Impact of Atomic Layer Deposition on the Photoluminescence of Colloidal Quantum Dots

Abstract

Colloidal QDs are hybrid objects that feature an inorganic, crystalline core surrounded by an organic ligand shell. The ligands are required for stabilising QD dispersions and, by passivating under-coordinated surface atoms, reducing the density of localized surface states that would quench their photoluminescence (PL). The integration of QDs in optoelectronic devices typically requires embedding of the QDs in a host material. Atomic layer deposition (ALD) is often considered as a deposition technique for such purpose. However, the ALD process chemistry can affect the surface of the QDs and their ligand shell, thus potentially leading to a reduction of the PL efficiency after embedding of the QDs. Hence, a fundamental understanding of the interplay between the ALD process chemistry and the QD photoluminescence stands out as a key challenge for turning ALD into a viable encapsulation method for QDs. In this work, a reactor for plasma enhanced ALD was equipped with the possibility to conduct in situ photoluminescence measurements during ALD encapsulation of QDs. This setup was used to investigate the influence of plasma treatment, precursor and reactant exposure on the PL of CdSe/CdS/ZnS core/shell/shell QDs during ALD of Al2O3, TiO2, HfO2, and ZnS. The in situ PL approach proved a versatile technique to screen suitable precursors, reactants and ALD processes for QD embedding and to investigate the interaction between QDs and reactive gaseous species in general.