Influence of interparticle electronic coupling on the temperature and size dependent optical properties of lead sulfide quantum dot thin films

Abstract

We report on the quantum dot (QD) size, temperature, and inter-dot coupling dependence on the optical absorption and emission for PbS QD thin films. Inter-dot coupling is induced by ligand exchange from oleic acid to 1,2-ethanedithiol, and the expected band gap red-shift observed for coupled QD thin films is accompanied by a modification to the temperature-dependence of the band gap energy. The amplitude and temperature dependence of the photoluminescence (PL) Stokes shift support recombination via a mid-gap state and also indicate that the application of band gap-specific models to fit the temperature dependence PL peak energy is inadequate. Electronically coupled QD thin films show PL quenching with decreasing temperature, following a Boltzmann model which is consistent with thermally activated carrier transport. Enhancing the inter-dot coupling results in the dynamic PL decay signal changing from single- to bi-exponential behavior, reveals a size-dependent transport activation energy, and yields a negative temperature dependent band gap energy for the smallest QD diameters.