Investigation of the Chemical Effect of Solvent during Ligand Exchange on Nanocrystal Thin Films for Wearable Sensor Applications

Ligand exchange processes have been attracting tremendous interest and are necessary when fabricating nanocrystal (NC) thin films for various applications. As ligand exchange processes are based on solution treatment processes, understanding solvents' effects on the ligand exchange process is necessary. Herein, we investigated the effects of exchanging solvents and rinsing solvents on silver (Ag) NC thin films during the ligand exchanging and rinsing steps. We studied the relationships between solvent properties, such as polarity and steric hindrance, and the structural, electronic, and electromechanical properties of NC thin films. A model system was proposed to explain the obtained relationships. We found that exchanging solvents and rinsing solvents during the ligand exchange process should be separated to regulate the ligand exchange process so that films with desired properties can be obtained. Films optimized for different purposes (highly conductive/highly electromechanically sensitive) were fabricated with the same materials and ligands using different solvents for each process. On the basis of these films, we fabricated a flexible strain sensor using an all-solution process at room temperature. This device exhibits excellent performance, including a high gauge factor up to 400, and high reliability, and stability; furthermore, it can detect minute human motion and sound.