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

Abstract

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 vacuumevaporated 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 effect's 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.

Towards enhanced optical sensor performance: SEIRA and SERS with plasmonic nanostars

“…Enhancement of IR absorption features was first demonstrated by Hartstein et al 6 for molecules adsorbed on nano-sized metal islands in comparison with smooth surfaces of the corresponding bulk metals. 7 Later, nanoparticles with different kinds of geometries and arrangements including linear nanorods, or nanoantennas, 8,9 split ring resonators, 10 and trapezoidal particles 11 were investigated as signal amplifiers at SEIRAS substrates. Since IR spectroscopy is a sensitive and complementary method to Raman spectroscopy, the combination of SERS and SEIRAS provides intriguing analytical capabilities from a practical perspective, as their complementarity provides a comprehensive 'chemical fingerprint' for the identification of molecules and substrates.…”

Versatile Analytical Platform Based on Graphene-Enhanced Infrared Attenuated Total Reflection Spectroscopy

“…To date, such enhancement effect has been observed on various nanostructured substrates, , including metallic nanoislands, , nanoshells, nanogaps, , nanoantennas, − and so on, which is commonly known as surface-enhanced infrared absorption (SEIRA) effect in analogy to surface-enhanced Raman scattering (SERS) and surface-enhanced fluorescence (SEF) . It is generally accepted that the enhancement of infrared absorption features at certain substrates may be attributed to at least two distinct mechanisms, that is, electromagnetic (EM) enhancement, an increase in the local electric field at the substrate surface, and chemical enhancement (CE), which is derived from chemical interactions between the target molecule and the substrate surface. − The majority of reported SEIRA substrates are based on noble metals (e.g., Au, Ag), as their plasmonic resonances can be extended to the infrared region via rational design of specific nanostructures . However, the plasmon frequencies of metals are fixed after structuring, thereby limiting SEIRA detection of multiple IR bands across a broad spectral range. , Additionally, the SEIRA effect is restricted to the immediate vicinity of the substrate surface and greatly depends on the surface morphology .…”

Periodic array-based substrates for surface-enhanced infrared spectroscopy

“…In contrast, Shimada et al investigated systematically the enhancement of periodic gold square column arrays on silicon [37]. The substrates were fabricated by electron beam lithography.…”