We report on the role of magic-sized clusters (MSCs) as key intermediates in the synthesis of indium phosphide quantum dots (InP QDs) from molecular precursors. Heterogeneous growth from the MSCs directly to InP QDs was observed without intermediate sized particles. These observations suggest that previous efforts to control nucleation and growth by tuning precursor reactivity have been undermined by formation of these kinetically persistent MSCs prior to QD formation. The thermal stability of InP MSCs is influenced by the presence of exogenous bases as well as choice of the anionic ligand set. Addition of a primary amine, a common additive in previous InP QD syntheses, to carboxylate terminated MSCs was found to bypass the formation of MSCs, allowing for homogeneous growth of InP QDs through a continuum of isolable sizes. Substitution of the carboxylate ligand set for a phosphonate ligand set increased the thermal stability of one particular InP MSC to 400 °C. The structure and optical properties of the MSCs with both carboxylate and phosphonate ligand sets were studied by UV-Vis absorption spectroscopy, powder XRD analysis, and solution 31 P{ 1 H} and 1 H NMR spectroscopy. Finally, the carboxylate terminated MSCs were identified as effective single source precursors (SSPs) for the synthesis of high quality InP QDs. Employing InP MSCs as SSPs for QDs effectively decouples the formation of MSCs from the subsequent second nucleation event and growth of InP QDs. The concentration dependence of this SSP reaction, as well as the shape uniformity of particles observed by TEM suggests that the stepwise growth from MSCs directly to QDs proceeds via a second nucleation event rather than an aggregative growth mechanism.

Magic-sized nanoclusters have been implicated as mechanistically relevant intermediates in the synthesis of group III-V quantum dots. Herein we report the single-crystal X-ray diffraction structure of a carboxylate-ligated indium phosphide magic-sized nanocluster at 0.83 Å resolution. The structure of this cluster, In37P20(O2CR)51, deviates from that of known crystal phases and possesses a non-stoichiometric, charged core composed of a series of fused 6-membered rings. The cluster is completely passivated by bidentate carboxylate ligands exhibiting predominantly bridging binding modes. The absorption spectrum of the cluster shows an asymmetric line shape that is broader than what would be expected from a homogeneous sample. A combination of computational and experimental evidence suggests that the spectral line width is a result of multiple, discrete electronic transitions that couple to vibrations of the nanocrystal lattice. The product of reaction of this nanocluster with 1 equiv of water has also been structurally characterized, demonstrating site selectivity without a drastic alteration of electronic structure.

We have developed a two-phosphine strategy to independently tune nucleation and growth kinetics based on the relative reactivity of each precursor in the synthesis of indium phosphide (InP) quantum dots (QDs). This approach was allowed by the exploration of the synthesis and reactivity of a series of sterically encumbered triarylsilylphosphines substituted at the para position of the aryl group, P(Si(C 6 H 4 -X) 3 ) 3 (X = H, Me, CF 3 , or Cl), as a contrast to P(SiMe 3 ) 3 , the P 3− source commonly employed in such syntheses. UV−vis absorption spectroscopy of aliquots taken during InP QD growth revealed a stark contrast between triarylsilylphosphines with electron-donating and electron-withdrawing groups in both the rate of InP formation and the final particle size. 31 P{ 1 H} nuclear magnetic resonance spectroscopy confirmed that precursor conversion remains rate-limiting throughout the nanocrystal synthesis when P(SiPh 3 ) 3 is incorporated as the sole phosphorus precursor; however, this is insufficient for effective separation of nucleation and growth in this system because of the slow nucleation rates that result. In all cases, syntheses that employ a single chemical species as the P 3− source were found to suffer from a poor match in reactivity with In(O 2 C(CH 2 ) 12 CH 3 ) 3 as they either fail to separate nucleation from growth because of slow precursor conversion rates [P(SiPh 3 ) 3 and P(Si(C 6 H 4 -Me) 3 ) 3 ] or preclude size selective growth from rapid precursor conversion [P(SiMe 3 ) 3 , P(Si(C 6 H 4 -Cl) 3 ) 3 , and P(Si(C 6 H 4 -CF 3 ) 3 ) 3 ]. To balance these two extreme cases, we developed a novel approach in which two different P 3− sources were introduced to segregate nucleation and growth based on the relative reactivity of each precursor.

We have studied the speciation of P(SiMe 3 ) 3 during the synthesis of colloidal InP quantum dots in the presence of proton sources. Using 31 P NMR spectroscopy, we show H 3-n P(SiMe 3 ) n formation on exposure of P(SiMe 3 ) 3 to a variety of protic reagents including water, methanol, and carboxylic acid, corroborating observations of P(SiMe 3 ) 3 speciation during the hot injection synthesis of InP QDs. Quantitative UV−vis comparisons between InP growth from P(SiMe 3 ) 3 and HP(SiMe 3 ) 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(SiMe 3 ) 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.

The reaction of primary amines with InP(OCR) is found to remove In(OCR) subunits from InP(OCR). This loss of Z-type ligands coincides with structural rearrangement to alleviate core strain and passivate phosphorus atoms. This result consolidates conflicting claims that primary amines both promote and retard precursor conversion rates for InP nanocrystals.