Synthesis and luminescence of Al based double perovskite quantum dots†

Abstract

Direct-bandgap AgIn based non-lead double perovskite quantum dots (DPQDs) face the challenge of low photoluminescence quantum yields (PLQYs). To address this issue, approaches such as ion doping and surface passivation have been developed, by which both emission color and intensity have been modulated. In this article, we selected (r_Al³⁺ = 0.053) to replace In³⁺ (r_In³⁺ = 0.081 nm) and further used Na⁺ (r_Na⁺ = 0.098 nm) to replace Ag⁺ (r_Ag⁺ = 0.126 nm), resulting in the synthesis of two new types of non-doped DPQDs, i.e. Cs₂AgAlCl₆ and Cs₂NaAlCl₆. The synthesized Al-based DPQDs have a hexagonal polycrystalline structure with average sizes of 8.84 nm and 5.76 nm, respectively. X-ray diffraction (XRD) data indicate the lattice contraction of Cs₂AgAlCl₆ and Cs₂NaAlCl₆ DPQDs in comparison to Cs₂AgInCl₆. X-ray photoelectron spectroscopy (XPS) data indicate the presence of all four elements Cs, Ag/Na, Al and Cl in the QDs. Compared with Cs₂AgInCl₆ DPQDs, replacement of In³⁺ with Al³⁺ increases the PLQY from 1.5% to 7.4% and further to 8.5% when Ag⁺ is further replaced with Na⁺. Doping the Cs₂AgAlCl₆ and Cs₂NaAlCl₆ DPQDs with Bi³⁺ ions further increases the PLQYs to 10.1% and 11.4%, respectively. The PLQY of Cs₂AgAlCl₆ DPQDs is again increased to 10.9% with the use of a ligand mixture of n-trioctylphosphine : oleylamine (40% : 60%). Our results demonstrate that the replacement of In³⁺ with small radius Al³⁺ is an effective strategy to enhance the emission of non-doped pristine direct-bandgap DPQDs and open an avenue for designing new types of DPQDs.