Disentangling the Role of Shape, Ligands, and Dielectric Constants in the Absorption Properties of Colloidal CdSe/CdS Nanocrystals

Abstract

We calculated the absorption coefficient of colloidal wurtzite CdSe/CdS nanocrystals (NCs), ranging from spherical to strongly elongated shapes (aspect ratio up to 21) by correlating the NC absorbance spectrum to the NC elemental composition and overall dimensions. We compared experimental data with numerical finite-element calculations of the NC absorption coefficient, which can account for the nonspheroidal NC shape and the influence of the organic ligand shell. The results unveiled that quantum confinement effects influence the NC absorption properties, even at relatively high photon energies (∼4.2 eV). From a practical point of view, we provide a general expression for the absorption coefficient that only requires the knowledge of the NC aspect ratio and CdSe/CdS volume ratio, giving access to a fast and nondestructive estimate of NC concentrations in solution and single-particle absorption cross sections, relevant for a wide range of photonic applications. More fundamentally, the unique aspect ratio-dependence of the absorption coefficient allowed us to derive the real (εr) and imaginary (εi) parts of the dielectric constant, demonstrating that even at energies far above the band gap the NC absorption coefficient differs from bulk due to a reduction of both εr and εi. As the methods presented are general, our analysis can be applied to a wide range of materials of varying composition and yields comprehensive insight into the optical constants of colloidal nanocrystals. (Graph Presented)

Band-edge oscillator strength of colloidal CdSe/CdS dot-in-rods: comparison of absorption and time-resolved fluorescence spectroscopy

“…In the case of non-analytical shapes such as our CdSe/CdS DiRs that are composed of a CdSe core located near one end of an elongated CdS shell, numerical models are required to evaluate the dielectric screening. 18 We performed 3D finite element numerical simulations (COMSOL multiphysics) to examine the near field optical properties of hNCs and calculate the electric field inside the CdSe core. The simulation domain comprised a spherical volume representing a CdSe/CdS DiR embedded in chloroform.…”

“…Values were taken from bulk data at the respective band edges (1.75 eV for CdSe, 21 2.42 eV for CdS 22 ), with the imaginary part of both set to zero, as we can assume ε imag ≪ ε real near the band edge, 23 and applying a reduction of 15% due to residual quantum confinement effects. 18,24 We previously found that the dielectric function in CdSe/CdS hNCs is somewhat reduced compared to bulk at 4.2 eV. 18 Since bulk ε real is nearly flat in the energy range between 4.2 eV and the band edge, 24 we applied the same 15% reduction to the CdS and CdSe optical constants at the band edge.…”

“…18,24 We previously found that the dielectric function in CdSe/CdS hNCs is somewhat reduced compared to bulk at 4.2 eV. 18 Since bulk ε real is nearly flat in the energy range between 4.2 eV and the band edge, 24 we applied the same 15% reduction to the CdS and CdSe optical constants at the band edge. In our simulations, we considered a CdSe core diameter of 4 nm, with a total diameter of 5.5 nm, representative for the experimental samples used.…”

“…We also added a 1.8 nm shell with a refractive index of 1.6 to account for the organic ligands. 18 The DiR aspect ratio (AR) was varied between 1 and 25. In addition, we have also evaluated the influence of the position of the CdSe core on the electric field profile, by moving it from 10% (i.e., near the DiR tip) to 50% (at the center of the DiR) of the total rod length.…”

Spectroelectrochemical Investigation of the Charge Carrier Kinetics of Gold‐Decorated Cadmium Chalcogenide Nanorods

“…The benefit of this particle is the higher specific absorption coefficient in relation to spherical particles of the same material as Angeloni et al. proved by their work . As a reference material CdSe/CdS nanorods are chosen.…”