Plasmonic extinction in gold nanoparticle-polymer films as film thickness and nanoparticle separation decrease below resonant wavelength

Abstract

Plasmonic nanoparticles embedded in polymer films enhance optoelectronic properties of photovoltaics, sensors, and interconnects. This work examined optical extinction of polymer films containing randomly dispersed gold nanoparticles (AuNP) with negligible Rayleigh scattering cross-sections at particle separations and film thicknesses less than (sub-) to greater than (super-) the localized surface plasmon resonant (LSPR) wavelength, λ LSPR . Optical extinction followed opposite trends in sub- and superwavelength films on a per nanoparticle basis. In ∼ 70 - nm -thick polyvinylpyrrolidone films containing 16 nm AuNP, measured resonant extinction per particle decreased as particle separation decreased from ∼ 130 to 76 nm, consistent with trends from Maxwell Garnett effective medium theory and coupled dipole approximation. In ∼ 1 - mm -thick polydimethylsiloxane films containing 16-nm AuNP, resonant extinction per particle plateaued at particle separations ≥ λ LSPR , then increased as particle separation radius decreased from ∼ 514 to 408 nm. Contributions from isolated particles, interparticle interactions and heterogeneities in sub- and super- λ LSPR films containing AuNP at sub- λ LSPR separations were examined. Characterizing optoplasmonics of thin polymer films embedded with plasmonic NP supports rational development of optoelectronic, biomedical, and catalytic activity using these nanocomposites.