Fast, High Yield, and High Solid Loading Synthesis of Metal Selenide

May 28, 2013 - The synthesis of metal selenide nanocrystals based on the injection of selenium powder in a hot mixture containing the metal cation pre...
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Fast, High Yield, and High Solid Loading Synthesis of Metal Selenide Nanocrystals Stijn Flamee,†,‡,∥ Marco Cirillo,†,‡,∥ Sofie Abe,†,‡ Kim De Nolf,†,‡ Raquel Gomes,†,‡,§ Tangi Aubert,†,‡ and Zeger Hens*,†,‡ †

Physics and Chemistry of Nanostructures and ‡Center for Nano- and Biophotonics (NB-Photonics), Ghent University, Belgium S Supporting Information *

ABSTRACT: The synthesis of metal selenide nanocrystals based on the injection of selenium powder in a hot mixture containing the metal cation precursor complexed by a carboxylic acid is demonstrated by means of the formation of CdSe and ZnSe nanocrystals. In both cases, the synthesis can reach reaction yields of 80−85% within 5 min. In the case of CdSe nanocrystals, a more extensive study shows that even without protective atmosphere, the synthesis leads to state-ofthe-art nanocrystals with low size dispersion. Importantly, the size of the nanocrystals at close to full yield can be changed by varying the carboxylic acid chain length, whereas the solid loading, that is, the amount of nanocrystals formed over the reaction volume, of the synthesis can be the 10-fold of typical literature syntheses. The potential of this reaction for the scaled up production of metal selenide nanocrystals is discussed and supported by the automated, parallel synthesis of CdSe nanocrystal batches using this heterogeneous selenium precursor where the standard deviation on the nanocrystal diameter is less than 1.5%. KEYWORDS: colloidal nanocrystals, quantum dots, hot injection, phosphine-free, ambient conditions

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INTRODUCTION The introduction of the hot injection method as a synthesis route for monodisperse colloidal nanocrystals (NCs) by Murray et al.1 has initiated a vast research activity around these materials that is driven by the exceptional combination of versatile synthesis methods, tunable materials properties, and a broad application potential. This is exemplified by the case of semiconductor nanocrystals or quantum dots (QDs) where the optical properties depend on size, shape, and heterostructure formation. Synthesis recipes for various II−VI, IV−VI, and III− V semiconductors have been developed,2−6 while shape control and the growth of heterostructures has become common practice.7−9 By now, the use of colloidal QDs as optical materials has been demonstrated in, for example, photovoltaics,10,11 lighting and display applications,12,13 photodetection,14,15 and fluorescent labeling.16 Following their increasing use, the supply of larger quantities of nanocrystals necessitates a scaling up of their production.17−20 As a result, synthesis cost, tunability of the NC size at full yield, and synthesis reproducibility have become key issues.21−24 Finding an optimal approach in this respect is a matter of methodology, involving the use of larger scale or automated batch reactors or continuous flow-line approaches,25 yet it also concerns a reassessment of the reagents used and the reaction conditions needed.26,27 The latter point is well exemplified by the case of CdSe NCs, which remain the most widely used colloidal QDs. The original synthesis by Murray et al.1 made use of dimethyl cadmium and bis(trimethylsilyl)selenium or selenium dissolved in trioctyl© 2013 American Chemical Society

phosphine (TOP), which are all expensive chemicals that are difficult to handle (pyrophoric, oxygen sensitive). Later, Peng et al., showed that CdSe NCs can be synthesized using less hazardous cadmium precursors, obtained by dissolving common cadmium salts or cadmium oxide in an organic solvent using carboxylic or phosphonic acids as a complexing agent.28 With respect to the chalcogen precursor, a number of phosphine-free alternatives have been proposed.26,29−35 For the specific case of selenium precursors, this mainly involves the approach introduced by Jasieniak et al. and Yang et al., where elemental selenium is dissolved in octadecene (ODE) at elevated temperature, either using a hot injection or a heating up approach.29,30 While elemental selenium is cheap and easy to handle, the formation of CdSe nanocrystals based on this homogeneous ODE-Se precursor is slow and does not allow opposite from TOP-Se based synthesesfor the full yield synthesis of CdSe NCs with different sizes.32 Moreover, the limited solubility of selenium in ODE makes that a relatively high amount of solvent is needed to synthesize a given weight of CdSe NCs, a ratio we refer to as the solid loading of the synthesis. Especially reaction yield and solid loading are important parameters determining the material cost of a synthesis, where both should be as high as possible to reduce the use of precursors and solvents. Received: March 11, 2013 Revised: May 16, 2013 Published: May 28, 2013 2476

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homogeneous ODE-Se precursor were injected. After injection, the reaction temperature was set to 235 °C. The reaction was stopped at the desired time by quenching with a water bath followed by the injection of 14 mL of toluene. During the synthesis, aliquots were taken in a time range from 30 s to 85 min to study the yield development of the reaction. Synthesis of ZnSe NCs. ZnSe NCs were prepared in a similar manner as described above using the heterogeneous Se precursor. In brief, 0.08 mmol of zinc carbonate (Zn5(CO3)2(OH)6) were mixed with 1.2 mmol of oleic acid and 10 mL of ODE. The solution was heated up to 270 °C to form a zinc carboxylate complex. Then 1 mL of the heterogeneous ODE-Se (0.2 M) precursor was injected. Aliquots were taken at fixed times (10 s, 20 s, 30 s, 1 min, 2 min, 4 min, 8 min, and 16 min). The reaction temperature was set to 260 °C. After 16 min, the reaction was stopped, and the reaction product purified as described above for CdSe QDs synthesized with the heterogeneous selenium precursor. Automated Synthesis. A set of experiments has been performed to investigate the reproducibility of the synthesis using an automated liquid handler (Tecan Liquid Handler type EVO 200 controlled through Tecan EVO-Ware software) working with 8 mL cylindrical reaction vials assembled in an aluminum block with cylindrical holes, heated through a hot plate with magnetic stirring. The experiments made use of a cadmium oleate stock solution that was prepared by adding 4 mmol of CdO to a flask containing 8 mmol of oleic acid and 200 mL of ODE. This solution was heated up to 250 °C until it became colorless and allowed to cool down afterward. A heterogeneous ODE-Se suspension was prepared by adding 9 mmol of Se to 89.5 mL of ODE. This solution was continuously stirred to maintain a homogeneous distribution of Se powder in the dispersion. For a single automated synthesis, the liquid handler was programmed to add 2.7 mL of the cadmium oleate stock solution (0.054 mmol Cd) to an 8 mL vial. The solution was then heated up to 250 °C followed by the addition of 270 μL of the ODE-Se solution (0.027 mmol Se), after which the temperature was kept constant. The reaction was stopped after 4 min by thermal quenching with a water bath and the injection of toluene. Finally, 1 mL of oleic acid was added to stabilize the QDs. The solutions did not receive any further purification. CdS Shell Growth. Core/shell CdSe/CdS NCs were obtained by adapting the synthesis developed by Li et al.38 In brief, S and Cd precursors were prepared as follows: 1 mmol of sulfur was dissolved in 10 mL of ODE at 100 °C; 1.38 mmol of CdO were dissolved in 12 mmol of oleic acid and 10 mL of ODE and heated up to 250 °C until complete dissolution. The synthesis was performed by adding 1.5 g of n-octylamine and 12 mL of ODE to a three neck flask. This solution was flushed under nitrogen while stirring at 100 °C for 1 h. Next, a solution containing 68.2 μmol of CdSe nanocrystals dissolved in hexane was injected. The temperature was raised and when 225 °C were reached the sulfur solution was added. After 10 min the cadmium precursor was also added. The process was repeated alternating the two precursors until the desired number of shells was reached. The amount of solution that was added during each injection was calculated to be equal to the amount of reagent necessary to grow one extra monolayer on the nanocrystals in the reaction mixture. The reaction was stopped by quenching with a water bath, after which 5 mL of toluene were added. For the purification isopropanol was added in a ratio 1:1 with respect to the amount of toluene, then methanol was used to precipitate the particles and the solution was centrifuged for 10 min at 3000 rpm. The purification was repeated twice more dispersing the particles in toluene and precipitating with methanol. Finally the particles were dispersed in toluene. Quantitative Analysis. For quantitative analysis, reaction aliquots were taken and injected in a known amount of chloroform, after which the resulting solution was weighed. The solutions were purified by addition of isopropanol and methanol, followed by centrifugation and redispersion of the obtained pellet in chloroform. UV−vis absorption spectra were taken with a Perkin-Elmer Lambda 950 spectrophotometer. The size of CdSe the nanocrystals was calculated from the position of the first exciton peak,39 while the size dispersion was estimated using the half width at half-maximum. The volume fraction

In this work, we propose an alternative approach to synthesize metal selenide NCs using a selenium precursor that adds high reactivity and high solid loading to the advantages of using homogeneous ODE-Se. The method involves the direct injection of a heterogeneous mixture of selenium powder dispersed in ODE in a hot solvent containing a metal carboxylate as the cation precursor and excess carboxylic acid. Both in the case of cadmium and zinc carboxylates, we find that the injection of this heterogeneous ODE-Se precursor is followed by the rapid formation of monodisperse nanocrystals, reachingdepending on the reaction conditionsreaction yields up to 80−85% within a few minutes. Moreover, the reaction can be run under air without compromising the quality of the end product, and the solid loading in the synthesis can be as high as 50 g/L. In the case of a fast and high yield synthesis as obtained here, reaction chemistry/nanocrystal property relations can be implemented to tune the size the nanocrystals reach at the end of the reaction.36,37 Here, we show that such size tuning can be achieved by changing the carboxylic acid chain length. Finally, we demonstrate that syntheses involving the heterogeneous ODE-Se precursor as proposed here can be reproducibly executed on an automated synthesis platform, thus showing the potential of this novel approach for scaling up the production of colloidal metal selenide nanocrystals.

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EXPERIMENTAL SECTION

Synthesis of CdSe NCs, Heterogeneous ODE-Se Precursor. For a standard synthesis, CdO (0.4 mmol) was added to 10 mL of ODE together with 1.2 mmol of myristic acid in a three neck flask with cooler under air (for suppliers and purity, see Supporting Information). The mixture was heated up to 270 °C to dissolve the red CdO in ODE by the formation of a cadmium carboxylate complex. The heterogeneous ODE-Se precursor was prepared by adding 2 mmol of Se powder to 10 mL of ODE at room temperature. The resulting unstable dispersion was left stirring, yet no attempt was made to dissolve the Se powder by heating. To initiate the reaction, 1 mL of the heterogeneous ODE-Se precursor was swiftly injected in the colorless reaction mixture containing the Cd precursor. Injection and growth temperature were set at 270 and 260 °C, respectively. The black color of the heterogeneous ODE-Se precursor disappeared upon injection, and the color of the mixture turned from yellow to orange to red depending on the size of the CdSe nanocrystals formed. For quantitative measurements, aliquots were taken after reaction times of 5 s, 10 s, 20 s, 30 s, 40 s, 1 min, 2 min, 3 min, 5 min, 7 min, and 10 min. The reaction was stopped by thermal quenching using a water bath followed by the injection of 10 mL of toluene. The reaction mixture was purified by the addition of isopropanol and methanol, both in a 1:1 ratio relative to the toluene added. The resulting turbid solution was centrifuged to obtain a pellet of NCs that was redispersed in toluene. Prior to a second purification step, oleic acid was added in a 10:1 ratio relative to the amount of acid originally used in the synthesis to replace the original carboxylic acid on the surface of the nanocrystals. Next, the purification was repeated twice using respectively toluene and methanol as solvent and nonsolvent to remove all residual reaction products. Synthesis of CdSe NCs, Homogeneous ODE-Se Precursor. CdSe QDs were synthesized using the method developed by Jaseniak et al.29 A homogeneous ODE-Se precursor was prepared by heating up 3.5 mmol of Se powder in 35 mL of ODE under nitrogen atmosphere for 2 h and 30 min. The cadmium oleate precursor was prepared by dissolving 1.55 mmol of CdO in 12.4 mmol of oleic acid and 11.6 mL of ODE and heating up to 250 °C until complete dissolution. The synthesis was performed by adding adding 3 mL of the Cd precursor solution to 7.75 mL of ODE in a three neck flask and flushing the mixture at room temperature for 10 min and at 100 °C for 30 min. Afterward, the temperature was raised to 260 °C, and 3 mL of the 2477

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Figure 1. Time development of the synthesis of CdSe nanocrystals by means of the heterogeneous ODE-Se precursor using the standard conditions as described in the Experimental Section. (a) Absorption spectra obtained for aliquots taken at the indicated time after injection. The spectra are normalized at the first exciton peak absorbance. (b) Time development of (red circles) average nanocrystal diameter and (blue squares) size dispersion as obtained from the absorption spectra. (c) Time evolution of (blue markers) the amount of CdSe formed as determined from UV−vis absorption spectroscopy, where the full blue line is a guide to the eye while the dashed blue line indicates the 100% yield level; time evolution of (red markers) the number of CdSe nanocrystals (nNC) in the reaction mixture, where the full red line indicates the average of nNC.

the first exciton peak allow for a detailed analysis of the reaction yield, the NC diameter (dNC), the size dispersion (σd), and the NC concentration. As shown in Figure 1b, dNC rapidly increases after injection and remains steady, in this case at about 3.1 nm, after 3 min of reaction. At the same time, σd remains low, at about 5−6%. Mimicking the development of dNC, also the reaction yield as determined using absorption spectroscopy36 evolves quickly from 40−45% after 5 s to 80−85% (i.e., close to full conversion) after 5 min (Figure 1c, blue markers). This conclusion is confirmed by XRF analysis of the selenium content in the supernatant obtained after purifying the final product once, where only 11.3% of the selenium originally used in the reaction is detected (see Supporting Information). At the same time, the number of CdSe nanocrystals (nNC) in the reaction mixture stays constant, which indicates that the formation of stable nuclei is stopped within the first 5 s of the reaction. Similar results are obtained working either under protective atmosphere or using a saturated hydrocarbon such as hexadecane (HDA) as the solvent. Figure 2a shows a representative bright field TEM image obtained from the purified reaction product made by the standard synthesis. The size histogram obtained by analyzing several TEM images (Figure 2b) confirms that quasi spherical nanocrystals are formed with an average diameter of 3.0 nm and a size dispersion of 6.7%, in good agreement with the figures estimated from the UV−vis spectra. The X-ray diffractogram (Figure 2c) demonstrates that the NCs formed have the crystal structure of zinc blende CdSe, a similar result as obtained with the homogeneous ODE-Se precursor.29 Finally, two signals appear in the Rutherford backscattering spectrum, which we attribute to Se and Cd (low and high energy signal in Figure 2d, respectively). The ratio of the Z2 corrected RBS signal intensities yields a Cd:Se ratio of 1.22 ± 0.05. Hence, similar to many other syntheses of binary nanocrystals such as CdSe, CdTe, PbSe, and PbS, the use of a heterogeneous Se precursor leads to CdSe nanocrystals that are cation rich.39,43−46 Figure 3 shows a quantitative 1H nuclear magnetic resonance (NMR) spectrum obtained on the purified reaction product, that is, involving a ligand exchange to oleic acid, dispersed in toluene-d8. Similar to previously published results on CdSe,41 PbSe,47 and PbS46 NCs, the spectrum contains the broad resonances characteristic of bound oleate ligands (peaks 2−6 in Figure 3), next to the sharp resonance of residual toluene-d8

of CdSe, and thus the yield of the reaction, was calculated from the absorbance at 300 nm assuming a Cd:Se ratio of 1.2.39 X-ray fluorescence (XRF) analysis was performed on the supernatant collected after the first purification step to detect unreacted selenium and validate the reaction yields obtained from absorption measurements. Materials Characterization. Samples for X-ray diffraction (XRD) were prepared by dropcasting a dispersion of QDs in an 80:20 hexane:heptane mixture on a silicon substrate. Samples for transmission electron microscopy (TEM) were prepared by dropcasting a solution on a carbon coated copper grid. Bright field TEM images were recorded using a Cs corrected JEOL 2200 FS microscope. Rutherford backscattering spectrometry was performed by measuring backscattered He+ ions accelerated to an energy of 3.71 MeV with an NEC 5SDH-2 Pelletron tandem accelerator on a thin film (approximately 2 monolayers) of nanocrystals spincoated on a silicon substrate. Samples for quantitative 1H NMR spectra were prepared by dissolving a known amount of QDs, as determined using published values for the CdSe absorption coefficient,39 in toluene-d8 as described before.40,41 They were analyzed using a Bruker Avance DRX 500 spectrometer at the frequency of 500.13 MHz. The presence of Se in the supernatant was analyzed using a Rigaku NEX CG energy dispersive XRF spectrometer on 4 mL (3.34 g) of the supernatant solution collected after the first purification. The analysis of the data was performed using the software (Fundamental Parameters) provided with the instrument after defining the weight, volume, and density of the solution. The software provides automatic recognition of the elements42 and their concentration as mg of element per kg of solution. The results were then related to the total amount of supernatant. Photoluminescence emission spectra were performed with an Edinburgh Instruments FLS920 fluorescence spectrometer. The photoluminescence quantum yields (PLQYs) were determined relatively to coumarin 2 (with a known PLQY of 93%) at an excitation wavelength of 365 nm.

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RESULTS Synthesis of CdSe Nanocrystals Using Heterogeneous ODE-Se. Figure 1 summarizes the characteristics of a standard synthesis where CdSe NCs are formed by injecting 1 mL of the standard heterogeneous ODE-Se precursor under air in a solution obtained by dissolving cadmium oxide (0.4 mmol) in a mixture of myristic acid (1.2 mmol) and octadecene (10 mL) at elevated temperature (270 °C). The UV−vis absorption spectra of aliquots taken between 5 s and 10 min after the injection (Figure 1a) are characteristic of CdSe QD dispersions with a narrow size distribution, where the absorbance at short wavelength and the maximum wavelength λ1Sh−1Se and width of 2478

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Tuning the Standard Synthesis. As indicated by Figure 4a, increasing the concentration of all reagents by a factor of 10 relative to the standard conditions does not change the general characteristics of the time development of the reaction. Also under these conditions, dNC and the reaction yield level off at 3−5 min of reaction time. As shown in the Supporting Information, the same behavior is observed for growth temperatures in the range of 220−260 °C and nCd:nSe ratios from 1.25:1 to 5:1. To analyze the tuning of the synthesis, we therefore limit ourselves to a study of the product obtained by stopping the reaction 10 min after the injection. Figure 4b shows the absorption spectra thus obtained for syntheses where nCd and nSe are systematically varied with respect to the standard values of 0.4 and 0.2 mmol, while keeping their ratio fixed at 2:1. A clear feature of the λ1Sh−1Se transition is discernible up to nCd:nSe equal to 10:5 mmol. We find that λ1Sh−1Se systematically shifts to longer wavelengths while the size dispersion of the NCs slightly increases from 4.5% for the most diluted to 7.5% for the most concentrated mixture analyzed. For nCd:nSe combinations up to 4:2 mmol, a reaction yield of 70% or more is obtained (Figure 4c). When increasing nCd:nSe to 10:5 mmol, the reaction yield drops to a value of about 55%. Nevertheless, this results in a CdSe solid loading in the reaction mixture of about 50 g/L. The increase of dNC with increasing precursor concentration is different from results obtained using a TOP-Se based approach to form zincblende CdSe NCs.36 However, this particular size tuning resulted from the increase of the reaction rate with increasing precursor concentration. As shown by Figure 4a, this link between reaction rate and precursor concentration is absent here and, concomitantly, a different relation between dNC and precursor concentration is obtained. Although changing the precursor concentration leads to a variation of dNC at the end of the reaction, we observe that the reaction yield goes down with increasing precursor concentration. As a result, size tuning at the optimal concentration is preferred over size tuning by changing the precursor concentration. A straightforward variable offered by the synthesis in this respect happens to be the chain length of the carboxylic acid used to complex the CdO and stabilize the resulting NCs. Executing the reactions under standard conditions, we find that again a constant diameter and a yield of 80−85% are obtained within 5 min of reaction, regardless of the acid chain length (see Supporting Information). Moreover, Figure 5 indicates that by changing the acid chain length from n = 22 (behenic acid) to n = 9 (nonanoic acid), dNC increases from 2.6 to 4.5 nm. Most tunability is found for ligands having n ≤ 16, although for the shortest ligands, this comes with a considerable deterioration of the size dispersion. CdS Shell Growth. In general, the photoluminescence quantum yield (PLQY) of CdSe core NCs is relatively low. For CdSe QDs synthesized under air using heterogeneous ODE-Se, typical values we measure after purification range between 5 and 10%. In line with their low size dispersion, the PL spectra are narrow, showing for example a full width at half-maximum of about 30 nm for NCs emitting at 533 nm (see Supporting Information). The PLQY of CdSe NCs is routinely enhanced by shell growth, where for instance the formation of a CdSe/ CdS core/shell nanocrystals by the successive ion layer adsorption and reaction (SILAR) approach is a well established method, which we use here as a benchmark to assess the quality of the CdSe NCs synthesized using heterogeneous ODE-Se

Figure 2. Structural characterization of CdSe NCs synthesized by the standard synthesis (see Experimental Section and Figure 1). (a) Bright field TEM image. The scale bar corresponds to 10 nm. (b) Size histogram obtained by analyzing 100 different NCs on TEM micrographs. (c) XRD pattern in combination with (vertical bars) the expected reflections for zinc blende CdSe. (d) Rutherford backscattering spectrum indicating (blue) backscattering of He+ by Se and Cd and (red) the integrated intensity used to calculate the Cd/ Se ratio.

Figure 3. 1D 1H NMR spectrum of a dispersion of CdSe NCs synthesized using the standard procedure (see Experimental Section and Figure 1). Indicated are the resonances of (2−6) bound oleate and (†) residual toluene-d8. See refs 47 and 41 for more details on the resonance assignment.

(†).41 The ratio between the integrated intensity of the resonance of the alkene protons of oleic acid at 5.6 ppm and the methyl protons at 1.0 ppm protons amounts to 2:3.3, indicating that at the most 10% of the original myristic acid ligands used in the synthesis, which contribute to the methyl proton resonance but not to the alkene proton resonance, remain after the oleic acid ligand exchange step in the purification procedure. From the same quantitative experiment, we obtain a ligand density of 3.4 ± 0.2 nm−2 based on the methyl resonance. In combination with the Cd:Se ratio as determined by RBS and the NC diameter, this yields a ratio between the number of ligands and the excess Cd atoms per nanocrystal of 1.9 ± 0.4. Hence, as previously found for CdSe NCs stabilized by oleate or phosphonate ligands,41,48 we find that the negative charge on the anionic oleate ligands (taken as −1) balances the positive charge on the excess cadmium (taken as +2). 2479

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Figure 4. (a) Time development of (blue) the reaction yield and (red) the NC diameter for a synthesis executed under standard conditions of injection/growth temperature, using 4.0 mmol of Cd and 2.0 mmol of Se (i.e., 10-fold the standard amounts). The full blue line is a guide to the eye. The light blue data give the yield development of the standard synthesis (see Figure 1a) as a reference. (b) Absorption spectra of CdSe NCs obtained by stopping the reaction after 10 min where the reactions have been run using nCd as indicated and proportionally scaling the amounts of all other reagents relative to the standard synthesis, except the solvent. In this case, oleic acid is used as the carboxylic acid. (c) Reaction yield as a function of nCd.

growth of 5 and 7 shells, respectively. This indicates an increase in dNC of 0.6 nm per CdS layer, in good correspondence with the 0.58 nm expected based on the lattice parameter of the zinc blende CdS unit cell. Upon CdS shell growth, the PLQY quickly increases to reach a value of 40−45% when 2−3 layers of CdS are grown (see Figure 6b). Further growth of CdS leads to a progressive reduction of the PLQY to values of about 20% for 6 CdS layers. A similar behavior has been described for SILAR procedures using CdSe cores synthesized using a protective atmosphere and has been attributed to the enhanced strain or the occurrence of crystal defects with increasing shell thickness.49 Synthesis of ZnSe Nanocrystals. Similar to the synthesis of CdSe NCs, ZnSe NCs are typically made using TOP-Se as the selenium precursor.50 Here, we inject 1 mL of the heterogeneous ODE-Se in a solution made by dissolving zinc carbonate (0.08 mmol of Zn5(CO3)2(OH)6) in a mixture of oleic acid (1.2 mmol) and ODE at 270 °C. Again, the absorption spectra of aliquots taken at different times after the reaction (Figure 7a) indicate the formation of ZnSe NCs that grow larger with increasing reaction time. The development of the wavelength of the first exciton transition as a function of time (Figure 7b) indicates that also in this case, dNC reaches a constant value within 8 min. The yield of the reaction, as determined by analyzing the supernatant after a first purification of the reaction mixture, amounts to ≈90%, that is, similar to what is achieved with CdSe NCs. As shown by Figures 7c and d, XRD and TEM confirm the formation of zinc blende ZnSe nanocrystals with, in this case, an average diameter of 3.4 nm. It should be noted that not all nanocrystals appear as quasi-spherical on the TEM micrograph in this case.

Figure 5. (a) Absorption spectra of CdSe NCs synthesized using different carboxylic acids, as characterized in the legend by the number of carbon atoms in the aliphatic chain, using the standard conditions of concentrations and temperature. (b) Variation of (marker) average NC diameter and (error bar) size dispersion as a function of the number of carbon atoms in the aliphatic chain of the saturated carboxylic acid.

under air. Figure 6a shows the evolution of the absorption and PL spectra when a successive ion-layer adsorption and reaction

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DISCUSSION The need for robust and scalable hot injection methods for the production of high quality colloidal nanocrystals has been addressed by various authors. This involves issues of synthesis methodology and of end product cost, especially in view of large volume applications such as photovoltaics or lighting. In this respect, various authors have raised the issue of chalcogen precursors based on phosphines, which are toxic, expensive, and oxidation sensitive and show batch to batch variations of purity.19,29,30,32,34,51 In the case of selenium precursors, most alternatives boil down to a precursor that is based on the dissolution of elemental selenium in an organic solvent such as octadecene,29,30,32 diesel,19 or paraffin.34,51

Figure 6. (a) Variation of the absorbance and photoluminescence spectra of CdSe/CdS core/shell NCs for dispersions with a number of CdS shells as indicated. (b) Photoluminescence quantum yield of CdSe/CdS NCs as a function of the number of shells grown.

(SILAR) approach is used to grow a CdS shell from initial, 2.7 nm CdSe cores NCs.38 The spectra, each taken after the completion of a single CdS layer, show the progressive red shift with increasing shell thickness characteristic of the delocalization of the conduction-band electron in the CdS shell.49 The formation of a CdS shell is further confirmed by TEM analysis (see Supporting Information), which shows that dNC increases from the initial core size of 2.7 nm to 5.7 and 6.9 nm after the 2480

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For scaling up, the combination of a slow reaction and a limited reaction yield is problematic, in particular for the synthesis of nanocrystals with small sizes. Using a reaction as the one shown in Figure 8, these are typically obtained by stopping the reaction in an early stage, where the reaction yield is low.52 A far more interesting approach is to use a fast, high yield reaction and tune the final size of the nanocrystals by the reaction conditions.36 This is exactly what the heterogeneous ODE-Se precursor as introduced here offers. The reaction runs to completion within 5−10 min, and the nanocrystals thus reach a constant diameter in the same time span. Moreover, we find that this final nanocrystal diameter can be tuned by changing the carboxylic acid used in the reaction or by changing the precursor concentrations. Finally, especially for small sizes, an extremely narrow size dispersion is obtained. An extensive study by Bullen et al.32 showed that the reactivity of the homogeneous ODE-Se precursor strongly depends on the temperature and time span used to dissolve selenium in ODE, where lower temperatures and shorter time spans yield a more reactive precursor. This was interpreted in terms of a vulcanization process where ODE molecules are first bridged by short Se chains and later on by single Se atoms. Since we obtain similar results using either ODE or hexadecane as a solvent, it appears that the reactive selenium species formed after the injection of heterogeneous ODE-Se is rather a dissolved Se compound that precedes these vulcanization reactions. Indeed, when using a 2-fold excess of selenium relative to cadmium in the reaction, 50% of the unreacted selenium is recovered as selenium powder when stopping the reaction after 8 min. Since heating up a reaction mixture containing Se powder from room temperature to the final reaction temperature, as proposed previously by Yang et al.,30 results in reaction rates comparable to that of homogeneous ODE-Se, it appears that the injection of Se powder at elevated temperature as introduced in this work is essential to obtain this highly reactive selenium species. A complication that comes with the synthesis of metal selenide nanocrystals using alkylphosphine-selenide precursors is the need to work under protective atmosphere since phosphines are highly sensitive to oxygen. In the literature, the use of such a protective atmosphere is typically extended to syntheses based on a homogeneous ODE-Se precursor. Here, we find that working under air with a heterogeneous ODE-Se precursor yields CdSe and ZnSe nanocrystals of, at least, comparable quality. Especially in the case of CdSe, size dispersions are excellent, the surface chemistry of the

Figure 7. (a) Absorption spectra of ZnSe nanocrystal dispersions obtained for aliquots taken at the indicated time after the start of the reaction. (b) Evolution of λ1Sh−1Se as a function of time as determined from the absorption spectra. (c) XRD, indicating the experimental data and the reflections expected for bulk zinc blende ZnSe. (d) Bright field TEM image of the resulting ZnSe nanocrystals. The scale bar corresponds to 10 nm.

Either used in a hot injection or a heating up procedure, the use of selenium homogeneously dissolved in organic solvents results in a relatively slow reaction with a limited reaction yield. For comparison, Figure 8 summarizes the synthesis development of a typical CdSe NC synthesis using such a homogeneous ODE-Se precursor as originally proposed by Jasieniak et al.29 The absorption spectra (Figure 8a) already indicate that the NC growth stage is considerably longer for this reaction since the spectra keep shifting to longer wavelengths even after 50 min. This concurs with a relatively slow increase of the chemical reaction yield up to ≈35% after 50 min, as compared to the steady diameter and the ≈85% yield achieved with the heterogeneous ODE-Se precursor within 5−10 min. A similarly slow reaction is obtained by Yang et al. when heating up a mixture of (heterogeneous) selenium and cadmium myristate in ODE.30

Figure 8. Time development of the synthesis of CdSe nanocrystals by means of the homogeneous ODE-Se precursor using the conditions as described in the Experimental Section. (a) Absorption spectra obtained for aliquots taken at the indicated time after injection. The spectra are normalized at the first exciton peak absorbance. (b) Time development of (red circles) average nanocrystal diameter and (blue squares) size dispersion as obtained from the absorption spectra. (c) Time evolution of the amount of CdSe formed, relative to the amount of Se injected, as determined from UV−vis absorption spectroscopy. 2481

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CONCLUSION We have demonstrated that the injection of selenium powder dispersed in octadecene in a reaction mixture containing either cadmium or zinc carboxylates dissolved in octadecene and an excess of carboxylic acid initiates the formation of CdSe and ZnSe nanocrystals. The reaction is fast, running to completion within 5−10 min, has a high yield even for small nanocrystals, and leads to high quality nanocrystals with low size dispersions. Moreover, in the case of CdSe, the final size of the nanocrystals can be tuned by changing the reaction conditions, where shorter chain carboxylic acids yield, for example, larger nanocrystals. The reactions can be executed under air and with solid loadings as high as 50 g/L. Importantly, the absence of a protective atmosphere during synthesis does not compromise the quality of the end product. In the case of CdSe, the surface chemistry of the nanocrystals is identical to what has been reported in the literature, and they can be used as cores to grow CdSe/CdS core/shell nanocrystals with photoluminescence quantum yields exceeding 40%. We argue that the combination of a fast and tunable, high yield reaction with atmospheric conditions and high solid loadings make the approach highly suited for the low cost, high volume production of colloidal metal selenide nanocrystals. This conclusion is stressed by demonstrating the reproducible synthesis of CdSe nanocrystals using the heterogeneous precursor by an automated liquid handler.

nanocrystals appears identical to those synthesized using a homogeneous ODE-Se precursor under protective atmosphere, and they can be used as cores to grow highly luminescent CdSe/CdS core/shell structures. An important cost factor in solution-based synthesis is the volume of solvents needed to produce a given amount of material. For the syntheses described here, we translate this into the solid loading of the reaction mixture, that is, the weight of the nanocrystals versus the volume of the solvent. With the homogeneous ODE-Se precursor, the limited solubility of selenium in the organic solvent restricts the solid loading. Typical values found in literature amount to 8 g/L.23,29 In the case of the heterogeneous ODE-Se precursor, the ODE is only a carrier for the powder, and the synthesis can be performed under conditions where 5 mmol of selenium is injected using only 1 mL of ODE. With the highest precursor amounts shown in Figure 4, over 0.5 g of CdSe is obtained using 11 mL of solvent. The resulting solid loading of 50 g/L is about 10 times higher than what is typically reported for TOP-Se36,53 or ODESe29 based syntheses for zinc blende CdSe NCs. As discussed above, the use of a heterogeneous ODE-Se precursor offers several advantages in the case of CdSe and ZnSe nanocrystals in terms of scaling up. The reaction does not use phosphines, it is fast and tunable, has a high reaction yield even for small nanocrystals, and it can be run under air with little use of solvents. Although heterogeneous precursors have been used before for the synthesis of colloidal nanocrystals by hot injection, most notably for the formation of PbS,46,54 the question remains as to whether they can be used for the reproducible synthesis of large quantities of colloidal nanocrystals. To check this, we used an automated liquid handler under air to run the same CdSe synthesis based on the injection of heterogeneous ODE-Se 16 times (see Experimental Section for details). Figure 9 shows the absorption spectra of the

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ASSOCIATED CONTENT

S Supporting Information *

More details on the chemicals used, the XRF analysis, the reaction development as a function of the synthesis conditions, the nanocrystals photoluminescence, and the TEM analysis of CdSe/CdS core/shell nanocrystals. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address §

(R.G.) Inorganic Chemistry Department, Karlsruhe Institute of Technology, Germany. Author Contributions ∥

Contributed equally to this work.

Notes

The authors declare no competing financial interest.

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Figure 9. (a) Normalized UV−vis absorption spectra of 16 CdSe nanocrystal dispersions obtained by executing the standard synthesis at 250 °C using an automated liquid handler. (b) The average nanocrystal diameter as a function of experiment number as calculated from the absorption spectrum. The solid line indicates the average diameter of 2.87(4) nm.

ACKNOWLEDGMENTS Z.H. acknowledges the FWO-Vlaanderen (G.0760.12), BelSPo (IAP 7.35, photonics@be) and the UGent Special Research Fund and Industrial Research Fund (Stepstone Project Selene) for funding. S.A. acknowledges the IWT-Vlaanderen (Agency for Innovation by Science and Technology in Flanders) for a scholarship. A. Vantomme and Q. Zhao are acknowledged for Rutherford Backscattering Spectrometry measurements. J. C. Martins and A. Hassinen are acknowledged for NMR measurements.

dispersions thus obtained, together with dNC as a function of the synthesis run. Disregarding the single outlier, the NCs have a batch-to-batch average diameter of 2.87 nm with a standard deviation of 0.04 nm, that is, 1.4%. Although no attempts have been made to optimize the automated procedure, this result is in between the 2.5% and 0.25% batch-to-batch variation reported for the manual and automated synthesis of CdSe NCs using a homogeneous TOP-Se precursor, respectively.55 Hence, the automated precision handling of a heterogeneous precursor is clearly possible.

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