Dielectric and optical properties of superlattices of epitaxially connected nanocrystals

Abstract

Colloidal semiconductor nanocrystals (NCs) are believed to strongly absorb resonant light; this property is key to ongoing applications of NCs as phosphor in light-emitting diodes (LEDs) for lighting, in displays, and photodetectors. However, it has been realized that NCs in a low dielectric medium suffer from a weak permeation of the electromagnetic field in the solid due to formation of depolarization fields, strongly reducing light absorption, as quantified by the local-field factor. For low-dimensional semiconductors, such as one-dimensional (1D) rods and two-dimensional (2D) superlattices, this depolarization field depends critically on the electromagnetic field direction versus the long direction of the material. Here we present atomistic calculations of the screening of the electromagnetic field for individual NCs and technologically important architectures, i.e., 1D and 2D superlattices of epitaxially connected NCs. We also investigate the combined effects of epitaxial connections and excitonic interactions in the particular case of dimers of NCs. Even if it can be improved by a matrix of high dielectric constant, the local-field factor of individual NCs is very low but can increase more than an order of magnitude in NC chains or NC superlattices for an in-line or in-plane electromagnetic field. The calculations of the optical absorptance agree with recent experimental results obtained on superlattices of PbSe and CdSe superlattices. The reduced screening of the electric field in the case of strong epitaxial connections between the NCs was recently invoked to explain that the absorptance of a 2D superlattice approaches the absorptance quantum (πα, where α is the fine structure constant) observed with semiconductor quantum wells [P.T. Prins et al., Nano Lett. 21, 9426 (2021)1530-698410.1021/acs.nanolett.1c02682]. We conclude that atomically coherent superlattices have a strong light absorption and emission, even in the NC monolayer limit, and are primary candidates for polarization-selective photodetectors and LEDs.