter external or internal to the medium: deformation, concentration and composition of a product, characteristics of a phase transition, degree of order of a structure, anharmonicity, and so on. The term "Raman sensor" is becoming increasingly popular. For this purpose, it is possible to take advantage of one or other of the characteristics of a Raman line: position of the maximum, width and intensity. Raman spectroscopy can also be used for multi-scale studies, thanks to the judicious choice of optical elements during measurement.

Recent technological innovations (lasers, rejection filters, detectors) have given Raman spectroscopy numerous advantages for both surface and volume measurements. The miniaturization of instruments has made it possible to diversify products (transportable and portable devices) and applications, which ten years ago were confined almost exclusively to laboratory research.

Raman spectroscopy is a non-destructive and non-invasive technique, enabling measurements both through a container and remotely via optical fibers, with timescales compatible with chemical reactions or industrial processes. As a result, it is now perfectly suited to studies in industrial environments, particularly in situ analyses, and even in hostile environments. In addition, ongoing improvements in data processing capabilities via software (e.g. chemometrics) are opening up major fields of study in real time. This has also led to significant progress in mapping studies, making Raman imaging much more readily available. Raman spectroscopy is therefore naturally a technique of choice for defect studies, providing useful information on dopant incorporation and localization, phase mixing, inhomogeneity and so on. This enables us to better understand and control the properties of materials, or to optimize the manufacturing process.

The aim of this article is to describe and explain the various possible effects of defects on the Raman spectra of solid materials. In the first part, the fundamental principles governing Raman spectroscopy...