Modified optical absorption of molecules on metallic nanoparticles at sub-monolayer coverage

TL;DR

This study provides direct measurements of the intrinsic optical absorption of dye molecules on silver nanospheres at sub-monolayer coverage, revealing shifts and broadening of molecular resonances crucial for understanding plasmonic interactions.

Contribution

It introduces a method to measure the intrinsic absorbance of molecules on metallic nanoparticles at very low coverage, isolating molecular properties from strong coupling effects.

Findings

Measured intrinsic absorbance of dye molecules on silver nanospheres.

Observed shifts and broadening of molecular resonances at sub-monolayer coverage.

Implications for interpreting surface-enhanced spectroscopies and molecular plasmonics.

Abstract

Enhanced optical absorption of molecules in the vicinity of metallic nanostructures is key to a number of surface-enhanced spectroscopies and of great general interest to the fields of plasmonics and nano-optics. Yet, experimental access to this absorbance has long proven elusive. We here present direct measurements of the intrinsic absorbance of dye-molecules adsorbed onto silver nanospheres, and crucially, at sub-monolayer concentrations where dye--dye interactions become negligible. With a large detuning from the plasmon resonance, distinct shifts and broadening of the molecular resonances reveal the intrinsic properties of the dye in contact with the metal colloid, in contrast to the often studied strong-coupling regime where the optical properties of the dye-molecules cannot be isolated. The observation of these shifts together with the ability to routinely measure them has broad implications in the interpretation of experiments involving resonant molecules on metallic surfaces, such as surface-enhanced spectroscopies and many aspects of molecular plasmonics.