Redox Reactions in Molten Indium Halide Salts Enable the Synthesis and Shape Control of In-Pnictide Nanocrystals

Abstract

Molten indium halide salts enable high-temperature and oxygen-free processing and production of III–V nanocrystals, but their redox chemistry can also induce nanocrystal decomposition. Here we show that redox potential of indium halide melts controls the chemical stability of InP, InAs, and InSb nanocrystals. Raman probes of halide speciation and powder XRD of recovered solids reveal that iodide melts, such as KInI4, can oxidize indium pnictide nanocrystals, coupling salt reduction to oxidative pnictide loss, whereas bromide melts, such as KInBr4, resist reduction and stabilize InP and InAs nanocrystals. This redox-tuned stability enables composition-preserving annealing that smooths (111) facets, converting round or tetrapod-like InP and InAs particles into highly faceted tetrahedra, and promotes atomically aligned self-assembly and preferential substrate orientation. Controlled ripening in KInBr4 yields larger III–V colloids inaccessible by synthesis conducted in conventional organic solvents. Furthermore, combining molten-salt annealing with monolayer ZnS growth by colloidal atomic layer deposition (c-ALD) produces significant photoluminescence enhancement, underscoring the chemical benefits of redox-tuned molten salt solvents for the synthesis of novel III–V nanocrystals. Finally, we demonstrate a strategy to enhance the reactivity of reduced indium halide salts through halide coordination with a strong Lewis acid, enabling direct synthesis of InP and InAs nanocrystals. Reaction environments with highly reactive reduced indium salt engineered in molten salts also reveal that under certain reaction conditions, unprecedented 2D InP nanoplatelets can be synthesized.