Role of Acid in Precursor Conversion During InP Quantum Dot Synthesis

We have studied the speciation of P(SiMe3)(3) during the synthesis of colloidal InP quantum dots in the presence of proton sources. Using P-31 NMR spectroscopy, we show H3-nP(SiMe3)(n) formation on exposure of P(SiMe3)(3) to a variety of protic reagents including water, methanol, and carboxylic acid, corroborating observations of P(SiMe3)(3) speciation during the hot injection synthesis of InP QDs. Quantitative UV-vis comparisons between InP growth from P(SiMe3)(3) and HP(SiMe3)(2) show unambiguously that when total H+-content is accounted for, particle size, size dispersity, and concentration are indistinguishable for these two reagents. The dual role of myristic acid in P-Si bond cleavage and as a source of the myristate anion, an essential component of the quantum dot surface, is interrogated using tetrabutylammonium myristate, confirming that it is the protons that are responsible for increased quantum dot polydispersity. Together these data support the existence of a competing acid-catalyzed pathway in the conversion of P(SiMe3)(3) to InP and demonstrate its impact. By preventing a constant solute supply and affecting the concentration of quantum dot surfactant over the course of the reaction, the existence of competing precursor conversion pathways is detrimental to formation of monodisperse colloids, explaining much of the irreproducibility in InP quantum dot syntheses to date.