Raman spectroscopy analysis of disordered and amorphous carbon materials: A review of empirical correlations

Raman spectroscopy is a widely used nondestructive technique for characterizing the structure of amorphous carbon materials. Its prominent spectral features, such as D and G bands, provide valuable insights into structural properties, including defects, graphitization, and disorder. Over decades, empirical correlations have been developed to relate Raman spectral features to carbon structures and properties, forming the foundation of quantitative Raman analysis. However, the accuracy and applicability of these correlations, derived under specific conditions, require further clarification. This review critically examines the derivation and limitations of key empirical correlations. The widely used three-stage model is addressed as a framework for understanding trends in spectral features and discussing correlations, such as I(D)/I(G) with crystalline domain size (L_a) and G band parameters with material structures and properties. We then address the correlation between photoluminescence (PL) background slope and hydrogen (H) content in hydrogenated amorphous carbon (a-C:H). Finally, we review the use of these correlations across diverse carbon materials, including graphite, amorphous carbon, soot, and activated carbon, emphasizing the limitations and complementary validation needs. This work aims to clarify the conditions for reliable utilization of empirical correlations, promoting more meaningful Raman-based structural analysis of carbon materials.