A Reduction Pathway in the Synthesis of PbSe Nanocrystal Quantum Dots

Abstract

Colloidal nanocrystal quantum dots (NQDs) of narrow band gap materials are of substantial general interest because of their unparalleled potential as infrared fluorophores. While PbSe NQDs are a promising class of infrared-active nanocrystals due to high emission quantum yields and a wide useful spectral range, typical synthetic methods are sensitive to a variety of factors, including the influence of solvent/ligand impurities that render reproducibility difficult. In this work, we specifically examine the effects of diphenylphosphine and 1,2-hexadecanediol, as surrogates for putative trioctylphosphine-based reducing impurities, on the synthesis of PbSe NQDs. Specifically, we compare their influence on NQD size, chemical yield, and photoluminescence quantum yield. While both additives substantially increase the chemical yield of the synthesis, they demonstrate markedly different effects on emission quantum yield of the product NQDs. We further examine the effects of reaction temperature and oleic acid concentration on the diol-assisted synthesis. Increased oleic acid concentration led to somewhat higher growth rates and larger NQDs but at the expense of lower chemical yield. Temperature was found to have an even greater effect on growth rate and NQD size. Neither temperature nor oleic acid concentration was found to have noticeable effects on NQD emission quantum yield. Finally, we use numerical simulations to support the conjecture that the increased yield is likely a result of faster monomer formation, consistent with the activation of an additional reaction pathway by the reducing species.

Low-Temperature Approach to High-Yield and Reproducible Syntheses of High-Quality Small-Sized PbSe Colloidal Nanocrystals for Photovoltaic Applications

“…Under such a condition but without DPP, nucleation and growth wasmuchlesssuccessful.Theresulting PbSe NCs with high particle yield exhibited optical properties (including PL QYs) comparable to those with low particle yield obtained in the absence of DPP. Note that, compared to those reported, 25,30 the different experimental parameters including the Pb-to-Se feed molar ratio and growth temperature were used in method (2), in order to promote the high particle yield together with the high quality of the small-sized PbSe NCs.…”

“…It has been acknowledged that one of the serious limitations of method (1) is low chemical/reaction/conversion yields or particle yield (in terms of NC concentration or the number of particles formed), 5,25 particularly for small-sized PbSe. The use of diphenylphosphine (DPP) and 1,2-hexadecanediol (HDD) in the synthesis of PbSe NCs was documented; however, with the assistance of DPP or HDD, the two hot-injection conditions explored were not able to produce small-sized PbSe NCs with high yield.…”

“…The use of diphenylphosphine (DPP) and 1,2-hexadecanediol (HDD) in the synthesis of PbSe NCs was documented; however, with the assistance of DPP or HDD, the two hot-injection conditions explored were not able to produce small-sized PbSe NCs with high yield. 25,30 For example, PbSe NCs with the first excitonic absorption peak at ∼1000 nm were obtained at 10 s/ 160°C with only 5% chemical yield, 25 as shown in Table 1. Very recently, Krauss and co-workers reported that pure tertiary phosphine selenide such as TOPSe did not react with Pb(OA) 2 , but secondary phosphine selenide SedPH(R a ) 2 did.…”

“…Again, our experimental results suggested that the presence of DPP could promote the PL QY; such a conclusion, however, is not in agreement with what was documented previously. 25 Here, the use of DPP, together with the optimized synthetic parameters such as the feed molar ratios of Pb-to-Se, DPP-to-Pb, and OA-to-Pb and the growth temperature, could lead to smallsized PbSe NCs with high particle/chemical yields and high QY, meeting successfully the requirement of the sizable nucleation at low temperature with consumption of a large amount of the monomer. Thus, the quality of the resulting PbSe NCs, including the size, size distribution, and emission brightness, is highly related to control of the experimental conditions for formation of the monomer and thus for balanced nucleation and growth.…”

Low-Temperature Noninjection Approach to Homogeneously-Alloyed PbSexS1−x Colloidal Nanocrystals for Photovoltaic Applications

“…This concept is represented by eq (bM) where DPP was used instead of TBP 97%. 19,22,23 Accordingly, for the present synthesis of homogeneously alloyed PbSe x S 1Àx with low-cost and air-stable TAA as the S precursor, we selected TBP 97% to prepare the Se precursor TBPSe (with the feed molar ratio of 1 Se-to-2.2 TBP). Also, we used DPP as a reducing agent to promote the formation of PbÀP complex which exhibited much higher reactivity than Pb(oleate) 2 and thus to further tune the reactivity match of the Se and S precursors at low temperature.…”