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Uncovering Active Precursors in Colloidal Quantum Dot Synthesis

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Uncovering Active Precursors in Colloidal Quantum Dot Synthesis Corrected: Publisher correction ARTICLE DOI: 10.1038/s41467-017-01936-z OPEN Uncovering active precursors in colloidal quantum dot synthesis Leah C. Frenette 1 & Todd D. Krauss 1,2 Studies of the fundamental physics and chemistry of colloidal semiconductor nanocrystal quantum dots (QDs) have been central to the field for over 30 years. Although the photo- physics of QDs has been intensely studied, much less is understood about the underlying 1234567890 chemical reaction mechanism leading to monomer formation and subsequent QD growth. Here we investigate the reaction mechanism behind CdSe QD synthesis, the most widely studied QD system. Remarkably, we find that it is not necessary for chemical precursors used in the most common synthetic methods to directly react to form QD monomers, but rather they can generate in situ the same highly reactive Cd and Se precursors that were used in some of the original II-VI QD syntheses decades ago, i.e., hydrogen chalcogenide gas and alkyl cadmium. Appreciating this surprising finding may allow for directed manipulation of these reactive intermediates, leading to more controlled syntheses with improved reproducibility. 1 Department of Chemistry, University of Rochester, Rochester, NY 14627-0216, USA. 2 Institute of Optics, University of Rochester, Rochester, NY 14627- 0216, USA. Correspondence and requests for materials should be addressed to T.D.K. (email: [email protected]) NATURE COMMUNICATIONS | 8: 2082 | DOI: 10.1038/s41467-017-01936-z | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-017-01936-z he thermolytic decomposition of organometallic pre- In the following study, we take a somewhat unique approach to Tcursors to form monomers has been the synthetic method our mechanistic studies by investigating the thermal decom- of choice for the nucleation of semiconductor nanocrys- position of each organometallic precursor separately, but – talline quantum dots (QDs) for decades1 3. Early synthetic importantly, under the high temperature reaction conditions that methods produced high-quality QDs through the injection of the are common in CdSe QD syntheses. Specifically, we focus on the phosphine-chalcogenide precursor trioctylphosphine selenide chemical reaction mechanism of the most common precursors for (TOPSe) into thermally decomposing dimethylcadmium2. In the making CdSe QDs: the reaction of cadmium carboxylate and past 20 years, synthetic methods have advanced substantially tertiary phosphine selenide. We hypothesize that complex reac- making the colloidal QD synthesis reaction safer, and with greater tion intermediates, where both cadmium and selenium precursors control over nanoparticle size, shape, and stoichiometry. For remain intact14, 16, 19, 20, are not likely to form given the extreme example, the highly toxic dimethylcadmium was replaced with temperatures necessary for CdSe QD formation thus suggesting cadmium carboxylates4, secondary phosphine chalcogenides an alternate reaction pathway. We find that at under conditions – allowed for control over QD surface composition5 7, and the use common to CdSe QD syntheses, both the tertiary phosphine- of single-source precursors drove improvements in reaction chalcogenide precursor and metal-carboxylate precursor ther- – yields8 10; all have allowed for improved size selectivity and mally decompose to extremely reactive species: hydrogen selenide monodispersity. Recent synthetic breakthroughs have focused and dialkylcadmium, respectively. On the basis of this observa- on designing precursors to better program size and reaction tion, we propose a mechanism for QD synthesis in which these – yield11 13. decomposition products react to form CdSe monomer, which Contrary to the advances made in developing improved QD then further react to nucleate nanoparticles3, 21. Importantly, this synthesis, elucidating the specific chemical reaction pathways mechanism includes the formation of the anhydride and the taking initial molecular precursors to QDs has been difficult and phosphine oxide products observed in previous mechanism stu- limited in scope. In an early example, it was shown that the dies14, 17, and can explain the relatively low chemical yields reactive chalcogenide precursor in lead chalcogenide QD synth- observed in the CdSe QD synthesis reactions of this type16. eses was not the tertiary phosphine-chalcogenide precursor, but 5 actually a secondary phosphine-chalcogenide impurity . With Results respect to CdSe QDs, it was proposed that QD molecular pre- Thermal decomposition of tri-n-butylphosphine selenide.We cursors (i.e., cadmium oleate and TOPSe) form a transition state – studied the decomposition of tri-n-butylphosphine selenide given by a Lewis acid-base complex14 16; which forms the CdSe 14, 16, 17 (TnBPSe) in the absence of a cadmium source to model the bond after an attack by an alkyl carboxylate . Noteworthy decomposition of tertiary phosphine selenides under the con- products from this reaction are trioctylphosphine oxide (TOPO) 14 ditions of typical CdSe QD syntheses. High-quality cadmium and oleic anhydride . Further mechanistic studies are motivated chalcogenide QDs are typically synthesized at temperatures by the need to control the kinetics of the reactive precursors in between 270 and 350 °C. Thus, tri-n-butylphosphine selenide solution and eliminate extraneous reactants, enhance reproduci- (TnBPSe), octanoic acid (a shorter chain analog for oleic bility, and obtain better overall control over nanoparticle 17, 18 acid), and tetradecane were combined and heated at 250 °C. growth . In addition, new precursors and synthetic methods We expected significant reactivity in the phosphine selenide, can be designed with controlled decomposition to yield more as phosphine chalcogenides are likely to decompose at highly engineered QDs of desired shapes, sizes, and 11 these temperatures given their reactivity under similar condi- compositions . tions22, 23. In addition, alkyl tertiary phosphine selenide bond ab31P NMR 100% TnBPSe TnBPO 80% 60% 20 min 15 min 40% Composition (%) 8 min 20% 5 min TnBPO TnBPSe 0% 0 5 10 15 20 25 60 40 (ppm) Time (min) Fig. 1 A NMR analysis following decomposition of TnBPSe after hot injection. a The 31P NMR spectrum of the reaction mixture over time, showing that the relative amount of TnBPSe decreases as the amount of TnBPO increases. b The percent composition of TnBPSe (navy blue circles) and TnBPO (royal blue squares) in the TnBPSe decomposition reaction over time. 31P and 77Se NMR of TnBPSe before reaction can be found in Supplementary Fig. 1. Trend lines are provided as guides to the eye 2 NATURE COMMUNICATIONS | 8: 2082 | DOI: 10.1038/s41467-017-01936-z | www.nature.com/naturecommunications NATURE COMMUNICATIONS | DOI: 10.1038/s41467-017-01936-z ARTICLE abc77Se NMR H2Se TPPSe –220 –240 –260 –280 (ppm) Fig. 2 Identifying the selenium decomposition product. a The experimental setup used to identify H2Se as the gaseous selenium source. This panel was 77 generated using templates in ChemDraw Professional 16.0. b The Se NMR spectrum of H2Se with TPPSe external standard. NMR characterization of TPPSe can be found in Supplementary Fig. 6. c Fluorescent CdSe nanocrystals under UV irradiation synthesized from the H2Se generated in situ, the absorbance spectrum is in Supplementary Fig. 7 fl dissociation energies are low, making breaking the phosphine- in the rightmost ask provides conclusive evidence that H2Se is chalcogenide bond accessible, as demonstrated by Ruberu et al. the selenium containing decomposition product of TnBPSe. through phosphine crossover experiments at temperatures Furthermore, we confirmed that the H Se produced from the – 2 above 250 °C24 26. TnBPSe decomposition can react to form CdSe nanoparticles, by As shown in Fig. 1, analysis of the reaction mixture by31P using the cannula apparatus in Fig. 2a with cadmium oleate in fl NMR (nuclear magnetic resonance spectroscopy) showed the tetradecane at 250 °C in the second ask replacing CS2.H2Se formation of tri-n-butylphosphine oxide (TnBPO), a product from the TnBPSe decomposition reaction in the first flask was universally seen in other studies of CdSe QD mechanism with bubbled through the cadmium oleate resulting in the synthesis of – cadmium present14, 27 29, and a clear indication that the TnBPSe CdSe nanoparticles shown in Fig. 2c. had decomposed. The TnBPSe decomposition reaction for the To quantify the amount of H2Se produced, a coaxial insert conversion of TnBPSe to TnBPO was further studied at various NMR tube was employed to allow the use of an external77Se time points after hot injection by 31P NMR (Fig. 1). The NMR standard. Triphenylphosphine selenide (TPPSe) was percentage composition of TnBPO after 15 min was calculated to chosen as the external standard because its chemical shift appears 77 be 30% on average, which is consistent with the reaction yields of near to, but distinct from, that of H2Se. The Se NMR showing 16 CdSe QDs using TnBPSe . Surprisingly, we were unable to the H2Se singlet and TPPSe standard doublet can be seen in observe any other molecular form of selenium in solution by 77Se Fig. 2b. From77Se NMR integrated peaks, we calculated an 31 or P NMR (Supplementary Figs. 2 and 3). Previous mechanistic average of 36% conversion of TnBPSe to H2Se after 15 min of studies attribute the lack of a Se signal in NMR to formation of reaction. The discrepancy between the conversion yields from CdSe QDs, which would be NMR silent5, 14, 17, 19, 27. However, in 77Se NMR (36%)
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