Plasmonic Metasurfaces as Surface-Enhanced Infrared Absorption Substrates for Optoelectronics: Alq₃ Thin-Film Study

Conventional infrared spectroscopy is widely used to analyze the structural properties of compounds in the fingerprint region. However, a precise spectroscopic study of thin films is a nontrivial task due to the small absorption cross sections of analytes. In this regard, surface-enhanced infrared absorption (SEIRA) spectroscopy can be applied to overcome the limitation by the near-field enhancement of plasmonic metasurfaces. Here we present for the first time the SEIRA study of the widely used organic optoelectronic material Tris(8-hydroxyquinoline) aluminum(III) (Alq3). A special design of the metasurfaces based on arrays of gold Y-shaped nanoantennas is developed for the SEIRA study of Alq3 thermally deposited layers with film thickness growth. This design supports two independent plasmonic resonances for each metasurface and makes it possible to use one SEIRA substrate for the precise study of several vibrational modes of the compound. Observed near-field enhancement of the fabricated metasurfaces revealed a saturation and a diminishing in signal with the analyte layer thickness from half the height of gold nanoantennas onward. This effect is associated with Alq3 side accumulation as shown by atomic force microscopy. A fabricated SEIRA substrate indicates the possibilities for further progress of spectroscopic chemical imaging in optoelectronics and is especially promising for organic electronic devices that consist of multiple layers of transport materials.