The Dielectric Function of Silver Nanoparticle Langmuir Monolayers Compressed through the Metal Insulator Transition

Abstract

Algorithms and experimental techniques for extracting the complex dielectric function of thin films (monolayers and bilayers) of quantum dots are developed. The algorithms are based on a combination of the so-called Newton−Raphson method, used in conjunction with a Kramers−Kronig analysis, and are used to analyze normal-incidence reflectance and transmission measurements of organically passivated silver quantum dot Langmuir monolayers. Single, unambiguous solutions to the dielectric function were determined for several particle monolayers and at various stages of compression. A transition of the quantum dot superlattice from the insulating to a metallic state was observed. When a monolayer of 6-nm-diameter Ag nanocrystals is compressed to an interparticle separation distance of ∼8−9 Å, a negative-valued featureless component of the real part of the dielectric function is detected. This indicates the onset of Drude-like behavior that is characteristic of a metallic film. The impact of superlattice disorder on this Drude response is also investigated.