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

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 (similar to 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 (epsilon(r)) and imaginary (epsilon(i)) 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 Er and epsilon(i). 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.