Synthesis of In<sub>1–<i>x</i></sub>Ga<sub><i>x</i></sub>P Quantum Dots in Lewis Basic Molten Salts: The Effects of Surface Chemistry, Reaction Conditions, and Molten Salt Composition
Margaret H. Hudson, Aritrajit Gupta, Vishwas Srivastava, Eric M. Janke, Dmitri V. Talapin
Abstract
Inorganic molten salts are emerging as versatile solvents for high-temperature processing of colloidal nanocrystals. Molten alkali bromide eutectics can serve as a convenient solvent for the transformation of InP quantum dots (QDs) to In1–xGaxP QDs, with simultaneous tuning of the composition and band gap. Here, we explore various aspects of this molten salt indium-to-gallium cation exchange in-depth, including the nanocrystal surface chemistry, reaction conditions, and salt composition, to obtain a more detailed understanding and finer control over the transformation. InP QDs capped with (DDA)2S, (NH4)2S, Li2Se, GaCl3, or InCl3 can be homogenously dispersed in a molten bromide salt mixture. We demonstrate that chalcogenide capping ligands improve the high temperature stability of InP QDs through the formation of a chalcogen-rich layer which prevents InP decomposition. For each surface chemistry studied, the indium-to-gallium cation exchange proceeds similarly, yielding colloidal In1–xGaxP QDs with an increased band gap and decreased lattice constant. By carefully engineering the reaction conditions and protecting the nanoparticles from oxidative exposure, we achieve a narrow emission linewidth of 41 nm full width at half maximum from the alloyed In1–xGaxP colloidal QDs. These insights provide the design space for colloidal synthesis in molten inorganic salts and introduce synthetic methods for In1–xGaxP QDs with tunable composition and properties. Our work demonstrates the development of nontoxic QD emitters with optimized stability, color purity, and luminescence quantum efficiency.