Spectrally-Resolved Dielectric Functions of Solution-Cast Quantum Dot Thin Films

Quantum confinement is the divergence, at small crystallite size, of the electronic structure of semiconductor nanocrystals, or quantum dots, from the properties of larger crystals of the same materials. Although the extinction properties of quantum dots in the dispersed state have been extensively studied, many applications for quantum dots require the formation of a solid material which nonetheless retains a size-dependent electronic structure. The complex index of refraction (or complex dielectric function), including the extinction coefficient, is critical information for interpretation of optoelectronic measurements and use of quantum dot solids in optoelectronic devices. Here, spectroscopic ellipsometry is used to provide an all-optical method to determine the thickness, complex index, and extinction coefficient of thin films made of quantum-confined materials through the visible and near-infrared spectral ranges. The characteristic, size-dependent spectral features in the absorption of monodisperse quantum dots are readily translated into spectral variations of the index of refraction. The complex indices of refraction of CdSe and PbS quantum dot solids depend strongly on quantum dot size and the processing conditions of the thin film, including ligand exchange and annealing. The dielectric functions of quantum dot solids are dominated by the fill fraction of quantum dots, with only secondary influence from interparticle interaction.