Label-free linear and non-linear vibrational spectroscopy for functional materials: state-ofthe-art and future perspectives

Functional materials, with their specific properties and functions, are instrumental in advancing technological applications across diverse fields such as biomedicine, energy, aerospace, and electronics. To drive innovation in these materials, it is essential to understand their molecular and structural compositions and to correlate these to their switchable macroscopic properties. This correlation is fundamental to their development and optimization. In this context, label-free linear and non-linear vibrational spectroscopy techniques such as infrared (IR) absorption and Raman spectroscopy are invaluable. These techniques offer molecular-level insights into the composition, structure, and dynamic molecular behavior of these functional materials. Moreover, they enable non-invasive, detailed analyses, which are critical for preserving the integrity of these materials during study. This article explores how state-of-the-art label-free linear and non-linear vibrational spectroscopy has effectively been applied in the study of functional materials. It highlights the necessity of these techniques and discusses their many advantages. Notably, their ability to deliver real-time, high-resolution, and non-destructive insights into the molecular and functional properties of functional materials makes them indispensable techniques. Consequently, these methods are accelerating innovation and paving the way for breakthroughs in the design and application of functional materials.