What is the Key Structural Parameter for Infrared Absorption Enhancement on Nanostructures?

The key structural parameter for greater enhancement of infrared absorption on metal nanostructures was identified by systematic and exhaustive surface-enhanced infrared absorption (SEIRA) measurements for well-defined periodic gold square column (SC) arrays on silicon wafers fabricated using electron beam lithography. The SC arrays have size parameters compatible with those of vacuum-evaporated thin metal films, which are used conventionally for SEIRA. The crucially important parameter determining the enhancement factor for infrared absorption is the ratio of the gap size to the nanoparticle size. An electrostatic SC model calculation based on a nonresonant-type electromagnetic mechanism shows good quantitative agreement with experimental observations of enhanced infrared absorption of adsorbed species. Furthermore, results show that the electrostatic SC model contains the previously proposed lightning-rod effects case. The clarified enhancement mechanism for infrared absorption of molecules adsorbed onto metal nanostructures is useful to elucidate applications such as designing SEIRA active substrates, analyzing reactions on electrodes, and detecting minute chemicals as highly sensitive chemical sensors and biosensors.