Indium – Polycarboxylic Acid Ligand Interactions Modify InP Quantum Dot Nucleation and Growth

Size control through precursor reactivity in InP quantum dot (QD) synthesis has been difficult due to the presence of kinetically persistent InP clusters when using conventional indium carboxylate and tris(trimethylsilyl)phosphine chemistry. However, the advent of the indium halide/aminophosphine-based synthesis creates new opportunities to harness precursor design to impact nucleation and growth. Driven to further explore indium coordination as a synthetic handle in InP QD synthesis, we have examined the effect of a strongly chelating anion on the nucleation and growth of InP QDs. Increasing the equivalents of metal-chelating aminopolycarboxylic acid EDTA ([CH2N(CH2CO2H)2]2) (0 to 0.75 equivalents per indium) is found to decrease the final diameter of InP QDs from 4.5 to 2.3 nm by lowering the initial InP growth rate. This size trend is rationalized by invoking a continuous nucleation model. Control experiments carried out with substoichiometric equivalents of indium do not exhibit a drastic size decrease, pointing to complex effects of EDTA on indium precursor reactivity. By 1H NMR spectroscopy, EDTA is identified to form an octahedral complex with indium that is less reactive. This competitive decrease in reactivity and in the effective concentration of indium precursor is proposed to suppress the initial InP growth rates and consequently decrease the final size of the nanocrystals.