(735e) The Effect of CdS Shell Thickness on the Complex Index of Refraction of CdSe/CdS Core/Shell Nanocrystal Films

Understanding how structural factors influence the complex refractive index of quantum dot (QD) solids is crucial to tailoring the light-matter interactions of QD-containing photonic and optoelectronic devices. QD films are challenging to accurately model as they are a mixture of the core/shell materials and the surrounding organic ligands. Furthermore, QD packing density varies based on particle size, ligand length, and the deposition process. Currently, effective medium approximations are often utilized to approximate the refractive index of QD solids. However, these models are inadequate to describe QD solids as they generally do not account for quantum confinement effects and are often optimized for dilute solutions rather than neat films. Here, we have undertaken a systematic study of the effect of shell thickness and ligand type on the refractive index of neat CdSe/CdS core/shell QD films by using variable-angle spectroscopic ellipsometry to derive the complex refractive index directly from fabricated films. This information allows us to more accurately predict and understand the behavior of QD based devices.

The use of CdSe/CdS heterostructures provides many advantages over bare CdSe QDs, including increased quantum yield and stability, as well tunable Stokes shifts. We have successfully synthesized zinc-blende CdSe QDs on a multigram scale through a reproducible, non-hot-injection synthesis, yielding monodisperse particles with an average diameter of 3.5 nm and standard deviation of 10%. The large scale of this synthesis allows us to maintain the same CdSe core particles while systematically varying the thickness of the CdS shell, leading to a family of CdSe/CdS QDs of varying diameters dependent only on the monolayers of CdS shell grown. We have realized between 3 and 12 monolayers of CdS growth, producing a range of CdSe/CdS particle sizes from 3.5 nm to 11 nm and emission peaks ranging from 585 nm to 645 nm.

Spin-coating a concentrated solution of the CdSe/CdS QDs onto an Al₂O₃ surface forms thin films with thicknesses ranging from 20 nm – 60 nm, depending on the coating conditions. Variable angle spectroscopic ellipsometry measurements on films containing CdSe QD cores and CdSe/CdS QDs with 3-9 monolayers of CdS shell have been carried out. For the case of CdSe surrounded by 5 monolayers of CdS, a refractive index, n, and extinction coefficient, k, of 1.925 and 0.022, respectively, were found at 600 nm with appropriate dispersion throughout the full spectrum, characterized from 400 to 1000 nm. The ellipsometry models include an empirical Sellmeier fit at energies below the band gap, and multiple Gaussian oscillators to model the QD absorption features throughout the visible. To model the strong UV absorption, the tail of a Tauc-Lorentz oscillator is used, which accounts for the bandgap of the QDs. These models show evidence of quantum confinement, having features where the first excitonic peak of the CdSe/CdS QDs would be expected based on absorption data taken by UV-Vis spectroscopy. We also have the ability to correlate ellipsometry measurements to 2D photoluminescent emission and lifetime maps of the QD films by using a piezo-controlled stage with nanometer control of sample position. This gives further insight into packing effects and other structural difference within the QD solid.

A systematic trend between the CdS shell thickness, ligand length, and the refractive index of the film will be presented to inform the development of computational models incorporating CdSe/CdS QDs, and to enable the design of tailored optoelectronic devices that use nanophotonic elements.