Determining the Complex Refractive Index of Neat CdSe/CdS Quantum Dot Films

Abstract

Understanding how structural and synthetic 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. However, neat QD films are challenging to accurately model as they are a mixture of inorganic core/shell materials and surrounding organic ligands. Furthermore, both the QD film morphology and the complex refractive index vary due to particle size, ligand chain length, and the deposition process. Here, we study the complex refractive index of neat CdSe/CdS core/shell QD films by using variable-angle spectroscopic ellipsometry to derive the effective complex refractive index using Kramers−Kronig consistent dispersion models. We use this information in conjunction with intrinsic refractive index data of CdSe and CdSe/CdS QDs extracted from solution-state absorption data and effective medium approximations (EMA) to describe neat QD films. We find that EMAs can successfully be used as a tool to approximate the complex refractive index of QD films. This information allows us to better understand packing variations between QD films and more accurately predict the absorption in QD thin films, including those made with core/shell heterostructures.

Intrinsic measurements of exciton transport in photovoltaic cells

“…For CdSe QD thin films, the QDs are zincblende nanocrystals (size ~4 nm) stabilized with 3-mercaptopropionic acid (3-MPA) after treatment via solid-state ligand exchange 66 . The resulting thin films have a very low extinction coefficient (k = 0.05 at excitonic peak, λ = 575 nm), and are therefore paired with a wider energy gap small molecule 2,3,6,7-tetrahydro-1,1,7,7,-tetramethyl-1H,5 H,11H-10-(2-benzothiazolyl)quinolizino[9,9a,1gh]coumarin (C545T), which does not absorb at λ = 575 nm.…”

“…This allows active materials to be selected solely on the basis of realizing a minimal overlap in optical absorption. The QD film thickness is controlled by the number of cycles for the ligand exchange process 66 . Figure 5e shows the C545T-CdSe QD η D ratio as a function of C545T thickness for two different QD film thicknesses (see Supplementary Figure 9 for broadband η EQE , η A and η IQE spectra).…”

“…Zincblende CdSe QDs with oleic acid capping ligands were synthesized as previously reported 66 , based off the original synthesis by Chen and et al 70 . Briefly, 1-octadecene (63 mL), cadmium myristate (570 mg) and selenium dioxide (110 mg) were added to a 3-neck round bottom flask and degassed at 75 °C for 1 h. After degassing, the reaction mixture was heated to 240 °C, at which point 1 mL of oleic acid was added dropwise to the solution once a red color was achieved.…”

Large-area patterning of full-color quantum dot arrays beyond 1000 pixels per inch by selective electrophoretic deposition

“…2d , with the increase of electric field force, the size of oligomer changes little, while the film thickness decreases and the packing density increases. In addition, the refractive index ( n ) associated with packing density 41 of films can also be tuned by an electric field (Supplementary Fig. 11 ), such as the n @630 nm (Fig.…”

Optical Gain in Ultrathin Self‐Assembled Bi‐Layers of Colloidal Quantum Wells Enabled by the Mode Confinement in their High‐Index Dielectric Waveguides

“…The complex refractive index of the thin CQW films was determined using ellipsometry in accordance with the previous literature. [ 41,42 ] Ellipsometric data was collected from a multilayered CQW sample deposited on silicon at 400–1000 nm range with 10 nm steps and at three different incidence angles of 65°, 70°, and 75°. The thickness of the sample was determined from the spectral region across which the CQWs are non‐absorbing (700–1000 nm) using a Cauchy model.…”