Photoactive Thin Films for Multi-modal Sensing

K.J. Loh
University of California, Davis, US

Keywords: sensing, thin films, nanocomposite

Summary:

In this work, a multifunctional nanocomposite was proposed for sensing different external stimuli, whether it be mechanical (strain), chemical (pH), or others. The specific objective was to design a flexible coating that does not rely on external power for operations but could instead generate electrical current proportional to the severity of the measurand. The design of the coating was based on a multilayered thin film architecture. One of the main components was a poly(3-hexylthiophene) (P3HT) and fullerene-copolymer-based photoactive layer fabricated by spin coating. This P3HT-based coating could generate electrical current in response to ambient broadband light illumination, but more importantly, it was engineered to be sensitive to applied strains. In addition, another polyaniline (PANI)-based thin film was incorporated using layer-by-layer assembly for pH sensing. The entire coating was then validated for multi-modal sensing and specifically to sense strain and pH. First, when the nanocomposite was illuminated with blue light (i.e., 410 to 490 nm), the film’s generated photocurrent was linearly correlated to only applied tensile strains (and not pH). Second, when illuminated by infrared light, generated photocurrent was only sensitive to different pH buffer solutions that the PANI-based film was exposed to. Multi-modal sensing was verified not only in that an individual sensor could measure different external stimuli, but it was validated that sensing was highly selective to strain or pH depending on the interrogating light source. Furthermore, optoelectronic characterization tests were conducted for explaining the observed phenomena. It is envisioned that this self-sensing coating could be used for monitoring cracks (strain) and corrosion (pH) without requiring an external power supply.