Mechanistic Insight and Optimization of InP Nanocrystals Synthesized

Article pubs.acs.org/cm

Mechanistic Insight and Optimization of InP Nanocrystals Synthesized with Aminophosphines Aude Buffard,†,∥ Sébastien Dreyfuss,‡,∥ Brice Nadal,† Hadrien Heuclin,† Xiangzhen Xu,† Gilles Patriarche,§ Nicolas Mézailles,*,‡ and Benoit Dubertret*,† †

Laboratoire de Physique et d’Etude des Matériaux, CNRS, ESPCI Paris, PSL Research University, Université Pierre et Marie Curie, Sorbonne-Universitiés, 10 Rue Vauquelin, 75005 Paris, France ‡ Laboratoire Hétérochimie Fondamentale et Appliquée CNRS, Université Paul Sabatier, 118 Route de Narbonne, 31062 Toulouse, France § Laboratoire de Photonique et Nanostructure, CNRS, Université Paris-Saclay, Route de Nozay, 91460 Marcoussis, France S Supporting Information *

ABSTRACT: We study the synthesis of indium phosphide quantum dots using InCl3, tris(dimethylamino)phosphine (P(NMe2)3), and oleylamine. We optimized the reaction conditions to reach high chemical yield (∼70%) and size control of the quantum dots with absorption maxima over all the visible range. Kinetic studies of the formation of the quantum dots show that, under certain conditions, the growth of nanoparticles seems to approach a LaMer type growth. We have used 31P NMR, mass spectroscopy, and DFT calculations to decipher the reaction mechanisms of InP formation at the molecular level. The mechanistic investigation is in good agreement with the conclusions drawn from the optimization of the synthetic conditions.

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INTRODUCTION

The case of InP synthesis is different. So far, attempts to obtain a well-defined LaMer type growth with precursor conversion, nucleation, and growth well-separated in time have failed. Several strategies have been tested to separate nucleation and growth. They include using several phosphorus precursors with different reactivities14,15 or using small InP clusters.16 Many different phosphorus precursors have been tested such as phosphine [PH3],17 trioctylphosphine,18 P4,19 and PCl3.20 However, the most widely used phosphorus precursor is tris(trimethylsilyl)phosphine (P(TMS)3) that reacts with indium carboxylate in a condensation-like reaction between In(III) and P(−III) compounds. Several publications have focused on the understanding of the mechanism of this reaction, with an aim to improve the synthesis of InP QDs. However, even with a better comprehension of mechanism pathway, none of them succeeds in obtaining very high quality InP NCs.14,16,21−25 Since 2013, a new methodology involving tris(dialkylamino)phosphine and indium halides has been developed.26,27 This synthesis provides a significant improvement because tris(dialkylamino)phosphines are much cheaper and easier to handle than P(TMS)3. Despite efforts to obtain high quality NCs, the full width at half-maximum is still broad compared to

The synthesis of indium phosphide quantum dots (QDs) has attracted large interest since the 1990s.1−5 In particular, they are considered as an interesting alternative to QDs containing cations such as cadmium.6 One of the main difficulties of InP QDs is that their size dispersion is not well-controlled. Poor size control produces inhomogeneous broadening of the QDs’ absorbance feature. It was shown that, at the single QD level, the full width at half-maximum (fwhm) of single InP QDs could be similar to that of CdSe QDs.7 There is thus a great motivation to improve the size control of InP QDs. In the case of cadmium chalcogenide QDs, the precursor conversion in monomers precedes the crystallization steps and is often irreversible.8,9 The conversion process is often slower than the consumption of monomers by growth and limits the rate of crystallization. In crystallization reactions that follow the LaMer model,10 as is the case of II−VI and IV−VI compounds, the precursor reaction determines the rate at which monomers supersaturate prior to nucleation. For these materials, the monomer supply kinetics is directly related to the nucleation and growth of QDs, and a nearly linear relationship between the final concentration of QDs versus the initial precursor conversion rate has been established.11 In addition, the outcome of a QD synthesis has been studied theoretically as a function of the precursor to monomer conversion kinetics.12,13 © 2016 American Chemical Society

Received: June 17, 2016 Revised: July 19, 2016 Published: July 21, 2016 5925

DOI: 10.1021/acs.chemmater.6b02456 Chem. Mater. 2016, 28, 5925−5934

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Chemistry of Materials

Figure 1. Influence of the amount of OLA. (A) Absorbance spectrum of InP QDs synthesized with 14 equiv of OLA (red) and of 50 equiv of OLA (black) compared to In at 220 °C. Evolution of the QD (B) diameter, (C) concentration, and (D) chemical yield with time. Error bars are presented for synthesis performed multiple times.

varying the reaction time,26 Tessier et al. obtain better size control by using different indium and zinc salts.27 In both cases, a large excess of aminophosphine (3.6 equiv) was used, and Tessier et al. showed that this excess was necessary to obtain syntheses with high chemical yield. The lack of understanding of the precise role of each reaction parameter prompted us to carry out a detailed study in order to propose an optimized synthesis of InP QDs using aminophosphines. First, we investigated the influence of the amount of primary amine as solvent in the reaction. Then, we studied the influence of the reaction temperature. Three reaction temperatures were tested: 180, 220, and 270 °C. After optimization of these two parameters, we studied the influence of the P:In ratio on the reaction kinetics and chemical yield. In our study the general protocol we used for InP QDs synthesis is the following. The indium precursor is dissolved in a hot degassed solvent. Then, the mixture is heated at the desired temperature before the injection of the phosphorus precursor in the solution. The heating is maintained, and the reaction is followed during the appropriate time. Past studies have shown that the addition of zinc salt in the reaction mixture of InP improves the excitonic feature in absorption spectra.26 As shown in Figure S1 in SI, this phenomenon is also observed in our case, and we choose to perform our study with the addition of a zinc salt. One hypothesis for this sharpening of the absorbance feature is the passivation of trap and defects of the InP NCs by zinc. Thanks to elemental cartography with EDX, we found an In:P:Zn 49:48:3 (atomic %) stoichiometry for our InP NCs (see EDX cartography in Figure S2). This elemental analysis proves that only a very small fraction of Zn, and thus one that is difficult to localize, is incorporated in the InP NCs despite the initial 1:1 ratio between In and Zn.

that of CdSe NCs, pointing to a polydispersed sample and poor control over the shape of the NCs. A mechanism of hydrolysis of P(NMe2)3 with the primary amine and yielding PH3 was proposed by Song et al., but no experimental evidence was provided.26 While our work was in progress, Tessier et al. made a mechanistic hypothesis based on a transamination reaction at the phosphorus center of the phosphine and on a “phosphorus nucleophilic substitution” to rationalize the formation of the aminophosphonium salt in addition to InP.28 In this paper, we explore further the use of this novel phosphorus precursor for the synthesis of InP QDs with the difficulty to react In(III) with P(III). Our goal was to optimize the reaction conditions with this precursor to see if we could reach a LaMer type synthesis with less inhomogeneous broadening. Using UV−vis spectroscopy, XRD, HAADFSTEM, TEM, and elemental analysis, we have studied the effect of different parameters such as the type and the quantity of amine involved in the reaction, the temperature of formation of QDs, and the ratio between indium and phosphorus precursors. We have also investigated the chemical mechanism of the formation of InP NCs with P(NMe2)3 thanks to 31P NMR, DFT calculations, and mass spectroscopy. This mechanistic study fully rationalizes the required stoichiometry of the reaction and confirms the role of primary amine as both solvent and reagent.

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DISCUSSION Study of the Reaction Parameters. The synthesis of InP QDs using P(NMe2)3 or P(NEt2)3 and an indium chloride salt dissolved in oleylamine (OLA) was pioneered by Song et al. in 2013.26 Later, Tessier et al. optimized the synthesis for different indium halide salts.27 Very recently, Kim et al. studied the surface passivation of InP QDs obtained with this method.29 Whereas Song et al. controlled the average size of the QDs by 5926


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Figure 2. Influence of the reaction temperature: (A) absorbance spectra for InP QDs synthesized at 180 °C (black), 220 °C (red), and 270 °C (blue) in a mixture of OLA/TOA (1/2). Evolution of the QDs: (B) diameter, (C) concentration, and (D) chemical yield with time. Error bars are presented for synthesis performed multiple times.

The first reaction parameter we studied is the influence of the amount of primary amine in the reaction medium. Indeed, primary amines are used to synthesize InP QDs using aminophosphine.26,27 The choice of OLA instead of another primary amine is described in Figure S3 and discussed in SI. In order to perform a systematic study of the kinetics of the formation of QDs, we analyzed the evolution with time of the concentration of QDs, of their size, and of the chemical yield of the reaction. This analysis was done by taking aliquots of the reaction mixture at different times and by recording their absorption spectra after precipitation and redispersion of the QDs. The washing step was necessary to remove absorbing species which are not QDs (ligands, solvent, unreacted inorganic species, and side products) from the solution. From the absorption spectra, we calculated the characteristic parameters of each reaction. In the literature, two methods are reported to compute the chemical yield and the concentration of QDs from absorption spectra. One is based on an empirical formula where the molar extinction coefficient depends on particle size.30 In this case, the yield of the reaction is calculated using estimated particle concentration. The other method uses the intrinsic absorption coefficient of bulk InP to estimate the QDs concentration.27 In the rest of this work, we used the first method to analyze our syntheses because it gave results which were in agreement with elemental analysis performed on benchmark syntheses (see details in SI). When pure OLA was used as a coordinating solvent (corresponding to 50 equiv to indium), we analyzed the evolution of the QDs growth for synthesis performed at 220 °C in excess of phosphorus precursor (P:In 4:1). In these conditions, we observed that the diameter of the QDs increased continuously during the reaction but the concentration of QDs decreased steadily after 10 min (Figure 1B,C), proof of Ostwald ripening. A small decrease of the chemical yield after 10 min was also observed, likely due to a dissolution of some growing InP crystals which are thus not associated with the Ostwald

ripening process. Since Ostwald ripening is usually associated with size defocusing and higher polydispersity of the QDs,8,31 we tested the addition of a less coordinating cosolvent to prevent this process. The total volume of the reaction mixture was kept the same as in pure OLA (5 mL). Because OLA is a strong coordinating ligand, we decided to replace a part of OLA by an equivalent volume of trioctylamine (TOA), which is more sterically encumbered and thus less coordinating. When 6 equiv of OLA with respect to indium was used, only aggregated and ill-formed NCs were obtained (see Figure S4 in SI). When this amount was increased to 14 equiv of OLA, the kinetics of the reaction changed compared to that of pure OLA, as shown in Figure 1B−D. Both the concentration and the average diameter of QDs increased throughout the synthesis, and no dissolution of QDs was observed. The nondissolution of the QDs with 14 equiv of OLA confirmed that the Ostwald ripening was prevented. In Figure 1A, absorbance spectra of InP solution with 14 and 50 equiv of OLA are presented. The feature of absorbance for the synthesis with 14 equiv of OLA is better defined than for 50 equiv of OLA. Thus, the use of TOA as cosolvent is a first step toward the optimization of monodisperse InP NCs. For the rest of the study, we decided to use the OLA/TOA (1/2) mixture described above. In a second step, we studied the influence of the temperature which is a key parameter in QD synthesis. For this purpose, we performed the synthesis at temperatures ranging from 130 to 270 °C (TEM images are presented in Figure S5 in SI). In Figure 2, we present a detailed study at three temperatures: 180, 220, and 270 °C. The corresponding absorbance spectra are presented in Figure 2A. For these reactions we kept the P:In ratio at 4:1, and we used the OLA/TOA mixture. In Figure 2B−D, kinetic parameters are presented. For temperatures lower than 220 °C, the concentration of QDs increases continuously with time while the chemical yields and the QDs’ diameter remain smaller than at higher temperatures over almost 30 min. These trends are further pronounced with the 5927


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Figure 3. Influence of the P:In ratio. (A) Evolution of the chemical yield with time using 2:1 (black), 4:1 (red), and 5:1 (blue) P:In ratio at 220 °C in a mixture of OLA/TOA (1/2). (B) Corresponding absorbance after 30 min of reaction. Error bars are presented for synthesis performed multiple times.

Figure 4. Characterization of InP QDs synthesized in optimized conditions. (A) Evolution of the absorbance of the excitonic peak from 490 to 610 nm with time. (B and C) TEM images of InP QDs synthesized in optimized conditions. (D) Atomic resolution HAADF-STEM image of InP QDs, with the crystal axis on the edges of the QD. (E) Fast Fourier transform of a single QD visualized on the HAADF measurement.

reduction of the temperature. For example, at 130 °C, after 30 min of reaction, the QD diameter is only 2.5 nm. For those temperatures, the growth process of particles is very different from a well-defined LaMer process with good separation of nucleation and growth, as in the case of II−VI QDs. However, the reaction kinetics changes drastically with temperatures higher than 220 °C. Indeed, at 270 °C, there is rapid nucleation burst upon injection of QDs that are around 5 nm in diameter, but this nucleation is rapidly followed by dissolution of the QDs, evidenced by a decrease of the concentration of QDs in solution. This slow dissolution of the QDs comes with a decrease of the chemical yield a few minutes after the onset of the reaction. It thus appears that, at high temperatures, the QDs dissolve rapidly after formation, despite the presence of a cosolvent. At 220 °C, the diameter steadily increases with time as the concentration of quantum dots increases to reach a constant value after 20 min. The chemical yield increases during the reaction up to 70%. Therefore, 220 °C is the optimal temperature to control the nucleation/growth kinetics, and to obtain QDs with high chemical yield, we decided to work at this temperature. Finally, the last parameter we optimized is the ratio of In and P precursors. The syntheses were done in the OLA/TOA (1/2) mixture at 220 °C. The absorbance spectra (Figure 3A) and the evolution of the QD diameter with time (Figure S6A) show

that the size of the NCs obtained is dependent on the quantity of the P:In ratio. As shown in Figure 3A, the P:In ratio has a strong influence on the reaction yield. The highest yield, around 70%, could be obtained when a 4:1 ratio was used, whereas a much lower yield was obtained when 2:1 and 5:1 ratios were used (respectively, 45% and 49%). It is to be noted that the best chemical yield obtained (70%) measured with our method is lower than the one presented by Tessier et al., who reported a yield of up to 80% in 2015,27 and 100% in 2016.28 However, in their protocol, the QDs had not been washed before measurement of the yield. When their protocol was reproduced, followed by a washing step, a chemical yield of 60% was measured, slightly lower than in our optimized conditions. The reaction is faster when the ratio P:In is lowered. Indeed, in the first minutes of the reaction, when P:In is 2:1, the kinetics of the reaction is very fast when compared to other ratios. Moreover, the initial reaction kinetics decreases when the P:In ratio increases. This faster kinetics for the low P:In ratio will be rationalized in the next section. With all other parameters equal, a lower chemical yield was observed when the P:In ratio of precursors was greater than 4:1 (5:1 for example). It can be rationalized thanks to the mechanistic study (vide infra). The optimized conditions to obtain high chemical yield and a better separation of nucleation and growth of the QDs are as 5928


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Figure 5. Transamination of P(NMe2)3 in OLA followed by 31P NMR (A) 31P{1H} NMR of (i) P(NMe2)3 in OLA (35 equiv), (ii) after heating for 12 h at 50 °C, and (iii) after applying vacuum for 30 min at RT and heating at 100 °C for 1 h. Inset: 31P NMR of product 3. (B) 31P NMR spectrum of the mixture of 2, 3, 4, 4′ obtained after reaction of P(NMe2)3 with 6 equiv of OLA. (C) Substitution of P(NMe2)3 by primary amines, tautomerization equilibrium, and formation of bridging products 4 and 4′.

follows: synthesis in the OLA/TOA mixture at 220 °C with a P:In ratio of 4:1. Using these conditions, we demonstrate that it is possible with a single synthesis of InP QDs to obtain QDs with a first absorption peak maximum from 490 to 610 nm just by tuning the reaction time (see Figure 4A). This range is slightly wider than the one reported by Song et al.26 (500−600 nm) and Tessier et al.27 (450−570 nm). InP QDs present a mean fwhm of the first excitonic peak in absorbance around 70 nm and can be as low as 64 nm which suggests that the sample is slightly polydisperse (see TEM images Figure 4B,C). Indeed, it is slightly larger than that for some P(TMS)3-based synthesis (fwhm in absorbance of 62 nm) but comparable with fwhm in absorbance of other InP synthesis, thus demonstrating the competitiveness of this methodology.14 The as-synthesized InP QDs are not fluorescent (fluorescence less than 1% as other bare InP QDs due to surface defects30,32,33), but as shown by Tessier et al. and Kim et al., passivating ZnS or ZnSe shells can be grown on the QDs in order to get highly fluorescent QDs.27,29 The NCs synthesized with this optimized synthesis present a cubic zinc blende structure confirmed by XRD pattern (see Figure S7 in SI) with a tetrahedral shape. Such shapes have already been reported when P(NMe2)3 is used as a phosphorus source, and seems to be a typical feature of these syntheses.29 Kim et al. recently speculated, using TEM and elemental analysis, that these tetrahedral NCs have indium rich (111) facets passivated with both chlorine and primary amine ligands.29 We confirm here that these tetrahedral NCs have indeed (111) facets as can be seen on the FFT of a single NC imaged using high angular annular dark field (HAADF) STEM as it is shown in Figure 4D,E. Moreover, the InP QDs present no oxidation during the synthesis (see Figure S9 in SI). However, the QDs are very sensitive to air and nonanhydrous solvents. Indeed, they oxidize very fast during postsynthetic treatment and exposure to air (see Figures S8−S10 in SI). Interestingly, the nonoxidized InP

QDs were not luminescent: PL could be detected only after exposure to air, as indicated in Figure S10. Reaction Mechanism. To further rationalize the role of the different parameters and in particular the role and fate of P(NMe2)3 and OLA, we investigated the reaction on a molecular level by using solution 31P NMR combined to mass spectroscopy. As mentioned above, our optimized experimental conditions are 1 equiv of InCl3, 1 equiv of ZnCl2, 4 equiv of P(NMe2)3, and OLA/TOA (1/2) as solvent. In this part of the work, for the sake of simplicity, OLA was used as solvent without the addition of a cosolvent. It is to be noted that a mechanism for this transformation was proposed recently by Tessier et al.28 In their mechanistic study, they used different experimental conditions from ours (1 equiv InCl3 and 5 equiv ZnCl2) which led to slightly different outcome for the reactions (see below). We found that, in the absence of InCl3 and ZnCl2, P(NMe2)3 (1) exchanged its dimethylamino groups with the OLA (RNH2, 35 equiv) to form P(NHR)3 (2, 98 ppm) as shown in Figure 5. The driving force of the reaction is the evaporation of dimethylamine which is gaseous at the temperature of the reaction (bp = 7 °C). The substitution reaction took place at temperatures as low as 50 °C. At this stage, intermediates were observed (see SI, Figure S11), P(NHR) (NMe2)2 at 116 ppm and P(NHR)2(NMe2) (108 ppm). The intermediates disappeared in favor of the fully transaminated product P(NHR)3 upon further heating: full conversion was achieved by heating at 100 °C and applying vacuum for 1 h. Concomitant with the formation of P(NHR)3, a new resonance (3) at 11.4 ppm (doublet in 31P NMR, 1JP−H = 550 Hz) was detected. This new product was the result of a tautomerization equilibrium, which is well-described in the literature;34 its structure is shown in Figure 5C. The formation of 2 and 3 was confirmed by MALDI-TOF mass spectroscopy (see SI). When a lower RNH2/P(NMe2)3 ratio was used (e.g., 6/1), products 4 and 4′ (respectively, 111 and 183 ppm in 31P NMR; 5929


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Chemistry of Materials see SI for mass spectrum) containing bridging amino groups were also obtained in important quantities, which is consistent with the literature.35,36 These experiments show that the first step in the mechanism involves a transamination step between the amino group of the starting phosphine and the solvent. Note here that Tessier et al. propose that the compound at 111 ppm corresponds to the fully transaminated product 2 while the compound at 98 ppm corresponds to P(NHR)2(NMe2) because of successive appearance during their reaction.28 Their hypothesis is contradicted by our experiment with a lower amount of oleylamine, in which the compound, 4, at 111 ppm is present in a high amount. We believe that, in their case, the bridging product 4 appears as major transaminated product in the reaction because of the large amount of zinc chloride present in addition to indium chloride. Indeed, zinc salts, being Lewis acidic, coordinate molecules of oleylamine (see below) and as a consequence modify the equilibria between compounds 2, 3, 4, and 4′. In a second stage, the reaction of 2 and 3 with InCl3 and in the presence of OLA was investigated. InCl3, being a potent Lewis acid, will bind the best Lewis base(s) in the reaction medium to complete its coordination sphere until it is typically hexacoordinated (in InCl3(H2O)3) or heptacoordinated: InCl3(R3N)3−4.37−42 On the basis of known isolated structures and our DFT calculations on models of complexes (vide infra), we propose that, in the presence of amine, the indium center is hexacoordinated by three chlorine atoms (X ligands) and three molecules of OLA (L ligands): InCl3(NH2R)3. In 31P NMR, the signals corresponding to compounds 2 and 3 did not change position or shape in the presence of InCl3(NH2R)3, which indicated that these species are weaker ligands than OLA (also present in much larger amounts). Nonetheless, to form P−In bonds, the displacement of an amine from the coordination sphere by the P donor molecule is a necessary step. DFT calculations were carried out to probe these hypotheses and observations (see SI for computational details). Long chain alkyl fragments in the amine (OA) and phosphine were modeled by the electronically equivalent CH3 moiety. First, penta- versus hexa- versus heptacoordination at In was compared in InCl3(NH2CH3)x complexes. Hexacoordination is more stable than pentacoordination by 2.5 kcal/mol (see SI), which nicely correlates with the fact that known crystal structures of InCl 3 (L) x complexes are only hexa- or heptacoordinated. Then, while the InCl3(NH2CH3)4 complex is slightly more stable than the InCl3(NH2CH3)3 complex and free amine (ΔG = −1.5 kcal/mol), the fourth amine molecule is located outside of the coordination sphere of indium. It appears that the H-bonding between two amines is slightly stronger than the entropic cost for an additional molecule (see ESI). Thus, the preferred coordination sphere at In is hexacoordinated, and it was kept for further calculations. Replacement of an amine molecule in InCl3(NH2R)3 by P(NHR)3 was envisaged in four different ways (Figure 6), because of the formation of compounds 2 and 3: (i) via P coordination in 2 (compound AI), (ii) via N coordination in 2 (compound AII), and (iii) via N coordination in 3 (compounds AIII and AIV). The most favorable of the four isomers is compound AI, followed by compounds AII (+5.3 kcal/mol), AIII (+7.8 kcal/ mol), and AIV (+15.3 kcal/mol). Nevertheless, coordination of the phosphine to the In center to form complex AI is an endergonic process (ΔG = +4.8 kcal/mol vs InCl3(CH3NH2)3, see SI), which nicely reproduces the experimental observations.

Figure 6. DFT calculations.

However, the complexes InCl3(CH3NH2)3 and AI are close enough in energy to allow complex AI (equlibrium constant of 4.8 × 10−3 at 180 °C) to be generated during the process (heating at 180 °C is necessary for the reaction to proceed). The probability of the presence for the two complexes AII and AIII is much smaller (equilibrium constant of 1.3 × 10−5 and 8.3 × 10−7, respectively, at 180 °C), and these are likely not involved in the reaction, but will be presented in the mechanism (Scheme 1) for the sake of completeness. To understand the next steps of the reaction, the QD formation was followed by performing the reaction in a J-Young NMR tube, under strict air and water free conditions. First, control experiments were carried out in the presence of 1 equiv of ZnCl2 (see Figures S17−S18). They show that the same P-containing species are present throughout the reaction, confirming that the Zn salt does not play a role in the molecular processes of the formation of the InP QDs. As a consequence, it is not included in the following discussion. In a typical experiment, the above-mentioned equilibrium mixture of 2 and 3 was preformed by heating to 80 °C for 12 h in the presence of InCl3, and then heated to 180 °C for 30 min to achieve InP QD synthesis. While no conversion of 2 and 3 was observed at 80 °C, the mixture started to darken at 180 °C, indicating the formation of the first InP QDs. Concomitant with the precursor conversion, a new resonance in the 31P{1H} spectrum at 30 ppm (compound 5) was observed, as shown in Figure 7. The spectrum did not change upon proton coupling, which proved the lack of P−H bound in compound 5. The only other species observed during the reaction is the starting aminophosphine 2. The 31P{1H} NMR chemical shift for 5 is consistent with a tetracoordinated P(V) fragment. The MALDI-TOF mass spectrum (see SI) showed an m/z of 1096 g/mol, consistent with the P(NHR)4+ moiety (R = C18H35, OLA). Together, these data prove product 5 to be a protonated phosphazene base (Figure 6B). The formation of this P(V) species shows that the aminophosphine 2 plays a dual role: as a P-source and as a reducing agent. Indeed, in terms of oxidation states of phosphorus, starting from In(III) and P(III) species, it is necessary to form 3 equiv of P(V) species per equivalent of InP. At this stage, the mechanistic study points out that the stoichiometry of the reaction requires 4 equiv of P(NMe2)3 for 12 equiv of OLA (or primary amine) per equivalent of InCl3 precursor, which is in very good accordance with our initial optimization reactions of the InP chemical yield. Indeed, in Figure 3, when 2 equiv of aminophosphine was used, a chemical yield of about 45% was obtained, whereas with 4 equiv, the efficiency of the reaction reached 70%. When 5 equiv of aminophosphine was used, the OLA:P ratio was 2.8. In these 5930


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Chemistry of Materials Scheme 1. Proposed Mechanism for the First In−P Bond Formation

lead to the same compound B, that can be written as different resonance structures. The next step involves the key redox reaction, which generates the P(V) species D as well as the “(RNH)2P−InCl2” fragment C. Compound D is a well-known strong base (pKa ∼ 30) and thus readily deprotonates the RNH3Cl salt to form the observed species 5 and RNH2.43 The next two In−P bond forming steps necessarily involve the same elementary reactions, acid−base and redox reactions, because of the overall stoichiometry. Most importantly, the fact that only the starting compounds and the final product are observed in the multiple condensation reactions that lead to the formation of InP QDs indicates that the rate-determining step is the one leading to intermediate C: all the other steps involve lower or similar activation barriers. In the overall mechanism, the role of OLA is key: it converts P(NMe2)3 into the active phosphorus species 2, coordinates InCl3, and is involved in the acid−base process leading to the formation of the P−In bonds. To underline this role and further prove the chemical mechanism, we performed the reaction in TOA, without OLA. After 2 h at 180 °C, no formation of InP QDs is observed, and the NMR signal is not altered, as shown in Figure S16. The mechanistic study indicates that the situation is very different compared to the classical P(TMS)3/indium carboxylate syntheses. Indeed, in the case of aminophosphines, the Psource 2 is present throughout the whole reaction, while in the case of P(TMS)3, Bawendi et al. and Cossairt et al. showed that the molecular precursors were rapidly depleted.44−46 Indeed, while the group of Bawendi showed that the presence of amine slowed down the precursor conversion in the P(TMS)3/indium carboxylate syntheses, this effect appears even more predominant in the case of aminophosphines, making the reaction limited by the precursor conversion rate. As a consequence, this shows that efficient LaMer type growth of InP QDs might be

Figure 7. Formation of product 5 observed by 31P NMR. (A) Typical 31 1 P{ H} NMR recording during the reaction between P(NMe2)3, OLA, and InCl3. Recorded after 12 h at 80 °C and 30 min at 180 °C. (B) Proposed chemical equation.

conditions, less reactive products 4 and 4′ were formed in higher quantities and trapped the OLA necessary for the reaction, thus leading to a lower yield. The characterization of products 2, 3, and 5 as well as the DFT calculations allowed us to propose a pathway for the formation of the P−In bonds of InP QDs (Scheme 1). The first step (endergonic) is the coordination of the aminophosphine 2 to the InCl3−OLA complex, forming the intermediates in equilibrium A1, A2, and A3 and liberating one molecule of OLA. Then, an acid−base reaction between the coordinated P(NHR) in A1 and A2 or PH function in A3 and the amine takes place, releasing intermediate B and the ammonium salt RNH3Cl. Note that the three intermediates 5931


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Ostwald ripening can be avoided by addition of a cosolvent. We also propose a fully consistent mechanism for the synthesis of InP QDs. This allowed us to rationalize the need for an excess of aminophosphine and to highlight the role of OLA as ligand and reagent in the chemical reaction. While this study was mainly centered on the role and fate of the phosphorus precursor, further studies will be devoted to the finding of new In precursors and ligand systems in order to obtain an efficient LaMer type growth and further improve the size control. In parallel, studies are devoted to calculating the potential energy surface leading to InP.

possible using aminophosphines, although the conditions still remain to be found. As mentioned above, Tessier and Hens et al. published a mechanism of this reaction while this manuscript was in preparation. Their proposed mechanism differs significantly from ours: they present a first InCl3−P(NHR)3 adduct, written as a zwitterionic compound where the P center is cationic while the In center possesses a negative charge. In the subsequent step, the cationic moiety favors a nucleophilic attack of the P atom of a second phosphine on the nitrogen atom of the coordinated phosphine. Unfortunately, this reasoning is based on a misleading representation of the dative bond and on related resonance structures. Indeed, in classical coordination chemistry, the interaction between a two-electron donor (neutral ligand, Lewis base) and a metal center (Lewis acid) is commonly represented by an arrow and does not result in a purely covalent or purely ionic bond. In order to illustrate this, we performed a natural population analysis on the phosphine and amine ligands, on the InCl3(CH3NH2)3 as well as on AI− AII complexes (Figure 8). First, because of a significant

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ASSOCIATED CONTENT

All reactions were performed under an inert atmosphere of dry argon. List of Chemicals. Indium chloride (Strem Chemicals anhydrous 99.99%), zinc chloride (Sigma-Aldrich anhydrous 99.995%), tris(dimethylamine)phosphine (Alfa Aesar 97%), oleylamine (Acros Organics 80−90%), trioctylamine (Merck 93%), hexane, and ethanol were purchased from Carlo Erba. Exchange of Amino Group from P(NMe2)3 with Oleylamine: Formation of 2 and 3. A 20 μL portion of P(NMe2)3 (0.11 mmol) and 1.25 mL of dried, degassed oleylamine (3.8 mmol) were introduced in a Schlenk flask inside a N2-filled glovebox. The reaction was followed by taking aliquots and preparing the NMR samples under argon flow in J-Young NMR tubes. After 12 h of heating at 50 °C, the Schlenk flask was put back to room temperature, and vacuum was applied for 30 min. The flask was successively heated to 100 °C for 1 h, leading to the full conversion into 2 and 3. 31P NMR (oleylamine): 2, δ = 98 ppm (s); 3, δ = 12 ppm (d of m, 1JP−H = 550 Hz). Observation of Product 5. A 33 mg portion of InCl3 (0.15 mmol) was weighted in a Schlenk flask inside a N2-filled glovebox. A 2.5 mL (7.6 mmol) portion of dried, degassed oleylamine was added, and the mixture was heated to 120 °C and vigorously stirred for 2 h to let InCl3 dissolve. Then, the Schlenk flask was taken back to the glovebox, and 114 μL of P(NMe2)3 (0.6 mmol) was added. A 0.5 mL aliquot of the mixture was transferred in a J-Young tube inside the glovebox. The mixture was then heated to 80 °C for 12 h and then to 180° for 30 min. 31P NMR (oleylamine): 2, δ = 98 ppm (s); 5, δ = 30 ppm (s). Optimized Synthesis of InP QDs. A 3.6 mL portion of trioctylamine (8.2 mmol) and 1.4 mL of oleylamine (4.3 mmol) were degassed at 100 °C under vacuum for 30 min under strong stirring. The mixture was then put under argon at 100 °C, and 66 mg of InCl3 (0.3 mmol) and 40 mg of ZnCl2 (0.3 mmol) coming directly from the glovebox were added to the mixture of solvents and then heated to 130 °C for 30 min to completely dissolve the salts. Then, the temperature was increased to 220 °C, and when the temperature was reached, 218 μL of P(NMe2)3 (1.2 mmol) was rapidly injected. The time of reaction depends on the size expected, and when it was reached, the temperature of reaction is cooled down. InP nanocrystals are washed by three cycles of precipitation/suspension with ethanol and hexane. With reaction at 220 °C, the NC diameter was between 3.6 and 4.6 nm for a reaction time between 30 s and 30 min.

Figure 8. NBO charges.

difference in electronegativities, we note that the charges at P and N in the phosphine ligand are high as expected (+1.30 and −1.02, respectively). Upon coordination of the ligands to the InCl3 center, the charges change because of electron transfer, but only to a small extent. For example, the charge at P increases from 1.30 to 1.39 in AI. Finally and most importantly, unlike what is presented by Tessier et al., both the charges at In and at P are positive (+1.52 and +1.39). We thus believe that the mechanism for the formation of InP using aminophosphine P(NHR)3 relies on the presence of a NH moiety, which allows proton transfer, and on possible hypervalence of P. This critical property results in possible oxidation of P(III) to P(V) and provides the pathway to formal transfer of the “NR” moiety from the P−In fragment to an incoming phosphine. The use of phosphines as reducing agents has been abundantly reported in the literature, for example, for desulfurization reactions.47−49

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CONCLUSION In conclusion we developed a methodology for the synthesis of size tunable InP QDs using aminophosphines with high chemical yield. The size tunability and dispersity are comparable to classical InP QD synthesis, but our methodology did not require the use of silylated phosphines which are dangerous and expensive or the change of indium precursor. We provide a kinetic study of the reaction, showing that a partial separation of nucleation and growth is possible and that

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b02456. Absorbance and photoluminescence spectrum, EDX cartography, TEM images, calculations, DRX, NMR spectra, MALDI-TOF spectrum, and DFT calculations (PDF) 5932


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(13) Rempel, J. Y.; Bawendi, M. G.; Jensen, K. F. Insights into the Kinetics of Semiconductor Nanocrystal Nucleation and Growth. J. Am. Chem. Soc. 2009, 131, 4479−4489. (14) Gary, D. C.; Glassy, B. a.; Cossairt, B. M. Investigation of Indium Phosphide Quantum Dot Nucleation and Growth Utilizing Triarylsilylphosphine Precursors. Chem. Mater. 2014, 26, 1734−1744. (15) Franke, D.; Harris, D. K.; Xie, L.; Jensen, K. F.; Bawendi, M. G. The Unexpected Influence of Precursor Conversion Rate in the Synthesis of III-V Quantum Dots. Angew. Chem., Int. Ed. 2015, 54, 14299−14303. (16) Gary, D. C.; Terban, M. W.; Billinge, S. J. L.; Cossairt, B. M. Two-Step Nucleation and Growth of InP Quantum Dots via MagicSized Cluster Intermediates. Chem. Mater. 2015, 27, 1432−1441. (17) Li, L.; Protière, M.; Reiss, P. Economic Synthesis of High Quality InP Nanocrystals Using Calcium Phosphide as the Phosphorus Precursor. Chem. Mater. 2008, 20, 2621−2623. (18) Lauth, J.; Strupeit, T.; Kornowski, A.; Weller, H. A Transmetalation Route for Colloidal GaAs Nanocrystals and Additional III−V Semiconductor Materials. Chem. Mater. 2013, 25, 1377− 1383. (19) Carenco, S.; Demange, M.; Shi, J.; Boissière, C.; Sanchez, C.; Le Floch, P.; Mézailles, N. White Phosphorus and Metal Nanoparticles: A Versatile Route to Metal Phosphide Nanoparticles. Chem. Commun. 2010, 46, 5578−5580. (20) Liu, Z.; Kumbhar, A.; Xu, D.; Zhang, J.; Sun, Z.; Fang, J. Coreduction Colloidal Synthesis of III−V Nanocrystals: The Case of InP. Angew. Chem., Int. Ed. 2008, 47, 3540−3542. (21) Buhro, W. E. Metallo-Organic Routes to Phosphide Semiconductors. Polyhedron 1994, 13, 1131−1148. (22) Gary, D. C.; Cossairt, B. M. The Role of Acid in Modulating Precursor Conversion in InP-QD Synthesis. Chem. Mater. 2013, 25, 2463−2469. (23) Cros-Gagneux, A.; Delpech, F.; Nayral, C.; Cornejo, A.; Coppel, Y.; Chaudret, B. Surface Chemistry of InP Quantum Dots: A Comprehensive Study. J. Am. Chem. Soc. 2010, 132, 18147−18157. (24) Virieux, H.; Le Troedec, M.; Cros-Gagneux, A.; Ojo, W.-S.; Delpech, F.; Nayral, C.; Martinez, H.; Chaudret, B. InP/ZnS Nanocrystals: Coupling NMR and XPS for Fine Surface and Interface Description. J. Am. Chem. Soc. 2012, 134, 19701−19708. (25) Gary, D. C.; Flowers, S. E.; Kaminsky, W.; Petrone, A.; Li, X.; Cossairt, B. M. Single-Crystal and Electronic Structure of a 1.3 Nm Indium Phosphide Nanocluster. J. Am. Chem. Soc. 2016, 138, 1510− 1513. (26) Song, W.-S.; Lee, H.-S.; Lee, J. C.; Jang, D. S.; Choi, Y.; Choi, M.; Yang, H. Amine-Derived Synthetic Approach to Color-Tunable InP/ZnS Quantum Dots with High Fluorescent Qualities. J. Nanopart. Res. 2013, 15, 1750. (27) Tessier, M. D.; Dupont, D.; De Nolf, K.; De Roo, J.; Hens, Z. Economic and Size-Tunable Synthesis of InP/ZnE (E = S, Se) Colloidal Quantum Dots. Chem. Mater. 2015, 27, 4893−4898. (28) Tessier, M. D.; De Nolf, K.; Dupont, D.; Sinnaeve, D.; De Roo, J.; Hens, Z. Aminophosphines: A Double Role in the Synthesis of Colloidal Indium Phosphide Quantum Dots. J. Am. Chem. Soc. 2016, 138, 5923−5929. (29) Kim, K.; Yoo, D.; Choi, H.; Tamang, S.; Ko, J.-H.; Kim, S.; Kim, Y.-H.; Jeong, S. Halide-Amine Co-Passivated Indium Phosphide Colloidal Quantum Dots in Tetrahedral Shape. Angew. Chem., Int. Ed. 2016, 55, 3714−3718. (30) Adam, S.; Talapin, D. V.; Borchert, H.; Lobo, A.; McGinley, C.; de Castro, A. R. B.; Haase, M.; Weller, H.; Möller, T. The Effect of Nanocrystal Surface Structure on the Luminescence Properties: Photoemission Study of HF-Etched InP Nanocrystals. J. Chem. Phys. 2005, 123, 084706. (31) Sugimoto, T. Underlying Mechanisms in Size Control of Uniform Nanoparticles. J. Colloid Interface Sci. 2007, 309, 106−118. (32) Mićić, O. I.; Sprague, J.; Lu, Z.; Nozik, A. J. Highly Efficient Band-Edge Emission from InP Quantum Dots. Appl. Phys. Lett. 1996, 68, 3150.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions ∥

A.B. and S.D. have contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Christian Pradel for recording the MALDI TOF spectra on the Waters MALDI Micro MX spectrometer provided by the Integrated Screening Platform of Toulouse (PICT, IBiSA). We thank Yannick Coppel (Laboratoire de Chimie de Coordination) for MAS NMR measurements. The STEM experiments have been done on a Titan Themis microscope from FEI acquired in the framework of the ANR project TEMPOS (reference ANR-10-EQPX-50 of the programm “Investissements d’avenir”). S.D. thanks the Ecole Polytechnique for a Ph.D. fellowship. N.M. acknowledges the Région Midi-Pyrénées for generous funding. B.D., N.M., G.P., and A.B. acknowledge funding from ANR project SNAP (ANR12-BS10-0011).

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