Plasmene nanosheets as optical skin strain sensors.

Skin-like optoelectronic sensors can have a wide range of technical applications ranging from wearable/implantable biodiagnostics, human-machine interfaces, and soft robotics to artificial intelligence. The previous focus has been on electrical signal transduction, whether resistive, capacitive, or piezoelectric. Here, we report on "optical skin" strain sensors based on elastomer-supported, highly ordered, and closely packed plasmonic nanocrystal arrays (plasmene). Using gold nanocubes (AuNCs) as a model system, we find that the types of polymeric ligands, interparticle spacing, and AuNC sizes play vital roles in strain-induced plasmonic responses. In particular, brush-forming polystyrene (PS) is a "good" ligand for forming elastic plasmenes which display strain-induced blue shift of high-energy plasmonic peaks with high reversibility upon strain release. Further experimental and simulation studies reveal the transition from isotropic uniform plasmon coupling at a non-strained state to anisotropic plasmon coupling at strained states, due to the AuNC alignment perpendicular to the straining direction. The two-term plasmonic ruler model may predict the primary high-energy peak location. Using the relative shift of the averaged high-energy peak to the coupling peak before straining, a plasmene nanosheet may be used as a strain sensor with the sensitivity depending on its internal structures, such as the constituent AuNC size or inter-particle spacing.