We have investigated the reaction mechanism responsible for QD nucleation using optical absorption and nuclear magnetic resonance spectroscopies. For typical II-VI and IV-VI quantum dot (QD) syntheses, pure tertiary phosphine selenide sources (e.g. trioctylphosphine selenide (TOPSe)) were surprisingly found to be unreactive with metal carboxylates and incapable of yielding QDs. Rather, small quantities of secondary phosphines, which are impurities in tertiary phosphines, are entirely responsible for the nucleation of QDs; their low concentrations account for poor synthetic conversion yields. QD yields increase to nearly quantitative levels when replacing TOPSe with a stoiciometric amount of a secondary phosphine chalcogenide such as diphenylphosphine selenide. Based on our observations, we have proposed potential monomer identities, reaction pathways and transition states, and believe this mechanism to be universal to all II-VI and IV-VI QDs synthesized using phosphine based methods.The significant potential of semiconductor nanocrystal quantum dots (QDs) for photovoltaics, bio-labeling/imaging, and light-emitting diodes has been well-reported. 1 Unfortunately, realizing this potential has yet to occur, in part due to a synthetic procedure that lacks scalability and high conversion yield. Current syntheses can produce high-quality QDs with respect to photoluminscent quantum yield (QY >50%) and size distribution (± 5%). However, growth temperatures are high (∼300 °C for CdSe), conversion yields are poor (< 2%) and the method suffers from well-known irreproducibilities. 2,3 We suggest that the lack of a rational synthetic mechanism has stalled advancements in QD synthetic design.The essential components in the synthesis of high-quality II-VI and IV-VI QDs have remained largely unchanged for 20 years; typically employing tertiary phosphine We studied reactions performed between high purity tertiary phosphine chalcogenides (e.g. triethylphosphine selenide, triisopropylphosphine selenide and triphenylphosphine selenide) and Pb(oleate) 2 heated to 120 °C, which were monitored by both optical absorption spectroscopy and 31 P NMR. In all casers, no reactivity was observed even after five hours of heating, in stark contrast to commercially obtained trioctylphosphine selenide (TOPSe), which reacts with Pb(oleate) 2 in minutes, forming a dark product. Upon addition of a secondary phosphine (diisopropylphosphine) to high purity tertiary phosphine chalcogenide precursors, reaction rates in all cases accelerated rapidly yielding PbSe QDs. Typical tertiary phosphine sources (tributylphosphine and trioctylphosphine (TOP)) used for QD syntheses are highly impure, and all contain measurable quantities of secondary phosphines (dibutylphosphine ( 31 P δ -69.5 ppm) and dioctylphosphine ( 31 P δ −69.7 ppm) respectively), as shown in the supporting information figures 1 and 2. The importance of secondary phosphines is further emphasized by Figure 1, which shows that growth kinetics and yield for PbSe magic-sized clusters (MS...

Colloidal CdS quantum dots (QDs) were synthesized with tunable surface composition. Surface stoichiometry was controlled by applying reactive secondary phosphine sulfide precursors in a layer-by-layer approach. The surface composition was observed to greatly affect photoluminescence properties. Band edge emission was quenched in sulfur terminated CdS QDs and fully recovered when QDs were cadmium terminated. Calculations suggest that electronic states inside the band gap arising from surface sulfur atoms could trap charges, thus inhibiting radiative recombination and facilitating nonradiative relaxation.

Using fluorescence-based measurements, the effect of confinement on the glass transition temperature (T g) was investigated in seven polymer systems of single-layer films supported on silicon substrates, all chosen such that they do not have substantial substrate interactions. Poly(vinyl chloride) exhibited the steepest reduction in T g with decreasing film thickness. The largest magnitude of T g reduction was observed in polysulfone where a 24-nm-thick film reported a T g reduced by 53 °C relative to bulk. A system was also selected which exhibited no confinement effect at a thickness of 13–14 nm. Differential scanning calorimetry was used to determine dynamic fragilities in order to test the hypothesis that fragility is a key variable governing the magnitude of T g-confinement effects. The systems in the present study show a one-to-one correlation between higher fragility and larger magnitudes of T g reduction with decreasing film thickness. In particular, T g/T g,bulk exhibits a universal nature as a function of a scaled film thickness for all seven systems. Fragility is a key variable which reflects the local packing efficiency in a polymer glass and which determines the susceptibility of a glass former to perturbations, in this case induced by confinement.

We present the synthesis and characterization of novel, ultrasmall PbSe magic-sized nanoclusters (MSCs). Unlike the syntheses of high-quality semiconductor nanoparticles, the MSC synthesis is straightforward, occurring at room temperature in air over several hours. MSCs have core diameters <2 nm and fluoresce in the near-infrared (900 nm) with remarkable quantum efficiencies consistently greater than 50%. The relatively slow growth of MSCs allows for a significant scale up of reaction volumes. Transfer to aqueous solvents has also been demonstrated, establishing MSCs as a potential new fluorophore for applications in biological imaging.