The dielectric function of silver nanoparticle Langmuir monolayers compressed through the metal insulator transition

Algorithms and experimental techniques for extracting the complex dielectri c function of thin films (monolayers and bilayers) of quantum dots are deve loped. The algorithms are based on a combination of the so-called Newton-Ra phson method, used in conjunction with a Kramers-Kronig analysis, and are u sed to analyze normal-incidence reflectance and transmission measurements o f organically passivated silver quantum dot Langmuir monolayers. single, un ambiguous solutions to the dielectric function were determined for several particle monolayers and at various stages of compression. A transition of t he quantum dot superlattice from the insulating to a metallic state was obs erved. When a monolayer of 6-nm-diameter Ag nanocrystals is compressed to a n interparticle separation distance of similar to 8-9 Angstrom, 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 characte ristic of a metallic film. The impact of superlattice disorder on this Drud e response is also investigated.